Materials Science and Engineering B-Optimizing the design of CIGS-based solar cells

Materials Science and Engineering B116(2005)293–296Optimizing the design of CIGS-based solar cells:a computational approachS.Al.Alagappan a,S.Mitra b,∗a Department of Electrical Engineering,600South College Avenue,University of Tulsa,Tulsa,OK74104-3189,USAb Department of Physics and Engineering Physics,600South College Avenue,University of Tulsa,Tulsa,OK74104-3189,USAReceived15November2003;received in revised form28April2004;accepted5May2004AbstractWe have used a program called analysis of microelectronic and photonic structures-one dimensional(AMPS-1D)to design two distinct types of CuIn(1−x)Ga x Se2(CIGS)-based solar cells and study their device performance.Thefirst type of design was a single layer CuIn(1−x)Ga x Se2 absorber layer.The thickness(d)varied from1.0to5␮m and device parameters likefill factor(FF),open-circuit voltage(V oc),short-circuit current density(J sc)and efficiency(η)were studied as a function of the thickness of the absorber layer and the gallium content,x.In the second type of design,the absorber layer was a bi-layered heterojunction with CuGaSe2(CGS)as the top layer and CuInSe2(CIS)as the bottom layer.Once again the device parameters were studied as a function of the total absorber layer thickness and the relative thicknesses of the absorber layers.In both cases,30nm of cadmium sulfide(CdS)and150nm of zinc oxide(ZnO)were used as window layer and top contact layer,respectively.The thicknesses of these layers were held constant.The front and the back contacts were assumed to be ohmic. It was found that the efficiencyηdepends both on the gallium content,x,and the thickness of the absorber layers.For bi-layer cells,it is also a function of the relative thicknesses of the layers.η,for single layered cells,was found to depend both on the absorber layer thickness and the gallium content,x.As the thickness was increased(x=constant),ηincreased from∼16%(1␮m thick absorber layer)to∼20%(for5␮m thick absorber layer).Any change ofηwas minimal for absorber layers containing between20and60%gallium.For bi-layersηdropped sharply when a thin layer of CGS was added as a top layer.ηthen increased steadily until the CGS thickness was about90%of the total thickness.©2004Elsevier B.V.All rights reserved.Keywords:Computer modelling;Solar cells;CIGS;Single-layer;Bi-layer;Device parameters1.IntroductionSolar cells based on CuIn(1−x)Ga x Se2(CIGS)are partic-ularly attractive since the device parameters can be changed continuously by changing the stoichiometry of the absorber material.CIGS is a direct band gap material with the bandgap (E g)ranging from0.98to1.68eV for x=0(CIS)and x=1 (CGS),respectively[1].Thus,the absorption coefficients for these materials are high.The CIGS based solar cells have largest efficiencies on the laboratory scale and as well as on the level of large-area modules.In addition to high effi-ciencies,CIGS thin-film modules exhibit excellent outdoor ∗Corresponding author.Tel.:+19186313029;fax:+19186312995.E-mail address:saibal-mitra@(S.Mitra).stability[2]and radiation hardness[3,4].Therefore,this com-bination of high efficiency coupled with stability and radia-tion hardness makes CIGS a promising material for the low cost,high efficiency solar cells.Recently,a conversion ef-ficiency of33.9%has been predicted for CGS/CIS tandem solar cells at global AM1.5illumination by Bloss et al.[5].A CGS/CIGS tandem solar cell has been modeled by Song et al.and they have shown that conversion efficiencies of25% were achievable when a high efficiency CGS top cell was used with an optimized CIGS bottom cell[6].Our understanding of this material is comparatively lim-ited compared to more common semiconducting materials like c-Si or a-Si.Most of the achievements accumulated over the years have been the result of intuition rather than knowledge-based design.Thus,in this work we have used a0921-5107/$–see front matter©2004Elsevier B.V.All rights reserved. doi:10.1016/j.mseb.2004.05.048294S.Al.Alagappan,S.Mitra /Materials Science and Engineering B 116(2005)293–296simulation program called analysis of microelectronic and photonic structures-one dimensional (AMPS-1D)to opti-mize the design of CIGS-based solar cells.Fig.1shows the schematic of the two cases of solar cell design studied in this work.The first type of design we studied had a single absorber layer of CIGS.The thickness (d )of this absorber layer was varied from 1.0to 5␮m.For each value of d ,the device parameters like fill factor (FF),open-circuit voltage (V oc ),short-circuit current density (J sc )and efficiency (η)were studied as a function of the Ga content,x .In the second type of design,the absorber layer was a simple bi-layered heterojunction with a CGS layer on top of a CIS layer.For this design,the important parameters were the total absorber layer thickness (d )and the relative thickness of CGS/CIS layer.We defined a parameter called r CGS ,which was the ratio of the CGS layer relative to the total thickness of the ab-sorber layer,d .Thus,r CGS could assume values ranging from 0to 1.0.The device parameters were studied as a function of both d and r CGS .In all cases,30nm of cadmium sulfide (CdS)and 150nm of zinc oxide (ZnO)were used as window layer and top contact layer,respectively.The thicknesses of these layers were kept constant so that we could study the trends in device performance as the absorber layer was changed.The top of the cell structure was designed to be a simple ZnO/CdS/CGS or a ZnO/CdS/CIGS structure due to the un-certainties in the interface and junction properties.The front and the back contacts were assumed to be ohmic.The main objective of this work was to study the trends in device parameters as a function of cell design rather than to predict absolute values.2.Modeling and simulation of CIGS and CGS/CIS solar cellsWe used the AMPS-1D simulation program to calculate the device performance of the solar cells as the design of the absorber layer was changed.The transport characteristics of an electronic device may be derived by solving the Pois-son’s equation and the continuity equation for the holesandFig.1.Schematic of the one-dimensional CIGS cell.Note for a single layer d 2=0and the absorber layer is CuIn (1−x )Ga x Se 2with 0≤x ≤1.For a bi-layer cell,d 1+d 2=d ,where d 1=0,d 2=0.The top layer is CGS and bottom layer is CIS.electrons.Thus,AMPS-1D solves three coupled differential equations each subject to three boundary conditions and then calculates the electrostatic potential and the quasi-Fermi level for holes and electrons at all point in the device.Once these variables are known as a function of depth,it is straightfor-ward to calculate the carrier concentrations,electric fields and currents,and device parameters like the open-circuit voltage (V oc ),short-circuit current density (J sc ),fill-factor (FF)and the efficiency (η).These parameters define the performance of a solar cell.The device performances were evaluated under global AM0illumination.The optical constants for the CIGS lay-ers were calculated from the available experimental data on extinction coefficients of CIS and CGS [7].The absorption coefficients at each wavelength and the band gaps (E g )were found by interpolation from the experimental data.We provided the semiconductor parameters of each layer to the AMPS-1D program.The top layer solar cell structure consisted of a 150nm ZnO film (see Fig.1).E g and the elec-tron affinity (χ)for ZnO were chosen to be 3.3and 4.0eV ,respectively.The ZnO layer was followed by a second layer made of a 30nm thick CdS film with E g and χequal to 2.4and 3.75eV ,respectively.The CdS layer was followed by the absorber layer,which was either a single layer of CIGS or a bi-layer of CGS/CIS.The value of χchosen for CGS and CIS was 3.4and 4.06eV ,respectively.For single layer CIGS cells,the value of χwas interpolated from the above values for the given Ga content,x .The recombination veloc-ity at the back contact was assumed to be 107cm/s and was kept constant.The electron and hole mobilities in the CIGS absorber layer were assumed to 300and 30cm 2/V s,respec-tively.The effective density of states for electrons and holes were taken to be 6.77×1017and 1.57×1019cm −3.As the material composition was changed,care was taken to ensure that the contact remained ohmic.3.Results and discussionThe results of our simulations for single layered CIS and CGS were found to be in rough agreement with the values quoted by Rau and Schock [8].This provided a check for the AMPS parameters chosen in this work.However,as men-tioned earlier,the motivation of this work is to study the trends in device performance rather than prediction of abso-lute values.3.1.Single layer resultsWe have studied the performance of a solar cell comprised of a single absorber layer.The thickness and the composition of the absorber layer were changed in a systematic manner.Fig.2shows the variation of the fill factor (FF)as the com-position and thickness are varied.It is clear that the initial addition of Ga had a beneficial effect on FF,which increased from 0.3(x =0)to 0.8(x =0.4)after which it flattened out.S.Al.Alagappan,S.Mitra /Materials Science and Engineering B 116(2005)293–296295Fig.2.Fill factor vs.x ,the Ga content in single-layered cells.Note the dependence of FF on the thickness of the absorberlayer.Fig.3.Efficiency vs.x ,the Ga content in single-layered cells.See text for details.For a given value of x ,the FF also increased with thickness.The short circuit current (J sc )and the open circuit voltage (V oc )were also calculated as a function of composition and thickness.As expected,J sc decreased steadily with increas-ing x .For example,J sc decreased from 49.8to 14.35mA/cm 2for x =0and x =1for the 5␮m thick film.This result was not unexpected since E g increases with increasing x and most of the radiation passed through the film to the back contact and was lost.However,V oc increased with increasing x .For the 5␮m film quoted above,it increased from 0.6V (x =0)to 1.06V (x =1).The conversion efficiency of the solar cell wascalculated from [9],η=J sc V oc FF P in(1)where P in is the input power of the AM0radiation.Fig.3shows the role of both x and d in determining the effi-ciency,η.It is clear from the plot that an initial addition of Ga increased ηregardless of the thickness of the absorber layer.However,as x continued to increase,ηsoon reached a max-imum and then started to decrease.The point at which this happened depended on the thickness.The results also sug-gest that there is a wide compositional range where changes in ηare small compared to changes in the composition of the absorbing layer.For the cell with an absorber layer thickness of 5␮m,ηreached a maximum at x ∼0.2and then remained constant over a wide compositional range before it started to decrease.This suggests that composition of the absorber layer is more forgiving as d increases.Note that for the 1␮m thick layer cell,ηreached a maximum at higher x and is less flat than its thicker counterparts.Also,ηincreased with increasing thickness regardless of the composition of the ab-sorbing layer.Thus,from a designer’s point of view,both x and d must be optimized for maximum efficiency.3.2.Bi-layered cellsThe second type of cell design that we studied was a bi-layered absorber.In this case,the ZnO and CdS layers were followed by a CGS layer which was on top of CIS layer.A bi-layer may be considered as the simplest case of a graded junction.It is well known that graded junctions improve the performance of the solar cells.Fabrication processes that in-volved the sequential deposition of absorber layers with dif-ferent Ga contents have been developed by various groups [10,11].In this work,the bi-layer consisted of a CGS layer on top of the CIS layer.The total absorber thickness was varied from 0.5to 7␮m.The ratio of the CGS/CIS layer within each absorber layer was varied.The top layers (ZnO and CdS)remained unchanged.Fig.4a and b shows the variation of J sc and V oc as a func-tion of r CGS ,the ratio of the CGS layer and the totalthicknessFig.4.Dependence of (a)J sc and (b)V oc on the CGS thickness in the absorber layer in CGS/CIS bi-layered structure for various absorber thicknesses.296S.Al.Alagappan,S.Mitra /Materials Science and Engineering B 116(2005)293–296Fig.5.The dependence of the (a)fill factor (FF)and (b)efficiency (η)on the CGS thickness in the CGS/CIS bi-layered structure for various absorber thicknesses.Note the initial drop in ηand the subsequent rise.For this design,the thickness of the absorber layer plays a critical role in determining η.of the CGS and CIS layer.Thus,the two extreme points on this plot represent single layer of CIS (r CGS =0)and CGS (r CGS =1).Note that addition of a small 10%CGS layer on top of a CIS layer resulted in a sharp decrease in J sc (Fig.3a).However,this drop in J sc was accompanied by a significant improvement of V oc (Fig.3b).Fig.5a and b shows the trend in FF and efficiency.It is clear that though an initial addi-tion of a small layer of CGS reduces solar cell performance,subsequent addition of thicker CGS layer has a beneficial ef-fect.For bi-layer cells,the efficiency has a strong dependence on the thickness of the absorber layers and from a designer point of view is an important result.For a 5-␮m cell,ηhas a peak value of 15.3%corresponding to a 4.5-␮m thick CGS layer on top of a 0.5-␮m CIS layer.All cells with different thickness showed similar trends.It is interesting to compare the performance of a single-layered cell with its bi-layer counterpart.For a 5-␮m thick cell,the bi-layered cell with 90%CGS layer on top of 10%CIS layer performed better than its 5-␮m CGS or CIS coun-terpart.For the bi-layered cell ηwas 15.3%while the corre-sponding value was 11.9and 12.3%for a CIS and CGS single layer cells,respectively.However,the maximum efficiency observed for any 5-␮m thick solar cell was a single layered cell with x =0.4.4.ConclusionWe have used AMPS-1D to study the dependence of de-vice parameters for single-layered and bi-layered solar cells as a function of cell design.We demonstrated that addition of Ga in a CIGS increased the performance for single layered cells.The efficiency ηincreased until it reached a maximum around x =0.2–0.4,depending on the thickness.As the thick-ness of the films is increased,ηstarted to plateau out at x ≥0.2suggesting any fine control of Ga content at this range is un-necessary.For bi-layered cells the results were more intrigu-ing and less intuitive.An addition of the thin CGS layer had a detrimental effect.The thickness of the CGS layer relative tothe CIS layer was important in determining the performance of the cell.The total thickness of the absorber layer in the bi-layered cells is also an important parameter.AcknowledgementsWe would like to acknowledge the use of AMPS-1D pro-gram that was developed by Dr.Fonash’s group at Pennsyl-vania State University (PSU).One of us (S.Al.A)gratefully acknowledges the training he received at PSU.This work was partially supported by a research grant from the Research Of-fice at The University of Tulsa and NASA grant no.NCC5-586.References[1]A.Rockett,R.W.Birkmire,J.Appl.Phys.70(1991)R82.[2]H.S.Ullal,K.Zweibel,B.V .Roedern,Proceedings of the 26th IEEESpec.Conference,IEEE,New York,1997,p.2012.[3]M.Yamaguchi,J.Appl.Phys.78(1995)1476.[4]H.W.Schock,K.Bogus,in:J.Schmid,H.A.Ossenbrink,P.Helm,H.Ehmann,E.D.Dunlop (Eds.),Proceedings of the 2nd World Confer-ence on Photovoltaic 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