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CIGS Friday 07:00-09:00 pm Textbook: Solar Cells edited by T. Markvart and L. Castaner Lecturer: Prof. Yeong-Cheol

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 Introduction 0.5 cm2 lab cell, 18.8% mini-modules with 20 cm2, 16.6% first CuInSe2 by Hahn in 1953

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CIGS Friday 07:00-09:00 pm Textbook: Solar Cells edited by T. Markvart and L. Castaner Lecturer: Prof. Yeong-Cheol

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  1. CIGS Friday 07:00-09:00 pm Textbook: Solar Cells edited by T. Markvart and L. Castaner Lecturer: Prof. Yeong-Cheol Kim

  2. Cu(In,Ga)Se2 thin-film solar cells 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

  3. 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

  4. 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 ZnSeare alternately occupied by Cu and In in the chalcopyrite structure..

  5. Fig. 2 Band-gap energies Eg vs. the lattice constant a of the Cu(In,Ga,Al)(S,Se)2 alloy system.

  6. ▶ tandem structure by composition control : CuGaSe2, CuInS2 ▶ high light absorption coefficient :105/cm

  7. CIGS 태양전지 동작원리 Window Back electrode Buffer Absorber Front electrode - + E E x

  8. 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 single-phase region.

  9. 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.

  10. 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 [24] 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 [34] Fig. 4

  11. Table 1. Electronic transition energies and formation energies ΔU of the 12 intrinsic defects in CuInSe2.

  12. 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.

  13. 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

  14. Fig. 5 Schematic layer sequence of a standard ZnO/CdS/Cu(In,Ga)Se2thin-film solar cell.

  15. 기본 공정도 -업체별 공정 특화: Wurth Solar, Show Shell -성막 방법, 사용 재료 Dip-Coating MO 증착 (Sputter) Patterning 1 Laser Scribe CIGS 성막 (co-evaporation & sputter후 Se/S 化) 버퍼층 형성 CBD 1) Co-evaporation 1) CdS 2) Zn(O,S,OH)x Cu, In, Ga, Se Evaporation Sources 2) Sputter法 + Se/S 化 Lamination & wiring Patterning 3 기계적 Scribe 후면 반사/전극 증착 (ZnO/Ag) Patterning 2 기계적 Scribe 1) Wurth Solar : SPT 2) Showa Shell : MOCVD

  16. SEM 단면도

  17. 구조 -광흡수층과버퍼층이 효율 좌우 Process Layer Material(Thickness) Window layer n-ZnO (500nm) / i-ZnO (50nm) • Sputtering • CBD (Chemical Bath Deposition) • Co-Evaporation Sputtering/Se • DC Sputtering • Substrate (Sodalime Glass) CdS (50nm) Buffer layer Absorber layer Cu(In,Ga)(Se,S)2 (2~3um) Back contact Mo (1um) Glass (2~3mm) Substrate

  18. 광흡수층 공정 비교 Selenization Sputter

  19. 동시 증발법 Cu/(In+Ga)

  20. Stoichiometric CIGS  Cu-rich CIGS Cu2-xSe : semi-metallic  Emissivity Cu/(In+Ga) ~ 1.0 Cu/(In+Ga) ~ 0.8 Cu/(In+Ga) ~ 1.25 In-situ Composition Monitoring Tech.  Precise composition control  High reproducibility End point of 2nd stage  Cu/(In+Ga) ~ 1.25

  21. 스퍼터링 법 -순차 스퍼터링법 채용 Cu/Ga합금 타겟+In 타겟 순차 스퍼터 -500C 이상 석영 전기로에서Se 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 2) Quartz furnace In Target Cu/Ga Target 고온 열처리 H2Se In Cu/Ga Cu/Ga CIGS

  22. Se 확산 방법 In In Cu/Ga Cu/Ga CIGS CIGS Selenization : Showa shell 적용 Quartz furnace H2Se gas H2Se CIGS Sputter Se evaporation : 유럽 장비 concept Furnace Se In Cu/Ga RTP Evaporation Sputter

  23. Q-cells Q.Smart UF 70-90

  24. Solar Frontier, Kunitomi공장 (Miyazaki 공장3) : CIGS 생산: 2011 초 자본금: 10억불 생산능력: 900MW/year 직원수: 700-800 일본 2곳 설치, 각 1MW

  25. 세계 CIGS 업체 현황 Solyndra, USA: cylindrically shaped solar panels, 500 MW, 2011년 Ascent Solar, USA: 플렉서블 플라스틱 기판 사용 TSMC: 10MW100MW 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% AvancisPowermax 130: 12.1%

  26. 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

  27. Fig. 6 Arrangement for the deposition of Cu(In,Ga)Se2 films on the laboratory scale by co-evaporation on a heated substrate. The rates of the sources are controlled by mass spectrometry.

  28. 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

  29. 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.

  30. 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 VSepassivated by O  reduced band bending, recombination probability Cu(In, Ga)Se2 surface, CdS/Cu(In,Ga)Se2 interface

  31. Fig. 8 Deposition and patterning sequence to obtain an integrated interconnect scheme for Cu(In,Ga)Se2 thin-film modules.

  32. Fig. 9 Sketch of an in-line deposition system for co-evaporation of Cu(In.Ga)Se2 absorber films from line-sources.

  33. Table 2. Comparison of efficiencies ηand areas A of laboratory cells, mini-modules, and commercial-size modules achieved with Cu(In,Ga)Se2thin 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)

  34. NREL CIGS conversion devices

  35. 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.

  36. Fig. 10 Band diagram of the ZnO/CdS/Cu(In,Ga)Se2heterojunction under bias voltage showing the conduction and valence band-edge energies ΔEcand Ev. The quantities ΔEcwb/badenote the conduction band offsets at the window/buffer and buffer/absorber inierfaces, respectively. An internal valence band offset ΔEvintexists between the bulk Cu(In,Ga)Se2 and a surface defect layer (SDL) on top of the Cu(In,Ga)Se2absorber film. The quantity ΔEFndenotes 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.

  37. Fig. 11 Optical and electronic losses of the short circuit current density Jsc of a high-efficiency ZnO/CdS/Cu(In,Ga)Se2heterojunction solar cell. The incident current density of 41.7mA/cm2corresponds to the range of the AM 1.5 solar spectrum that has a photon energy larger than the band gap energy Eg=1.155 eVof 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 ZnOwindow 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 Jscof 34.6 mA/cm2of the cell stems almost exclusively from the Cu(In,Ga)Se2 absorber and only to a small extend from the CdS buffer layer.

  38. 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)Se2absorber and the CdS buffer layer. Back contact recombination is reduced by the conduction band offset ΔEcbackbetween 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 ΔEvintbetween the bulk of the Cu(In.Ga)Se2absorber and the Cu-poor surface layer. The quantity Φ*bpdenotes 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.

  39. 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)2solar cells.

  40. 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)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)Se2cells.

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