CIGS2 Thin-Film Solar Cells on Flexible Foils for Space Power

PROGRESS IN PHOTOVOLTAICS:RESEARCH AND APPLICATIONS

Prog.Photovolt:Res.Appl.2002;10:407–416(DOI:10.1002/pip.447)

CIGS2Thin-Film Solar Cells on Flexible Foils for Space Power

Neelkanth G.Dhere 1,*,y ,Shantinath R.Ghongadi 1,Mandar B.Pandit 1,Anant H.Jahagirdar 1

and David Scheiman

Florida Solar Energy Center,1679Clearlake Road,Cocoa,FL 32922-5703,USA 2Ohio Aerospace Institute,NASA Glenn Research Center,Cleveland,Ohio 44135,USA

CuIn 1àx Ga x S 2(CIGS2)thin-?lm solar cells on ?exible stainless steel foil are of

interest for space power because of the near optimum bandgap,potential for higher

speci?c power,and superior radiation resistance.This paper describes development

of CIGS2solar cells on 127-m m and 20-m m-thick,?exible SS foils for space power.A

large-area,dual-chamber,deposition system has been fabricated.Cu-rich Cu-Ga/In

layers were sputter-deposited on unheated Mo-coated foils.p -type CIGS2thin ?lms

were obtained by etching Cu-rich surface layer from CIGS2?lms prepared by

sulfurization in Ar:H 2S 1:0á04mixture at 475 C for 60min with an intermediate

30-min annealing step at 135 C.Solar cells were completed by deposition of CdS/

ZnO/ZnO:Al layers and Ni/Al contact ?ngers.AM0PV parameters of a CIGS2solar

cell on 127-m m-thick SS foil measured at NASA-GRC were:V OC ?802á9mV ,

J SC ?25á07mA/cm 2,FF ?60.06%,and ef?ciency ?8.84%.For this cell,AM1á5PV

parameters measured at NREL were V OC ?763mV ,J SC ?20á6mA/cm 2,

FF ?67á4%,and ef?ciency ?10á4%.Copyright #2002John Wiley &Sons,Ltd.

INTRODUCTION

L ightweight,?exible,thin-?lm solar cells have many promising applications in space and terrestrial

photovoltaic power systems.1Future space missions are likely to include very large satellites,such as solar power satellites and very small satellites.Long-term plans envisage swarms of distributed,autonomous,small satellites,termed microsats or even nanosats,to perform speci?c tasks.Some missions will use solar electric propulsion (SEP)instead of rockets.CIGS2thin-?lm solar cells on ?exible stainless steel (SS)may be able to increase the speci?c power by an order of magnitude from the current level of 65Wkg à1.Thin-?lm technology could conservatively reduce the array-manufacturing cost of a medium-sized 5-k W satellite from the current level of $2000k to less than $500k.2Weight bene?ts of higher-ef?ciency cells are decreased and high costs become less affordable in the case of ?exible thin-?lm blanket arrays that can be easily rolled out.Non-rigid cells also have an advantage in stability.Because of the low initial velocities and steady accelera-tion,SEP satellites must spend long periods in intense regions of trapped radiation belts.CIGS solar cells are superior to the conventional silicon and gallium arsenide solar cells in the space radiation environment.The Received 27February 2002

Revised 27March 2002*Correspondence to:Neelkanth G.Dhere,Florida Solar Energy Center,1679Clearlake Road,Cocoa,FL 32922-5703,USA.y E-mail:dhere@https://www.360docs.net/doc/0117176185.html,

Contract/grant sponsor:NASA Glenn Research Center;contract/grant number:NCC 3712.

Contract/grant sponsor:Air Force Research Lab through Jackson and Tull and Universities Space Research Association;contract/grant number:09503-01.

Contract/grant sponsor:National Renewable Energy Laboratory;contract/grant number:NDJ-2-30630-03.

Special Issue

potential for improved radiation resistance of thin-?lm solar cells relative to single-crystal cells,could extend mission lifetimes substantially.Recent studies have shown that12.6%ef?cient,thin-?lm cells would start to become cost-competitive in GEO and LEO missions.3However,signi?cant technological hurdles remain before thin-?lm technology could be implemented as the primary power source for spacecraft.

There is a recent interest in the development of CIGS2solar cells on?exible substrates.Several groups have reported fabrication of polycrystalline CuIn1àx Ga x Se2(CIGS)solar cells on?exible foils substrates.4The CIGS cell with an ef?ciency exceeding17%has been obtained using SS substrate.4

The objective of the present research is to develop ultra-lightweight,radiation-resistant,highly ef?cient, high-speci?c-power CuIn1àx Ga x S2(CIGS2)thin-?lm solar cells for space electric power.A small proportion of gallium is incorporated so as to obtain bene?ts of improved adhesion,slightly higher bandgap,and incorpora-tion of a back-surface?eld,as has been done with CIGS cells.Initially,CIGS2thin-?lm solar cells are being fabricated on127-m m-thick SS substrates.A few CIGS2cells have also been prepared on20-m m-thick SS substrates.The main thrust is towards development of fundamental understanding and baseline processes rather than attaining the highest ef?ciencies.

EXPERIMENTAL TECHNIQUE

Bright annealed stainless steel foils with thickness of127m m and20m m were used as substrates.Copper–gal-lium and indium layers were sputter-deposited to achieve the elemental ratio of Cu/(IntGa)$1á4.CIGS2thin ?lms were prepared by a two-step process.The?rst step involved deposition of alternate layers of CuGa(22%) and In on a molybdenum-coated?exible foil substrate.This formed a stacked elemental layer sequence that produced a predominant Cu11In9precursor phase.This layer was sulfurized in a Ar:H2S1:0á04gas mixture and argon?ow rate of650sccm,using a three zone furnace.The temperature of initial dwell was changed from 120to135 C.After annealing the samples in?owing argon for25min at this temperature,the?ow of H2S gas was initiated.The maximum sulfurization temperature was475 C,while the sulfurization time was varied from 20to60min.It was found that the binary Cu11In9precursor phase reacted in the H2S:Ar gas environment to form a good crystalline pseudo-quaternary phase of CIGS2?lm.Some samples were sulfurized at475 C for 30min,followed by annealing at500 C without H2S gas?ow in an Ar atmosphere for10min.This annealing step favored grain growth and recrystallization.The Cu-rich stoichiometry during the growth of CIGS2?lms results in an improved morphology,i.e.,enhanced grain sizes of the polycrystalline?lms.For selenide system, this phenomenon is attributed to CuSe liquid formed on top of Cu-rich?lms.5For the sulfur system,according to the phase diagram of Cu–S,the respective liquid phase is not expected in the substrate temperature range of T sub<600 C.However,owing to the high cation mobility in the Cu–S compounds,the cation lattice in a binary Cu–S phase at the surface of a growing CIS2?lm behaves as a quasi-liquid.The Cu x S phase was etched in10%aqueous KCN solution.Solar cells were completed by deposition of CdS heterojunction partner layer by chemical bath deposition,transparent-conducting ZnO/ZnO:Al window bilayer by RF sputtering,and

vacuum deposition of Ni/Al contact?ngers through a metal mask(Figure1).Bright annealed stainless

steel

https://www.360docs.net/doc/0117176185.html,yer sequence of CIGS2thin?lm-solar cell

408N.G.DHERE ET AL. Copyright#2002John Wiley&Sons,Ltd.Prog.Photovolt:Res.Appl.2002;10:407–416

foils of 20and 127m m thickness were evaluated as possible substrate materials for polycrystalline CIGS2solar cell.

The phases,surface morphologies and elemental depth pro?les of the CIGS2?lms prepared on stainless steel ?exible foils substrates were characterized.Films were examined visually for their appearance,color and any tendency to peeling.X-ray diffraction (XRD)was used to identify the crystalline phases,using a RIGAKU dif-fractometer.The 2 -range for the diffractometer was set from 10to 80 with a step size of 0á02 .Surface mor-phology of the CIGS2thin ?lm was studied using scanning electron microscopy (SEM).Chemical composition was analyzed by electron probe microanalysis (EPMA).Depth pro?ling was performed by secondary ion mass spectroscopy (SIMS)and Auger electron spectroscopy (AES)with simultaneous sputter etching.The substrate surface roughness and thickness of thin ?lms were measured using the DEKTAK surface pro?le measuring system.3

Current–voltage characteristics of CIGS2solar cells were measured under AM0and AM1á5conditions at the NASA Glenn Research Center and National Renewable Energy Laboratory (NREL).Quantum ef?ciency was measured at NREL.

RESULTS AND DISCUSSION

To avoid the formation of voids before the initiation of sulfurization,the initial dwell of 30min at 120 C was changed to 135 C,and the ?ow of H 2S gas was started after 25min at 135 C.6The XRD spectrum of the as-deposited (Cu tGa)/In metallic precursors processed at 135 C indicated the presence of a highly oriented Cu 11In 9phase without any elemental or alloy phases.The XRD pattern of a near stoichiometric,slightly Cu-poor,etched CIGS2thin ?lm is shown in Figure 2;it showed (101),(112),(103),(004)/(200),(213),(204)/(200),(116)/(312),(008)/(400),and (332)re?ections of a CuIn 0á7Ga 0á3S 2phase.The calculated lattice parameters were a ?5á67A

?and c ?11á34A ?.A Mo peak was obtained at 40á22 .The (112)peak of CuGaS 2was also observed.This is attributed to Ga diffusion towards the Mo back-contact,creating a Ga-rich phase near the back,and a Ga-poor phase at the surface and in the bulk of the CIGS2thin ?lm.7In the case of random orientation,the intensity ratio of (112)with respect to (220)/(204)peaks,should be 1á5.The intensity ratio of I 112/I 220/204,for these ?lms,was remarkably higher,showing a high degree of (112)preferred orientation.It may be noted that (112)orientation would be bene?cial for achieving a good lattice match with CdS,and consequently for ef?cient device fabrication.

Despite the very Cu-rich ?lm of the unetched sample,with Cu/(In tGa)ratio up to 1á68,no secondary phases could be detected in the XRD pattern for the unetched sample.8As discussed earlier,a Cu-rich

stoichiometry

Figure 2.XRD pattern from a near stoichiometric,slightly Cu-poor,etched CIGS2thin ?lm

CIGS2THIN-FILM SOLAR CELLS ON FLEXIBLE FOILS 409Copyright #2002John Wiley &Sons,Ltd.Prog.Photovolt:Res.Appl.2002;10:407–416

during the growth of CIGS2?lms results in an improved morphology,i.e.,enhanced grain size of the polycrys-talline ?lms.This excess Cu x S was etched in 10%KCN solution.

The surface scanning electron micrograph (SEM)of a near stoichiometric,slightly Cu-poor,etched CIGS2thin ?lm is shown in Figure 3.SEM image for the sample showed large,well-faceted grains with slight porosity.The porosity was observed at the grain boundaries from where the Cu x S phases have been etched.The grain size measured by an intercept method was 3m m i.e.,comparable to the ?lm thickness.

An AES survey (Figure 4)of near stoichiometric,slightly Cu-poor,etched CIGS2thin ?lm sample was per-formed over a range of kinetic energies (50–2250eV),using the primary electron beam of energy 5keV .A representative area of sample was chosen and an AES survey was carried out at magni?cation of 1000?,equivalent to an area of 102?102m m.An AES survey over the selected area showed the presence of copper at 60,777,850,and 924eV,sulfur at 151,and 2131eV,indium at 345and 410eV ,gallium at 1071eV,carbon at 273eV,potassium at 252eVand oxygen at 515eV.The surface atomic concentrations calculated from the peak-to-peak height from the AES survey and relative sensitivities of the respective elements were as follows:copper 20á67at.%,sulfur 31á62at.%,indium 12á3at.%,gallium 6á92at.%,potassium 7á72at.%,oxygen 11á92at.%and carbon 15á04at.%.

The AES depth pro?le was obtained by sputtering an area of 1?1mm 2with energetic argon ions at a rate of 475A

?/min for 72min.Figure 5shows peak heights at different depths (time)for different elements.The peak heights were obtained for the following elements:copper,indium,gallium,oxygen,carbon,

sulfur,

Figure 3.SEM image of a near stoichiometric,slightly Cu-poor,etched CIGS2thin

?lm

Figure 4.Surface AES survey of near stoichiometric,slightly Cu-poor,etched CIGS2thin ?lm

410N.G.DHERE ET AL.

molybdenum,iron and potassium.Copper and sulfur concentration showed the same trend;it decreased near the Mo back-contact.Potassium was detected at the surface.Its concentration decreased rapidly in the bulk.Potas-sium was attributed to the KCN treatment that was used to etch away excess Cu x S phase.Indium concentration is constant in most of the thickness of CIGS2layer and falls near the Mo back-contact.Concentration of gallium at the surface was approximately 10%,while at the CIGS/Mo interface it was 30%.This showed that Ga con-centration increased towards the Mo back-contact.Indium concentration decreased with depth,and was negli-gible at the CIGS/Mo interface.Beyond the thickness of the Mo back-contact layer,the concentration of iron rises,due to iron in the stainless steel foil.The apparent concentration of iron within the CIGS2?lm thickness would create an erroneous impression of iron diffusion.This is discussed further below.Apparent humps in the depth pro?les were caused by a non-uniform sputter-etching rate.It can be seen that the ?lm thickness of etched CIGS2sample from the depth pro?le was approximately 2á4m m.

SIMS depth pro?ling (Figure 6)was performed on an etched sample by positive SIMS,using a CAMECA IMS-3F system with oxygen primary beam current 150nA,impact energy 5á5keV ,angle of incidence 42 ,ras-tered over 250?250m m,with source at 10keV and sample at 4á5keV .The corresponding unetched sample had Cu/(In tGa)$1á4,and was sulfurized at 475 C for 30min,and annealed for 10min each at 475and 500 C.The slightly Cu-poor,etched CIGS2thin-?lm sample was monitored for eight species.To achieve high-sensitivity measurements,secondary positive cluster ions such as 23Na,34S,K,54Fe,65Cu,(69Ga tO),92Mo,and (113In tO)were used for detection of Na,S,K,Fe,Cu,Ga,Mo,and In respectively.Cu concentration

was

Figure 5.AES depth pro?le of near stoichiometric,slightly Cu-poor,etched CIGS2thin

?lm

Figure 6.SIMS depth pro?le of near stoichiometric,slightly Cu-poor,etched CIGS2thin ?lm

CIGS2THIN-FILM SOLAR CELLS ON FLEXIBLE FOILS 411

mostly constant over the depth of the?lm.Ga concentration remained constant to1m m,and then increased to the Mo back-contact.Indium concentration remained constant through most of?lm thickness and decreased near the Mo back-contact.Na was not added intentionally.S concentration was uniform throughout?lm thick-ness.Na concentration at the surface of the substrate was attributed to the residue from cleaning with soap solu-tion.Potassium incorporation was due to etching of the Cu x S phase present on the grains and near the grain boundaries with KCN solution.Careful SIMS analysis,described below,showed that the concentration of iron was below the detection limit.Thus there was no signi?cant diffusion of iron.No sharp interfaces were observed at CIGS2/Mo and Mo/substrate.This was probably because of the interdiffusion of species and porosity formed due to etching with10%KCN treatment.

The mass spectrum recorded from masses of1–120is shown in Figure7.Sharp peaks were observed for

elemental and molecular species such as Ot

2,St,Kt,Cat,SOt,Cut,Gat,GaOt,CuOt,CuO2t,Int,InOt,

and InOt

2.Na was not detected during analysis of mass spectra.Because of the large number of isotopes of Cu,

In,Mo,Ga,and S,a number of mass interferences were observed.The mass spectrum clearly indicated the presence of Cu,In,Ga,S,Mo and K.

The chemical composition of CIGS2?lms was analyzed by EPMA.Average atomic concentrations measured at10kV and20kV for an unetched sample showed Cu:In:Ga:S proportion of51á52:7á80:1á83:38á83and 41á90:10á92:2á56:44á63,respectively.The Cu:In:Ga:S atomic concentrations for the etched sample at10and 20kV were found to be27á38:21á61:3á30:47á71and23á96:19á36:5á70:50á99,respectively.The compound for-mulae were Cu1á09In0á87Ga0á13S2and Cu0á96In0á77Ga0á23S2,respectively.The Cu:In:Ga:S atomic concentrations for the etched sample deposited on20-m m-thick SS foil at20kV were found to be28á60:14á90:11á42:45á10. The compound formula was Cu1á10In0á60Ga0á40S2.

Surface roughness of the substrate was measured with the DEKTAK surface pro?le measuring system.3The average roughness R a value for127-m m-thick SS foil(Figure8)was62á3A?and average waviness W a was 141á6A?.The average roughness R a for20-m m-thick SS foil(Figure9)was396á4A?and surface waviness W a was773á2A?.

PV parameters of a CIGS2solar cell on127-m m-thick SS?exible foil measured under AM0conditions (Figure10)at NASA GRC were:V OC?802á9mV,J SC?25á07mA/cm2,FF?60.06%,and ?8.84%.For this cell,AM1á5PV parameters measured at NREL were:V OC?763mV,J SC?20á26mA/cm2,FF?67á04%, ?10á4%.The quantum ef?ciency curve(Figure11)showed a sharp QE cut-off,equivalent to a CIGS2band-gap of$1á50eV,fairly close to the optimum value for ef?cient AM0PV conversion in space.PV parameters for a cell fabricated on20-m m-thick SS foil measured at NREL under AM1á5conditions were:V OC?740mV, J SC?13á129mA/cm2,FF?41á63%, ?4.06%.

Foils with high defect density,in the form of surface roughness,showed an increase in Ga content in the bulk of material.Fill factor,i.e.,the squareness of the I–V curve also decreased with increase in defect density. Ef?ciency showed a decreasing trend with increasing surface roughness.The loss in ef?ciency was attributed to surface roughness of the substrate,increase in Ga content in the bulk of the material and decrease in?ll factor. The solar ef?ciency of a photovoltaic system depends critically on the spectral distribution of the radiation.

At AM0the solar spectrum has an irradiance of1353W/m2.At the PV Materials Laboratory of FSEC,the

goal

Figure7.SIMS mass spectra of near stoichiometric,slightly Cu-poor,etched CIGS2thin?lm

412N.G.DHERE ET AL. Copyright#2002John Wiley&Sons,Ltd.Prog.Photovolt:Res.Appl.2002;10:407–416

Figure 8.Surface roughness measurement of 127-m m-thick SS

foil

Figure 9.Surface roughness measurement of 20-m m-thick SS

foil

Figure 10.AM0I –V curve of CIGS2thin ?lm solar cell (NASA GRC)

CIGS2THIN-FILM SOLAR CELLS ON FLEXIBLE FOILS 413

is to achieve an ef?ciency under AM0conditions in the range of 10–15%.Table I provides the projected speci?c power in W/kg of ?exible metallic substrate at AM0for 10and 15%ef?cient CIGS2solar cells.

https://www.360docs.net/doc/0117176185.html,RGE-AREA,DUAL-CHAMBER MAGNETRON-SPUTTERING AND SULFURIZATION SYSTEMS

Earlier,the substrate size was limited to 1?1inch (2á5?2á5cm).A large-area,dual-chamber magnetron-sputtering unit has been fabricated.The chambers are equipped with cryopumps,two-stage mechanical vacuum pumps,throttled-gate valves,mass-?ow controllers for argon and oxygen,and convectron and Bayard–Alpert ionization gauges (Figure 12).A large number of feed-through ports have been provided to both the chambers for rotation and electrical feed-through.This will permit addition of in situ diagnostic tools.Three 4?12inch (10?30cm)DC magnetron sputtering sources have been installed for sputter deposition from molybdenum,indium,and copper,CuGa (22%)or CuGa (67%)targets in the larger chamber.Two 4?12inch RF magnetron sputtering sources have been installed into the smaller chamber for deposition of a transparent conducting ZnO/ZnO:Al bilayer window.The thickness uniformity along the 12inch dimension is expected to be better than ?2%over the center width of 5inch (12á5cm)and better than ?3%over the center width of 6inch (15cm)for linear substrates motion along the 4inch dimension.Moreover,the sputtering sources are expected to provide excellent (>40%)target utilization.A linear substrate movement set-up has been fabricated for in-line deposition of molybdenum back-contact and Cu-Ga/In metallic precursors.The vacuum system and chambers were designed at the Florida Solar Energy Center (FSEC).Vacuum chambers were built elsewhere.The complete system was constructed at FSEC.A four-hearth electron-beam source has also been installed in a high-vacuum chamber for vacuum evaporation of Ni/Al contact grids.Siemens Solar Industries has donated

Figure 11.QE curve of CIGS2thin ?lm solar cell (NREL)

Table I.Projected speci?c power for CIGS2solar cells

Substrate Projected speci?c power (W/kg)

?10%at AM0

?15%at AM0127-m m (5mil)SS foil

33á0199á620-m m (<1mil)SS foil

768á81153á125á4m m (1mil)Ti foil 1015á81523á6

414N.G.DHERE ET AL.

selenization–sulfurization furnace,reactor and control system for preparation of large (4?4inch,10?10cm)CuIn 1àx Ga x Se 2ày S y (CIGSS)thin ?lms and solar cells.

CONCLUSIONS

Sputtered copper,gallium and indium precursors with Cu/(In tGa)ratio $1á4formed a predominant Cu 11In 9precursor phase,free from inhomogeneous secondary phases.The temperature of the initial 30-min dwell of 120 C was changed to 135 C.H 2S gas ?ow was begun after 25min of the dwell.The binary Cu 11In 9precursor phase reacted in an H 2S:Ar gas environment to form a good crystalline pseudo-quaternary phase of CIGS2?lm.After etching in KCN,CIGS2?lms became stoichiometric,regardless of the ratio of Cu/(In tGa)of unetched ?lm.CIGS2?lms grew with (112)texture of the chalcopyrite structure.A highly Ga-rich phase was formed near the back-contact while a Ga-poor phase was formed in the bulk of the ?lm and in the electrically active part of the device,because of the tendency of gallium to diffuse towards the back-contact.The bandgap of this ?lm was narrow at the surface and in the bulk,and widened towards back-contact,leading to formation of a back-surface ?eld.This built-in back-surface electric ?eld was expected to improve the solar cell performance.9,10

PV parameters of a CIGS2solar cell on 127-m m-thick SS ?exible foil measured under AM0conditions at the NASA GRC were:V OC ?802á9mV ,J SC ?25á07mA/cm 2,FF ?60á06%,and ?8á84%.For this cell,AM1á5PV parameters measured at NREL were:V OC ?763mV ,J SC ?20á26mA/cm 2,FF ?67á04%, ?10á4%.Quantum ef?ciency curve showed a sharp QE cut-off,equivalent to a CIGS2bandgap of $1á50eV ,fairly close to the optimum value for ef?cient AM0PV conversion in space.PV parameters for a cell fabricated on 20-m m-thick SS foil measured at NREL under AM1á5conditions were:V OC ?740mV ,J SC ?13á129mA/cm 2,FF ?41á63%, ?4á06%.

Foils with high defect density in the form of surface roughness showed an increase in Ga content in the bulk of material.Fill factor,i.e.,the squareness of the I –V curve also decreased with increase in defect density.Ef?ciency decreased with increasing surface roughness.

With the construction of large-area,dual-chamber magnetron-sputtering unit,samples of thickness uniformity ?2%over the central 5inch (12á5cm)width and ?3%over the central 6inch (15cm)width will be fabricated.

Acknowledgements

This work was supported by the NASA Glenn Research Center,contract NCC 3712,the Air Force Research Laboratory through Jackson and Tull and Universities Space Research Association,contract 09503-01,and

the Figure https://www.360docs.net/doc/0117176185.html,rge-area,dual-chamber magnetron-sputtering unit

CIGS2THIN-FILM SOLAR CELLS ON FLEXIBLE FOILS 415

416N.G.DHERE ET AL. National Renewable Energy Laboratory,contract NDJ-2-30630-03.The authors are thankful to Dr.Kannan Ramnathan,Tom Moriarty and Bobby To of NREL for help with Ni/Al grid deposition,I–V and QE measurements,and EPMA analysis and discussions.

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