CIGSFriday 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 properties2.1 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.

▶ high light absorption coefficient :105/cm

5.2 5.4 5.6 5.8 6.0 6.20

1

2

3

4

CuGaTe2 CuInTe2

AgInSe2

AgGaSe2

CdS

AgGaS2

AgInS2

InP

GaAsSi

CuInS2

GaP

CuInSe2

CuGaSe2

CuAlSe2CuGaS2

Band

gap

ene

rgy

(eV

)Lattice constant (A)

CuAlS2

▶ tandem structure by composition control : CuGaSe2, CuInS2

E

E

+

Back electrodeFront electrode

WindowBuffer Absorber-

x

)()( xoeIxI x

)()( xoeIxI

II

doln1

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 (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 sin-gle-phase region.

Fig. 3 Quasi-binary phase diagram of CuInSe2 along the tie-line that connects the bi-nary 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 [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

Table 1. Electronic transition energies and formation energies ΔU of the 12 intrinsic de-fects 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 posi-tion 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 technology3.1 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.

16/117

MO 증착(Sputter)

Patterning 1

Laser Scribe

CIGS 성막(co-evaporation & sputter 후

Se/S 化 )

버퍼층 형성CBD

후면 반사 / 전극 증착

(ZnO/Ag)

Patterning 3기계적 Scribe

Lamination & wiring

Patterning 2

기계적 Scribe

1) CdS2) Zn(O,S,OH)x

1) Co-evaporation

2) Sputter 法 + Se/S 化

1) Wurth Solar : SPT2) Showa Shell : MOCVD

Dip-Coating

Cu, In, Ga, SeEvaporation

Sources

기본 공정도- 업체별 공정 특화 : Wurth Solar, Show Shell- 성막 방법 , 사용 재료

SEM 단면도

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) 1) Sputtering

2) CBD (Chemical Bath Deposi-

tion)

3) Co-Evaporation Sputtering/Se

4) DC Sputtering

5) Substrate (Sodalime Glass)

Layer Material(Thickness) Process

구조- 광흡수층과 버퍼층이 효율 좌우

Co-evapration SPT + 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

1st stage,(In,Ga)xSey formation

In +Ga + Se

(In,Ga)xSey

Glass

Mo

Cu + Se Cu(In,Ga)Se2

Glass

(In,Ga)xSey

Mo

After 2nd stage,Cu-rich Cu(In,Ga)Se2 ( 1.25 )

Semi-metallic

Glass

Cu(In,Ga)Se2

Mo

2nd stage,Cu(In,Ga)Se2 direct formation

3rd stage, Composition change

Cu-poor layer formation.

Glass

Cu(In,Ga)Se2

In +Ga + Se

Mo

After 3rd stageCu-poor CIGS ( 0.9 )

Adjusting doping conc.

Glass

Cu(In,Ga)Se2

Mo

1st stage

2nd stage3rd stageEvaporation Time

In + Ga+ Se

Cu+ Se

~600

Subs

trat

e Te

mp

(o C)

350

In +

Ga

+ Se

1st

2nd 3rd

Se

Only Se

Evaporation Time

In + Ga+ Se

Cu+ Se

~600

Subs

trat

e Te

mp

(o C)

350

In +

Ga

+ Se

1st

2nd 3rd

Se

Only Se

Cu/(In+Ga)

동시 증발법

44 46 48 50 52 54 56 58690

695

700

705

710

715

720

Tem

p. (o C)

Time ( min )

Glass

Back contact: Mo, 1m

CIGS

Cu2-xSe

Cu2-xSe : 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 2nd stage Cu/(In+Ga) ~ 1.25

Cu/(In+Ga) ~ 0.8

H2Se

2) Quartz furnace

고온 열처리

CIGS

스퍼터 後 세렌化

Cu/Ga Target In Target

Cu/Ga Cu/GaIn

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 fur-nace

Sputter

CIGS

Cu/GaIn

Cu/GaIn

Se evaporation : 유럽 장비 concept

Selenization : Showa shell 적용

Cu/GaIn

Evaporation

Se

H2Se gas

Sputter

H2Se

Furnace

CIGS

RTP

CIGS

Se 확산 방법

Q-cells Q.Smart UF 70-90

Solar Frontier, Kunitomi 공장 (Miyazaki 공장 3): CIGS

생산 : 2011 초자본금 : 10 억불생산능력 : 900MW/year직원수 : 700-800

일본 2 곳 설치 , 각 1MW

세계 CIGS 업체 현황Solyndra, USA: cylindrically shaped solar panels, 500 MW, 2011 년Ascent Solar, USA: 플렉서블 플라스틱 기판 사용TSMC: 10MW100MW 증설아반시스 : 100MW 증설솔리브로 : Q-cells 자회사서퍼셀 , 미아솔 , AQT, 누보선 , 헬리오볼트텔리오솔라 , LG 이노텍 (13% 효율 , 80% 수율 ), 삼성전자 (11% 효율 ), 대양금속 (SS)

CIGS 모듈 제품Wurth 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 techniques3.3.1 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 de-fect distribution

during film growth, Na forms NaSex, slows down CIS growth, facilitate in-corporation 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 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

- 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 de-posited by sputtering on to a Mo-coated glass. In the second step. this stack is se-lenised 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 probabilityCu(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 ab-sorber 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-evapo-ration and the selenisation process. NREL denotes the National Renewable Energy Lab-oratories (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 Met-zger,1 Jian Li,1 Steve Johnston,1 Brian Egaas,1 Clay DeHart,1 John Scharf,1 Brian E. McCandless,2 and Rom-mel 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 ΔEc

wb/ba denote the conduction band offsets at the window/buffer and buffer/absorber inierfaces, respectively. An internal valence band offset ΔEv

int 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 conductionband 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-effi-ciency 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. Op-tical 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 con-tact. Electronic losses are recombination losses in the window, buffer, and in the ab-sorber layer. The finally measured Jsc of 34.6 mA/cm2 of the cell stems almost exclu-sively 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 cir-cuit. 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 con-tact recombination is reduced by the conduction band offset ΔEc

back 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 in-ternal valence band offset ΔEv

int 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-cir-cuit current density Isc, fill factor FF, and area A of the best Cu(In,Ga)Se2, CuInSe2, Cu-GaSe2, 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 band-gap energies of the absorber layers. Full symbols correspond to Cu(In,Ga)Se2 al-loys 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 banddiagram (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 bar-

rier Φbp in the device that was prepared Cu-rich (b). The difference is the internal va-lence band offset ΔEv

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-ra-tio towards the front minimise recombination in critical regions at the back contact (re-combination 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 ener-gies of a non-graded device.