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
CuAlS2
Band gap energy (eV)
▶ high light absorption coefficient
:105/cm
3
CuAlSe2
CuGaS2
2
AgGaS2
CdS
GaP
CuGaSe2 AgInS2
InP
CuInS2
AgGaSe2
AgInSe2
GaAs
1
0
5.2
Si
5.4
CuInSe2
5.6
5.8
CuGaTe2 CuInTe
2
6.0
Lattice constant (A)
6.2
CIGS 태양전지 동작원리
Front
electrode
Window Buffer
Back electrode
Absorber
E
I ( x)  I o e
x
I ( x )  I o e (  x )
1
d
  ln
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 [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 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)
Process
1) Sputtering
2) CBD
(Chemical Bath Deposition)
3) Co-Evaporation
Sputtering/Se
4) DC Sputtering
Back contact
Substrate
Mo
(1um)
Glass
(2~3mm)
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, 1m
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: 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%
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.