Cu(InGa)Se solar cells

Solar Cells© F.-J. Haug, ETH Zürich

Cu(InGa)Se2 solar cells

n-ZnO

glass

Mo

p-CIGS

n-CdS

Efficiencies up to 18.8% (NREL, USA), 15.7% (ETHZ)


Crystallographic properties of CIGS

Diamond (element semiconductors: Ge and Si)

Zincblende (optoelectronic materials: GaAs, InP, GaN)

Photovoltaic materials: CdTe (Zincblende)CuInSe2 (Chalcopyrite) Cu

In,GaSe

a

c


Phase diagram of CuInSe2

15 20 25 30

100

300

500

700

900

Cu [at%]

t [°C

]

α

α

α+β

α+δ

δ

β

β+δ

δ

α+Cu2Se (HT)

α+Cu2Se

Cu In

Se

In2Se3

Cu2Se

CuInSe2

CuIn3Se5CuIn5Se8

Pseudo-binary cut through ternary diagram

Device quality: Cu-poor; Cu-rich: Cu2-xSe seggregations

Fearheiley, SC, 16, p. 91


Substrate and back contact

Substrate:Soda lime glass (supply of Na)steel foils (Kovar, Invar; rough surfaces)Polyimides (Kapton, Upilex, other; poor adhesion)

Back contact:Mo (formation of MoSe2, back surface field)Cr (Na barrier)Cu (CIS Cut process)TCOs (superstrate cells, reversed configuration)


Absorber layer

CuInSe2 with Ga addition Cu(In,Ga)Se2 (CIGS)

Compound formation ~400-500°C; growth ~550°C

“one stage” process: low quality material

Cu-poor material: single phase chalcopyriteCu-rich material: Cu2-xSe segregates,

conducting (undesired), flux (large grain)

“bilayer” or “three stage” superior quality


Spectral absorption

Direct bandgap (CuInSe2) Indirect bandgap (Si) (additional phonon required)

1.0 1.5 2.0 2.5 3.0

0.0

2.0x104

4.0x104

6.0x104

8.0x104

1.0x105

1.2x105

1.4x105

1.6x105

Photon energy [eV]

Abs

orpt

ion

coef

ficie

nt [c

m-1]

CuInSe2

c-Si

1.0 1.5 2.0 2.5 3.0101

102

103

104

105

Abs

orpt

ion

coef

ficie

nt [c

m-1]


Transmission (absorber layer)

1.0 1.5 2.0 2.5 3.0 3.5 4.00

20

40

60

80

100

0.2

0.25

x=0.3

CuGa3Se5

CuIn1-xGaxSe2

CuGaSe2

Tra

nsm

issi

on [%

]

Band gap energy [eV]

1200 900 600 300

Wavelength [nm]

Bandgap depends on I/(I+III) and Ga/(Ga+In) ratio


Deposition methods

co-evaporation(Best results)

Selenisation of precursors(low cost, difficulty of complete conversion)

Vacuum Multilayers(sputtering, evaporation)

Pastes(Screen printing)

Se or H2Se


Morphology of absorber layers

Cu-rich Cu-poor

“one step”

“two step”


Band offsets

Gereference

-0.32 eV

1.04 eV1.68 eV

CuIn1-xGaxSe

-0.6 eV

1.3 eV

OVC

-1.4 eV

CdS Zn1-xMgxO

-2.7 eV

3.3 eV

2.4 eV

-0.6 eV

1.5 eV

CuInS1-xSex

Schmid et.al. SEM 41/42, p. 281Klein et.al. APL 70(19), p. 1299Minemoto et.al. JAP 89(12), p. 8327

Turcu et.al. APA 73, p. 769


Band alignment (equilibrium)

ZnO CdS

OVC

CuIn0.7Ga0.3Se

“Spike” type, ~0.2 eV

Spike <0.3 uncritical

Niemegeers et.al.APL, 67(6), p. 843

Spike <0.4 favourableMinemoto et.al.SEM, 67, p. 83


Buffer layer

Formation of pn-junction

(Historically: also front contact)

Deposition of CdS in chemical bath process (CBD)

Cd(Ac)2, (H2N)2CS (Thiourea), NH3, precipitation of CdS

Alternate:In(OH)xSy, ZnSe, ZnS; (CBD process)ZnS, ZnSe; (evaporation)Cd, Zn; (partial electrolyte treatment)


Transmission (window layer)

CdS buffer absorbs at 600 nm => reduced photocurrent

1.0 1.5 2.0 2.5 3.0 3.5 4.00

20

40

60

80

100

AM 1.5 CdS (400nm)

ITO

CdS (50nm)

ZnO

Tra

nsm

issi

on [%

]


1200 900 600 300

Wavelength [nm]


Buffer layer and cell efficiency

ZnO direct

CdS/ZnO

Cd-treatment

Zn-treatment

Performance of CdS buffer layer is unsurpassed

Canava et.al.TSF 361/362, p. 187


Alternate buffers

Thickness of CdS may be reduced

ZnSe and ZnS are promising

Engelhardt et. al., PPV 7, p 423


Transmission (solar cell)

1.0 1.5 2.0 2.5 3.0 3.5 4.00

20

40

60

80

100

CdS (50nm)

ZnO

x=0.3

CuIn1-xGaxSe2

Tra

nsm

issi

on [%

]


1200 900 600 300

Wavelength [nm]

Absorption in CdS does not contribute to current


Transparent conducting oxides(TCOs)

Suppress absorption in visible range => Eg>3.3 eV

Oxides: ZnO, CdO, SnO2, In2O3, (CdSnO4)

Doping: ZnO: B, Al, Ga, In; SnO2: F (FTO)

Multinary compounds: In2O3-SnO2 (ITO)

n-type

ZnO:N, ZnO:N,Ga

p-typeCuGaO2, CuInO2, SrCu2O2

Difficulty to dope p-type


Spectral response

Higher mobility in ZnO:BHigher Transmission in IRHigher current density

Hagiwara et.al, SEM 67, p. 267


Module interconnectionMo P1

CIGS P2

ZnO/CdS P3

Problem: corrosion of ZnO/Mo contact


Superstrate solar cells

Reversed structure

n-ZnO

glass

Au

p-CIGS

Challenges:•thermally stable ZnO:Al•no CdS buffer layer•no Na (ZnO diffusion barrier)

n++-ZnO:Al

Efficiencies up to 12.8% (AGU, Japan), 11.2% (ETHZ)

Prospects:•low cost encapsulation•dry processing


Flexible solar cells

Challenges:•processing on polymer•low temperature•no Na supply

Prospects:•flexibility (“wearable”)•light weight

Efficiencies up to 12.8% (ETH)


Tandem solar cells

CuInSe2 and CuGaSe2 are an ideal combination


Mechanically stacked tandem

no current matching necessary separate HT from LT processes

Top cell: 15% minimum, present CGS cell ~9% on Mo

Also: Bifacial approach

Four terminal device


Monolithically integrated tandem

“blue” absorber

p++n++ tunnel junction

“red” absorber

Two terminal device

Monolithic GaInP-GaAs cells: >30%

Disadvantage: current matching required


n++

III-V tandem solar cells

Top cell (InGaP):•blue: absorption (Eg~1.7 eV)•red: transmission

Bottom cell (GaAs):•red absorption (Eg~1.0 eV)

n

pp++

n++

np

Junction:• transparent

Documents

Cu(InGa)Se solar cells