CIGS Friday 07:00-09:00 pm Textbook: Solar Cells edited by T. Markvart and L. Castaner Lecturer: Prof. Yeong-Cheol Kim Cu(In,Ga)Se2 thin-film solar cells I. Introduction 0.5 cm2 lab cell, 18.8% mini-modules with 20 cm2, 16.6% first CuInSe2 by Hahn in 1953 single-xtal SC with 12% in 1974 poly films SC with 10% by Boeing Co in 1983-84. thin-film SC with 14.1% by Arco Solar in 1987 first commercial CIGS solar modules by Shell Solar in 1998 process that avoids H2Se by Shell Solar other substrate by Global Solar and ISET co-evaporation process by Wurth Solar in 2003 H2Se by Showa Shell and co-evaporation by Matshushita II. Material properties 2.1 Chalcopyrite lattice CuInSe2, CuGaSe2: I-III-VI2 materials family, tetragonal zinc blende structure of II-VI materials such as ZnSe strengths of I-VI and III-VI bonds are different c/a is not 2 2-c/a: measure of tetragonal distortion 2.2 Band gap E 1.04-2.4 eV, CuInSe2 – CuGaS2, 2.7 eV in CuAlS2 direct BG, PV absorber Fig. 2: no miscibility gap Fig. 1 Unit cells of chalcogenide compounds. (a) Sphalerite or zinc blende structure of ZnSe (two unit cells) (b) Chalcopyrite structure of CuInSe2. The metal sites in the two unit cells of the sphalerite structure of ZnSe are alternately occupied by Cu and In in the chalcopyrite structure.. Fig. 2 Band-gap energies Eg vs. the lattice constant a of the Cu(In,Ga,Al)(S,Se)2 alloy system. ▶ tandem structure by composition control : CuGaSe2, CuInS2 4 C u A lS 2 B and gap energy (eV ) ▶ high light absorption coefficient :105/cm 3 C u A lS e 2 AgG aS2 CuG aS2 CdS G aP 2 C uG aSe2 A g In S 2 A gG aSe2 In P C u In S 2 A g In S e 2 G aAs 1 Si C u In S e 2 C u G a T e 2 C u In T e 2 0 5.2 5.4 5.6 5.8 6.0 L attice con stan t (A ) 6.2 CIGS 태양전지 동작원리 Front electrode Window Buffer Back electrode Absorber E + I ( x) I oe I ( x) I oe 1 d ln x ( x ) Io I ( x ) E x 2.3 The phase diagram CIGS: most complicated phase diagram among thin-film PV Fig. 3: alpha-phase (CIS2), beta-phase (CI3S5), CuySe all phases have similar structure beta-phase: ordered array of defect pairs (VCu and InCu) CuySe: CuIn and Cui sphalerite phase existence range of alpha-phase in pure CIS2: 24~24.5% typical Cu content: 22~24% at growth T, single-phase region at room T, two-phase alpha+beta region phase separation in CuInSe2 after deposition partial replacement of In with Ga, Na-containing substrates: widens singlephase region. Fig. 3 Quasi-binary phase diagram of CuInSe2 along the tie-line that connects the binary compounds In2Se3 and Cu2Se established by Differential Thermal Analysis (DTA) and microscopic phase analysis. 2.4 Defect physics of CIGS Cu-chalcopyrite compounds: dope with native defects, large off-stoichiometries, electrically neutral nature p-type: Cu-poor, annealed under high Se vapor pressure n-type: Cu-rich, Se deficient VSe: dominant donor in n-type, VCu: dominant acceptor in p-type calculation of metal-related defects in CIS and CGS by Zhang negative formation E for Vcu in Cu-poor and stoichiometric material low Ef for CuIn in Cu-rich, shallow acceptor strong self-compensation, difficult extrinsic doping ref  table 1. ionisation E and defect formation E of 12 intrinsic defects in CIS Ef of defect complexes, (2VCu,InCu), (CuIn, InCu), (2Cui,CuIn) (2Cui,CuIn): no electronic transition with BG, occur in In-rich Turcu  Fig. 4 Table 1. Electronic transition energies and formation energies ΔU of the 12 intrinsic defects in CuInSe2. Fig. 4 Band gap evolution diagram of the CuIn(Se,S)2 (a) and the Cu(In,Ga)Se2 (b) alloy system with the trap energy ET(N2, open diamonds) taken as an internal reference to align the conduction band and the valence band energies Ec and Ev. The energy position of an additional defect state in Cu(In,Ga)Se2 (full diamonds) as well as that of an interface donor (open triangles) in Cu(In,Ga)(Se,S)2 is also indicated. III. Cell and module technology 3.1 Structure of the heterojunction SC ZnO/CdS/CIGS heterojunction SC Fig. 5 1 um Mo on soda-lime glass, back contact 1-2 um CIGS, PV absorber 50 nm CdS by chemical bath deposition 50-70 nm i-ZnO sputter deposition heavily doped ZnO, 3.2 eV band gap, window layer Fig. 5 Schematic layer sequence of a standard ZnO/CdS/Cu(In,Ga)Se2 thin-film solar cell. 기본 공정도 -업체별 공정 특화: Wurth Solar, Show Shell -성막 방법, 사용 재료 MO 증착 (Sputter) Patterning 1 Laser Scribe CIGS 성막 (co-evaporation & sputter후 Se/S 化) 1) Co-evaporation Cu, In, Ga, Se Evaporation Sources 버퍼층 형성 CBD 1) CdS 2) Zn(O,S,OH)x 2) Sputter法 + Se/S 化 Dip-Coating Lamination & wiring Patterning 3 기계적 Scribe 후면 반사/전극 증착 (ZnO/Ag) 1) Wurth Solar : SPT 2) Showa Shell : MOCVD 16/117 Patterning 2 기계적 Scribe SEM 단면도 구조 -광흡수층과 버퍼층이 효율 좌우 Layer Material(Thickness) Window layer n-ZnO (500nm) / i-ZnO (50nm) Buffer layer CdS (50nm) Absorber layer Cu(In,Ga)(Se,S)2 (2~3um) Back contact Mo (1um) Substrate Glass (2~3mm) Process 1) Sputtering 2) CBD (Chemical Bath Deposition) 3) Co-Evaporation Sputtering/Se 4) DC Sputtering 5) Substrate (Sodalime Glass) 광흡수층 공정 비교 Co-evapration □ 금속원료(Cu,In,Ga,Se) 동시 증착 SPT + Selenization □ SPT (Cu,Ga,In) 후 Se diffusion Process Sputter 적용업체 장점 단점 □ Wurth Solar, Johanna (독일) □ 최고 효율 달성 (19.2% @NREL) □ 학계 연구 자료 多 Selenization □ Showa Shell, Honda (일본) □ 대형화에 유리 □ Throughput 유리 □ SPT 공정 사용 (LCD Normal 공정) □ 대형화 어려움 (現 60*120 이하) □ Showa shell 특허 등록 □ LCD 비사용 공정 □ 국내 학계 경험 적음 2nd ~600 1st 350 In + Ga + Se 3rd Cu+ Se Se Only Se 1st stage In + Ga + Se Substrate Temp (oC) 동시 증발법 In +Ga + Se (In,Ga)xSey Mo Glass 1st stage, (In,Ga)xSey formation Evaporation Time 3rd stage 2nd stage In +Ga + Se Cu + Se Cu(In,Ga)Se2 Cu(In,Ga)Se2 Cu(In,Ga)Se2 Cu(In,Ga)Se2 (In,Ga)xSey Mo Mo Mo Mo Glass Glass Glass Glass After 3rd stage Cu-poor CIGS ( 0.9 ) Adjusting doping conc. 3rd stage, Composition change Cu-poor layer formation. After 2nd stage, Cu/(In+Ga) 2nd stage, Cu-rich Cu(In,Ga)Se2 ( 1.25 ) Cu(In,Ga)Se2 direct formation Semi-metallic Stoichiometric CIGS Cu-rich CIGS Cu2-xSe : semi-metallic Emissivity 720 Cu/(In+Ga) ~ 0.8 Cu/(In+Ga) ~ 1.0 Cu/(In+Ga) ~ 1.25 710 o Temp. ( C) 715 Cu2-xSe CIGS Back contact: Mo, 1m Glass 705 700 695 690 44 46 48 50 52 Time ( min ) In-situ Composition Monitoring Tech. Precise composition control High reproducibility End point of 2nd stage Cu/(In+Ga) ~ 1.25 54 56 58 스퍼터링 법 -순차 스퍼터링법 채용 Cu/Ga 합금 타겟+In 타겟 순차 스퍼터 -500C 이상 석영 전기로에서 Se 침투 -양산성 우수, LCD 공정의 스퍼터러 설비 사용 가능 -Showa Shell, Honda, 독일 Sulfur cell 적용 중 -기업체 기반 업체에서 주로 채택 스퍼터 後 세렌化 1) Inline sputter Cu/Ga Target 2) Quartz furnace In Target 고온 열처리 H2Se In Cu/Ga Cu/Ga CIGS Se 확산 방법 Selenization : Showa shell 적용 Quartz furnace In H2Se gas H2Se Cu/Ga CIGS Sputter Se evaporation : 유럽 장비 concept Furnace CIGS In Se In Cu/Ga Cu/Ga RTP CIGS Sputter Evaporation Q-cells Q.Smart UF 70-90 Solar Frontier, Kunitomi 공장 (Miyazaki 공장3) : CIGS 생산: 2011 초 자본금: 10억불 생산능력: 900MW/year 직원수: 700-800 일본 2곳 설치, 각 1MW 세계 CIGS 업체 현황 Solyndra, USA: cylindrically shaped solar panels, 500 MW, 2011년 Ascent Solar, USA: 플렉서블 플라스틱 기판 사용 TSMC: 10MW100MW 증설 아반시스: 100MW 증설 솔리브로: Q-cells 자회사 서퍼셀, 미아솔, AQT, 누보선, 헬리오볼트 텔리오솔라, LG이노텍(13% 효율, 80% 수율), 삼성전자(11% 효율), 대양금속(SS) CIGS 모듈 제품 Wurth WSG0036E092: 12.6% Avancis Powermax 130: 12.1% 3.2 Key elements for high-efficiency CIGS SC 4 technological innovations in 1990-2000 - improved film quality by CuySe (y<2) - Na-containing soda-lime glass: efficiency, reliability, process tolerance - partial replacement of In with Ga, 1.04 to 1.1-1.2 eV, 20-30% of Ga - 50 nm CdS by CBD, ZnO window layer 3.3 Absorber preparation techniques 3.3.1 Basics Na diffuse from glass through Mo into growing absorber blocking layers, SiNx, SiO2, Cr, NaF, Na2Se, Na2S deposition other substrates like metal or polymer foils Na effect: better film morphology and higher conductivity, change in defect distribution during film growth, Na forms NaSex, slows down CIS growth, facilitate incorporation of Se widening existence range of alpha phase, larger tolerance to Cu/(In+Ga) ratio MoSe2 forms at Mo surface MoSe2, layered semiconductor with p-type, 1.3 eV BG, weak van der Waals bond along c-axis larger BG low-recombinative back surface for e’s, low-resistance contact for h’s Fig. 6 Arrangement for the deposition of Cu(In,Ga)Se2 films on the laboratory scale by coevaporation on a heated substrate. The rates of the sources are controlled by mass spectrometry. 3.3.3 Selenisation processes - separation of deposition and compound formation into 2 processing steps - sputtering, selenisation in H2Se - Shell Solar Inc. - Fig. 7 - 2nd thermal process in H2S, Cu(In,Ga)(S,Se)2 - avoid toxic H2Se, RTP, Se is incorporated in layer - better performance when annealed in S-containing atm. - sequential processes need 2 or 3 stages for absorber completion counterbalance the advantage of sputtering Fig. 7 Illustration of the sequential process. First a stack of metal (Cu.In.Ga) layers deposited by sputtering on to a Mo-coated glass. In the second step. this stack is selenised in H2Se atmosphere and converted into CuInSe2. 3.3.4 Other absorber deposition processes - MBE, MOCVD not suitable for high efficiency - electrodeposition, annealing process, recrystallisation vs. decomposition - electrodeposition of Cu-rich CuInSe2, vacuum evaporation of In(Se) - particle deposition by printing, 13% 3.3.4 Post-deposition anneal - air annealing - positive VSe passivated by O reduced band bending, recombination probability Cu(In, Ga)Se2 surface, CdS/Cu(In,Ga)Se2 interface Fig. 8 Deposition and patterning sequence to obtain an integrated interconnect scheme for Cu(In,Ga)Se2 thin-film modules. Fig. 9 Sketch of an in-line deposition system for co-evaporation of Cu(In.Ga)Se2 absorber films from line-sources. Table 2. Comparison of efficiencies η and areas A of laboratory cells, mini-modules, and commercial-size modules achieved with Cu(In,Ga)Se2 thin films based on the co-evaporation and the selenisation process. NREL denotes the National Renewable Energy Laboratories (USA), ZSW is the Center for Solar Energy and Hydrogen Research (Germany), EPV is Energy Photovoltaics (USA), ASC is the Angstrom Solar Centre (Sweden) NREL CIGS conversion devices CHARACTERIZATION OF 19.9%-EFFICIENT CIGS ABSORBERS Ingrid Repins,1 Miguel Contreras,1 Manuel Romero,1 Yanfa Yan,1 Wyatt Metzger,1 Jian Li,1 Steve Johnston,1 Brian Egaas,1 Clay DeHart,1 John Scharf,1 Brian E. McCandless,2 and Rommel Noufi3 1National Renewable Energy Laboratory, Golden, CO 80401 2Institute for Energy Conversion, Newark, DE 19716 3Solopower, San Jose, CA 95138 we document the properties of high-efficiency (19.9%) CIGS by a variety of characterization techniques, with an emphasis on identifying near-surface properties associated with the modified processing. Fig. 10 Band diagram of the ZnO/CdS/Cu(In,Ga)Se2 heterojunction under bias voltage showing the conduction and valence band-edge energies ΔEc and Ev. The quantities ΔEcwb/ba denote the conduction band offsets at the window/buffer and buffer/absorber inierfaces, respectively. An internal valence band offset ΔEvint exists between the bulk Cu(In,Ga)Se2 and a surface defect layer (SDL) on top of the Cu(In,Ga)Se2 absorber film. The quantity ΔEFn denotes the energy distance between the electron Fermi level EFn and the conduction band at the CdS buffer/Cu(ln,Ga)Se2 absorber interface, and Фn denotes the neutrality level of interface states at this heterointerface. Fig. 11 Optical and electronic losses of the short circuit current density Jsc of a high-efficiency ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cell. The incident current density of 41.7mA/cm2 corresponds to the range of the AM 1.5 solar spectrum that has a photon energy larger than the band gap energy Eg=1.155 eV of the Cu(In,Ga)Se2 absorber. Optical losses consist of reflection losses at the ambient/window, at the window/buffer, the buffer/absorber, and at the absorber/back contact interface as well as of parasitic absorption in the ZnO window layer (free carrier absorption) and at the Mo back contact. Electronic losses are recombination losses in the window, buffer, and in the absorber layer. The finally measured Jsc of 34.6 mA/cm2 of the cell stems almost exclusively from the Cu(In,Ga)Se2 absorber and only to a small extend from the CdS buffer layer. Fig. 12 Recombination paths in a ZnO/CdS/ (low-gap) Cu(In,Ga)Se2 junction at open circuit. The paths A represent recombination in the neutral volume. A' recombination at the back contact, B recombination in the space-charge region, and C recombination at the interface between the Cu(In,Ga)Se2 absorber and the CdS buffer layer. Back contact recombination is reduced by the conduction band offset ΔEcback between the Cu(In,Ga)Se2 absorber and the MoSe2 layer that forms during absorber preparation on top of the metallic Mo back contact. Interface recombination (C) is reduced by the internal valence band offset ΔEvint between the bulk of the Cu(In.Ga)Se2 absorber and the Cu-poor surface layer. The quantity Φ*bp denotes the energy barrier at the CdS/absorber interface and ET indicates the energy of a recombination centre in the bulk of the Cu(In,Ga)Se2. Table 3. Absorber band-gap energy Eg , efficiency η, open-circuit voltage Voc, short-circuit current density Isc, fill factor FF, and area A of the best Cu(In,Ga)Se2, CuInSe2, CuGaSe2, Cu(In,Al)Se2, CuInS2, Cu(In.Ga)S2, and Cu(In,Ga)(S.Se)2 solar cells. Fig. 13 Open-circuit voltages of different Cu-chalcopyrite based solar cells with various bandgap energies of the absorber layers. Full symbols correspond to Cu(In,Ga)Se2 alloys prepared by a simple single layer process (squares), a bi-layer process (triangles down), and the three-stage process (triangles up). Cu(In,Ga)Se2 cells derived from an in-line process as sketched in Fig. 9 are denoted by diamonds. Open triangles relate to Cu(In,Ga)S2, open circles to Cu(In,Ga)(S,Se)2, and the crossed triangles to Cu(In,Al)Se2 cells. Fig. 14 Energy band diagram of a ZnO/CdS/(wide-gap) Cu(In,Ga)(Se.S)2 heterojunction. The band diagram (a) that includes the surface defect layer (SDL) of a Cu-poor prepared film shows that the interface recombination barrier Φ*bp = Φbp + ΔEvint is larger than the barrier Φbp in the device that was prepared Cu-rich (b). The difference is the internal valence band offset ΔEvint between the SDL and the bulk of the absorber. The larger value of Φ*bp reduces interface recombination. Fig. 15 Band diagram of a ZnO/CdS/Cu(In,Ga)(Se,S)2 heterojunction with a graded-gap absorber. The minimum band gap energy is in the quasi neutral part of the absorber. An increasing Ga/In ratio towards the back surface and an increasing Ga/In or S/Se-ratio towards the front minimise recombination in critical regions at the back contact (recombination path A'), in the space charge region (path B), and at the hetero interface (path C). The dotted lines correspond to the conduction and valence band edge energies of a non-graded device.