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TARGET ARTICLEThe “What” and “Why” of Goal Pursuits: Human Needs and the Self-Determination of Behavior Edward L. Deci and Richard M. RyanDepartment of PsychologyUniversity of RochesterSelf-determination theory(SDT)maintains that an understanding of human motiva-tion requires a consideration of innate psychological needs for competence,auton-omy,and relatedness.We discuss the SDT concept of needs as it relates to previous need theories,emphasizing that needs specify the necessary conditions for psycholog-ical growth,integrity,and well-being.This concept of needs leads to the hypotheses that different regulatory processes underlying goal pursuits are differentially associ-ated with effective functioning and well-being and also that different goal contents have different relations to the quality of behavior and mental health,specifically be-cause different regulatory processes and different goal contents are associated with differing degrees of need satisfaction.Social contexts and individual differences that support satisfaction of the basic needs facilitate natural growth processes including intrinsically motivated behavior and integration of extrinsic motivations,whereas those that forestall autonomy,competence,or relatedness are associated with poorer motivation,performance,and well-being.We also discuss the relation of the psycho-logical needs to cultural values,evolutionary processes,and other contemporary mo-tivation theories.Most contemporary theories of motivation assume that people initiate and persist at behaviors to the ex-tent that they believe the behaviors will lead to desired outcomes or goals.Beginning with the work of Lewin (1936)and Tolman(1932),this premise has led moti-vation researchers to explore the psychological value people ascribe to goals(e.g.,T.Kasser&Ryan,1996; Vroom,1964),people’s expectations about attaining goals(e.g.,Abramson,Seligman,&Teasdale,1978; Bandura,1989;Rotter,1966),and the mechanisms that keep people moving toward selected goals(e.g., Carver & Scheier, 1998).Whereas initially this approach assumed that any two equally valued goals with the same expectancies for attainment would yield the same quality of perfor-mance and affective experience,recent work on goal-directed behavior has begun to distinguish among types of goals or outcomes.Researchers have, for example,contrasted ability-development goals with ability-demonstration goals(Dweck,1986; Nicholls,1984)and approach goals with avoidance goals(Carver&Scheier,1998;Elliot&Church, 1997;Higgins,1996),suggesting that the different types of goals have different behavioral and affective consequences.Like these other theories,self-determination theory (SDT;Deci&Ryan,1980,1985b,1991)has differen-tiated the concept of goal-directed behavior,yet it has taken a very different approach.SDT differentiates the content of goals or outcomes and the regulatory pro-cesses through which the outcomes are pursued,mak-ing predictions for different contents and for different processes.Further,it uses the concept of innate psy-chological needs as the basis for integrating the differ-entiations of goal contents and regulatory processes and the predictions that resulted from those differentia-tions.Specifically,according to SDT,a critical issue in the effects of goal pursuit and attainment concerns the degree to which people are able to satisfy their basic psychological needs as they pursue and attain their val-ued outcomes.The concept of needs was once widely employed in empirical psychology to organize the study of motiva-tion.Although variously defined at the physiological or psychological levels and as innate or learned,the concept of needs specified the content of motivationPsychological Inquiry Copyright © 2000 by 2000, Vol. 11, No. 4,227–268Lawrence Erlbaum Associates, Inc.and provided a substantive basis for the energization and direction of action.Beginning around the1960s, however,the dramatic shift toward cognitive theories led to the concept of needs being repudiated and re-placed by the concept of goals as the dominant motiva-tional concept.The focus became the processes of goal selection and pursuit rather than the content of the goals being selected and pursued.The concept of va-lence(or psychological value)of outcomes was de-fined functionally(and thus was not related to need satisfaction),much as the concept of reinforcement had been defined functionally in operant psychology (B.F.Skinner,1953),ignoring the needs that had pro-vided the underpinning of reinforcements in drive the-ories (e.g., Hull, 1943).Since the time of the shift toward cognitive theories, most motivation theorists remained unwilling to con-sider needs,focusing instead on goal-related efficacy. SDT has,in contrast,maintained that a full understand-ing not only of goal-directed behavior,but also of psy-chological development and well-being,cannot be achieved without addressing the needs that give goals their psychological potence and that influence which regulatory processes direct people’s goal pursuits. Specifically,in SDT,three psychological needs—for competence,relatedness,and autonomy—are consid-ered essential for understanding the what(i.e.,content) and why(i.e.,process)of goal pursuits.Before outlin-ing the SDT perspective on the content and process of goal-directed behavior,however,we begin with an his-torical consideration of the concept of needs as a foun-dation for our subsequent discussion.The Concept of NeedsEarly Needs TheoriesTwo very different intellectual traditions in the em-pirical psychology of motivation employed the con-cept of needs.In experimental psychology,Hull (1943)suggested that the task of psychology is to un-derstand molar behavior by linking it to the organism’s primary needs and the conditions in the environment relevant to them.He specified a set of innate physio-logical needs(e.g.,for food,water,sex)that are based in non-nervous-system tissue deficits,give rise to drive states,push the organism into action,and must be satis-fied for the organism to remain healthy.The drive states,when reduced,produce learning by linking drive stimulations to the responses that led to drive re-duction(e.g.,Hull,1943;Spence,1956).Drive states and the stimulus–response associations were used to predict subsequent behavior.This tradition produced a rich array of findings based on the drive theory as-sumptions,but among its shortcomings was that it could not provide a meaningful account of a large class of behaviors such as curious exploration,investigatory manipulation,vigorous play,and other spontaneous activities that had no apparent ties to the dynamics of drive reduction.Indeed,it was partly the drive theo-rists’attempts to account for such behaviors that gave rise to the recognition of intrinsic motivation(see White,1959)and ultimately led to specification of the psychological needs.The second tradition focusing on needs stems from the work of Murray(1938).Murray addressed needs at the psychological rather than physiological level and viewed them primarily as acquired rather than innate.In this approach the concept of needs was very broadly construed,as we see here in Murray’s oft-cited definition:A need is a construct(a convenient fiction or hypothet-ical concept)that stands for a force(the physico-chem-ical nature of which is unknown)in the brain region,aforce that organizes perception,apperception,intellection,conation and action in such a way as totransform in a certain direction an existing,unsatisfy-ing situation. (pp. 123–124).Indeed,this definition is so broad that one could substi-tute terms like motive,desire,or goal for need without losing any meaning.By this definition,almost any-thing that moves one to action is a need,a fact that is highlighted by Murray’s inclusion of such psychologi-cal needs as abasement(self-degradation),acquisitive-ness(greed),and dominance within his extensive list. We maintain,however,that,although motives such as these may energize action,they are certainly not needs in either the Hullian or the SDT sense of specifying necessary nutriments for healthy functioning.Rather, Murray’s needs represent an array of salient motives whose pursuit may or may not conduce to optimal functioning:motives that reflect ambient social values and the dynamics of their transmission.In the Murray tradition the focus of empirical stud-ies has been on individual differences in need strength, particularly those for achievement,power,and affilia-tion.These individual differences are the foci of the-matic(or implicit)and questionnaire(or explicit) methods of assessment(Ryan&Manly,in press)and are used as the basis for predicting affective and behav-ioral outcomes(e.g.,McAdams,1989;McClelland, 1985).The Nature of Needs in SDTTo explicate the meaning of needs in SDT,we con-sider not only the theoretical concept but also the or-ganismic-dialectical metatheory that underlies it.In so doing,we contrast SDT with theories in the traditions of Hull and Murray.Although these theories do not228DECI & RYANhave a strong presence in current motivation research, they provide a useful contrast with SDT because,un-like most current theories,they are macrotheories of motivation that explicitly considered the concept of needs and clearly specified their metatheoretical ter in the article we examine the relation of SDT to a number of current theories.As in the Hullian tradition,we define needs as in-nate,organismic necessities rather than acquired mo-tives,and as in the Murray tradition,we define needs at the psychological rather than physiological level. Thus,in SDT,needs specify innate psychological nu-triments that are essential for ongoing psychological growth,integrity,and well-being.As noted,we identi-fied three,the needs for competence,relatedness,and autonomy.This definition can be considered in organismic and functional terms.It assumes a fundamental human tra-jectory toward vitality,integration,and health,and fur-ther assumes that this organismic tendency will be actualized so long as the necessary and appropriate nu-triments are attainable but will give way to the emer-gence of nonoptimal psychological outcomes under conditions of threat or deprivation.In other words,hu-man needs specify the necessary conditions for psy-chological health or well-being and their satisfaction is thus hypothesized to be associated with the most effec-tive functioning.A further claim is that each of these three needs plays a necessary part in optimal develop-ment so that none can be thwarted or neglected without significant negative consequences.This claim cannot be made for most psychological needs that were stud-ied,for example,in the Murray tradition,because there are countless instances in which people achieve psy-chological integrity and health without having the so-called needs for power,acquisitiveness,or self-abasement well satisfied.However,we assert that there are not instances of optimal,healthy develop-ment in which a need for autonomy,relatedness,or competence was neglected,whether or not the individ-uals consciously valued these needs.In short,psycho-logical health requires satisfaction of all three needs; one or two are not enough.Functionally,we expect to observe optimal devel-opment and well-being under facilitating conditions that support need satisfaction,and to observe degrada-tion or ill-being under conditions that thwart basic need satisfaction.Just as one can conclude that plants need water by noting that they flourish when they are hydrated but that impoverished growth and,ulti-mately,a breakdown of integrity results when they are systematically deprived of water,SDT maintains that a psychological need can be identified by observing that positive psychological consequences results from con-ditions that allow its satisfaction and negative conse-quences accrue in situations that thwart it. Accordingly,if motives or goals were not linked di-rectly to basic needs,their fulfillment versus thwarting would not be expected to result invariantly in the en-hancement versus diminishment of growth and well-being.Our definition is congruent with Hullian thought in that both approaches specify a set of innate or essential nutriments and with Murray’s personologic approach in that his and ours focus at the psychological level,but our approach is quite different from those previous tra-ditions because it is embedded in an organismic-dia-lectical metatheory.Accordingly,as we will show,the concept is used to address different issues and to pro-vide different types of interpretations.The organismic dialectic.The starting point for SDT is the postulate that humans are active, growth-oriented organisms who are naturally inclined toward integration of their psychic elements into a uni-fied sense of self and integration of themselves into larger social structures.In other words,SDT suggests that it is part of the adaptive design of the human organ-ism to engage interesting activities,to exercise capaci-ties,to pursue connectedness in social groups,and to integrate intrapsychic and interpersonal experiences into a relative unity.Our organismic-dialectical perspective further pro-poses that these natural organismic activities and the integrative propensities that coordinate them require fundamental nutriments—namely,ambient supports for experiencing competence,relatedness,and auton-omy.As such,the natural processes such as intrinsic motivation,integration of extrinsic regulations,and movement toward well-being are theorized to operate optimally only to the extent that the nutriments are im-mediately present,or,alternatively,to the extent that the individual has sufficient inner resources to find or construct the necessary nourishment.To the degree that these organismic processes are hindered by nonfavorable conditions—specifically when one’s context is excessively controlling,overchallenging,or rejecting—they will,to that degree,be supplanted by alternative,often defensive or self-protective pro-cesses,which no doubt also have functional utility un-der nonsupportive circumstances.Such processes would include,for example,the capacity to compart-mentalize rather than integrate psychological struc-tures,the tendency to withdraw concern for others and focus on oneself,or,in more extreme cases,to engage in psychological withdrawal or antisocial activity as compensatory motives for unfulfilled needs.Accordingly,innate psychological needs for com-petence,relatedness,and autonomy concern the deep structure of the human psyche,for they refer to innate and life-span tendencies toward achieving effective-ness,connectedness,and coherence.The presence ver-sus absence of environmental conditions that allow229GOAL PURSUITSsatisfaction of these basic needs—in people’s immedi-ate situations and in their developmental histories—is thus a key predictor of whether or not people will dis-play vitality and mental health.As we argue later,the existence of these basic psychological needs and their phenomenological salience appear to yield consider-able adaptive advantage at the level of individual and group selection(Ryan,Kuhl,&Deci,1997).Further-more,basic needs play an essential role in cultural transmission,helping to account for how memes are assimilated and maintained in and across diverse hu-man groups (Inghilleri, 1999).A direct corollary of the SDT perspective is that people will tend to pursue goals,domains,and rela-tionships that allow or support their need satisfaction. To the extent that they are successful in finding such opportunities,they will experience positive psycho-logical outcomes.Needs in SDT versus drive theories.We,like drive theorists,consider needs to be innate rather than learned and therefore to give motivational content to life.However,although we acknowledge physiological drives,we give primacy to the core psychological needs in our exploration of issues such as human learn-ing,interpersonal relations,and the general mastery and management of people’s physical and social envi-ronments.By positing a set of basic psychological needs,SDT specifies psychological elements of human nature,much as Hull’s work specified physiological el-ements of human nature.Further,we suggest that the drive-based behaviors that Hull(as well as Freud)de-scribed are typically regulated by psychological pro-cesses and therefore interface with the issues of auton-omy, competence, and relatedness.Our focus at the psychological level within the or-ganismic-dialectical metatheory leads to a set of very important differences between our approach and that of drive theories.From the latter perspective,needs are understood as physiological deficits that disturb the or-ganism’s quiescence and push the organism to behave in ways that were learned because they satisfied the needs and returned the organism to quiescence.Thus, in drive theories,the set point of the human organism is quiescence or passivity;need satisfaction is a process of replenishing deficiencies;and the purpose of behav-ior is need satisfaction.By contrast,in SDT,the set point is growth-oriented activity.That is,rather than viewing people as passively waiting for a disequilib-rium,we view them as naturally inclined to act on their inner and outer environments,engage activities that in-terest them,and move toward personal and interper-sonal coherence.Thus,they do not have to be pushed or prodded to act.Further,and importantly,their be-havior does not have to be aimed at need satisfaction per se,it may simply be focused on an interesting ac-tivity or an important goal if they are in a context that allows need satisfaction.If,however,need satisfaction is not forthcoming while they are acting,nonoptimal or dysfunctional consequences typically follow.Con-sider several important implications of this viewpoint.From the perspective of drive theory,all behaviors are based in drive reduction processes;in other words, the functional aim of all behavior can be understood as need satisfaction.Hungry people act to get food, pained people act to get relief,and all behavior can be traced back to disequilibria.From the perspective of SDT,however,innate life processes and their accom-panying behaviors can occur naturally,without the prod of a need deficit.Much as Piaget(1971)sug-gested that it is inherent in the assimilation schema to function,we suggest that it is inherent in people’s na-ture to act in the direction of increased psychological differentiation and integration in terms of their capaci-ties,their valuing processes,and their social connectedness.These inherent integrative tendencies require the nutriments of need satisfaction to be sus-tained and for positive consequences to follow,but need satisfaction is not necessarily the aim of these ac-tions.Thus,for example,it is adaptive for children to play,but they do not play to feel competent.Similarly, curiosity-based exploration,openness to the sensory experiences of nature,and assimilation of values ex-tant in one’s social milieu—all natural activities—re-quire the nutriments of basic need satisfaction to operate optimally,but these activities are not necessar-ily(indeed they may seldom be)consciously intended to satisfy the basic needs.Of course,we recognize that many behaviors are specifically aimed at satisfaction of the basic needs, particularly when little satisfaction has been forthcom-ing.When lonely,people may explicitly seek out com-panionship;when controlled,people may explicitly seek out autonomy;and when feeling ineffective,peo-ple may explicitly work to become more competent. But,when people are experiencing reasonable need satisfaction,they will not necessarily be behaving spe-cifically to satisfy the needs;rather,they will be doing what they find interesting or important.As we argue later,finding an activity either interesting(intrinsic motivation)or important(well-internalized extrinsic motivation)is influenced by prior experiences of need satisfaction versus thwarting,but doing what one finds interesting or important does not have the explicit in-tent of satisfying the basic needs in the immediate situ-ation.A man who,in the evening,sits at the keyboard and begins to play a piece of music,may become lost in its beauty and experience great pleasure.He would not experience the pleasure if coerced to play,or if he felt unable to master the music.Thus,need satisfaction, which in this case means experiences of autonomy and competence,is necessary for the enjoyment of the ac-tivity,but his explicit purpose in playing the music is230DECI & RYANnot likely to be need satisfaction.He would be doing what interests him,and he would experience spontane-ous pleasure as long as the activity was self-organizing and the task appropriately challenging.There is another very important way that psycho-logical needs differ from physiological needs.When a physiological need is thwarted,people typically step up their efforts to satisfy it.Indeed,the longer they are deprived,the more salient and consuming the need be-comes.When hungry enough,people are likely to think of little else and to engage in few behaviors that are not intended to satisfy the hunger.With psycholog-ical needs,lack of satisfaction may also tend to focus people’s efforts on getting the needs satisfied,but with psychological need thwarting people more readily make accommodations that lessen their direct attempts to satisfy needs.For example,thwarting of psychologi-cal needs can promote the development of defenses and need substitutes that may,over time,lead to further thwarting of need satisfaction,as,for instance,when a woman becomes self-controlling in her eating behav-iors against the backdrop of having been controlled by the contingent regard and evaluations of significant others(Strauss&Ryan,1987).Rather than staying on the natural track toward healthy development,people may instead become controlled(either complying or defying)or amotivated(either being out of control or acting helpless).And these responses can,as we will see later,become self-perpetuating.According to SDT,however,such defensive adaptations,regardless of whether individuals claim to value them,will have significant negative consequences for the individuals’vitality, integrity, and health.Needs in SDT versus personality theories.By defining needs at the psychological level we suggested a commonality between SDT and some personality the-ories that use the concept of needs.However,because empirically based personality theories that investigate needs(e.g.,McClelland,1985;Murray,1938)tend to view them as learned,our conception of needs is,in some ways,closer to that of the less empirically derived theories that view psychological needs as innate(e.g., Kohut, 1977; Maslow, 1943).The most direct predecessor of our approach to psy-chological needs is the work of White(1959)who as-serted that an understanding of behavior and development requires that drive motivation be supple-mented with a different type of innate motivation,one conceptualized at the psychological level.White spoke of a primary propensity for competence,suggesting that there is an energy source in humans(and other mammals)that operates between episodes of homeostatic crisis and does not follow deficit princi-ples.For White,this energy source was a direct mani-festation of a deeply structured effectance-focused motivation—a propensity to have an effect on the envi-ronment as well as to attain valued outcomes within it. Although White used the term motive to describe this motivational propensity,his formulation was fully consistent with our definition of a psychological need. Indeed,as noted,we consider competence or effectance to be one of the three fundamental psycho-logical needs that can energize human activity and must be satisfied for long-term psychological health.As also noted,we further proposed the innate needs for relatedness and autonomy.Relatedness refers to the desire to feel connected to others—to love and care, and to be loved and cared for(Baumeister&Leary, 1995;Bowlby,1958;Harlow,1958;Ryan,1993).Like us,Baumeister and Leary argued that relatedness is a fundamental need,and the idea of relatedness as a need is central to,although not widely discussed in the field of attachment(Ainsworth,Blehar,Waters,&Wall, 1978).Indeed,many empirically based theories as-sume a desire or tendency for relatedness even if they do not explicitly formulate it as a need.Autonomy refers to volition—the organismic desire to self-organize experience and behavior and to have activity be concordant with one’s integrated sense of self(Angyal,1965;deCharms,1968;Deci,1980;Ryan &Connell,1989;Sheldon&Elliot,1999).The con-cept of autonomy is far less prevalent in empirical psy-chology than are the ideas of competence and relatedness.And indeed,when it is discussed it is often incorrectly equated with the ideas of internal locus of control,independence,or individualism(see, e.g., Deci,Koestner,&Ryan,1999b;Ryan,1995).For us, however,autonomy concerns the experience of inte-gration and freedom,and it is an essential aspect of healthy human functioning.According to SDT,these three needs can be satis-fied while engaging in a wide variety of behaviors that may differ among individuals and be differentially manifest in different cultures,but in any case their sat-isfaction is essential for the healthy development and well-being of all individuals regardless of culture.Defining psychological needs as inherent to human nature has led to a research focus that is very different from that of other empirical personality theorists such as McClelland(1965)who maintained that needs are largely learned and thus differ in strength as a function of that learning.Specifically,McClelland(1985)and others assessed individual differences in need strength and used that as the primary basis for predicting behav-ior.Research in that tradition predicted variation in need strengths from the social conditions theorized to created them,and then,even more importantly,used need strengths to predict various outcomes.Re-searchers,for example,examined the consequences of different levels of achievement motivation(Atkinson, 1958)and power motivation(Winter,1973),and the outcomes that result form different combinations of231GOAL PURSUITSneed strengths.In so doing they have not made any im-plicit assumption that need satisfaction would be asso-ciated with healthier functioning.We,on the other hand,viewing needs as universal, innate,and essential for well-being,do not generally focus on variation in need strength.Instead,our re-search has focused primarily on an examination of the degree to which individuals experience basic psycho-logical need satisfaction in different social contexts and of the consequences of various degrees of satisfac-tion.We do assume that there are important individual differences that affect the degree to which people will experience need satisfaction in different contexts,so we use both characteristics of the social environment and individual differences to predict people’s need sat-isfaction and,in turn,the quality of their experience, behavior,and health.However,these individual differ-ences do not concern need strength.Rather,the type of individual-difference concepts used in SDT and other theories that assume innate,psychological needs are regulatory or interactive styles.These are regarded as outcomes of the ongoing dialectic between people’s needs and their ambient social contexts that have either fulfilled or frustrated the needs,and they describe the way people orient toward the social environment and thus affect its potential for providing them further need satisfaction.In SDT,we refer to these as causality ori-entations(Deci&Ryan,1985a)at the broadest level of generality,and as regulatory styles(Ryan&Connell, 1989)at a more domain-specific level of generality (see also Vallerand, 1997).In selecting this focus for examining individual dif-ferences,we do not maintain that there are no differ-ences in need strength.Rather,we suggest that a focus on the strength of innate needs does not get at the issues we consider most important.In this regard,there is an-other similarity between our approach and that of the physiological-need theories.Just as it is probable that people have innate differences in the strength of their need for food,it is as well probable that there are innate differences in their needs for competence.Human characteristics tend to be normally distributed.None-theless,psychologists do not typically focus on innate individual differences in hunger,instead treating such differences as givens and focusing instead on the ef-fects of food deprivation versus availability on con-sumptive patterns.From that perspective,the critical issue is not to identify innate differences in the strength of hunger,but rather to see how hunger has been af-fected by the interaction of the basic need for food and the environment in which it is or is not supported.Similarly,although there may be individual differ-ences in the strength of people’s needs for competence, autonomy,and relatedness,we believe that these in-nate differences are not the most fruitful place to focus attention.Instead,greater benefits will be reaped from focusing on individual differences in motivational ori-entations and in the importance of goal contents,these being individual differences that result from the inter-action of the basic needs with the social world—that is, from past experiences of need satisfaction versus thwarting.As with the case of an unusually strong de-sire for food,we would consider an unusually strong desire to be with other people not to be a reflection of a strong innate need for relatedness but instead to be a re-sult,in part at least,of previous experiences in which the basic needs were thwarted.Similarly,an unusually strong desire to be in control of a situation would be viewed as resulting not from a strong need for compe-tence or autonomy but rather from experiences of those needs being thwarted.Like an unusually strong desire for food,an unusually strong desire to be in control is likely to be compensatory.Herein,lies one of the most important implications of proposing innate needs.They are the basis for—in-deed,they require—dynamic theorizing that links var-ied phenotypic desires and goals to underlying needs that the person may not even be directly aware of at the time.Without the concept of innate needs,all desires are equal in functional importance if they are equal in strength.In other words,every set of closely related behaviors would have its own need(e.g.,achievement behaviors would imply a need for achievement),and there would be no basis for predicting the qualities of performance or the degree of well-being that would be associated with different ones of these so-called needs. The concept of basic needs,in contrast,implies that some desires are linked to or catalyzed by our psycho-logical design,as it were,whereas others are not.These others,often being derivative or compensatory,can be the by-products of past need thwarting,and,as defen-sive adaptations,they may even form the basis for fu-ture need thwarting.Needs, Goals, and RegulatoryProcessesThe specification by SDT of the three fundamental needs for competence,relatedness,and autonomy was not simply an assumptive or a priori process but in-stead emerged from inductive and deductive empirical processes.We found that without the concept of needs we were unable to provide a psychologically meaning-ful interpretation and integration of a diverse set of re-search results in the areas of intrinsic motivation, which we consider to be a basic,lifelong psychological growth function(Deci&Ryan,1980),and internaliza-tion,which we consider to be an essential aspect of psychological integrity and social cohesion(Ryan, Connell,&Deci,1985).We now review the research on intrinsic motivation that led to the postulate of psy-chological needs,and then we move on to review the research on internalization,discussing its relevance to232DECI & RYAN。

The effect of mandatory audit firm rotation on audit quality and audit fees- Empirical

Auditing:A Journal of Practice&Theory American Accounting Association Vol.33,No.4DOI:10.2308/ajpt-50814 November2014pp.167–195The Effect of Mandatory Audit Firm Rotation on Audit Quality and Audit Fees:Empirical Evidence from the Korean Audit Market Soo Young Kwon,Youngdeok Lim,and Roger Simnett SUMMARY:Using a unique setting in which mandatory audit firm rotation wasrequired from2006–2010,and in which both audit fees and audit hours were disclosed(South Korea),this study provides empirical evidence of the economic impact of thispolicy initiative on audit quality,and the associated implications for audit fees.Thisstudy compares both pre-and post-policy implementation and,after the implementa-tion of the policy,mandatory long-tenure versus voluntary short-tenure rotationsituations.Where audit firms were mandatorily rotated post-policy,we observe thataudit quality(measured as abnormal discretionary accruals)did not significantlychange compared with pre-2006long-tenure audit situations and voluntary post-rotation situations.Audit fees in the post-regulation period for mandatorily rotatedengagements are significantly larger than in the pre-regulation period,but arediscounted compared to audit fees for post-regulation continuing engagements.Wealso find that the observed increase in audit fees and audit hours in the post-regulationperiod extends beyond situations where the audit firm was mandatorily rotated,suggesting that the introduction of mandatory audit firm rotation had a much broaderimpact than the specific instances of mandatory rotation.Keywords:auditﬁrm rotation;audit fees;audit quality;audit hours.Data Availability:Most of theﬁnancial data used in the present study are available fromthe KIS Value Database.The data for audit hours and fees weredrawn from statements of operating resultsﬁled with the FinancialSupervisory Services(FSS)in Korea.Soo Young Kwon is a Professor at Korea University Business School,and Youngdeok Lim is a Senior Lecturer and Roger Simnett is a Professor,both at UNSW Australia.We are grateful for insightful comments from Michael Ettredge,Brian T.Carver,Anna Huggins,and participants at the 2011American Accounting Association Annual Meeting,as well as at seminars at the University of New South Wales and the8th Annual ANCAAR Audit Research Forum.Soo Young Kwon is the corresponding author (sykwon@korea.ac.kr).Editor’s note:Accepted by Marleen Willekens.Submitted:April2012Accepted:May2014Published Online:May2014167INTRODUCTIONWhether audit ﬁrm rotation should be mandatory is an issue that has been debated foralmost ﬁve decades.Proponents of mandatory audit ﬁrm rotation argue that auditorindependence may be enhanced by increased professional skepticism that comes withfresh eyes.By contrast,opponents of this policy argue that incoming auditors may lack industry expertise and detailed knowledge of the client’s particular situation,which may result in higher fees for initial engagements and a greater incidence of problem audits in the early years of a new engagement.To the extent that these increased costs are passed on to clients,increased audit fees will be observed across the relationship due to a limited ability to amortize these familiarization costs over an extended period (J.Myers,L.Myers,and Omer 2003;Carey and Simnett 2006).This study provides empirical evidence on this debate,utilizing the unique setting of Korea where this policy ﬁrst took effect in 2006and where both audit fee and audit hours information is available.A Public Company Accounting Oversight Board concept paper (PCAOB 2011)and European Commission Green Paper (EC 2010)both reinvigorated discussions about the desirability of mandatory audit ﬁrm rotation policies.Like most regulators/standard-setters,both the PCAOB and the EC require an analysis of the economic impact of any proposed policy,and our research has the ability to provide empirical evidence of the beneﬁts and costs associated with the introduction of the mandatory audit ﬁrm rotation policy,constituting a timely contribution.The introduction of such a rotation policy continues to be contentious,as evidenced by the U.S.House of Representatives legislation introduced in July 2013that prevents the PCAOB from implementing a system of mandatory rotation for audit ﬁrms.This can be compared with the European Union (EU 2013)agreement in December 2013,which contains requirements for the mandatory rotation of auditors after ten years for public interest entities (PIEs).Member states may allow the auditor to continue to audit the same PIEs up to a maximum duration of 20years where a public tendering is conducted and up to 24years in the case of a joint audit.It is possible to provide direct empirical evidence on the economic impact of an audit ﬁrm rotation policy in instances where countries have introduced such a rotation policy.In an attempt to provide the most appropriate empirical evidence regarding the potential impact of the introduction of this policy initiative,we examine the recent South Korean initiative of mandatory audit ﬁrm rotation.The background to the South Korean initiative is that,in the wake of the 1997Asian ﬁnancial crisis,and further stimulated by the Sarbanes-Oxley Act debate,in 2003South Korea’s regulators,the Financial Supervisory Services (FSS),proposed an accounting reform bill that required audit ﬁrm rotation.This bill was implemented and,starting in 2006,listed public entities were required to rotate audit ﬁrms after six consecutive years of audit engagement.Signiﬁcantly,in terms of allowing us to assess the economic impact of this policy,the Korean regulator requires disclosures of both audit fees and audit hours.This context provides an appropriate setting for providing empirical evidence of the potential beneﬁts,improvements in audit quality associated with implementing this policy,along with the potential costs in terms of increased audit effort and fees.Using a unique database that includes South Korean public companies both before (2000–2005)and after (2006–2009)the introduction of the mandatory rotation policy,this study examines the effect of mandatory audit ﬁrm rotation on audit quality (measured by discretionary accruals in the ﬁrst instance)and audit fees.After controlling for audit hours,we ﬁnd little impact on audit quality after the introduction of mandatory audit ﬁrm rotation in 2006,either in the ﬁrst year of an engagement with a new auditor or in subsequent years.This is in comparison to voluntary rotations pre-2006,as well as voluntary (below ﬁrm tenure limit)rotations post-2006.However,in our examination of audit quality,audit hours is signiﬁcantly negative,showing that168Kwon,Lim,andSimnettAuditing:A Journal of Practice &TheoryNovember 2014more time spent on the audit is associated with decreased accruals and therefore increased audit quality.With regard to fees,audit fees in the post-regulation period forﬁrms’mandatorily auditor-rotated engagements increase signiﬁcantly compared with audit fees in the pre-regulation period.We alsoﬁnd that the observed increase in audit fees in the post-regulation period extends to all situations,irrespective of whether mandatory rotation of the auditﬁrm occurred or not, suggesting that audit fees increased for all types of engagements after the introduction of the mandatory rotation requirement.This observed increase is after controlling for increased audit hours,with the increased audit effort signiﬁcantly positively associated with audit fees.Our study contributes to the literature by empirically examining the impact of the introduction of a mandatory auditﬁrm rotation requirement on audit quality and audit fees.Previous studies (e.g.,Davis,Soo,and Trompeter2009;Myers et al.2003)have examined either the effects of auditor tenure on earnings quality or the characteristics ofﬁrms changing auditors under a voluntary rotation system,but not a mandatory system.There are also a number of studies that have attempted to infer the possible impact of a mandatory rotation policy by examining forced auditor change in other settings(e.g.,Nagy2005;Blouin,Grein,and Rountree2007;Kim and Yi2009;Chen,Su, and Wu2009).In contrast to these prior studies,this is theﬁrst study that examines the direct effects associated with a mandatory auditﬁrm rotation requirement.The remainder of this paper proceeds as follows.The following section outlines theoretical considerations and the central research question.The third section describes the research design, and the fourth reports the empirical results.Theﬁfth section provides additional analyses,and the ﬁnal concludes with a summary.THEORETICAL BACKGROUND AND RESEARCH QUESTIONSA Refocus on Mandatory Audit Firm Rotation PolicyThe Enron collapse in late2001refocused attention on the audit profession’s effectiveness in protecting the public interest.Subsequently,the2002Sarbanes-Oxley Act(SOX)required the United States General Accounting Ofﬁce(GAO)to study the potential effects of mandatory rotation of auditﬁrms registered under the Act.The GAO’s2003study concluded that mandatory auditﬁrm rotation might not be the most efﬁcient way to strengthen auditor independence.Some legislatures thus settled on rotating lead partners.The GAO,however,left the issue of revisiting the mandatory auditﬁrm rotation requirement open if the other requirements of the Sarbanes-Oxley Act did not lead to improved audit quality.In2011,Public Company Accounting Oversight Board(PCAOB) Chairman James Doty rekindled the debate by suggesting that mandatory auditﬁrm rotation could be part of overhauling the auditing profession(Chasan2011).1The PCAOB(2011)released a concept paper soliciting public comment on mandatory auditﬁrm rotation and convened three public roundtable meetings in2012to hear views about the implications of introducing such a policy.However,in July2013,the U.S.House of Representatives approved legislation that prohibits the PCAOB from requiring mandatory auditﬁrm rotation(Chasan2013).The American Institute of Certiﬁed Public Accountant’s President and CEO,Barry Melancon,stated that‘‘in the absence of evidence that mandatory auditﬁrm rotation would enhance audit quality,the House has sent regulators in the United States and Europe a clear message that the time has come to end the debate over rotation’’(AICPA2013).1PCAOB Chairman James Doty said the accounting industry watchdog would consider instituting a mandatory audit rotation requirement for panies.The idea was to force companies to switch external auditors every few years so that the auditor-client relationship does not get too comfortable.Such a comfortable relationship,according to Doty,threatens to undermine audit quality.The Effect of Mandatory Audit Firm Rotation on Audit Quality and Audit Fees169Auditing:A Journal of Practice&Theory November2014The aftermath of the ﬁnancial crisis in 2008also triggered the European Commission (EC)to consider the policy of mandatory audit ﬁrm rotation in its Green Paper (EC 2010)as a way to enhance auditor independence and a catalyst to introduce more dynamism into the audit market.The Green Paper called for more research to further inform this potential policy initiative.The EC is proposing major reforms to the European audit profession in response to perceived issues it identiﬁed during the global ﬁnancial crisis (EC 2011).In this proposal,they state,‘‘With a view to addressing the threat of familiarity that results from the audited undertaking often appointing and re-appointing the same audit ﬁrm for decades,the regulation introduces mandatory rotation of audit ﬁrms after a maximum period of six years that may be,under certain exceptional circumstances,extended to 8years ’’(EC 2011,section 3.3.3).In the U.K.,the House of Lords Select Committee on Economic Affairs (2011)suggested that regulators need to achieve greater rotation of audit ﬁrms of FTSE 350companies in order to improve audit quality.However,in its ﬁnal summary report on this issue,the United Kingdom Competition Commission (2013)inquiry into the competitiveness of its statutory audit services market moved away from plans to mandate audit ﬁrm rotation for listed entities on the basis of insufﬁcient empirical evidence.It has proposed instead a requirement that audit committees of U.K.listed companies must place their audits up for tender after ten years.As mentioned earlier,the approaches in the U.S.and U.K.differ from the European Union (EC 2013)agreement in December 2013,which contains requirements for the mandatory rotation of auditors after ten years for PIEs.Our study contributes empirical evidence on the logic behind these developments by examining the impact on audit quality and audit costs associated with the introduction of an audit ﬁrm rotation policy.International Instances of Audit Firm Rotation and the Korean Audit MarketIn determining whether we can learn about the impact of this policy from the experiences gained in other countries,we need to ﬁrst identify those countries that have implemented this policy,and the availability of data.The countries with an audit ﬁrm rotation system currently in place are Italy,Brazil,and Singapore.Italy has a statutory requirement for audit ﬁrm rotation every nine years.In Brazil,companies are required to change audit ﬁrms every three years.In Singapore,banks are required to change audit ﬁrms every ﬁve years,but there is no such requirement for other listed companies.Spain also introduced mandatory rotation in 1988after a maximum audit ﬁrm tenure of nine years,but abolished the requirement in 1995.South Korea also implemented this policy from 2006–2010,and there are appropriate and unique data available in the form of audit fees and audit hours spent in this setting.It is important to understand the context in which this reform was implemented to gain a better understanding and assess the generalizability of any lessons learned from the Korean experience to other national settings.From the early 1970s,the Korean economy experienced signiﬁcant growth,which along with a rapid development in capital markets and an opening up to international markets,increased the demand for credible ﬁnancial reporting and external auditing.These inﬂuences prompted the regulatory authority in Korea to introduce the Act on External Audit (AEA),implemented in 1980.The AEA led to many changes in the accounting and auditing professions in Korea.On the demand side,the AEA increased signiﬁcantly the number of ﬁrms that were subject to external audits by requiring the ﬁnancial statements of a ﬁrm whose total assets exceed a regulatory limit (currently 100billion Korean won or about US$8.5million)to be audited by an independent auditor.On the supply side,the AEA relaxed restrictive licensing procedures for certiﬁed public accountants (CPAs),resulting in an increase in the number of CPAs.The Korean Institute of Certiﬁed Public Accountants (KICPA)was established in 1954to improve CPA skills and monitor the professional170Kwon,Lim,andSimnettAuditing:A Journal of Practice &TheoryNovember 2014conduct of its members.One hundred and thirteen auditﬁrms were practicing as of2011and many of them have a memberﬁrm relationship with an international accountingﬁrm,including the Big4 auditﬁrms who have maintained between50–60percent of the listed company market share for the last ten years.The focus of the current research is the South Korean mandatory auditﬁrm rotation policy.In 2002,the South Korean government formed a task force composed of experts from both the public and private sectors.The group was mandated with formulating robust reform proposals to further strengthen Korea’s corporate regulatory standards.In April2003,South Korean regulators proposed a reform bill that would require listed companies to rotate auditﬁrms periodically.The bill passed the National Assembly,and consequently the mandatory rotation rule,which took effect in2006,required auditﬁrm rotation after six consecutive years of audit engagement.2This law was intended to prevent auditors from compromising their duty or independence because ofﬁnancial interests or a long-term relationship with the same client.Following signiﬁcant discussion and anecdotal comments about its cost and effectiveness,the mandatory auditﬁrm rotation policy was abolished in2010.This paper provides an empirical analysis of the issues of cost and effectiveness that underpinned this decision,and informs other policy makers who are considering introducing such a policy.Literature ReviewThere are two major strands of research that can inform the discussion with regard to mandatory auditﬁrm rotation.In theﬁrst strand,most of the major published research has attempted to infer conclusions about mandatory auditﬁrm rotation policies by looking at the audit quality associated with long-tenure versus short-tenure auditor-client relationships in settings where mandatory auditﬁrm rotation is not required,on the basis that a rotation policy would not allow instances of long-tenure relationships.Some studies have reported results consistent with the rationale for the introduction of this policy,noting that audit quality deteriorates as the length of audit tenure increases(Deis and Giroux1992;Bazerman,Loewenstein,and Moore2002;Davis et al.2009).Other studies have,by contrast,provided conﬂicting results(Geiger and Raghunandan 2002;Johnson,Khurana,and Reynolds2002;Carcello and Nagy2004;J.Myers,L.Myers, Palmrose,and Scholz2004;Davis et al.2009),or have found results against the rationale of the policy(Myers et al.2003;Ghosh and Moon2005).Clearly,prior research on the effect of audit ﬁrm tenure on audit quality has been mixed.Results obtained from these other settings,however,may not easily extend to a mandatory auditﬁrm change requirement and thus may not accurately inform the policy debate.For example, in the voluntary settings used in these studies,a company is free to either choose a different audit ﬁrm or remain with its current auditor.There is,therefore,the possible concern of endogeneity in extending theﬁndings from voluntary settings as,for example,troubledﬁrms may change auditors more often than do sound ones.Furthermore,incentives may exist for managers to switch to lower quality auditors the moment a problem appears,rather than identifying and disclosing the problem. Lower quality auditors may not identify the problem or,if they do identify the problem, management may dissuade the auditors from disclosing it(DeAngelo1981b).Therefore the association between auditor tenure and audit quality examined in these studies has a potential self-selection bias(i.e.,clients with long tenure tend to be better performers with fewer incentives to 2Under this rule,aﬁrm could keep the same auditor beyond six consecutive years under certain exemption conditions.Mandatory auditﬁrm rotation was not required if:(1)as a foreign controlledﬁrm,it was necessary to retain the same auditﬁrm as the overseas parent company;or(2)aﬁrm was listed on a foreign stock exchange.We found eight of these observations in our sample.We conduct a sensitivity analysis excluding these eight observations from ourﬁnal sample andﬁnd that our main results hold.The Effect of Mandatory Audit Firm Rotation on Audit Quality and Audit Fees171Auditing:A Journal of Practice&Theory November2014manage earnings).Any generalization of such ﬁndings,therefore,to a regime with mandatory audit rotation should be treated with caution.The second relevant stream of research studied the effect of forced auditor change in other settings (i.e.,mandatory rotation per se rather than a mandatory rotation requirement ).These other settings include the failure of Arthur Andersen (AA)in the U.S.(Nagy 2005;Blouin et al.2007),the failure of eight Chinese audit ﬁrms in 2001(Chen et al.2009),and the auditor designation rule in Korea (Kim and Yi 2009).Signiﬁcantly,however,none of these studies examine data from a context in which a general mandatory audit ﬁrm rotation policy was being implemented,which underscores an important incremental contribution of the current study.Perhaps the study that is most similar to the present study is Kim and Yi’s (2009)examination of the impact of the ‘‘auditor designation ’’rule in South Korea from 1991–2000.The auditor designation rule was a forerunner to the mandatory rotation requirement whereby ﬁrms that were deemed by the relevant regulatory authority to be ‘‘problematic ’’in the sense that they had ‘‘strong incentives and/or great potential for opportunistic earnings management ’’were required to have designated auditors replace the extant auditors and be retained for a speciﬁed period (Kim and Yi 2009).The authors found that ﬁrms with designated auditors from 1991–2000had signiﬁcantly lower levels of discretionary accruals than ﬁrms with a free selection of auditors,and compared to ﬁrms with voluntary auditor changes.However,the ﬁndings from this study may not translate to a broader mandatory audit ﬁrm rotation since the sample is restricted to ‘‘problematic ’’ﬁrms,highlighting the distinction between a general mandatory rotation policy and speciﬁc mandatory rotation policy situations.Other prior studies have examined the effect of audit partner rotation on audit quality (Carey and Simnett 2006;Chen,C.Lin,and Y.Lin 2008;Chi,Huang,Liao,and Xie 2009).3However,audit partner rotation differs considerably from audit ﬁrm rotation;although the former increases the risk of audit failures during a partner’s initial years on an engagement and brings fresh eyes to an engagement (thereby increasing audit quality),the extent of the fresh view and the increased costs incurred are likely to be less than that of the latter because of the potential knowledge transfer and staff sharing within the audit ﬁrm.Thus,it is not clear whether the results from these studies can be extended to a mandatory audit ﬁrm rotation setting.Furthermore,E.Bamber and L.Bamber (2009)suggested that,compared with audit ﬁrm rotation,audit partner rotation is likely to yield second-order effects.To date,two studies have examined the effect of audit ﬁrm rotation under a mandatory rotation ing the number of suspended partners imposed by the Italian National Commission as a proxy for audit quality,Cameran,Livatino,Pecchiari,and Vigano `(2007)concluded that mandatory auditor rotation was detrimental to audit quality because it increased start-up costs and disrupted the appointment phase.Ruiz-Barbadillo,Gomez-Aguilar,and Carrera (2009)examined the impact of mandatory audit ﬁrm rotation on auditor behavior in the Spanish context.They used the likelihood of issuing going-concern opinions as a proxy for audit quality and focused on ﬁnancially distressed ﬁrms from 1991–2000.They found no evidence that the mandatory audit ﬁrm rotation policy (which was in effect for part of this period,1991–1995)had a positive impact on audit quality.The results of these two studies should be interpreted with caution,because both partner suspensions and going-concern opinions are rare and occur only in speciﬁc circumstances.They therefore may not provide the 3Carey and Simnett (2006)found that audit quality,proxied by the propensity to issue going-concern opinions and the incidence of just beating (missing)earnings benchmarks,decreased under long partner ing audit data from Taiwan,Chi et al.(2009)found no support for the claim that mandatory audit partner rotation enhances audit quality,whereas Chen et al.(2008)found that audit quality increased with partner tenure.172Kwon,Lim,andSimnett Auditing:A Journal of Practice &TheoryNovember 2014complete story about the cost and effectiveness of a general mandatory auditﬁrm rotation requirement.Research QuestionsDeAngelo(1981a)deﬁned audit quality as the joint probability of detecting and reporting material misstatements,suggesting that auditor independence and auditor competence are important audit quality components.Even though audit quality is a complex concept and cannot be reduced to a simple deﬁnition(Francis2011),we can identify the expected impact of a mandatory auditﬁrm rotation policy on audit quality by examining its likelihood of enhancing auditor independence and/or impairing auditor competence.The most widely used arguments in favor of auditﬁrm rotation,some of which were foreshadowed in the introduction to this study, are as follows.First,if auditﬁrms continue to audit a particular entity for a long period,they risk developing a close relationship with the client and compromising independence.Second, periodically engaging a new auditor brings a fresh look to the company’sﬁnancial reporting, helping the auditor deal appropriately withﬁnancial reporting issues(Carey and Simnett2006; EC2010).In relation to audit quality,the main argument against mandatory auditﬁrm rotation is that in the initial years of an auditﬁrm’s tenure,new auditors may miss problems because they lack adequate experience with the client to notice either unusual events or important changes in the client’s environment.Because of the lack of client familiarity,the incoming auditor may increasingly rely on the client’s estimates and representations in the initial years of the engagement.Also,the beneﬁts associated with engaging industry specialist auditors may be lost,as auditﬁrm rotation will mean that the audit client will,after a time,have to rotate away from the auditﬁrm they deem to be the most appropriate specialist for their business.4Client-speciﬁc knowledge of items including operations,accounting systems,and internal control structure is crucial for auditors to detect material errors and misstatements, indicating that mandatory auditﬁrm rotation could harm auditor competence.Thus,it is of interest to empirically test the overall effect of mandatory auditﬁrm rotation on audit quality. In this analysis,we include audit hours as a control variable to identify the extent to which auditors expend additional effort to reduce any adverse effects of lack of client familiarity on audit quality,and to identify the effects of the rotation policy beyond additional hours on audit quality.Consequently,we extend prior studies by examining ourﬁrst research question, RQ1:RQ1:What is the impact of introducing a mandatory auditﬁrm rotation policy on audit quality?As well as questions regarding the impact of the introduction of mandatory auditﬁrm rotation on audit quality,there are questions as to its impact on audit fees.It is anticipated that in the initial years of an auditﬁrm engagement with a new client,the auditﬁrm will have to work harder to build up client familiarization to a satisfactory level to achieve the appropriate level of audit quality.This will involve greater auditor exertion(audit hours),and greater audit costs.4For companies limited to using one of the Big Nﬁrms,the selection may be further limited by an auditﬁrm providing certain non-audit services or serving as a company’s internal auditor because independence rules prohibit that audit ﬁrm from also serving as its auditor of record.In some cases,a company may also be limited in its choice ofﬁrms if an auditﬁrm audits one of the company’s major competitors and the public company decides not to use thatﬁrm as its auditor of record(GAO2003).The Effect of Mandatory Audit Firm Rotation on Audit Quality and Audit Fees173Auditing:A Journal of Practice&Theory November2014However,competition among public accounting ﬁrms to provide audit services may impact the audit fees that the client has to pay.Audit ﬁrms may offer audit fee discounts on initial audit engagements as a tactic to gain access to a continuing audit revenue stream or fees for providing other services (commonly referred to as low-balling).5DeAngelo (1981b)suggested that a learning curve in auditing can lead to auditors pricing below cost when bidding to perform a new engagement.Simon and Francis (1988)suggested that price-cutting exists in early periods and that fee discounting occurs when clients incur considerable incremental costs when changing auditors.Deis and Giroux (1996)provided empirical evidence that initial audits in particular are associated with lower audit fees.The incidence of offering audit fee discounts is more likely to occur when the client has the opportunity to remain with its incumbent audit ﬁrm for a longer period (that is,the audit ﬁrm has a longer period over which it can recoup any initial fee discounts).Petty and Cuganesan (1996)argued that under a mandatory audit ﬁrm rotation policy,audit fees are likely to increase because auditors would have a shorter time to absorb the familiarization costs associated with the ﬁrst year of auditing.For engagements under a mandatory audit ﬁrm rotation regime,the period over which economic beneﬁts can be realized is truncated due to an inability to extend the relationship beyond a deﬁned period.Arru˜n ada and Paz-Ares (1997),through their detailed review and mathematical modeling of audit costs,argued that mandatory rotation could cause decreased competition and a subsequent increase in cost to the client as there would be a reduction in the incentives for audit ﬁrms to be efﬁcient.However,mandatory audit ﬁrm rotation may intensify price competition due to increased dynamics in the audit market.Due to audit fee pressure in an environment where every audit ﬁrm must continuously compete to ﬁnd new companies to audit,audit fees may decrease in the short term.In the case of auditing,which is generally considered a public interest activity,such competition may be considered as inappropriate.The 2003GAO study suggested that if intense price competition occurs,the expected beneﬁts of mandatory audit ﬁrm rotation can be undermined if audit quality suffers as a result of audit fees that do not support an appropriate level of audit work.If this were the case,audit fees under the mandatory audit ﬁrm rotation regime would be less than those under the voluntary auditor change regime.In this analysis we include audit hours as a control variable to identify the relationship between audit effort and audit fees,and to identify the effects of the rotation policy beyond additional hours on audit fees.Thus,our second research question will examine the impact of mandatory audit ﬁrm rotation on audit fees:RQ2:What is the impact of introducing a mandatory audit ﬁrm rotation policy on audit fees?The two research questions considered together inform the beneﬁt-cost analysis that is undertaken by regulators/standard-setters when making policy decisions.As we have discussed,and as depicted in Figure 1,there is tension both within and between these two research questions.As stated above,if we take the central aim of mandatory audit ﬁrm rotation as improving audit quality,there is tension between the potential positive impacts associated with the fresh eyes of a new auditor,and the potential negative impacts arising from the loss of auditor familiarity with the client and the client’s industry.The potential costs associated with decreased auditor familiarity can potentially be overcome by increased audit effort.As previously discussed,greater audit effort will usually result in clients being charged higher audit fees,except where audit fee discounting occurs.Thus,to examine RQ2,audit fee data are 5These practices may also have a detrimental effect on audit quality by resulting in insufﬁcient audit work being completed,with auditors working within the constraints of lower budgets (lower audit quality).174Kwon,Lim,andSimnettAuditing:A Journal of Practice &TheoryNovember 2014。

Aou-2007-Roles of Conformatio

Roles of Conformational and Configurational Defects on the Physical Aging of Amorphous Poly(lactic acid)Kaoru Aou†and Shaw Ling Hsu*Department of Polymer Science and Engineering and Materials Research Science and Engineering Center,Uni V ersity of Massachusetts,Amherst,Massachusetts01003Lothar W.Kleiner and Fuh-Wei TangAbbott Vascular,Cardiac Therapies,3200Lakeside Dri V e,Santa Clara,California95054Recei V ed:June11,2007;In Final Form:August8,2007The roles of poly(lactic acid)chain conformation and configuration on the enthalpy relaxation kinetics ofamorphous poly(lactic acid)were examined.Enthalpic relaxation data,which were scaled to the samesupercooling from the initial fictive temperature,were taken for three types of the polymer containing variousD-lactyl monomers(5.7%,13.0%,50%)to assess the effects of configurational defects.The kinetics datawere very similar from sample to sample.The effects of configurational defects were assessed using thegeneralized Kohlrausch-Williams-Watts(KWW)equation solved by the Tool-Narayanaswamy-Moynihan(TNM)equation.The major effect of increasing the D-lactyl contents was to lower the PLA glass transitiontemperature,thereby accelerating the kinetics of enthalpic relaxation.Configurational defects showed nosignificant effect on the other KWW/TNM fit parameters(x,∆h,ln A).A slightly larger KWW stretchedexponential parameter is observed for greater(50%D)than for lower(5.7%D)amount of D-lactyl monomer,although these differences are just within the experimental error.Raman spectroscopy showed that conformationdoes not change appreciably during physical aging.IntroductionThe changes in mechanical and transport properties,which accompany physical aging,1,2are critically relevant to both industrial and medical applications of poly(lactic acid),or PLA. This polymer,derived from renewable resources,3,4was recently introduced in commercial applications as a viable commodity-scale thermoplastic.5-7PLA has also been studied as a drug-carrier matrix in drug delivery systems as well as the bulk component of other medical devices as it has been established to be biocompatible and bioresorbable.8-11It is important to understand the kinetics of PLA physical aging and its impact on the performance of PLA-based drug delivery systems,as it impacts device shelf life,which needs to be evaluated for any pharmaceutical device.Physical aging is the densification,or isotropic volumetric shrinkage,of amorphous materials below their glass transition temperature.This is in contrast to anisotropic shrinkage,in particular fiber shrinkage,which we previously studied using a spectroscopic method and in relation to molecular deformation in PLA fibers.12The same method,originally developed in another study,7is used here to evaluate any changes in the rotational isomeric populations of PLA during physical aging, in order to determine any changes in PLA conformation during physical aging.Conformational changes have been reported to cease below the glass transition for polystyrene and poly(methyl methacrylate),13,14as reported from wide-angle X-ray scattering, whereas others have used a model that assumes conformational changes during physical aging.15,16There are also studies based on thermal density fluctuations,which show that physical aging is accompanied by an increase in local order.17,18Therefore,it was important to study whether or not conformational changes are observable during the physical aging of PLA.We are also interested in the effect of configurational defects, or polymer tacticity,on the kinetics of physical aging.To our knowledge,there are only a few studies,namely,on poly(methyl methacrylate),that report an effect of polymer tacticity on the activation enthalpy.19,20The effect of stereochemical defects on physical aging is a relatively unexplored and important aspect that we pursue through this study.From our past investigations, we know that D-lactyl defects perturb the distribution of rotational isomer populations at D-L junctions,thereby render-ing the overall chain more flexible.21We know that a greater fraction of configurational defects leads to lower glass transition temperatures(T g),which speeds up the kinetics of physical aging.We wish to see any effects of these defects above and beyond this change in the T g.PLA amorphous dynamics have previously been studied using dielectric spectroscopy and calorimetric methods.22-27However, no well-known physical aging models were used to analyze those aging kinetics data.We use the generalized Kohlrausch-Williams-Watts(KWW)equation,solved by the Tool-Narayanaswamy-Moynihan(TNM)description of relaxation time,to characterize the physical aging kinetics of PLA,as it is an accepted model for analysis of physical aging.28-30In this study,calorimetric methods are employed to monitor enthalpic relaxation of PLA,which is a measure of physical aging. Materials and Experimental TechniquesMaterials.Poly(DL-lactic acid),or PLA50,was obtained from Abbott Vascular,Inc.Poly(L-lactic acid)with 5.7%D,or*To whom correspondence should be addressed.Phone:413-577-1125.E-mail:slhsu@.†Current address:PU Global Product R&D,The Dow ChemicalCompany,Freeport,TX77541.12322J.Phys.Chem.B2007,111,12322-1232710.1021/jp074509t CCC:$37.00©2007American Chemical SocietyPublished on Web10/04/2007PLA5.7,and 13.0%D -lactyl content,or PLA13,were re-ceived from NatureWorks and were the same samples used in previous studies.21The D -lactyl contents were chosen such that quiescent crystallization was slow and crystallization could be readily suppressed by a rapid thermal quench,at the same time covering a wide range of tacticity.Residual lactides were found for the as-received PLA50sample,as indicated in its Raman spectra by the extra peak at 656cm -1,as shown in Figure 1.DL -Lactide was used as received from Aldrich.The PLA50sample was therefore purified by first preparing a solution in dichloromethane at a concentration of 10%by weight and then precipitating in methanol.The sample was dried in ambient atmosphere and then further dried in vacuum at 60°C for 2h.Raman spectra of the resulting samples were taken to ensure that no solvent peaks are present.The same treatment was performed for the PLA13and PLA5.7samples.Molecular Weight Characterization.An in-house-as-sembled size exclusion chromatograph was used to measure the molecular weight parameters of various PLA samples.A Knauer HPLC pump K-501and a Polymer Laboratories PL DataStream detector were used on a Polymer Laboratories 5µm mixed-D column,with chloroform at 30°C as the eluting medium.Narrow molecular weight polystyrene standards ranging from 2.7to 390kg mol -1were used for calibration.Differential Scanning Calorimetry.A TA Instruments model Q100differential scanning calorimeter equipped with an RCS cooling system and nitrogen gas purge set at 50mL/min flow rate was used for all calorimetric measurements.Aluminum pans with PLA samples were hermetically sealed before measurement.Polymer sample masses varied from 6to 13mg.The top part of the pan was pressed down to improve thermal contact with the sample,taking care that the flatness of the bottom face contacting the temperature sensor in the differential scanning calorimetry (DSC)chamber was maintained.Indium and water were used as a standard to calibrate the temperature at their onset melting points of 156.6and 0.0°C,respectively.The indium heat of fusion (28.6J g -1)was used to calibrate heat flow.31Heat flow and not absolute heat capacity was the quantity measured,since for the purposes of enthalpy relaxation kinetics,it was only necessary to measure the total heat release.19The difference between heat flows of aged samples and quenched glass was integrated over a temperature interval of 10-100°C to compute the excess enthalpy that characterizes the extent of physical aging,in accordance with the method used by Petrie.32The “glass transition temperature”in heating scans was taken as the halfway point of the tangent line containing the inflection point and hereafter referred to as T g,h .The fictive temperature option available on the Universal Analysis software from TA Instruments was not used,in largepart because in our measurements,the value thus measured did not differ by more than 0.05°C compared with T g,h .The onset of glass transition upon cooling,or T g,onset ,has been taken to be the temperature of the intersection point between the liquid baseline and the glass transition inflection point in constant rate cooling scans.Dispersive Raman Microspectroscopy.A Jobin-Yvon Horiba LabRam HR800dispersive Raman spectrometer was used to obtain Raman spectra of PLA50.The 632.8nm line of the helium -neon gas laser was used for excitation.A spectral resolution of 4cm -1was maintained near the laser line.Circularly polarized incident light was obtained with a quarter-wave retardation plate in order to perform a signal aver-aging over all possible orientations.For the purposes of acquiring temperature-dependent Raman spectra,an in-house-built aluminum heating block was used.A copper -constantan thermocouple (diameter ∼0.08mm),calibrated with boiling water (100.0°C)and ice water (0.0°C),was placed close to the sample for temperature measurement.The PLA50sample was further wrapped with aluminum foil for good thermal conduction,with a small hole at the top (∼2mm)to allow for Raman spectral acquisition.The sample was conditioned at 55°C (well above both its glass transition and fictive temperatures)for 1.5h to initialize thermal history to the equilibrium liquid state.Computer Program for Curve Fitting to KWW/TNM Equations.A computer program code written in Visual Basics for Applications (VBA)was implemented in a spreadsheet written in Microsoft Excel 2000.This VBA program was used to fit the data to the nonlinear,generalized KWW/TNM equations.A standard Levenberg -Marquardt algorithm was used to fit the experimental aging data to those equations.33,34ModelingIn order to evaluate the effect of configurational defects,the theories on physical aging kinetics are reviewed.The aging behavior of small molecular glass and atactic polymers has been studied extensively in the past.Many phenomenological models of sub-glass-transition amorphous dynamics exist.28-30,35,36One that is commonly encountered in literature is the Cowie -Ferguson equation,37or the linear KWW equation,which requires a simple linear three-parameter fit and has been used most often with small molecular and inorganic glasses.38,39Its theoretical explanation has been presented with minimal as-sumptions,40which is the major reason for its adoption in the interpretation of physical aging in many systems.However,the linear KWW equation has problems in theory and in application in the context of enthalpic relaxation.41In fact,the asymmetry-of-approach and the memory effect experiments by Kovacs showed that experimental phenomena could not be explained by a constant relaxation time,36,42which means that a variable relaxation time has to be incorporated in any aging kinetics model.A recent study also indicates that the fit value of ∆H endo approaching time infinity from a linear KWW analysis of short-time data cannot predict the equilibrium value of ∆H endo that was obtained experimentally.43In order to reconcile nonlinear effects,the generalized KWW equation and TNM formalism for relaxation time are used to analyze the physical aging most commonly of polymeric glasses.28-30,36The TNM model modifies the Arrhenius equation by incorporating the fictive temperature as a measure of the instantaneous structure,in addition to the annealing temperature,of the material during aging.The generalized KWW equation is expressed as follows:Figure 1.Raman spectra of DL -lactide,PLA50pellet as-received,and PLA50after purification.Physical Aging of Amorphous Poly(lactic acid)J.Phys.Chem.B,Vol.111,No.42,200712323whereδ/δ0represents the deviation from thermodynamic equilibrium,t is the aging time,τ0is the characteristic relaxation time(variable over the course of enthalpic relaxation),and is a stretched exponential parameter.The calorimetricδ/δ0is calculated from experimental quantities as follows:where∆H∞is the endotherm at long time(equilibrium)and ∆H t is the endotherm at time t.As eq1is not solvable analytically for known expressions of the relaxation time,it is numerically solved.The solution uses the well-known TNM equation,28,29,36which has temperature-(T a)and structure-dependent(T f)terms:where ln A is a pre-exponential factor,R is the universal gas constant,T a is the annealing temperature,∆h is the activation energy for enthalpy relaxation,and x partitions the temperature and structure terms.T f is the instantaneous fictive temperature of the material that represents the instantaneous structure of the glass during aging,as shown:where T a is the aging temperature and∆C p is the change in heat capacity as the system passes through the glass transition temperature.Although there are criticisms concerning the shortcoming of the model,37,44,45this is the preferred model usedin studies of polymer physical aging.46,47In the actual application of the TNM formalism to the analysis of physical aging,finding a stable solution can be problematic if a four-parameter fit( ,x,∆h,ln A)is adopted.This owes to the fact that the solution can be overly sensitive to experimental error.This aspect is remedied by reducing the data analysis to a three-parameter fit.This is done by fixing one of the parameters out of the four-parameter fit.It has been noted previously that,due to the fact that T a/T f is close to unity on the absolute temperature scale,then eq3simplifies to where T a∼T f∼T so that two of the fitting parameters(ln Aand∆h)would be correlated linearly.19Owing to this correlation, the curve fit becomes extremely sensitive to small changes in δ(of the order of magnitude of experimental uncertainty).To address this issue,the following approximation was used:48where q is the cooling rate and T g,onset is the onset of glass transition.The parameter∆h was thus determined from a set of cooling rate dependence experiments,the result of which is summarized in Figure2.The activation energy for enthalpy relaxation obtained through eq6for amorphous PLA of varying tacticity was in the range of750-900kJ/mol for the three PLA samples,which was on the same order of magnitude as,or higher than,for other polymers.47,49-51With the value of∆h determined,the analysis of enthalpic relaxation now involves a stable three-parameter fit.Results and DiscussionThe PLA samples were measured using size exclusion chromatography in order to assess their molecular weight and molecular weight distribution.Mark Houwink parameters of (K/mL g-1,a))(4.9×10-3,0.794)were used for polysty-rene,52whereas(5.45×10-2,0.73)and(2.27×10-2,0.77) were used for all-L and DL-PLA,respectively.53The result is shown in Table1.It is noted that the molecular weights are all above the entanglement molecular weight of8700and10500 g mol-1of a PLA melt(reported at180and200°C,respec-tively).54,55DSC was used to obtain the T g,h of all the samples so that subsequent physical aging studies are done for each samples at nearly the same supercooling from the respective T g,h values.In addition,with different heating rate,the heat capacity change at the glass transition(∆C p)did not vary more than2%,2,19and the aging endotherm(∆H endo)at a given time and temperature also did not vary significantly(see Table2). The average of the∆C p value for the different heating rates was used in the analysis of enthalpy relaxation kinetics.We then further studied the role of conformational and configura-tional defects of these samples associated with physical aging. From past studies we know that the conformer distribution is perturbed by D-lactyl configurational defects.21Here,in the context of physical aging,we were more concerned with the change in the conformer distribution during physical aging rather than the difference in the initial values of the tg’t conformer population of the PLA samples of varying tacticity.We used aδ(Ta ,t)/δ(Ta,0))exp[-(∫0t d t/τ0(T,t)) ](1)δ(Ta ,t)/δ(Ta,0))(∆H∞-∆Ht)/∆H∞(2)lnτ0)ln A+x∆hRTa+(1-x)∆hRTf(δ)(3)T f (δ))Tf(Ta,t))Ta+δ(Ta,t)∆Cp(4)lnτ≈ln A+∆hRT(5)d ln|q|d(1/Tg,onset)≈-∆hR(6)Figure2.Determination of the TNM∆h parameter by the coolingrate dependence of the onset of the glass transition for(a)PLA5.7%D,(b)13.0%D,and(c)50%D.The uncertainty of∆h is stated as onestandard deviation.12324J.Phys.Chem.B,Vol.111,No.42,2007Aou etal.method from our previous studies,which relied on the fact that ratio of the Raman spectral intensities at the 1128and 1044cm -1can be related linearly with the tg ’t conformer popula-tion.7,56The conformer population of the PLA50was observed as a function of time at 37°C during the physical aging process.In the annealing time range and beyond where the DSC results from Figure 3show that physical aging is underway,Raman spectra from Figure 4shows no discernible difference during physical aging at 37°C.This is especially true in the present case where there are no detectable differences in the tg ’t population during physical aging.This is consistent with what has been observed for other amorphous polymers,where neither interchain spacing nor conformation change below their glass transition temperatures.13,14However,we should recognize the possibility that the changes in conformer population are small and therefore experimentally undetectable.This may result from the fact that the tg ’t conformer (∼80%)is dominant in PLA.7,56Therefore,conformational defects have little or no detectable role in physical aging of PLA.On the other hand,configurational defects are known to lower the T g of PLA by as much as 10°C and therefore are expected to have significant impact on theaging kinetics.In addition,any impact of configurational defects beyond their effect on the T g was studied.To analyze these effects of configurational defects,DSC was used,as relatively small samples needed to be tested.46When referring specifically to physical aging measured through DSC,the term enthalpic relaxation would be used.The experimental details and analysis of excess enthalpy using calorimetry is not as trivial as in the case of crystallinity analysis,and the issues are discussed here.Enthalpic relaxation kinetics is very sensitive to its thermal history,and therefore certain precautions needed to be taken.After every heating scan,the sample was quenched to 0°C at the maximum cooling rate accessible by the equipment (∼40°C/min near T g ),before reheating to the annealing temperature for physical aging.This was done to approach the annealing temperature from the low-temperature side,which would minimize the extent of aging prior to reaching isothermal conditions,46as well as to suppress crystallization for the PLA5.7sample,which can crystallize when cooled down slowly from the melt.Also,in order to initialize the thermal history of the sample at the melt state,heating scans were performed in the temperature range of 0to 120,160,and 160°C for PLA50,PLA13,and PLA5.7,respectively.The difference between annealing temperature and the glass transition is a very important parameter to consider,since enthalpic relaxation kinetics accelerates as the temperature approaches the glass transition.The purpose of measuring T g,h was to determine the set of supercooling temperatures to use for the annealing temperatures with respect to the approximate T g,h of the ter measurement of T g,h upon cooling at 5°C min -1yielded values that were separated by about the same amount from sample to sample.In order to evaluate the enthalpy relaxation kinetics,the initial fictive temperature is necessary,and this is determined by the equilibrium excess enthalpy for samples annealed at 5°C lower than the T g,h .46,47It should be noted that true glass transition temperature in practice is determined during cooling scans;57however,as our purpose does not lie in the determination of the true glass transition temperature,this was not done.As annealing required isothermal conditions,stability of temperature as well as reproducibility of data were examined.Sub-T g annealing was performed in the DSC cell for the same sample at temperatures 14,12,10,7,and 5°C lower than the T g,h ,and aging experiments conducted under identical conditions (annealing time and temperature)were repeated three times.Data were reproducible as shown in Figure 5.The cell temperature was stable to (0.02°C for isothermal tests programmed at 30.0,40.0,and 50.0°C for 10min,and the temperature range covers the set of annealing temperatures used in the present study.Heating scans of melt-quenched states were compared before and after the set of aging experiments were carried out toTABLE 1:Summary of Molecular Weights and Glass Transition Temperatures of PLA SamplesPLLA standardPDLLA standardM n /kg mol -1M w /kg mol -1PDI M n /kg mol -1M w /kg mol -1PDI T g,h /°C at 10°C min -1PLA 5.7%D 3569 1.964687 1.9059.3PLA 13.0%D 3365 1.954382 1.9058.4PLA 50%Dn/an/an/a21401.9253.0TABLE 2:Summary of ∆C p and ∆H endo of the Three PLA Samples at 5,10,and 20°C Min -1Heating RatesPLA 5.7%D (T a )46.0°C)PLA 13.0%D (T a )45.0°C)PLA 50%D (T a )40.0°C)heating rate/°C min -1510205102051020∆H endo after 30min aging at T a 1.1 1.2 1.2 1.1 1.2 1.2 1.0 1.2 1.2∆C p at T g,h /J g -1°C -10.540.540.540.530.520.520.530.540.53T g,h /°C58.359.361.357.258.460.751.853.054.9Figure 3.DSC traces of PLA 50%D annealed at 37.0°C.Conditioning took place after initializing thermal history at 120°C for 1min.Heating rate is 20°C min -1.Figure 4.Raman spectra of PLA50during physical aging at 37.0°C.The sample was preconditioned at 55°C for 1.5h to remove thermal history.Physical Aging of Amorphous Poly(lactic acid)J.Phys.Chem.B,Vol.111,No.42,200712325confirm the absence of sample degradation due to the DSC heating scans.In the context of physical aging/enthalpic relaxation,the most significant effect of configurational defects on the PLA back-bone may be their effect on the T g,58which in turn affects the enthalpic relaxation paring the samples of PLA5.7 and PLA50,we find that the addition of more D-lactyl units resulted in the lowering of T g,h by about6°C,as seen from Table2.Consequently,the initial fictive temperature(T f,o)is also lowered by about6°C,as shown in Table3.Since physical aging occurs only below T g,where glassy behavior is observed, then for a given annealing temperature,enthalpic relaxation of PLA50should proceed faster than that of PLA5.7,all other things being equal.So a difference in T g in fact should have a very large impact on aging kinetics.In order to investigate effects of configurational defects above and beyond their impact on T g,the experiments were set up such that a series of approximately constant supercoolings relative to T g,h,which is related to the T f,o,were applied to the three types of PLA samples to facilitate comparison of enthalpic relaxation kinetics.The KWW/TNM equations require T f,o to be included as a parameter so that using these equations allow us to separate the two effects of configurational defects on the enthalpic relaxation kinetics:one on the T g,and thus T f,o,and the other on the KWW/TNM fit parameters.The data and curve of best fit to the KWW/TNM equations are presented in Figure5.We note that it is difficult to discern an obvious difference in enthalpy relaxation kinetics between the three different samples.The KWW/TNM fitting parameters are shown in Figure6.The error bars of∆h in that figure correspond to the propagation of the curve-fitting error combined with the experimental error,as shown in Figure2.The error bars for the other three parameters in Figure6represent the lower and higher values obtained when the∆h parameter is used in the KWW/TNM fit is fixed to its lower or higher value.For clarity,the actual values are tabulated in Table3. We note that smaller error bars may be achieved if both the curve-fitting and experimental errors were reduced,which can be accomplished if a greater range of cooling rates were covered in obtaining the value of∆h.In future attempts,one must still keep in mind that too slow a cooling rate would be affected by noise of the calorimeter,while too fast a cooling rate could suffer from thermal lags that would render the measured values of T g,onset meaningless.That is,there is a limited range of cooling rates under which meaningful values of∆h can be determined. Figure6shows that aside from ,none of the fitting param-eters are different beyond the uncertainty limits.Since the values of∆h and ln A are correlated to each other,then their un-certainties would also be correlated.Unless the∆h were deter-mined to higher accuracies,the sample-to-sample differences for these two parameters would not be distinguishable.The x parameter,which partitions the relaxation time into temperature-and structure-dependent terms,also does not vary beyond significance across the different PLA samples.Since the physical meaning of x is not well understood,further analysis is difficult at this point.From Figure6,we find that the parameter is all within experimental error for the three PLA samples.The relative magnitude for PLA13compared with the other two samples is not obvious owing to its large uncertainty.It should be kept in mind that a value of about0.5has been found in polystyrene where KWW/TNM analysis was used,with the∆h determined separately as in the present study.43However,as the value of is just within the error bar for PLA5.7and PLA50,it merits some discussion,since it is conceivable that difference may be significant if the uncertainty of∆h can be reduced in futureFigure5.Extent of enthalpy relaxation as a function of time for(a) PLA5.7%D,(b)PLA13.0%D,and(c)PLA50%D.The black dotted lines represent the curve fit to the generalized KWW/TNM equations. The gray dotted lines correspond to the upper and lower uncertainty limits of the∆h of each sample.TABLE3:TNM Fitting Parameters for PLA5.7%D,13.0%D,and50%DPLA5.7%D PLA13.0%D PLA50%D T f,o/°C56.955.950.4∆h/104J mol-176(1591(1687(15x0.63-0.150.47-0.110.48-0.11 0.41(0.050.49(0.070.51(0.08 ln(A/s)-272-55-328-59-319-56Figure6.KWW/TNM fitting parameters of PLA5.7%D,13.0%D, and50%D.The error bars correspond to the upper and lower uncertainty limits of the∆h of each sample.12326J.Phys.Chem.B,Vol.111,No.42,2007Aou etal.experiments.There is the possibility that the difference is simply due to the fact that PLA5.7can crystallize,whereas PLA50 cannot.59However,as care was taken to suppress crystallization by quenching rapidly from the melt,and subsequent DSC scans showed no detectable crystallinity,there is the possibility that difference in crystallinity does not exist and cannot explain the difference in .In order to explain the difference in the parameter,it has been suggested that in glassy polymers there is a crossover size limit(“Gaussian blobs”)between Rouse dynamics( )0.5)and R-relaxation dynamics( <0.5).60-63 In those studies,it is proposed that Rouse dynamics is dominant above,and R-relaxation dynamics below,the crossover limit.64,65 If the precision obtained for the values of are improved through better estimation of∆h and there are significant differences between the values,then this crossover size limit concept should certainly be revisited in greater depth. ConclusionsConformational analysis using Raman spectroscopy reveals little or no change in helical tg’t conformer population during the physical aging process,which is consistent with previous observations of other polymers.We find that the use of cooling rate dependence of T g,onset to obtain the activation energy for enthalpy relaxation(∆h)is required in order to have a stable and reliable curve fit.The values of∆h were therefore calculated for each PLA sample and had similar magnitude as other polymeric glasses.The effects of polymer tacticity on the KWW/ TNM fit parameters were analyzed.The major effect of configurational defects was to lower the T g,h of PLA,thereby accelerating the enthalpic relaxation kinetics at a given annealing temperature.All KWW/TNM fit parameters were found not to change significantly from sample to sample.Further attempts to improve accuracy of the∆h and thus of the curve-fitting parameters should be done in the future. 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TPER

of the polymers were soluble in most common organic solvents even at room temperature, and some were soluble on heating. The degradation temperatures of the resultant polymers fell in the range of 260–500C in nitrogen (with only 10% weight loss). The speciﬁc heat capacity at 200C ranged from 1.0 to 2.21 J gÀ1 KÀ1. The temperatures at which the maximum degradation of the polymer occurred ranged from 510 to 610C. The glass-transition temperatures of the polyimides ranged from 182 to 191C. The activation energy and enthalpy of the polyimides ranged from 44.44 to 73.91 kJ/mol and from 42.58 to 72.08 kJ/mol K, respectively. The moisture absorption was found in the range of 0.23–0.71%. VC 2009 Wiley Periodicals, Inc. J Appl
1Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan 45320 2Institut fu¨ r Anorganische Chemie, J. W. Goethe-Universita¨t Frankfurt, Max-Von-Laue-Strasse 7, Frankfurt/Main, Germany 60438 3Nescom, P.O. Box 2801, Islamabad, Pakistan

PbI2

Remnant PbI2, an unforeseen necessity in high-efficiency hybrid perovskite-based solar cells?a)Duyen H. Cao, Constantinos C. Stoumpos, Christos D. Malliakas, Michael J. Katz, Omar K. Farha, Joseph T. Hupp, and Mercouri G. KanatzidisCitation: APL Materials 2, 091101 (2014); doi: 10.1063/1.4895038View online: /10.1063/1.4895038View Table of Contents: /content/aip/journal/aplmater/2/9?ver=pdfcovPublished by the AIP PublishingArticles you may be interested inParameters influencing the deposition of methylammonium lead halide iodide in hole conductor free perovskite-based solar cellsAPL Mat. 2, 081502 (2014); 10.1063/1.4885548Air stability of TiO2/PbS colloidal nanoparticle solar cells and its impact on power efficiencyAppl. Phys. Lett. 99, 063512 (2011); 10.1063/1.3617469High efficiency mesoporous titanium oxide PbS quantum dot solar cells at low temperatureAppl. Phys. Lett. 97, 043106 (2010); 10.1063/1.3459146Near-IR activity of hybrid solar cells: Enhancement of efficiency by dissociating excitons generated in PbS nanoparticlesAppl. Phys. Lett. 96, 073505 (2010); 10.1063/1.3292183Effects of molecular interface modification in hybrid organic-inorganic photovoltaic cellsJ. Appl. Phys. 101, 114503 (2007); 10.1063/1.2737977APL MATERIALS2,091101(2014)Remnant PbI2,an unforeseen necessity in high-efﬁciency hybrid perovskite-based solar cells?aDuyen H.Cao,1Constantinos C.Stoumpos,1Christos D.Malliakas,1Michael J.Katz,1Omar K.Farha,1,2Joseph T.Hupp,1,band Mercouri G.Kanatzidis1,b1Department of Chemistry,and Argonne-Northwestern Solar Energy Research(ANSER)Center,Northwestern University,2145Sheridan Road,Evanston,Illinois60208,USA2Department of Chemistry,Faculty of Science,King Abdulaziz University,Jeddah,Saudi Arabia(Received2May2014;accepted26August2014;published online18September2014)Perovskite-containing solar cells were fabricated in a two-step procedure in whichPbI2is deposited via spin-coating and subsequently converted to the CH3NH3PbI3perovskite by dipping in a solution of CH3NH3I.By varying the dipping time from5s to2h,we observe that the device performance shows an unexpectedly remark-able trend.At dipping times below15min the current density and voltage of thedevice are enhanced from10.1mA/cm2and933mV(5s)to15.1mA/cm2and1036mV(15min).However,upon further conversion,the current density decreases to9.7mA/cm2and846mV after2h.Based on X-ray diffraction data,we determinedthat remnant PbI2is always present in these devices.Work function and dark currentmeasurements showed that the remnant PbI2has a beneﬁcial effect and acts as ablocking layer between the TiO2semiconductor and the perovskite itself reducingthe probability of back electron transfer(charge recombination).Furthermore,weﬁnd that increased dipping time leads to an increase in the size of perovskite crys-tals at the perovskite-hole-transporting material interface.Overall,approximately15min dipping time(∼2%unconverted PbI2)is necessary for achieving optimaldevice efﬁciency.©2014Author(s).All article content,except where otherwisenoted,is licensed under a Creative Commons Attribution3.0Unported License.[/10.1063/1.4895038]With the global growth in energy demand and with compelling climate-related environmental concerns,alternatives to the use of non-renewable and noxious fossil fuels are needed.1One such alternative energy resource,and arguably the only legitimate long-term solution,is solar energy. Photovoltaic devices which are capable of converting the photonﬂux to electricity are one such device.2Over the last2years,halide hybrid perovskite-based solar cells with high efﬁciency have engendered enormous interest in the photovoltaic community.3,4Among the perovskite choices, methylammonium lead iodide(MAPbI3)has become the archetypal light absorber.Recently,how-ever,Sn-based perovskites have been successfully implemented in functional solar cells.5,6MAPbI3 is an attractive light absorber due to its extraordinary absorption coefﬁcient of1.5×104cm−1 at550nm;7it would take roughly1μm of material to absorb99%of theﬂux at550nm.Further-more,with a band gap of1.55eV(800nm),assuming an external quantum efﬁciency of90%,a maximum current density of ca.23mA/cm2is attainable with MAPbI3.Recent reports have commented on the variability in device performance as a function of perovskite layer fabrication.8In our laboratory,we too have observed that seemingly identicalﬁlmsa Invited for the Perovskite Solar Cells special topic.b Authors to whom correspondence should be addressed.Electronic addresses:j-hupp@ and m-kanatzidis@2,091101-12166-532X/2014/2(9)/091101/7©Author(s)2014FIG.1.X-ray diffraction patterns of CH3NH3PbI3ﬁlms with increasing dipping time(%composition of PbI2was determined by Rietveld analysis(see Sec.S3of the supplementary material for the Rietveld analysis details).have markedly different device performance.For example,when ourﬁlms of PbI2are exposed to MAI for several seconds(ca.60s),then a light brown coloredﬁlm is obtained rather than the black color commonly observed for bulk MAPbI3(see Sec.S2of the supplementary material for the optical band gap of bulk MAPbI3).23This brown color suggests only partial conversion to MAPbI3and yields solar cells exhibiting a J sc of13.4mA/cm2and a V oc of960mV;these values are signiﬁcantly below the21.3mA/cm2and1000mV obtained by others.4Under the hypothesis that fully converted ﬁlms will achieve optimal light harvesting efﬁciency,we increased the conversion time from seconds to2h.Unexpectedly,the2-h dipping device did not show an improved photovoltaic response(J sc =9.7mA/cm2,V oc=846mV)even though conversion to MAPbI3appeared to be complete.With the only obvious difference between these two devices being the dipping time,we hypothesized that the degree of conversion of PbI2to the MAPbI3perovskite is an important parameter in obtaining optimal device performance.We thus set out to understand the correlation between the method of fabrication of the MAPbI3layer,the precise chemical compositions,and both the physical and photo-physical properties of theﬁlm.We report here that remnant PbI2is crucial in forming a barrier layer to electron interception/recombination leading to optimized J sc and V oc in these hybrid perovskite-based solar cells.We constructed perovskite-containing devices using a two-step deposition method according to a reported procedure with some modiﬁcations.4(see Sec.S1of the supplementary material for the experimental details).23MAPbI3-containing photo-anodes were made by varying the dipping time of the PbI2-coated photo-anode in MAI solution.In order to minimize the effects from unforeseen variables,care was taken to ensure that allﬁlms were prepared in an identical manner.The composi-tions ofﬁnal MAPbI3-containingﬁlms were monitored by X-ray diffraction(XRD).Independently of the dipping times,only theβ-phase of the MAPbI3is formed(Figure1).9However,in addition to theβ-phase,allﬁlms also showed the presence of unconverted PbI2(Figure1,marked with*) which can be most easily observed via the(001)and(003)reﬂections at2θ=12.56◦and38.54◦respectively.As the dipping time is increased,the intensities of PbI2reﬂections decrease with a concomitant increase in the MAPbI3intensities.In addition to the decrease in peak intensities of PbI2,the peak width increases as the dipping time increases indicating that the size of the PbI2 crystallites is decreasing,as expected,and the converse is observed for the MAPbI3reﬂections.This observation suggests that the conversion process begins from the surface of the PbI2crystallites and proceeds toward the center where the crystallite domain size of the MAPbI3phase increases and that of PbI2diminishes.Interestingly,the remnant PbI2phase can be seen in the data of other reports, but has not been identiﬁed as a primary source of variability in cell performance.8,10 Considering that the perovskite is the primary light absorber within the device,we wantedto further investigate how the optical absorption of theﬁlm changes with increasing dipping timeFIG.2.Absorption spectra of CH3NH3PbI3ﬁlms as a function of unconverted PbI2phase fraction.FIG.3.(a)J-V curves and(b)EQE of CH3NH3PbI3-based devices as a function of unconverted PbI2phase fraction.(Figure2).11,12The pure PbI2ﬁlm shows a band gap of2.40eV,consistent with the yellow color of PbI2.As the PbI2ﬁlm is gradually converted to the perovskite,the band gap is progressively shifted toward1.60eV.The deviation of MAPbI3’s band gap(1.60eV)from that of the bulk MAPbI3 material(1.55eV)could be explained by quantum conﬁnement effects related with the sizes of TiO2and MAPbI3crystallites and their interfacial interaction.13,14Interestingly,we also noticed the presence of a second absorption in the light absorber layer,in which the gap gradually red shifts from1.90eV to1.50eV as the PbI2concentration is decreased from9.5%to0.3%(Figure2—blue arrow).Having established the chemical compositions and optical properties of the light absorberﬁlms, we proceeded to examine the photo-physical responses of the corresponding functional devices in order to determine how the remnant PbI2affects device performance.The pure PbI2based device remarkably achieved a0.4%efﬁciency with a J sc of2.1mA/cm2and a V oc of564mV (Figure3(a)).Upon progressive conversion of the PbI2layer to MAPbI3,we observe two different regions(Figure4,Table I).In theﬁrst region,the expected behavior is observed;as more PbI2is converted to MAPbI3,the trend is toward higher photovoltaic efﬁciency,due both to J sc and V oc, until1.7%PbI2is reached.The increase in J sc is attributable,at least in part,to increasing absorption of light by the perovskite.We speculate that progressive elimination of PbI2,present as a layer between TiO2and the perovskite,also leads to higher net yields for electron injection into TiO2and therefore,higher J values.For a sufﬁciently thick PbI2spacer layer,electron injection would occur instepwise fashion,i.e.,perovskite→PbI2→TiO2.Finally,the photovoltage increase is attributable toFIG.4.Summary of J-V data vs.PbI2concentration of CH3NH3PbI3-based devices(Region1:0to15min dipping time, Region2:15min to2h dipping time).TABLE I.Photovoltaic performance of CH3NH3PbI3-based devices as a function of unconverted PbI2fraction.Dipping time PbI2concentration a J sc(mA/cm2)V oc(V)Fill factor(%)Efﬁciency(%) 0s100% 2.10.564320.45s9.5%10.10.93352 4.960s7.2%13.40.96052 6.72min 5.3%14.00.964557.45min 3.7%14.70.995578.315min 1.7%15.1 1.036629.730min0.8%13.60.968648.51h0.4%12.40.938657.62h0.3%9.70.84668 5.5a Determined from the Rietveld analysis of X-ray diffraction data.the positive shift in TiO2’s quasi-Fermi level as the population of photo-injected electrons is higher with increased concentration of MAPbI3.The second region yields a notably different trend;surprisingly,below a concentration of2% PbI2,J sc,V oc,and ultimatelyηdecrease.Considering that the light-harvesting efﬁciency would increase when the remaining2%PbI2is converted to MAPbI3(albeit to only a small degree),then the remnant PbI2must have some other role.We posit that remnant PbI2serves to inhibit detrimental electron-transfer processes(Figure5).Two such processes are back electron transfer from TiO2to holes in the valence band of the perovskite(charge-recombination)or to the holes in the HOMO of the HTM(charge-interception).This retardation of electron interception/recombination observation is reminiscent of the behavior of atomic layer deposited Al2O3/ZrO2layers that have been employed in dye-sensitized solar cells.15–18It is conceivable that the conversion of PbI2to MAPbI3occurs from the solution interface toward the TiO2/PbI2interface and thus would leave sandwiched between TiO2and MAPbI3a blocking layer of PbI2that inhibits charge-interception/recombination.For this hypothesis to be correct,it is crucial that the conduction-band-edge energy(E cb)of the PbI2be higher than the E cb of the TiO2.19–21 The work function of PbI2was measured by ultraviolet photoelectron spectroscopy(UPS)and was observed to be at6.35eV vs.vacuum level,which is0.9eV lower than the valence-band-edge energy(E vb)of MAPbI3(see Sec.S7of the supplementary material23for the work function of PbI2);the E cb(4.05eV)was calculated by subtracting the work function from the band gap(2.30eV).solar cell.FIG.6.Dark current of CH3NH3PbI3-based devices as a function of unconverted PbI2phase fraction.The E cb of PbI2is0.26eV higher than the E cb of TiO2and thus PbI2satisﬁes the conditions of a charge-recombination/interception barrier layer.In order to probe the hypothesis that PbI2acts as a charge-interception barrier,dark current measurements,in which electronsﬂow from TiO2to the HOMO of the HTM,were made.Consistent with our hypothesis,Figure6illustrates that the onset of the dark current occurs at lower potentials as the PbI2concentration decreases.In the absence of other effects,the increasing dark current with increasing fraction of perovskite(and decreasing fraction of PbI2)should result in progressively lower open-circuit photovoltages.Instead,the photocurrent density and the open-circuit photovoltage bothincrease,at least until to PbI2fraction reaches1.7%.As discussed above,thinning of a PbI2-basedFIG.7.Cross-sectional SEM images of CH3NH3PbI3ﬁlm with different dipping time.sandwich layer should lead to higher net injection yields,but excessive thinning would diminish the effectiveness of PbI2as a barrier layer for back electron transfer reactions.Given the surprising role of remnant PbI2in these devices,we further probed the two-step conversion process by using scanning-electron microscopy(SEM)(Figure7).Two domains of lead-containing materials(PbI2and MAPbI3)are present.Theﬁrst domain is sited within the mesoporous TiO2network(area1)while the second grows on top of the network(area2).Area2initially contains 200nm crystals.As the dipping time is increased,the crystals show marked changes in size and morphology.The formation of bigger perovskite crystals is likely the result of the thermodynamically driven Ostwald ripening process,i.e.,smaller perovskite crystals dissolves and re-deposits onto larger perovskite crystals.22The rate of charge-interception,as measured via dark current,is proportional to the contact area between the perovskite and the HTM.Thus,the eventual formation of large, high-aspect-ratio crystals,as shown in Figure7,may well lead to increases in contact area and thereby contributes to the dark-current in Figure6.Regardless,we found that the formation of large perovskite crystals greatly decreased our success rate in constructing high-functioning,non-shorting solar cells.In summary,residual PbI2appears to play an important role in boosting overall efﬁciencies for CH3NH3PbI3-containing photovoltaics.PbI2’s role appears to be that of a TiO2-supported blocking layer,thereby slowing rates of electron(TiO2)/hole(perovskite)recombination,as well as decreasing rates of electron interception by the hole-transporting material.Optimal performance for energy conversion is observed when ca.98%of the initially present PbI2has been converted to the perovskite. Conversion to this extent requires about15min.Pushing beyond98%(and beyond15min of reaction time)diminishes cell performance and diminishes the success rate in constructing non-shorting cells.The latter problem is evidently a consequence of conversion of small and more-or-less uniformly packed perovskite crystallites to larger,poorly packed crystallites of varying shape and size.Finally,the essential,but previously unrecognized,role played by remnant PbI2 provides an additional explanation for why cells prepared dissolving and then depositing pre-formed CH3NH3PbI3generally under-perform those prepared via the intermediacy of PbI2.We thank Prof.Tobin Marks for use of the solar simulator and EQE measurement system. Electron microscopy was done at the Electron Probe Instrumentation Center(EPIC)at Northwestern University.Ultraviolet Photoemission Spectroscopy was done at the Keck Interdisciplinary SurfaceScience facility(Keck-II)at Northwestern University.This research was supported as part of theANSER Center,an Energy Frontier Research Center funded by the U.S Department of Energy, Ofﬁce of Science,Ofﬁce of Basic Energy Sciences,under Award No.DE-SC0001059.1R.Monastersky,Nature(London)497(7447),13(2013).2H.J.Snaith,J.Phys.Chem.Lett.4(21),3623(2013).3M.M.Lee,J.Teuscher,T.Miyasaka,T.N.Murakami,and H.J.Snaith,Science338(6107),643(2012).4J.Burschka,N.Pellet,S.J.Moon,R.Humphry-Baker,P.Gao,M.K.Nazeeruddin,and M.Gratzel,Nature(London) 499(7458),316(2013).5F.Hao,C.C.Stoumpos,D.H.Cao,R.P.H.Chang,and M.G.Kanatzidis,Nat.Photonics8(6),489(2014);F.Hao,C.C. Stoumpos,R.P.H.Chang,and M.G.Kanatzidis,J.Am.Chem.Soc.136,8094–8099(2014).6N.K.Noel,S.D.Stranks,A.Abate,C.Wehrenfennig,S.Guarnera,A.Haghighirad,A.Sadhanala,G.E.Eperon,M.B. Johnston,A.M.Petrozza,L.M.Herz,and H.J.Snaith,Energy Environ.Sci.7,3061(2014).7H.S.Kim,C.R.Lee,J.H.Im,K.B.Lee,T.Moehl,A.Marchioro,S.J.Moon,R.Humphry-Baker,J.H.Yum,J.E.Moser, M.Gratzel,and N.G.Park,Sci.Rep.2,591(2012).8D.Y.Liu and T.L.Kelly,Nat.Photonics8(2),133(2014).9C.C.Stoumpos,C.D.Malliakas,and M.G.Kanatzidis,Inorg.Chem.52(15),9019(2013).10J.H.Noh,S.H.Im,J.H.Heo,T.N.Mandal,and S.I.Seok,Nano Lett.13(4),1764(2013).11Diffuse reﬂectance measurements of MAPbI3ﬁlms were converted to absorption spectra using the Kubelka-Munk equation,α/S=(1-R)2/2R,where R is the percentage of reﬂected light,andαand S are the absorption and scattering coefﬁcients, respectively.The band gap values are the energy value at the intersection point of the absorption spectrum’s tangent line and the energy axis.12L.F.Gate,Appl.Opt.13(2),236(1974).13O.V oskoboynikov,C.P.Lee,and I.Tretyak,Phys.Rev.B63(16),165306(2001).14X.X.Xue,W.Ji,Z.Mao,H.J.Mao,Y.Wang,X.Wang,W.D.Ruan,B.Zhao,and J.R.Lombardi,J.Phys.Chem.C 116(15),8792(2012).15E.Palomares,J.N.Clifford,S.A.Haque,T.Lutz,and J.R.Durrant,J.Am.Chem.Soc.125(2),475(2003).16C.Prasittichai,J.R.Avila,O.K.Farha,and J.T.Hupp,J.Am.Chem.Soc.135(44),16328(2013).17A.K.Chandiran,M.K.Nazeeruddin,and M.Gratzel,Adv.Funct.Mater.24(11),1615(2014).18M.J.Katz,M.J.D.Vermeer,O.K.Farha,M.J.Pellin,and J.T.Hupp,Langmuir29(2),806(2013).19M.J.DeVries,M.J.Pellin,and J.T.Hupp,Langmuir26(11),9082(2010).20C.Prasittichai and J.T.Hupp,J.Phys.Chem.Lett.1(10),1611(2010).21F.Fabregat-Santiago,J.Garcia-Canadas,E.Palomares,J.N.Clifford,S.A.Haque,J.R.Durrant,G.Garcia-Belmonte, and J.Bisquert,J.Appl.Phys.96(11),6903(2004).22Alan D.McNaught and Andrew Wilkinson,IUPAC Compendium of Chemical Terminology(Blackwell Scientiﬁc Publica-tions,Oxford,1997).23See supplementary material at /10.1063/1.4895038for experimental details,absorption spectrum of bulk CH3NH3PbI3,fraction,size,absorption spectrum,work function of unconverted PbI2,and average photovoltaic perfor-mance.。

Highly Efficient Solar Cell Polymers Developed

Highly Efﬁcient Solar Cell Polymers Developed viaFine-Tuning of Structural and Electronic Properties Yongye Liang,†Danqin Feng,†Yue Wu,‡Szu-Ting Tsai,‡Gang Li,*,‡Claire Ray,†and Luping Yu*,†Department of Chemistry and the James Franck Institute,The Uni V ersity of Chicago, 929E.57th Street,Chicago,Illinois60637,and Solarmer Energy Inc.,3445Fletcher A V enue,El Monte,California91731Received February27,2009;E-mail:lupingyu@;gangl@Abstract:This paper describes synthesis and photovoltaic studies of a series of new semiconducting polymers with alternating thieno[3,4-b]thiophene and benzodithiophene units.The physical properties of these polymers wereﬁnely tuned to optimize their photovoltaic effect.The substitution of alkoxy side chains to the less electron-donating alkyl chains or introduction of electron-withdrawingﬂuorine into the polymer backbone reduced the HOMO energy levels of polymers.The structural modiﬁcations optimized polymers’spectral coverage of absorption and their hole mobility,as well as miscibility with fulleride,and enhanced polymer solar cell performances.The open circuit voltage,V oc,for polymer solar cells was increased by adjusting polymer energy levels.It was found thatﬁlms withﬁnely distributed polymer/fulleride interpenetrat-ing network exhibited improved solar cell conversion efﬁciency.Efﬁciency over6%has been achieved in simple solar cells based onﬂuorinated PTB4/PC61BMﬁlms prepared from mixed solvents.The results proved that polymer solar cells have a bright future.IntroductionSemiconducting polymers have shown physical properties similar to those of typical inorganic semiconductors,although the underlying mechanisms are usually different from each other.1Photovoltaic effect is one of these properties that stimulate people’s enthusiasm because solar energy conversion into electricity is the cleanest way to harvest this vast renewable energy source.So far,the most efﬁcient architecture to build polymeric photovoltaic solar cells is the bulk heterojunction (BHJ)structure prepared by mixing electron-rich polymers and electron-deﬁcient fullerides.2This approach can be easily implemented and allows a quick survey of the best composition of polymeric activeﬁlms.Detailed studies by many groups in the past years have identiﬁed poly(3-hexylthiophene)(P3HT) as the most attractive donor material.Power conversion ef-ﬁciency(PCE)of about5%has been achieved in solar cells based on P3HT/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)derivatives.3Although it is signiﬁcant progress in a relatively short period of research time,it is still far away from commercial viability.4After an exhaustive research effort,it becomes increasingly apparent that the PCE of solar cells based on P3HT/PC61BM is approaching its limit.New materials exhibiting better performance are needed in order to achieve the desired performance in these types of solar cells for practical application.5The performance of polymer solar cells is characterized by three parameters:open-circuit voltage(V oc),short-circuit current density(J sc),andﬁll-factor(FF),all of which are related to the PCE by the following equation:PCE)(V oc×J sc×FF)/ (I p×M),where I p is the power density of the incident lightirradiation and M is spectral mismatch factor.It has been realized that the ideal polymer in BHJ structure should exhibit a broad absorption with high coefﬁcient in the solar spectrum,high hole mobility,suitable energy level matching to fulleride,and appropriate compatibility with fulleride to form bicontinuous interpenetrating network on a nanoscale.6It is difﬁcult to design a polymer to fulﬁll all these requirements.Current polymer solar cells often suffer from small values in some or all of these parameters due to a variety of issues related to the nature of materials and device engineering.So far,besides the P3HT system,there are very few polymer solar cell systems reported which exceed5%in power conversion efﬁciency.7†The University of Chicago.‡Solarmer Energy Inc.(1)Skotheim,T.A.;Reynolds J.Handbook of conducting polymers;CRC:London,2007.(2)(a)Yu,G.;Gao,J.;Hummelen,J.C.;Wudl,F.;Heeger,A.J.Science1995,270,1789.(b)Gnes,S.;Neugebauer,H.;Sariciftci,N.S.Chem.Re V.2007,107,1324.(3)(a)Li,G.;Shrotriya,V.;Huang,J.S.;Yao,Y.;Moriarty,T.;Emery,K.;Yang,Y.Nat.Mater.2005,4,864.(b)Ma,W.L.;Yang,C.Y.;Gong,X.;Lee,K.H.;Heeger,A.J.Ad V.Funct.Mater.2005,15, 1617.(c)Li,G.;Shrotriya,V.;Yao,Y.;Yang,Y.J.Appl.Phys.2005, 98,043704.(4)Scharber,M.;Muhlbacher,D.;Koppe,M.;Denk,P.;Waldauf,C.;Heeger,A.J.;Brabec,C.Ad V.Mater.2006,18,789.(5)Thompson,B.C.;Frechet,J.M.J.Angew.Chem.,Int.Ed.2008,47,58.(6)Roncali,J.Macromol.Rapid Commun.2007,28,1761.(7)(a)Muhlbacher,D.;Scharber,M.;Morana,M.;Zhu,Z.G.;Waller,D.;Gaudiana,R.;Brabec,C.Ad V.Mater.2006,18,2884.(b)Peet,J.;Kim,J.Y.;Coates,N.E.;Ma,W.L.;Moses,D.;Heeger,A.J.;Bazan,G.C.Nat.Mater.2007,6,497.(c)Wang,E.G;Wang,L;Lan,L.F.;Luo,C.;Zhuang,W.L.;Peng,J.B.;Cao,Y.Appl.Phys.Lett.2008,92,33307.(d)Hou,J.H.;Chen,H.Y.;Zhang,S.Q.;Li,G.;Yang,Y.J.Am.Chem.Soc.2008,130,16144.Published on Web05/19/200910.1021/ja901545q CCC:$40.75 2009American Chemical Society 77929J.AM.CHEM.SOC.2009,131,7792–7799Recently,we have developed a new polymer,namely PTB1,based on alternating thieno[3,4-b ]thiophene and benzodithiophene units (Figure 1).Simple single-layer polymer solar cells exhibited solar conversion efﬁciency of 4.8%based on PTB1/PC 61BM BHJ structure and 5.6%on PTB1/PC 71BM structure.8The J sc and FF obtained from such polymer solar cells are among the highest values reported for solar cell system based on low-band-gap polymers.However,the V oc of the polymer solar cells is relatively small,just about 0.56-0.58V.In this paper,we describe our results in the development of new polymers which exhibit higher solar cell conversion efﬁciencies.These polymers were developed via synthetic ﬁne-tuning of their structural and electronic properties.Experimental SectionMaterials.Otherwise stated,all of the chemicals are purchased from Aldrich and used as received.The 4,6-dihydrothieno[3,4-b ]thiophene-2-carboxylic acid (1),94,6-dibromothieno[3,4-b ]thiophene-2-carboxylic esters,81,5-bis(trimethyltin)-4,8-dioctylbenzo[1,2-b :4,5-b ’]dithiophene,7and benzo[1,2-b:4,5-b ′b ′]dithiophene-4,8-dione 10were synthesized according to the procedures reported in the literature.Other monomers were synthesized according to Scheme 1.3-Fluoro-4,6-dihydrothieno[3,4-b ]thiophene-2-carboxylic acid (2).The 4,6-dihydrothieno[3,4-b ]thiophene-2-carboxylic acid (1.46g,7.85mmol)was dissolved in 60mL of THF and cooled in an acetone/dry ice bath under nitrogen protection.Butyllithium solution ((6.9mL,17.3mmol)was added dropwise with stirring.The(8)Liang,Y.Y.;Wu,Y.;Feng,D.Q.;Tsai,S.T.;Son,H.J.;Li,G.;Yu,L.P.J.Am.Chem.Soc.2009,131,56.(9)Yao,Y.;Liang,Y.Y.;Shrotriya,V.;Xiao,S.Q.;Yu,L.P.;Yang,Y.Ad V .Mater.2007,19,3979.(10)Beimling,P;Ko mehl,G.Chem.Ber.1986,119,3198.Figure 1.Synthetic routes for polymers PTB1-PTB6and structure of PC 61BM.Scheme 1.Synthesis Routes forMonomersJ.AM.CHEM.SOC.9VOL.131,NO.22,20097793Highly Efﬁcient Solar Cell Polymers A R T I C L E Sresulting mixture was kept in a dry ice bath for1h.Then N-ﬂuorobenzenesulfonimide(3.22g,10.2mmol)in20mL of THF was added dropwise,and the solution was stirred at RT overnight. The reaction was quenched with50mL of water,and the organic solvent was removed by evaporation at reduced pressure.The solid residue was collected byﬁltration and puriﬁed by chromatography on silica with ethyl acetate.A mixture of 1.30g containing ﬂuorinated product and unﬂuorinated reactant with a4:1ratio was obtained.The calculated mass ofﬂuorinated product is1.04g,65%. 1HNMR(D6-DMSO):δ4.01-4.05(2H,t,J)3Hz),4.20-4.24 (2H,t,J)3Hz).MS(EI):Calcd,204.0;found(M-1)-,202.9. Octyl3-Fluoro-4,6-dihydrothieno[3,4-b]thiophene-2-carboxyl-ate(3).The raw materials of2(1.30g,6.3mmol),DCC(1.58g), and DMAP(260mg)were added to a50mL round-bottomﬂask with CH2Cl2(15mL).1-Octanol(8.22g,63mmol)was added to theﬂask and then stirred for20h under N2protection.The reaction mixture was poured to100mL of water and extracted with CH2Cl2. The organic phase was dried by sodium sulfate,and the solvent was removed.Column chromatography on silica gel using hexane/ CH2Cl2)1/1yielded the title compound as an oil(1.42g,71%). 1HNMR(CDCl3):δ0.85-0.91(3H,t,J)6Hz),1.22-1.45(10H, m),1.68-1.76(2H,m),3.99-4.02(2H,t,J)3Hz),4.15-4.18 (2H,t,J)3Hz),4.24-4.29(2H,t,J)6Hz).MS(EI):Calcd, 316.1;found(M+1)+,317.0.Octyl3-Fluorothieno[3,4-b]thiophene-2-carboxylate(5).A solution of compound3(1.42g,4.5mmol))in100mL of ethyl acetate was stirred and cooled in a dry ice bath.MCPBA(0.78g, 4.5mmol)in30mL of ethyl acetate was added dropwise to the reaction solution.The resulting mixture was stirred overnight.The solvent was removed by evaporation,and the residue contained a crude product of4and3-chlorobenzoic acid.The residue was reﬂuxed in acetic anhydride for2.5h.The mixture was cooled, and the solvent was removed by evaporation.The residue was puriﬁed byﬂash chromatography on silica gel with hexanes/ dichloromethane(2:1)to give compound5(0.95g,67%).1HNMR (CDCl3):0.85-0.92(3H,t,J)7Hz),1.23-1.48(10H,m), 1.70-1.80(2H,m),4.30-4.35(2H,t,J)7Hz),7.27-7.29(1H, d,J)3Hz),7.65-7.67(1H,d,J)3Hz).Octyl4,6-Dibromo-3-ﬂuorothieno[3,4-b]thiophene-2-carboxyl-ate(6).To a solution of compound5(0.95g,3.0mmol)in10mL of DMF was added dropwise a solution of NBS(1.34g,7.56mmol) in10mL of DMF under nitrogen protection in the dark.The reaction mixture was stirred at RT for24h.Then it was poured into saturated sodium sulﬁte solution in an ice-water bath and extracted with dichloromethane.The organic phase was collected and dried by sodium sulfate.Removal of the solvent and column puriﬁcation on silica get using dichloromethane/hexane(1/4)yielded the target product(0.98g,69%)as a light yellow solid.1HNMR (CDCl3):0.85-0.92(3H,t,J)7Hz),1.23-1.48(10H,m), 1.70-1.80(2H,m),4.30-4.35(2H,t,J)7Hz).MS(EI):Calcd, 469.9;found(M+1)+,471.9.4,8-Bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene(8).The benzo[1,2-b:4,5-b′]dithiophene-4,8-dione(1.0g,4.5mmol)was mixed with zinc dust(0.65g,10mmol)in aﬂask.Ethanol(4mL) and NaOH solution(15mL,20%)were added,and the mixture was reﬂuxed for1h.2-Ethylhexyl p-toluenesulfonate(4.3mL)was added in portions with stirring until the color changed to red.The resulting precipitate wasﬁltered;theﬁltrate was diluted with100 mL of water and extracted with chloroform(100mL).The organic extraction was dried with anhydrous sodium sulfate and evaporated in vacuo.Column chromatography on silica gel using dichlo-romethane and hexanes mixed eluents yielded the compound8as a light yellow oil(0.80g,40%).1HNMR(CDCl3):δ0.72-0.90 (12H,m),1.10-1.38(16H,m),1.50-1.62(2H,m),3.84-3.97(4H, m),7.32-7.36(2H,d,J)5Hz),7.46-7.50(2H,d,J)5Hz). MS(EI):Calcd,446.2;found(M+1)+,447.2.2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene(9).Compound8(0.62g,1.4mmol)was dissolved in20 mL of anhydrous THF and cooled in an acetone/dry ice bath under nitrogen protection.Butyllithium solution(1.4mL,3.5mmol)was added dropwise with stirring,after the addition the mixture was kept in a dry ice bath for30min and at RT for30min.The mixture was cooled in the dry ice bath and trimethyltin chloride solution(4.2mL, 4.2mmol,1M in hexane)was added,and the mixture was stirred at RT overnight.The mixture was quenched with50mL of water and extracted with hexanes.The organic extraction was dried with anhydrous sodium sulfate and evaporated in vacuo.Recrystallization of the residue from isopropanol yield the compound9as colorless needles(0.8670g,80%).1HNMR(CDCl3):δ0.43(18H,s),0.90-1.10 (12H,m),1.33-1.85(18H,m),1.54-1.58(4H,m),4.15-4.23(4H, d,J)5Hz),7.51(2H,s).4,8-Dioctynbenzo[1,2-b:4,5-b′]dithiophene(10).Isopropylmag-nesium chloride(2M solution,12mL,24mmol)was added dropwise to a solution of1-octyne(2.97g,27mmol)at RT.The reaction mixture was heated up to60°C and stirred for100min. It was cooled to RT,and benzo[1,2-b:4,5-b′]dithiophene-4,8-dione (1g,4.53mmol)was added.The reaction mixture was heated up to60°C and kept for60min.It was then cooled to RT,and7g of SnCl2in HCl solution(16mL10%)was added dropwise to the reaction mixture.The reaction mixture was heated at65°C for60 min,then cooled down to room temperature,and poured into100 mL of water.It was extracted with50mL of hexanes twice.The organic phase was combined and dried with anhydrous Na2SO4, and the organic solvent was removed by vacuum evaporation.The residue was puriﬁed by column chromatography on silica with hexanes/dichloromethane(3/1,volume ratio),yielding compound 10(1.66g,90%).1HNMR(CDCl3):δ0.88-0.96(6H,t,J)5 Hz),1.32-1.42(8H,m),1.53-1.63(4H,m),1.68-1.77(4H,m), 2.61-2.66(4H,t,J)7Hz),7.48-7.51(2H,d,J)6Hz), 7.56-7.58(2H,d,J)6Hz).MS(EI):Calcd,406.2;found(M+ 1)+,407.1.4,8-Dioctylbenzo[1,2-b:4,5-b′]dithiophene(11).To the solution of compound2(1.66g,4.07mmol)in75mL of THF was added Pd/C(0.45g,10%),and the reaction mixture was kept in a hydrogen atmosphere for18h at RT.The mixture wasﬁltered with Celite, and the solvent was removed by vacuum evaporation.The residue was puriﬁed by column chromatography on silica with hexane as the eluent,yielding compound11(0.95g,56%)as white solids. 1HNMR(CDCl3):δ0.85-0.92(6H,t,J)7Hz),1.20-1.40(16H, m),1.40-1.50(4H,m),1.76-1.84(4H,m),3.13-3.21(4H,t,J)8Hz),7.43-7.45(2H,d,J)6Hz),7.46-7.48(2H,d,J)6 Hz).MS(EI):Calcd,414.2;found(M+1)+,415.2.2,6-Bis(trimethyltin)-4,8-dioctylbenzo[1,2-b:4,5-b′]dithiophene (12).Compound3(0.95g,2.3mmol)was dissolved in20mL of anhydrous THF and cooled in an acetone/dry ice bath under nitrogen protection.Butyllithium solution(2.3mL,5.7mmol)was added dropwise with stirring.The mixture was kept in a dry ice bath for 30min and then at RT for30min.The mixture was cooled in the dry ice bath,and6.5mL(6.5mmol)of trimethyltin chloride solution (1M in hexane)was added and stirred at RT for overnight.The mixture was quenched with50mL of water and extracted with hexanes.The organic extraction was dried with anhydrous sodium sulfate and evaporated in vacuo.Recrystallization of the residue from isopropanol yields the titled compound12(0.50g,88%). 1HNMR(CDCl3):δ0.45(18H,s),0.87-0.91(6H,t,J)7Hz), 1.25-1.42(16H,m),1.42-1.51(4H,m),1.76-1.85(4H,m), 3.17-3.23(4H,t,J)8Hz),7.49(2H,s).Synthesis of Polymers.PTB4.Octyl-6-dibromo-3-ﬂuo-rothieno[3,4-b]thiophene-2-carboxylate(6)(236mg,0.50mmol) was weighted into a25mL round-bottomﬂask.2,6-Bis(trim-ethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophe-ne(9)(386mg,0.50mmol)and Pd(PPh3)4(25mg)were added. Theﬂask was subjected to three successive cycles of vacuum followed by reﬁlling with argon.Then,anhydrous DMF(2mL) and anhydrous toluene(8mL)were added via a syringe.The polymerization was carried out at120°C for12h under nitrogen protection.The raw product was precipitated into methanol and collected byﬁltration.The precipitate was dissolved in chloro-7794J.AM.CHEM.SOC.9VOL.131,NO.22,2009A R T I C L E S Liang et al.form and ﬁltered with Celite to remove the metal catalyst.The ﬁnal polymers were obtained by precipitating in hexanes and drying in vacuum for 12h,yielding PTB4(309mg,82%).1HNMR (CDCl 2CDCl 2):δ0.80-2.40(45H,br),3.90-4.70(6H,br),7.00-7.90(2H,br).GPC:M w (19.3×103g/mol),PDI (1.32).PTB2,PTB3,PTB5,and PTB6are synthesized according to the same procedure as PTB4with respective monomers.The 1HNMR and gel permeation chromatography (GPC)data of the polymers are listed below.PTB2.1HNMR (CDCl 2CDCl 2):δ0.70-2.42(45H,br),3.90-4.80(6H,br),6.70-8.00(3H,br).GPC:M w (23.2×103g/mol),PDI (1.38).PTB3.1HNMR (CDCl 2CDCl 2):δ0.70-2.35(45H,br),2.90-3.40(4H,br),4.20-4.70(2H,br),6.70-8.20(3H,br).GPC:M w (23.7×103g/mol),PDI (1.49).PTB5.1HNMR (CDCl 2CDCl 2):δ0.90-2.40(45H,br),3.90-4.70(6H,br),7.00-7.60(2H,br),7.60-8.10(1H,br).GPC:M w (22.7×103g/mol),PDI (1.41).PTB6.1HNMR (CDCl 2CDCl 2):δ0.70-2.42(53H,br),3.90-4.80(6H,br),6.70-8.00(3H,br).GPC:M w (25.0×103g/mol),PDI (1.50).Characterization.1H NMR spectra were recorded at 400or 500MHz on Bruker DRX-400or DRX-500spectrometers,respectively.Molecular weights and distributions of polymers were determined by using GPC with a Waters Associates liquid chromatograph equipped with a Waters 510HPLC pump,a Waters 410differential refractometer,and a Waters 486tunable absorbance detector.THF was used as the eluent and polystyrene as the standard.The optical absorption spectra were taken by a Hewlett-Packard 8453UV -vis spectrometer.Cyclic voltammetry (CV)was used to study the electrochemical properties of the polymers.For calibration,the redox potential of ferrocene/ferrocenium (Fc/Fc +)was measured under the same conditions,and it is located at 0.09V to the Ag/Ag +electrode.It is assumed that the redox potential of Fc/Fc +has an absolute energy level of -4.80eV to vacuum.11The energy levels of the highest (HOMO)and lowest unoccupied molecular orbital (LUMO)were then calculated according to the following equationswhere φox is the onset oxidation potential vs Ag/Ag +and φred is the onset reduction potential vs Ag/Ag +.Hole mobility was measured according to a similar method described in the literature,12using a diode conﬁguration of ITO/poly(ethylenedioxythiophene)doped with poly(styrenesulfonate)(PEDOT:PSS)/polymer/Al by taking current -voltage current in the range of 0-6V and ﬁtting the results to a space charge limited form,where the SCLC is described bywhere ε0is the permittivity of free space,εr is the dielectric constant of the polymer,µis the hole mobility,V is the voltage drop across the device,L is the polymer thickness,and V )V appl -V r -V bi ,where V appl is the applied voltage to the device,V r is the voltage drop due to contact resistance and series resistance across the electrodes,and V bi is the built-in voltage due to the difference in work function of the two electrodes.The resistance of the device was measured using a blank conﬁguration ITO/PEDOT:PSS/Al andwas found to be about 10-20Ω.The V bi was deduced from the best ﬁt of the J 0.5versus V appl plot at voltages above 2.5V and is found to be about 1.5V.The dielectric constant,εr ,is assumed to be 3in our analysis,which is a typical value for conjugated polymers.The thickness of the polymer ﬁlms is measured by using AFM.Device Fabrication.The polymers PTB1-PTB6were codis-solved with PC 61BM in 1,2-dichlorobenzene (DCB)in the weight ratio of 1:1,respectively.PTB1,PTB2,and PTB6concentrations are 10mg/mL,while PTB3,PTB4,and PTB5concentrations are 13mg/mL.For the last three polymer solutions,we also studied mixed solvent effect with about 3%(volume)1,8-diiodooctance,also used to further improve the ﬁnal device performances.ITO-coated glass substrates (15Ω/0)were cleaned stepwise in detergent,water,acetone,and isopropyl alcohol under ultrasoni-cation for 15min each and subsequently dried in an oven for 5h.A thin layer (∼30nm)of PEDOT:PSS (Baytron P VP A14083)was spin-coated onto ITO surface which was pretreated by ultraviolet ozone for 15min.Low-conductivity PEDOT:PSS was chosen to minimize measurement error from device area due to lateral conductivity of PEDOT:PSS.After being baked at 120°C for ∼20min,the substrates were transferred into a nitrogen-ﬁlled glovebox (<0.1ppm O 2and H 2O).A polymer/PCBM composites layer (ca.100nm thick)was then spin-cast from the blend solutions at 1000rpm on the ITO/PEDOT:PSS substrate without further special treatments.Then the ﬁlm was transferred into a thermal evaporator which is located in the same glovebox.A Ca layer (25nm)and an Al layer (80nm)were deposited in sequence under the vacuum of 2×10-6torr.The effective area of ﬁlm was measured to be 0.095cm 2.Current -Voltage Measurement.The fabricated device was encapsulated in a nitrogen-ﬁlled glovebox by UV epoxy (bought fromEpoxyTechnology)andcoverglass.Thecurrentdensity -voltage (J -V)curves were measured using a Keithley 2400source-measure unit.The photocurrent was measured under AM 1.5G illumination at 100mW/cm 2under the Newport Thermal Oriel 911921000W solar simulator (4in.×4in.beam size).The light intensity was determined by a monosilicon detector (with KG-5visible color ﬁlter)calibrated by National Renewable Energy Laboratory (NREL)to minimize spectral mismatch.External quantum efﬁciencies (EQEs)were measured at UCLA by using a lock-in ampliﬁer (SR830,Stanford Research Systems)with current preampliﬁer (SR570,Stanford Research Systems)under short-circuit conditions.The devices were illuminated by mono-chromatic light from a xenon lamp passing through a monochro-mator (SpectraPro-2150i,Acton Research Corporation)with a typical intensity of 10µW.Prior to incident on the device,the monochromic incident beam is chopped with a mechanical chopper connected to the lock-in ampliﬁer and then focused on the testing pixel of the device.The photocurrent signal is then ampliﬁed by SR570and detected with SR830.A calibrated mono silicon diode with known spectral response is used as a reference.Conductive Atomic Force Microscopy (CAFM)Measure-ment.All CAFM measurements were done under ambient condi-tions using a commercial scanning probe microscope (Asylum Research,MFP-3D).Platinum-coated,contact-mode AFM canti-levers with spring constant of 0.2N/m and tip radius of ca.25nm (Budget Sensors)were used to map out the hole-current of ﬁlms in the dark using contact mode.The deﬂection set point is 0.3V and bias voltage is -2V for all the sample measurements,and the conditions used to prepare the ﬁlms are the same to make the solar cell device.Results and DiscussionSynthesis.The encouraging results of polymer PTB1leadus to select the polymer backbone as the structural platform to investigate structure/property relationship and to search for new polymers with improved solar cell performance.8The PTB1(11)Pommerehne,J.;Vestweber,H.;Guss,W.;Mahrt,R.F.;Bassler,H.;Porsch,M.;Daub,J.Ad V .Mater.1995,7,551.(12)(a)Malliaras,G.G.;Salem,J.R.;Brock,P.J.;Scott,C.Phys.Re V .B 1998,58,13411.(b)Goh,C.;Kline,R.J.;McGehee,M.D.;Kadnikova,E.N.;Frechet,J.M.J.Appl.Phys.Lett.2005,86,122110.E HOMO )-(φox +4.71)(eV)E LUMO )-(φred +4.71)(eV)J )9ε0εr µV 2/8L 3J.AM.CHEM.SOC.9VOL.131,NO.22,20097795Highly Efﬁcient Solar Cell Polymers A R T I C L E Swas designed based on the concept that the thienothiophene moiety can support the quinoidal structure and lead to narrow polymer band gap,which is crucial to efﬁciently harvesting solar energy.Since the thienothiophene moiety is very electron-rich,an electron-withdrawing ester group is introduced to stabilize the resulting polymers.Indeed,the results conﬁrmed our design idea.9However,we also noticed that the long n -dodecyl side chain is grafted on the thieno[3,4-b ]thiophene ester in PTB1,which may decrease the miscibility of the conjugated polymer with PC 61BM and affect the formation of effective interpenetrat-ing network.13The small V oc value indicates the need to adjust the HOMO -LUMO energy level relative to fullerides.4With these considerations in mind,we have synthesized six related polymers as shown in Figure 1.The polymerization was carried out via the Stille polycon-densation reaction.14The corresponding monomers were syn-thesized according to Scheme 1.To shorten the dodecyl ester chain in PTB1,an n -octyl side chain substituted polymer was synthesized.However,this polymer exhibits poor solubility,which limits its processing ability,and was not studied further.Soluble PTB2was synthesized with shortened and branched side chains.For comparison,a bulkier branched side chain,2-butyloctyl,was used in PTB6.The branched side chain can also be grafted to the benzodithiophene,which leads to PTB5with two 2-ethylhexyloxy side chains attached to the ben-zodithiophene ring.The alkoxy groups grafted on ben-zodithiophene ring are strong electron-donating groups that can raise the HOMO energy level of the polymer.15This will lead to the reduction in V oc ,detrimental to the performance of polymer solar cells.4In order to further adjust the polymer’s electronic properties,PTB3with less electron-donating alkyl chains in benzodithiophene was synthesized.To further lower the HOMO level,a second electron-withdrawing group can be introduced to the 3position of the thieno[3,4-b ]thiophene ring.Fluorine is a good candidate to functionalize the 3position because ﬂuorine has a high electronegativity.The size of the ﬂuorine atom is small,which will introduce only small steric hindrance for the conﬁgurationand packing of the polymer.16The ﬂuorinated thieno[3,4-b ]thiophene was synthesized via a modiﬁed route previously reported for ester-substituted thieno[3,4-b]thiophene (Scheme 1).9The ﬂuorine was introduced to the fused ring unit from 4,6-dihydrothieno[3,4-b ]thiophene-2-carboxylic acid after depro-tonation by using BuLi and reacting with PhSO 2NF.The ﬂuorinated acid was ﬁrst converted to ester and then dibromo-substituted thieno[3,4-b ]thiophene.Initially,we attempted to introduce ﬂuorine atom to PTB1(R 1)n -dodecyl,R 2)n -octyloxy).The obtained polymer exhibited poor solubility and only dissolves in dichlorobenzene over 100°C,which makes it difﬁcult to prepare uniform ﬁlms.To increase the solubility,benzodithiophene substituted with branch side chains was used,and the ﬂuorinated polymer,PTB4,was obtained.The structures of polymers were characterized with 1HNMR spectroscopy,all consistent with the proposed ones.Gel permeation chromatography (GPC)studies showed that these polymers have similar weight-averaged molecular weights between 19.3and 25.0kg/mol with a relatively narrow polydispersity index (PDI)between 1.25and 1.50.The results indicate that the changes in monomer structures did not lead to signiﬁcant changes in polymerization reaction.These polymers have good solubility in chlorinated solvents,such as chloroform and chlorobenzene.Thermogravimetric analyses (TGA)indicate that the polymers are stable up to about 200°C.Electrochemical and Optical Properties.The HOMO and LUMO energy levels of the polymers were determined by cyclic voltammetry (CV),and the results are summarized in Figure 2a.The HOMO energy levels of the polymers are very close except for PTB3and PTB4.From the comparison of PTB2and PTB3,it was noticed that the substitution of octyloxy side chain to octyl side chain lowered the HOMO energy level of the polymer from -4.94to paring PTB4and PTB5,polymers with same side chain patterns,it is clear that the introduction of the electron-withdrawing ﬂuorine in the polymer backbone signiﬁcantly lowered the HOMO level.The ﬁlm absorption spectra of the polymers are showed in Figure 2b,and characteristics of the polymer absorption are summarized in Table 1.All these polymers show very similar absorption spectra;the changes of the absorption peak and onset point among the polymers are within 25nm.(13)Thompson,B.C.;Kim,B.J.;Kavulak,D.F.;Sivula,K.;Mauldin,C.;Frechet,J.M.J.Macromolecules 2007,40,7425.(14)Bao,Z.N.;Chan,W.K.;Yu,L.P.J.Am.Chem.Soc.1995,117,12426.(15)Daoust,G.;Leclerc,M.Macromolecules 1991,24,455.(16)Babudri,F.;Farinola,G.M.;Naso,F.;Ragni,mun.2007,1003.Figure 2.(a)HOMO and LUMO energy levels of the polymers.Energy levels of PC 61BM are listed for comparison.(b)UV -vis absorption spectra of thepolymer ﬁlms.7796J.AM.CHEM.SOC.9VOL.131,NO.22,2009A R T I C L E S Liang et al.Hole Mobility.The hole mobility of the polymers is measured according to method based on the space charge limited current (SCLC)model,12and the results are plotted in Figure3.The hole mobilities of4.7×10-4,4.0×10-4,7.1×10-4,7.7×10-4,4.0×10-4,and2.6×10-4cm2/V·s are found for PTB1, PTB2,PTB3,PTB4,PTB5,and PTB6,respectively.A small decrease of the polymer hole mobility is observed after the introduction of bulky branched side chains to the polymer backbones.It is expected that the bulky side chains may increase the steric hindrance for intermolecular packing,so the hole mobility decreases.This explains that the largest decrease of the hole mobility happens to PTB6,which has the bulkiest 2-butyloctyl side chain on the ester group.It is interesting to note that the alkyl-grafted PTB3has higher mobility than the alkoxy-grafted PTB2,though they both have similar side chain patterns.PTB4has the largest hole mobility of7.7×10-4cm2/ V·s among these polymers.It has been reported that there is a strongπ-stacking interaction between the electron-deﬁcientﬂuorinated aromatic rings and the electron-rich nonﬂuorinated ones in theﬂuorine-substituted aromatic moieties.17The increase of mobility inﬂuorinated PTB4is probably due to the increase in intermolecular packing between theﬂuorinated backbone. Detailed studies by using grazing angle X-ray diffraction are in progress to elucidate polymer structures and will be presented in a future publication.Photovoltaic Properties.Photovoltaic properties of the poly-mers were investigated in solar cell structures of ITO/PEDOT: PSS/polymer:PC61BM(1:1,wt ratio)/Ca/Al.The polymer active layers were spin-coated from a dichlorobenzene solution.Figure 4shows the photo J-V curves of the polymer solar cells under AM1.5condition at100mW/cm2.Representative characteristics of the solar cells are summarized in Table2.Generally,the bulky side chain grafted polymers show larger V oc than PTB1, as they have lower HOMO energy levels.The alkyl-substituted PTB3has an enhanced V oc compared to PTB2,which is expected from the HOMO energy level difference.Theﬂuori-nated polymer PTB4devices showed a larger V oc than PTB5. However,except for PTB2and PTB3,the other polymer solar cells suffer obvious decrease in short-circuit current(J sc)and ﬁll factor(FF)compared to the PTB1solar cell.Further studies by using transmission electron microscopy(TEM)indicated that the poor solar cell performances in PBT3-PTB6are related to the nonoptimized morphology,which has a large effect on the BHJ polymer solar cell performance(Figure5).18The TEM images of PTB2/PC61BM blendﬁlm showﬁner features comparable to the PTB1one,which may be due to the increase in the miscibility of the polymer with PC61BM after shortening the dodecyl side chain into the2-ethylhexyl side chain.As a result,J sc and FF in the PTB2solar cell are slightly larger than the PTB1one.However,large domains are observed in their PC61BM blendﬁlms of PTB5or PTB6.The bulky side chains reduce the miscibility of polymer with PC61BM,leading to better phase separation between polymer chains and PC61BM mol-ecules.As a result,the interfacial areas of charge separation in PTB5or PTB6are reduced,and the polymer solar cell performances are diminished.It is not coincident that the PTB6 has the largest feature sizes(150-200nm)in the TEM image and its solar cell performance is the worst.With the same side chain patterns as PTB5,theﬂuorinated PTB4also suffers the nonoptimized morphology,as shown by the large features(over 100nm)in the TEM image of PTB4/PC61BM blendﬁlm. Although PTB4shows the lowest HOMO energy level and the largest hole mobility,its photovoltaic performance in simple polymer/PC61BM solar cells is modest(3.10%).Comparing (17)Feast,W.;Lovenich,P.W.;puschmann,H.;Taliani, C.Chem.Commun.2001,505.(18)(a)Yang,X.N.;Loos,J.Macromolecules2007,40,1353.(b)Li,G;Yao,Y.;Yang,H.C.;Shrotriya,V.;Yang,G.W.;Yang,Y.Ad V.Funct.Mater.2007,17,1636.Table1.Molecular Weight and Absorption Properties of the Polymerspolymers M w(kg/mol)PDIµpeak(nm)µonset(nm)E g opt(eV) PTB122.9 1.25687,638786 1.58 PTB223.2 1.38683,630780 1.59 PTB323.7 1.49682,628777 1.60 PTB419.3 1.32682,627762 1.63 PTB522.7 1.41677,623764 1.62PTB625.0 1.50675,6307681.61Figure3.J0.5vs V plots for the polymerﬁlms.The solid lines areﬁts of the data points.The thickness of theﬁlms is indicated in theplots.Figure4.Current-voltage characteristics of polymer/PC61BM solar cells under AM1.5condition(100mW/cm2).Table2.Characteristic Properties of Polymer Solar Cells polymers V oc(V)J sc(mA/cm2)FF(%)PCE(%)PTB10.5812.565.4 4.76PTB20.6012.866.3 5.10PTB30.7413.156.8 5.53PTB40.769.2044.5 3.10PTB50.6810.343.1 3.02PTB60.627.7447.0 2.26PTB3a0.7213.958.5 5.85PTB4a0.7413.061.4 5.90(6.10b) PTB5a0.6610.758.0 4.10a Devices prepared from mixed solvents dichlorobenzene/diiodooctance (97/3,v/v).b Value after spectral correction.J.AM.CHEM.SOC.9VOL.131,NO.22,20097797Highly Efﬁcient Solar Cell Polymers A R T I C L E S。

Computational Analysis of Enhanced Cooling Performance and Pressure Drop

CONTENTS
9.1 Background Information................................................................................ 250 9.1.1 Introduction........................................................................................ 250 9.1.2 Review of Experimental Evidence..................................................... 251 9.1.3 Review of Computational Analyses................................................... 253 9.1.4 Mixture Viscosity Models.................................................................. 254 9.2 Theory............................................................................................................ 254 9.2.1 Governing Equations......................................................................... 254 9.2.2 Mixture Properties............................................................................. 256 9.2.2.1 Effective Dynamic Viscosity............................................... 256 9.2.2.2 Effective Thermal Conductivity.......................................... 258 9.2.3 Entropy Generation............................................................................ 263 9.2.4 Thermal Performance Comparisons..................................................264 9.2.5 Numerical Method.............................................................................264 9.3 Results and Discussion.................................................................................. 265 9.3.1 Flow Friction Validation Studies....................................................... 265 9.3.2 Convective Heat-Transfer Validation Results....................................266 9.3.3 Friction Factor and Pressure Drop Results........................................266 9.3.4 Convective Heat Transfer................................................................... 269 9.3.5 Entropy Generation............................................................................ 269 9.4 Conclusion..................................................................................................... 271 Acknowledgment.................................................................................................... 273 References............................................................................................................... 273

JVB2016-2017年(97-101卷)Journal of Vocational Behavior文献目录及摘要摘取

表格 1题⽬目作者Highlights AbstractVolume-1011Cross-culturalwork andfamilyresearch: Areview of theliterature跨⽂文化⼯工作与家庭研究：⽂文献综述KristenM.Shockley a, , , JillDouek b,ChristineR.Smith b,Peter P.Yu b,SonerDumani c,1,KimberlyA.French d, 1Highlights•Anglo regions have receivedthe most attention, followed byEastern Europe.•Many studies focus on meandifferences across cultures inwork-family constructs.•The review of theory suggeststhat collectivism is the mostcommonly used theoreticalexplanation.•There is a great deal ofinconsistency in the way theoryhas been applied.AbstractResearch aimed at understanding the intersection of employees'work and family lives has blossomed over the past few years, and,in more recent times, has begun to have an increasingly globalfocus. Conducting research in diverse cultural settings is importantgiven that work and family dynamics are entrenched in largersocietal contexts, such as gender role norms, national policies, andcultural values. However, the literature has not developed in aprogrammatic way, making it difficult to build upon the currentknowledge base. The goal of this study is to review extant cross-cultural work and family published research in an effort tosynthesize and assess the current state of the literature, with afocus on theoretical logic and methodology. We do this by reportingdescriptive statistics regarding which global regions have receivedthe most attention, the way culture is considered analytically andtheoretically, and information about measurement. Through thisreview we aim to provide scholars with a more completeunderstanding of the state of cross-cultural work-family knowledgeand offer recommendations for future research that will facilitatetheoretical advancement.KeywordsCross-cultural; Work-family; Global; Collectivism; Genderegalitarianism本⽂文旨在了了解员⼯工⼯工作与家庭⽣生活的交叉点的研究在过去⼏几年年⾥里里已经蓬勃发展，⽽而且在最近的⼀一段时间⾥里里，全球范围内的⼯工作重⼼心开始逐渐扩⼤大。

Dye-Sensitized Solar Cells with Conversion Efficiency of 11.1%

Dye-Sensitized Solar Cells with Conversion Eﬃciency of11.1%Yasuo C HIBA,Ashraful I SLAM,Yuki W ATANABE,Ryoichi K OMIYA,Naoki K OIDE and Liyuan H ANÃSolar Systems Development Center,Sharp Corporation,282-1Hajikami,Katsuragi,Nara639-2198,Japan(Received June7,2006;accepted June9,2006;published online June23,2006)Dye-sensitized solar cells(DSCs)using titanium dioxide(TiO2)electrodes with diﬀerent haze were investigated.It was found that the incident photon to current eﬃciency(IPCE)of DSCs increases with increase in the haze of the TiO2electrodes, especially in the near infrared wavelength region.Conversion eﬃciency of11.1%,measured by a public test center,was achieved using high haze TiO2electrodes.This indicates that raising the haze of TiO2electrodes is an eﬀective technique for improvement of conversion eﬃciency.[DOI:10.1143/JJAP.45.L638]KEYWORDS:dye-sensitized solar cells,haze,internal resistance,TiO2electrode,IPCEDye-sensitized solar cells(DSCs)have been widely investigated as a next-generation solar cell because of their simple structure and low manufacturing cost.1,2)In general, a DSC comprises a nanocrystalline titanium dioxide(TiO2) electrode modiﬁed with a dye fabricated on a transparent conducting oxide(TCO),a platinum(Pt)counter electrode, and an electrolyte solution with a dissolved iodide ion/tri-iodide ion redox couple between the electrodes.Although certiﬁed conversion eﬃciency using black dye has been reported to be10.4%by the Swiss Federal Institute of Technology in Lausanne(EPFL),3)eﬃciency of over10% has rarely been achieved due to insuﬃcient understanding of the mechanism of DSCs,which is diﬀerent from that of conventional solar cells.It is well known that the conversion eﬃciency( )of solar cells can be represented as follows:4)¼FFÂI SCÂV OC=P in;ð1Þwhere FF,I SC,V OC,and P in areﬁll factor,short circuit current,open circuit voltage,and incident power,respec-tively.Equation(1)suggests that it should be necessary to improve these three parameters in order to raise conversion eﬃciency.In our previous studies,we investigated the internal resistance of DSCs using electrochemical impe-dance spectroscopy(EIS)measurement as a means of investigating DSC mechanisms and proposed an equivalent circuit for modeling DSCs based on the results of EIS analysis.5)We also found that the series resistance of DSCs consists of three resistance elements,namely,the sheet resistance of the TCO,the resistance of ionic diﬀusion in the electrolyte,and the resistance at the interface between the counter electrode and the electrolyte.We discovered that FF increases with decrease in the internal resistance elements and reported a resulting rise in certiﬁed conversion eﬃ-ciency to10.2%.6)In order to further improve the eﬃciency, we then considered ways of elevating the other factors such as short circuit current density(J SC)and V OC.There are two approaches to improving J SC.One is to develop a new dye which can absorb incident light of longer wavelengths,and the other is to increase the extent of light trapping within the TiO2electrodes.Many panchromatic dyes have been developed such as -diketonato Ru(II) complex.7,8)Unfortunately,the certiﬁed eﬃciency has not been improved as there is no dye superior to black dye.As for the latter approach,Usami made the theoreticalﬁnding that light scattering magnitude in DSCs could be controlled by adding submicron particles to TiO2electrodes composed of nanocrystalline particles.9)Most related experimental work involves making a rough estimate of the light-scattering magnitude from the light-harvesting eﬃciency (LHE)of the dye-treated TiO2electrodes.10)However,it is very diﬃcult to obtain the exact magnitude and to elevate it because LHE is aﬀected by the absorption characteristics of the dye.An eﬀective index of the light-trapping eﬀect of TiO2electrodes is therefore needed.In addition,it is very important to conﬁrm that the J SC obtained from current–voltage(I–V)measurement is con-sistent with the value estimated from incident photon to current eﬃciency(IPCE)spectra,because J SC can be described by integrating the product of the incident photon ﬂux density[Fð Þ]and IPCEð Þof the cell over the wavelength( )of the incident light,expressed asJ SC¼ZqFð Þð1Àrð ÞÞIPCEð Þd ;ð2Þwhere q is the electron charge and rð Þthe incident light loss in light absorption and reﬂection by the conducting glass.It is diﬃcult to obtain accurate IPCE spectra of DSCs because IPCE strongly depends on measurement conditions such as chopping frequency and bias light.Recently,Hishikawa et al.reported that IPCE spectra increase with decrease of chopping frequency under AC mode.They suggested that the IPCE of DSCs should be measured under white bias light and in AC mode with low chopping frequency of1–2Hz or in DC mode and that J SC measured from the I–V curve should be veriﬁed with the correct IPCE measurement.11) In the present letter,we attempt to introduce the concept of haze to estimate the optical path length in TiO2electrodes and discuss the relationship between haze in the infrared region of TiO2electrodes and IPCE spectra.We veriﬁed J SC not only by I–V measurement but also IPCE measurement. We also report the achievement of high performance in DSCs.TiO2electrodes with diﬀerent haze were prepared using a screen-printing method.DSCs were fabricated using a published procedure.6)The TiO2electrodes on TCO sub-strates were placed in a mixture solution of tert-butyl alcohol and acetonitrile(1:1)at a concentration of2Â10À4M of black dye.Deoxycholic acid was added to the dye solution as a coadsorbent at a concentration of20mM.Pt-coatedÃE-mail address:han.liyuan@sharp.co.jpglass was used for the counter electrodes.The composition of the electrolyte solution in acetonitrile was:dimethyl propyl imidazolium iodide (0.6M),lithium iodide (0.1M),iodine (0.05M),and tert -butylpryidine (0.5M).IPCE spectra were measured with monochromatic incident light of 1Â1016photon/cm 2under 100mW/cm 2white bias light in DC mode (CEP-2000,Bunkoh-Keiki).11)Current–voltage (I –V )characteristics of DSCs were measured using a digital source meter (2400,Keithley)under a standard air mass (AM 1.5)with simulated solar illumination at 100mW/cm 2(WXS-155S-10,Wacom).12)Haze was controlled by the addition of submicron particles (400nm diameter)to the TiO 2electrodes.Haze,deﬁned as the ratio of diﬀused transmittance to total optical transmittance,was measured at the wavelength of 800nm using an integral sphere.Figure 1shows the dependence of IPCE spectra on haze,which varied in the range from 3to 76%.Here,the haze at 800nm was used as an index as this is the wavelength in which we want to improve IPCE .Figure 1indicates that IPCE is greatly increased by increase in haze,especially in the infrared region,where it increased from 10to 50%with increase in haze from 3to 76%.On the other hand,IPCE spectra in the visible region,for example around 600nm,increased only gradually,from 65to ca.80%,with increase in haze from 3to 53%and reached saturation with further increase in haze.These results suggest that IPCE of 80%in the visible wavelength region is easily obtained using electrodes of medium haze TiO 2because of the large extinction coeﬃcient of the dye,while high IPCE in the infrared region demands high haze TiO 2because of the small extinction coeﬃcient of the dye.For example,black dye in ethanol has an extinction coeﬃcient of about 7Â103mol À1cm À1at around 600nm,while extinction coef-ﬁcient is about 200mol À1cm À1at around 800nm.3)Exper-imental evidence was thus found that the IPCE of DSCs is eﬀectively improved by increase in the haze of TiO 2electrodes.Figure 2shows the relationship between J SC calculated from eq.(2)and J SC measured from I –V characteristics at varying haze at 800nm.In the calculation,F ð Þis assumed to be AM 1.5standard spectrum.A linear relationship between measured J SC and calculated J SC is observed.The slope of the line is very close to 1,suggesting that IPCE could be accurately measured under DC mode.The reasonfor using DC mode can be explained as follows:in the equivalent circuit model proposed in our previous study,5)there are two large capacitance elements (1F and 2m F)parallel to the series resistance.It is these capacitance elements that delay the response of the DSCs.Since it allows the eﬀect of the capacitance elements to be excluded,the DC mode as used here is preferable for IPCE measurement.As a result,the highest J SC of 21mA/cm 2was achieved when using TiO 2electrodes with haze of 76%.This result indicates that haze is a useful index of the improvement of J SC and energy conversion eﬃciency in DSCs.When using TiO 2electrodes with haze of 76%,we almost always obtained high J SC of 20.8–21.1mA/cm 2.The cell gap was also investigated.It was found that the resistance of the ionic diﬀusion in the electrolyte decreases when the counter electrode is placed directly on the TiO 2electrode,so that the total series resistance was successfully decreased to 1.6 cm 2.For the sake of fairness,I –V characteristics were independently measured by the Research Center for Photo-voltaics,the National Institute of Advanced Industrial Science and Technology (AIST,Japan),using a black metal mask with an aperture area of 0.219cm 2under standard AM 1.5sunlight (100.0mW/cm 2),as shown in Fig.3.An overall conversion eﬃciency of 11.1%was achieved,which is the highest eﬃciency conﬁrmed by a public test center.In conclusion,we investigated the improvement of DSC performance using TiO 2electrodes with high haze.It was found that the increase of haze in TiO 2electrodes at 800nm eﬀectively improves the IPCE of DSCs.It is therefore proposed that haze is a useful index of improvement in conversion eﬃciency.Moreover,high J SC was veriﬁed not only by I –V measurement but also by precise IPCE measurement in DC mode.The highest DSC eﬃciency recorded so far,11.1%(0.219cm 2),was obtained and conﬁrmed by AIST.These ﬁndings appear to emphasize the important role of the haze of TiO 2electrodes in device performance and should aid the further development of DSCs for photovoltaic applications.0204060801004006008001000Wavelength (nm)I P C E (%)Fig.1.Dependence of IPCE spectra on haze of TiO 2electrodes.Haze in the ﬁgure was measured at 800nm.1015202510152025Measured Jsc (mA/cm 2)C a l c u l a t e d J s c (m A /c m 2)Fig.2.Relationship between J SC calculated from eq.(2)and J SC measured from I –V characteristics.The solid line shows a good ﬁt to the data,y ¼0:99x À0:008.Percentage values indicate the haze of each TiO 2electrode at 800nm.1) B.O’Regan and M.Gra ¨tzel:Nature 353(1991)737.2)M.K.Nazeeruddin,A.Kay,I.Rodicio,R.Humphry-Baker,E.Mu ¨ller,P.Liska,N.Vlachopoulos and M.Gra ¨tzel:J.Am.Chem.Soc.115(1993)6382.3)M.K.Nazeeruddin,P.Pe´chy,T.Renouard,S.M.Zakeeruddin,R.Humphry-Baker,te,P.Liska,L.Cevey,E.Costa,V.Shklover,L.Spiccia,G.B.Deacon,C.A.Bignozzi and M.Gra ¨tzel:J.Am.Chem.Soc.123(2001)1613.4)See for example,S.M.Sze:Physics of Semiconductor Devices (Wiley,New York,1981)2nd ed.,p.807.5)L.Han,N.Koide,Y.Chiba and T.Mitate:Appl.Phys.Lett.84(2004)2433.6)L.Han,N.Koide,Y.Chiba,A.Islam,R.Komiya,N.Fuke,A.Fukuiand R.Yamanaka:Appl.Phys.Lett.86(2005)213501.7)A.Islam,H.Sugihara,M.Yanagida,K.Hara,G.Fujihashi,Y.Tachibana,R.Katoh,S.Murata and H.Arakawa:New J.Chem.26(2002)966.8) A.Islam,F.A.Chowdhury,Y.Chiba,R.Komiya,N.Fuke,N.Ikeda and L.Han:Chem.Lett.34(2005)344.9) ami:Sol.Energy Mater.Sol.Cells 64(2000)73.10)Y.Tachibana,K.Hara,K.Sayama and H.Arakawa:Chem.Mater.14(2002)2527.11)Y.Hishikawa,M.Yanagida and N.Koide:Proc.31st IEEE Photo-voltaic Specialists Conf.,Florida,2005,p.67.12)N.Koide and L.Han:Rev.Sci.Instrum.75(2004)2828.Fig.3.Current–voltage characteristics of DSC sensitized with black dye.Results were measured at 25 C with an aperture area of0.219cm 2using a black metal mask and irradiance of 100.0mW/cm 2.Short circuit current I SC ¼4:57mA;short circuit current density J SC ¼20:9mA/cm 2;open circuit voltage V OC ¼736mV;ﬁll factor FF ¼72:2%;maximum power voltage V PMAX ¼583mV;maximum power current I PMAX ¼4:16mA;maximum power P MAX ¼2:429mW;eﬃciency =11.1%.The solid line and dashed line indicate current and power,respectively.This measurement was independently carried out by the Research Center for Photovoltaics,AIST.。

yantubbs-Discrete_element_simulation_of_crushable_soil_-

In view of the importance of soil crushability to stresslevel effects, further studies have been carried out to understand the statistics of individual grain breakages. Results of single-particle crushing tests have been compared with the one-dimensional compression of samples of the same u et al., 2001). They showed
de la vitesse de chargement sur la position des courbes de compression sous un angle pratique : arriver a` mener
des simulations DEM aussi rapidement que possible sans cre´er d’erreurs d’inertie. D’autres chemins de contrainte, incluant celui de compression triaxiale ‘draine´e’, celui a` contrainte moyenne effective constante et celui a` volume constant ont e´galement e´te´ simule´s a` partir de diffe´rentes contraintes initiales le long de la courbe vierge e–log p9. Nous comparons les re´sultats nume´riques de manie`re quantitative avec les donne´es des essais sur le sable siliceux. Ces re´sultats sont e´galement e´tudie´s aﬁn de caracte´riser les sols sujets au broyage lors d’une compression normale, a` la rupture et a` l’e´tat critique. Les simulations de sols sujets a` l’e´crasement, utilisant la me´thode DEM, donnent des informations pre´cieuses sur les origines microme´caniques de la plasticite´ des sols.

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TheoreticalStudiesofSolidBicyclo-HMX:EffectsofHydrostaticPressureandTemperature

LingQiu,†,‡Wei-HuaZhu,†Ji-JunXiao,†andHe-MingXiao*,†InstituteforComputationinMolecularandMaterialScience,SchoolofChemicalEngineering,NanjingUniVersityofScienceandTechnology,Nanjing210094,People’sRepublicofChina,andJiangsuInstituteofNuclearMedicine,KeyLaboratoryofNuclearMedicine,MinistryofHealth,Wuxi214063,People’sRepublicofChina

ReceiVed:January31,2007;InFinalForm:January5,2008

Onthebasisofdensityfunctionaltheory(DFT)andmoleculardynamics(MD),thestructural,electronic,andmechanicalpropertiesoftheenergeticmaterialbicyclo-HMXhavebeenstudied.ThecrystalstructureoptimizedbytheLDA/CA-PZmethodcompareswellwiththeexperimentaldata.Bandstructureanddensityofstatescalculationsindicatethatbicyclo-HMXisaninsulatorwiththebandgapofca.3.4eVandtheN-NO2bond

isthereactioncenter.Thepressureeffectonthebulkstructureandpropertieshasbeeninvestigatedintherangeof0-400GPa.Thecrystalstructureandelectroniccharacterchangeslightlyasthepressureincreasesfrom0to10GPa;whenthepressureisover10GPa,furtherincrementofthepressuredeterminessignificantchangesofthestructuresandlargebroadeningoftheelectronicbandstogetherwiththebandgapdecreasingsharply.Thereisalargercompressionalongthec-axisthanalongthea-andb-axes.Toinvestigatetheinfluenceoftemperatureonthebulkstructureandproperties,isothermal-isobaricMDsimulationsareperformedonbicyclo-HMXinthetemperaturerangeof5-400K.Itisfoundthattheincreaseoftemperaturedoesnotsignificantlychangethecrystalstructure.Thethermalexpansioncoefficientscalculatedforthemodelindicateanisotropicbehaviorwithslightlylargerexpansionalongthea-andc-axesthanalongtheb-axis.

1.IntroductionSolidenergeticmaterialshavelongplayedanimportanttechnologicalroleasexplosivesandfuels.Cyclicnitramines,suchasRDX(1,3,5-trinitro-1,3,5-triazacyclohexane)andHMX(1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane),arepowerfulsolidexplosivesthatareusedextensivelyinbothcivilianandmilitaryfields.Inthepastdecades,however,thereisanincreasinginterestinlookingforenergeticmaterialswithsuperiordetonationperformancesoverthecurrentlyusedmaterials.1-4Thepolycyclicandcagednitraminesattractedparticularattentionsincetheywouldgiveabettercombinationofstability,oxygenbalance,heatofformation,density,anddetonationperformancethantheopencounterpartsorcarbocy-clicanalogues,forexample,TNAD(trans-1,4,5,8-tetranitro-1,4,5,8-tetraazadecalin)5andHNIW(2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane).6TheyappeartobethefuturecandidatestocompetewithHMX,whichisoneofthemostpowerfulwidelyusedexplosives.Thecompoundcis-2,4,6,8-tetranitro-1H,5H-2,4,6,8-tetraazabicyclo[3.3.0]octane,com-monlycalled“bicyclo-HMX”becauseofitsstructuralanalogywithHMX,isanimportantpolycyclicnitramineexplosiveandcanbeemployedinsolidpyrotechniccompositions,forexample,insolidpropellantsorinplastic-bondedexplosives.Figure1showsaschematicillustrationofthemolecularandcrystalstructuresofbicyclo-HMX,andthenumberingoftheatomsthatwillbeusedinsubsequentdiscussionsisalsoshown.Ascanbeseen,itcontainsfewerHatomsandlargerringstrainthanHMXandwouldexhibitsuperiorexplosiveperformancetoHMX.Therefore,therehasbeenasignificanthistoryofinterestinitsstructuralandexplosivepropertiessinceitwasfirstreported.7-19Materialssuchaspropellantsandexplosivescontaintightlybondedgroupsofatomsthatretaintheirmolecularcharacteruntilasufficientstimulusisappliedtocauseexothermicdissociation.Thisinturntriggersfurtherdissociationleadingtoinitiationorignition.Themacroscopicbehaviorisultimatelycontrolledbymicroscopicpropertiessuchastheelectronicstructureandinteratomicforces.Thus,adesiretoprobemorefundamentalquestionsrelatingtothebasicpropertiesofbicyclo-HMXasasolidenergeticmaterialisgeneratingsignificantinterestinthebasicsolid-statepropertiesofsuchenergeticsystems.Althoughthedetaileddecompositionmechanismbywhichbicyclo-HMXreleasesenergyundermechanicalshockisstillnotwell-understood,ithasbeensuggestedthatitsdecompositionmayresultfromtransferringthermalandme-chanicalenergyintotheinternaldegreesoffreedomoftightlybondedgroupsofatomsinsolid.8,20-22Therefore,theknowledge

ofitsbulk,electronic,andmechanicalpropertiesappearstobeveryimportantinunderstandingitsexplosiveproperties.Theinvestigationofthemicroscopicpropertiesofenergeticmaterials,whichpossessacomplexchemicalbehavior,remainsachallengingtask.Animportantchallengeforexperimentalmeasurementsistopredictthepressureandtemperatureeffectsonexplosivematerialsunderextremeconditions.Pressureandtemperaturearetwoimportantvariablesthatcanbeusedtotunethephysicalpropertiesofmaterials.Considerableamountsofexperimentaleffort23-33havebeendevotedtoinvestigatethe