(Ebooks) DIY - Energy - High Perfomance Solar Cells, diy
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//-->Research Toward High Performance Epitaxial and Low-temperature Cu(In,Ga)Se2Solar CellsA. Rockett, D.X. Liao, and C.M. MuellerUniversity of Illinois, Department of Materials Science and Engineering,1-107 Engineering Sciences Building, MC-233, 1101 W. Springfield Ave., Urbana, IL 61801ABSTRACTThe CIGS research effort at the University of Illinoisrepresents a three-pronged approach to understanding andsolving some of the most critical issues in CIGS device.These three prongs are: (1) development of a basicunderstanding of the issues limiting performance in CIGSdevices, (2) advancing the performance of the devicesthrough single crystal epitaxial layers for integration intohigh-performance cells, and (3) developing novel growthprocesses that will allow lower deposition temperaturesnecessary to multijunction devices. This paper presents anapproach for CIGS/GaAs and CIGS/Ge heterojunction solarcells for multijunction high-efficiency devices. In addition,application of ionized physical vapor deposition to low-temperature deposition of CIGS is described. The twoprojects will be coupled and results from one used toenhance progress in the other as part of the Beyond theHorizon and High Performance PV programs now starting.1. IntroductionPhotovoltaic devices based on Cu(In,Ga)Se2(CIGS) havethe highest performance of any thin film technology.However, the possibilities for even higher performances aresignificant. Multijunction devices involving CIGS either inconjunction with III-V compound semiconductors (GaAsand related materials) or various Cu chalcopyritecompounds (CuGaSe2, CuInS2, or others) remain to beexploited. The projects described here take two approachesto the study of such devices -- novel processing methodsrequired for multijunction devices, and direct application ofthe existing methods for deposition to multijunctionepitaxial solar cells. These new projects are just beginningat the University of Illinois under the Beyond the Horizonand High Performance Photovoltaics programs funded bythe National Renewable Energy Labroatory. The formerfocuses on developing a novel low-temperature depositionprocess for production of CIGS. This will be necessary inany application where a CIGS device is to be fabricated on atemperature-sensitive existing junction. The latter involvesgrowth of CIGS epitaxial layers on GaAs and Ge substratesand demonstration of the performance of resulting devicesfor application with III-V materials. These two projects arebriefly summarized below.2. Ionized Physical Vapor Deposition for CIGS DevicesUnder this new program, we will develop a unique next-generation method for low-temperature deposition of CIGSbased on the ionized physical vapor deposition (IPVD)method.[1,2] This technique has been shown to dramaticallyreduce required deposition temperatures in other thin filmcoatings. It supplies energy to the growing film surfacethough the working gas rather than by heating the substrate.substrateIonized atom fluxdc biassupply-+rf plasmaSputtered neutralatom source fluxdc sputteringplasmasmaterial source(eg: dc magnetron)rf coilIonization eventFigure 1: The basic IPVD process.The basic process is shown schematically in Figure 1. An rfplasma near the substrate ionizes up to 80% of the species inthe gas phase.[1] A dc bias voltage (typically 0 to 25 eV)applied between the rf coil and the substrate determines theenergy for particles striking the growth surface. Thethreshold energy for displacement cascades in solids leadingto formation of vacancies and interstitials is ~25 eV. Biasbelow ~50 V keeps the energy transferred to surface atomsbelow the threshold necessary to damage the film. With80% of particles striking the growth surface having 10,000times the thermal energy (i.e. 25 eV), surface atomicmobilities are greatly enhanced and the heat input needed tomaintain a given film quality is reduced. Furthermore, theaccelerated particles include a number of inert gas specieswhich further contribute to surface adatom motion and filmgrowth. This technique has been used to deposit a variety offilms at reduced temperatures. We anticipate a 100-400°Creduction in needed deposition temperature of CIGSepitaxial or polycrystalline layers while retaining device-quality material. We expect to see significantly alteredincorporation probabilities for some of the elements in theprocess, especially an increased Se incorporation rate.3. CIGS For Multijunction High Performance DevicesAs part of the High-performance PV initiative, we aredeveloping CIGS as a narrow-gap component ofmultijunction solar cells. We currently plan to participate inboth the single crystal epitaxial and polycrystalline highperformance cell programs. In previous efforts, we havedeveloped a well-characterized and reproducible method fordeposition of single-crystal epitaxial layers of Cu(In,Ga)Se2alloys on GaAs substrates of each of the three major surfaceorientations, (111), (100), and (110). The technique,[c.f.Refs 3,4] consists of sputtering Cu or Cu-Ga and In targets227in Ar gas and simultaneously evaporating molecular Se(and/or S) from an effusion cell or cells.The present work will begin with a detailed study of theelectrical properties of CIGS-GaAs heterojunctions. This iscritical to application of CIGS in high efficiency cells fortwo reasons. First, because the only way to produce a two-contact multijunction solar cell involving CIGS is to use oneof the surrounding semiconductors as the heterojunctionpartner. Therefore, it is necessary to establish theperformance of junctions of candidate materials with theCIGS. Second, because the CIGS epitaxial layers are high-quality single crystals, growth of multilayer structures willbe possible. Such growth is required in current designs ofnon-mechanically-stacked high efficiency devices where the1.0 eV gap device is surrounded both above and below byadditional devices. Our preliminary studies will concentrateon demonstration of solar cells based on p-CIS/n+-GaAs andp-CIS/n-Ge heterojunctions.Other aspects of the program will include study of methodsto control interdiffusion of elements across theheterojunction and low-temperature deposition processes,which will reduce the chance of damage to previously-fabricated III-V heterojunction solar cells. This portion ofthe program will be closely coupled to the beyond-the-horizon portion of the program, described above.Finally, we will supply epitaxial layers of CIS on GaAs toNREL for use as substrates for test growth experiments fordeposition of III-V semiconductor layers on the CIS films.These efforts correspond largely to the focus of the single-crystal high-performance program at NREL. We will,however, also be collaborating with the polycrystalline highperformance project through supply of materials and growthof device structures. In particular, we will use low-temperature growth to deposit additional junctions onpreviously grown solar cell layers to test multijunctionstructures.4. Thin Film PartnershipWhile we have, as yet, no indication of funding under thethin film partnership, should this program be funded we willbe analyzing solar cell materials gathered from a widevariety of sources by transmission electron microscopy.The objective is to determine the microstructural andmicrochemical nature of a good CIGS solar cell and how todistinguish it from a poor solar cell. This will assist inoptimizing cell performance. This work will be coupledwith intensive modeling of device performances, probablybased on the AMPS computer code to draw a directcorrelation between cell performance and microstructure.AcknowledgementsThe work is being conducted in collaboration with theNational Renewable Energy Laboratory and the Institute forEnergy Conversion at the University of Delaware, whosehelp we greatly appreciate.REFERENCES[1] C.A. Nichols, S.M. Rossnagel, S. HamaguchiJ. Vac SciTechnB 14(5), 3270 (1996).[2] S.M. RossnagelJ. Vac Sci TechnB 16(6), 3008 (1998),and S.M. RossnagelJ. Vac Sci TechnB 16(5), 2585 (1998).[3] David J. Schroeder, Gene D. Berry, and A. Rockett,Applied Physics Letters69(26), 1 (1996).[4] L. Chung Yang, L.J. Chou, A. Agarwal, and A. Rockett,"Single Crystal and Polycrystalline CuInSe2 by the HybridSputtering and Evaporation Method," 22nd IEEEPhotovoltaic Specialists Conference, Las Vegas, October 7-11, 1991 (Institute of Electrical and Electronics Engineers,New York, 1991), p 1185.[5] D. Liao and A. Rockett,J. Appl. Phys.,submitted.228
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