intrinsic dx centers in ternary chalcopyrite semiconductors … · electron-trapping due to dx...
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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)