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 CuInSe 2. 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 E g vs. the lattice constant a of the Cu(In,Ga,Al)(S,Se) 2 alloy system.
▶ high light absorption coefficient :10 5 /cm ▶ tandem structure by composition control : CuGaSe 2, CuInS 2
E E + Back electrode Front electrode Window BufferAbsorber - x CIGS 태양전지 동작원리
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 (V Cu and In Cu ) Cu y Se: Cu In and Cu i 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 single-phase region.
Fig. 3 Quasi-binary phase diagram of CuInSe 2 along the tie-line that connects the binary compounds In 2 Se 3 and Cu 2 Se 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 V Se : dominant donor in n-type, V Cu : dominant acceptor in p-type calculation of metal-related defects in CIS and CGS by Zhang negative formation E for V cu in Cu-poor and stoichiometric material low E f for Cu In 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 E f of defect complexes, (2V Cu,In Cu ), (Cu In, In Cu ), (2Cu i,Cu In ) (2Cu i,Cu In ): 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 CuInSe 2.
Fig. 4 Band gap evolution diagram of the CuIn(Se,S) 2 (a) and the Cu(In,Ga)Se 2 (b) alloy system with the trap energy E T (N 2, open diamonds) taken as an internal reference to align the conduction band and the valence band energies E c and E v. The energy position of an additional defect state in Cu(In,Ga)Se 2 (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)Se 2 thin-film solar cell.
16/117 MO 증착 (Sputter) Patterning 1 Laser Scribe CIGS 성막 (co-evaporation & sputter 후 Se/S 化 ) 버퍼층 형성 CBD 후면 반사 / 전극 증착 (ZnO/Ag) Patterning 3 기계적 Scribe Lamination & wiring Patterning 2 기계적 Scribe 1) CdS 2) Zn(O,S,OH)x 1) Co-evaporation 2) Sputter 法 + Se/S 化 1) Wurth Solar : SPT 2) Showa Shell : MOCVD Dip-Coating Cu, In, Ga, Se Evaporation Sources 기본 공정도 - 업체별 공정 특화 : Wurth Solar, Show Shell - 성막 방법, 사용 재료
Substrate Back contact Absorber layer Buffer layer Window layer Glass (2~3mm) Mo (1um) Cu(In,Ga)(Se,S) 2 (2~3um) CdS (50nm) n-ZnO (500nm) / i-ZnO (50nm) Sputtering CBD (Chemical Bath Deposition) Co-Evaporation Sputtering/Se DC Sputtering Substrate (Sodalime Glass) LayerMaterial(Thickness) Process 구조 - 광흡수층과 버퍼층이 효율 좌우
Co-evaprationSPT + Selenization Process □ 금속원료 (Cu,In,Ga,Se) 동시 증착 □ SPT (Cu,Ga,In) 후 Se diffusion 적용업체 □ Wurth Solar, Johanna ( 독일 ) □ Showa Shell, Honda ( 일본 ) 장점 □ 최고 효율 달성 (19.2% @NREL) □ 학계 연구 자료 多 □ 대형화에 유리 □ Throughput 유리 □ SPT 공정 사용 (LCD Normal 공정 ) 단점 □ 대형화 어려움 ( 現 60*120 이하 ) □ LCD 비사용 공정 □ Showa shell 특허 등록 □ 국내 학계 경험 적음 Sputter Selenization 광흡수층 공정 비교
Cu 2-x Se : semi-metallic Emissivity Stoichiometric CIGS Cu-rich CIGS Cu/(In+Ga) ~ 1.0 Cu/(In+Ga) ~ 1.25 In-situ Composition Monitoring Tech. Precise composition control High reproducibility End point of 2 nd stage Cu/(In+Ga) ~ 1.25 Cu/(In+Ga) ~ 0.8
H 2 Se 2) Quartz furnace 고온 열처리 CIGS 스퍼터 後 세렌化 Cu/Ga Target In Target Cu/GaCu/Ga In 1) Inline sputter 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
Quartz furnace Sputter CIGS Cu/GaIn Cu/GaIn Se evaporation : 유럽 장비 concept Selenization : Showa shell 적용 Cu/Ga In Evaporation Se H 2 Se gas Sputter H 2 Se Furnace CIGS RTP CIGS Se 확산 방법
Solar Frontier, Kunitomi 공장 (Miyazaki 공장 3) : CIGS 생산 : 2011 초 자본금 : 10 억불 생산능력 : 900MW/year 직원수 : 700-800 일본 2 곳 설치, 각 1MW
세계 CIGS 업체 현황 Solyndra, USA: cylindrically shaped solar panels, 500 MW, 2011 년 Ascent Solar, USA: 플렉서블 플라스틱 기판 사용 TSMC: 10MW 100MW 증설 아반시스 : 100MW 증설 솔리브로 : Q-cells 자회사 서퍼셀, 미아솔, AQT, 누보선, 헬리오볼트 텔리오솔라, LG 이노텍 (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 Cu y Se (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)Se 2 films on the laboratory scale by co- evaporation 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 - 2 nd 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 H 2 Se atmosphere and converted into CuInSe 2.
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 V Se 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)Se 2 thin-film modules.
Fig. 9 Sketch of an in-line deposition system for co-evaporation of Cu(In.Ga)Se 2 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)Se 2 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)
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)Se 2 heterojunction under bias voltage showing the conduction and valence band-edge energies ΔE c and Ev. The quantities ΔE c wb/ba denote the conduction band offsets at the window/buffer and buffer/absorber inierfaces, respectively. An internal valence band offset ΔE v int exists between the bulk Cu(In,Ga)Se 2 and a surface defect layer (SDL) on top of the Cu(In,Ga)Se 2 absorber film. The quantity ΔE Fn denotes the energy distance between the electron Fermi level E Fn and the conduction band at the CdS buffer/Cu(ln,Ga)Se 2 absorber interface, and Ф n denotes the neutrality level of interface states at this heterointerface.
Fig. 11 Optical and electronic losses of the short circuit current density J sc of a high-efficiency ZnO/CdS/Cu(In,Ga)Se 2 heterojunction solar cell. The incident current density of 41.7mA/cm 2 corresponds to the range of the AM 1.5 solar spectrum that has a photon energy larger than the band gap energy E g =1.155 eV of the Cu(In,Ga)Se 2 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 J sc of 34.6 mA/cm 2 of the cell stems almost exclusively from the Cu(In,Ga)Se 2 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)Se 2 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)Se 2 absorber and the CdS buffer layer. Back contact recombination is reduced by the conduction band offset ΔE c back between the Cu(In,Ga)Se 2 absorber and the MoSe 2 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 ΔE v int between the bulk of the Cu(In.Ga)Se 2 absorber and the Cu-poor surface layer. The quantity Φ * bp denotes the energy barrier at the CdS/absorber interface and E T indicates the energy of a recombination centre in the bulk of the Cu(In,Ga)Se 2.
Table 3. Absorber band-gap energy E g, efficiency η, open-circuit voltage V oc, short-circuit current density I sc, fill factor FF, and area A of the best Cu(In,Ga)Se 2, CuInSe 2, CuGaSe 2, Cu(In,Al)Se 2, CuInS 2, Cu(In.Ga)S 2, and Cu(In,Ga)(S.Se) 2 solar cells.
Fig. 13 Open-circuit voltages of different Cu-chalcopyrite based solar cells with various band-gap energies of the absorber layers. Full symbols correspond to Cu(In,Ga)Se 2 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)Se 2 cells derived from an in-line process as sketched in Fig. 9 are denoted by diamonds. Open triangles relate to Cu(In,Ga)S 2, open circles to Cu(In,Ga)(S,Se) 2, and the crossed triangles to Cu(In,Al)Se 2 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 = Φ bp + ΔE v int is larger than the barrier Φ bp in the device that was prepared Cu-rich (b). The difference is the internal valence band offset ΔE v int 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.