Intrinsic DX centers in ternary chalcopyrite semiconductors

Stephan Lany and Alex Zunger

“Why metastable intrinsic defects cause open-circuit-voltage limitationand 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-590-43272Presented 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]

1.7eVCGS

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 DXZnSe:GaZn CuGaSe2:GaCu

In II-VI, DX centers require extrinsic impuritiesIn 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

InCuVCu

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 isolatedor in complexes, e.g.,(InCu-2VCu) [1]

occursabove

Transition EF > EV +

InCu2+ + 2e → InDX

0 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 reconfigurationVSe-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 limitingmetastable 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[ ] [ ] qE E q E− + µ − µ + ⋅D, F( , )

qH E∆ µ =

● 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 addressother issues and trade-offs

ReferencesS. 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