Intrinsic DX centers in ternary 

chalcopyrite semiconductors

Stephan Lany and Alex Zunger

“Why metastable intrinsic defects cause open-circuit-voltage limitation

and how they can be avoided”

This work was supported by the U.S. Department of Energy 

under Contract No. DE-AC36-99GO10337 with the National 

Renewable Energy Laboratory.

NREL/PR-590-43272

Presented at the 33rd IEEE Photovoltaic Specialist Conference held May 11-16, 2008 in San Diego, California
VOC =

VOC saturation in CIGS

[1] W.N. Shafarman and L. Stolt, in: 

Handbook of Photovoltaic 

Science and Engineering

[2] R. Kniese, M. Lammer, U. Rau , 

M. Powalla, TSF 451-452, 430 

(2004).

[3] G. Hanna, A. Jasenek, U. Rau, 

H.W. Schock, TSF 387, 71 (2001).

[1]

[2]

1.7eV

CGS

VOC =

1.2eV

Higher VOC:

• Higher η for single-junction

• Needed for TF tandem

• Reason: Recombination due to 

deep defects [3]

VOC = Eg − 0.5 eV
Defects levels (I) – Example: Orbital interaction in the H2 molecule

anti-bonding

superposition

bonding

H1 H2
Defects levels (II): Se-vacancy in CuInSe2

Se

Cu

In

VSe

VBM

CBM

a

b
Intrinsic DX centers in CIGS
DX centers: Electron traps formed due to lattice relaxations

CB

VB

GaCu donor relaxed DX

ZnSe:GaZn CuGaSe2:GaCu

In II-VI, DX centers require extrinsic impurities

In CIGS, native defects (InCu, GaCu) exhibit DX behavior

Extrinsic DX in II-VI Intrinsic DX in CIGS

εDX

S. Lany and A. Zunger, Phys. Rev. Lett. 100, 016401 (2008). 
Evolvement of ionic structure, electron-level, and energy 

during the transition into the deep DX state

CB

VB

Se

InVCu

Ini

resonant level

(shallow InCu)

gap level

(deep InDX)

InCu InDX
Critical Fermi levels for electron-trapping

Electron-trapping due to DX centers occurs mainly 

in wider-gap CuIn1–xGaxSe2 alloys with x ≥ 0.3

InCu (GaCu) exists isolated

or in complexes, e.g.,

(InCu-2VCu) [1]

occurs

above

Transition EF > EV +

InCu

2+ + 2e → InDX0 0.9 eV

(InCu-VCu)+ + 2e → (InDX-VCu)

– 1.1 eV

(InCu-2VCu)0 + 2e → (InDX-VCu)

2– 1.3 eV

[1] S.B. Zhang, S.-H. Wei, and A. Zunger, Phys. Rev. Lett. 78, 4059 (1997).
VOC limitation by InCu, GaCu, VSe and their complexes with VCu

InCu, GaCu: VOC is limited by the transition that causes atomic reconfiguration

VSe-VCu: The negative (acceptor) configuration exhibits deep trap level

Both types of defects limit VOC below ~1 eV 

[1] S. Lany and A. Zunger, Phys. Rev. Lett. 100, 016401 (2008). 

[2] S. Lany and A. Zunger, J. Appl. Phys. 100, 113725 (2006). 

[1] [2]
How to avoid VOC limiting

metastable defects?
Formation energies vs growth conditions

CuInSe2 stability condition

∆μCu+ ∆μIn+ 2∆μSe

= ∆Hf(CIS)

Competing phases 

e.g., 3∆μCu + 2∆μSe ≤ ∆Hf

(Cu3Se2)

D, host host D F [ ] [ ] D, F (, ) E E q − + μ −μ + ⋅ q E

 

∆ μ= H E q

● Minimize InCu, GaCu,

(InCu-2VCu)

● Minimize VSe, (VSe-VCu)

● Cu-rich / Se-rich growth
Trade-offs for minimizing VOC limiting defects

Minimizing defects: Se-rich / Cu-rich 

e.g., phase-equilibrium with Cu3Se2

Type inversion: Se-poor / III-rich (Cu-deficient) [1]

Other causes of VOC limit. : band-offset [2], ...?

CBO < 0

[1] S. Lany et al., Appl. Phys. Lett. 86, 042109 (2005)

[2] M. Morkel et al., Appl. Phys. Lett. 79, 4482 (2001)
Conclusions

● Intrinsic donor-type defects InCu, GaCu, and VSe, 

and their complexes with VCu cause metastability,

but also act to limit VOC 

● Growth conditions which minimize these defects

(Cu-rich/Se-rich)

are very different from those currently used

● Overcoming VOC limitation requires to address

other issues and trade-offs

References

S. Lany and A. Zunger, Phys. Rev. Lett. 100, 016401 (2008)

S. Lany and A. Zunger, J. Appl. Phys. 100, 113725 (2006)

Stephan_Lany@NREL.gov