Advances in Materials Modeling of Kesterite Thin-film ... · NREL is a national laboratory of the...
Transcript of Advances in Materials Modeling of Kesterite Thin-film ... · NREL is a national laboratory of the...
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
Advances in Materials Modeling of Kesterite Thin-film Solar Cell
2012 World Renewable Energy Forum
Su-Huai Wei1, Shiyou Chen2, Aron Walsh3, Xingao Gong2
1National Renewable Energy Laboratory 2Fudan University, China 3University of Bath, UK
May 16, 2012
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Searching new PV absorber through cation mutation
Strong need for new solar cell absorbers:
With direct band gap around 1.2~1.5 eV.
No or less expensive In, Ga and toxic Cd.
Good material/defect properties.
Despite great success of current solar cell technologies,
large scale application of PV is still quite challenging:
They depends on materials that has limited efficiency
(a-Si), high cost (c-Si, III-V), limited abundance (In, Ga
in CIGS or Te in CdTe), or could be toxic (Cd in CdTe).
One approach to systematical
search of new PV absorbers is
through cation-mutation
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ZnSe CuGaSe2 Cu2ZnGeSe4 mutation
ZnSe (2.82 eV)
CuGaSe2 (1.68 eV)
KS-Cu2ZnGeSe4 (1.50 eV)
KS-Cu2ZnSnS4 (1.50 eV)
KS-Cu2ZnSnSe4 (1.00 eV)
S. Chen et al., Appl. Phys. Lett. 94, 041903 (2009);
Phys. Rev. B 79, 165211 (2009).
The stable structure obey the octet rule.
Chalcopyrite structure is most stable for
ternary compounds and kesterite structure
usually has the lowest energy especially for the
intra-row mutated quaternary compounds
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Band gap decreases in ZnS CuGaS2 Cu2ZnGeS4 mutation
Band gap decreases in the
binary to ternary to quaternary
mutation process.
The VBM increases due to
large Cu, d – S, p level repulsion.
The CBM has large cation s
character, its decrease in energy is
due to the localization of
wavefunction on Ge site.
CBM CBM
VBM VBM
Chen, Gong, Walsh and Wei, Phys. Rev. B 79, 165211 (2009); Appl. Phys. Lett. 94, 041903 (2009)
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Chemical potential range that gives stable CZTS
Narrow stable region.
ZnS, CuS, Cu2S, SnS
forms very easily.
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Defect formation energy and transition energy level
Acceptors have lower
energy than donors, explaining
the p-type conductivity in the
system;
CuZn antisite has a deeper
accepter level than Vcu.
Chen‚ Gong‚ Walsh‚ and Wei‚ Appl. Phys. Lett. 96‚ 021902 (2010).
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Transition energy levels
SnCu can have shallow donor levels but SnZn has deeper levels
VS creates deep donor levels inside band gap
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Ordered vacancy compounds and charge separation
New phases caused by CuZn+ZnCu pair create disordered kesterite
phase, but won’t produce a hole barrier relative to the kesterite phase;
Other complexes like VCu+ZnCu can produce a hole barrier, but Zn rich
and Cu poor condition is necessary to stabilize this complex.
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Composition dependence of properties of the Cu2ZnSn(S,Se)4 alloy
The calculated formation enthalpy is small, indicates that the mixed anion
alloys are highly miscible.
The cations maintain the same ordering preferences as in pure kesterite
structured constituents.
The band gaps of the random alloy decrease with Se content with only a small
bowing parameter. Chen, Gong, Walsh and Wei, Phys. Rev. B (in press)
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Why can Cu2ZnSn(S,Se)4 alloy with high Se concentration have high solar cell efficiency?
The band alignment between Cu2ZnSnS4 and Cu2ZnSnSe4 is of type-I.
Cu2ZnSn(S,Se)4 alloys with high Se concentration is easier to be doped both n-
and p-type. The balance between the band gap size and the doping ability
determines the optimal alloy composition to achieve high efficiency
Cu2ZnSn(S1-xSex)4 based solar cells.
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Conclusion
There are clear mutation trends in the properties of binary, ternary
and quaternary chalcogenides. Cu2ZnSn(Se,S)4 crystallize either in
kesterite or partially disordered kesterite structures.
(i) The chemical potential region that Cu2ZnSnS4 can stoichiometrically
form is very narrow. It is difficult to obtain high quality Cu2ZnSnS4;
(ii) The p-type defects have lower formation energy than n-type defects,
and the dominant acceptor is CuZn which has relatively deep level;
(iii)The most-popular defect pair is CuZn+ZnCu, but it does not enhance
carrier separation;
(iv)To avoid the issues in (ii) and (iii) in solar cell application, non-
equilibrium techniques at Cu-poor/Zn-rich conditions should be used
to grow the Cu2ZnSnS4, so VCu and VCu +ZnCu become the dominant
defects, because they will produce shallow levels and enhance the
carrier separation. Grow Cu2ZnSn(S,Se)4 alloy at relatively high Se
concentration can also improve alloy defect properties.
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Code: Vienna Ab-initio Simulation Package (VASP)
Exchange-correlation potential: GGA (PW91)
Basis functions: PAW
Energy cutoff : 300 eV
k-point meshes: 4x4x4 Monkhorst–Pack
Band gap correction HSE hybrid functional
Calculation Methods
First-principles band structure and total energy
calculations are performed within the density
functional theory (DFT).
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Cu+Zn disorder (displacement along [110])
doesn’t violate the octet rule, hence small
energy cost (0.8 meV/atom for Cu2ZnSnS4)
It has the same symmetry as the ST, thus
difficult to be distinguished by x-ray diffraction.
Cu+Zn disorder decreases the band gap of KS by
0.04 eV for Cu2ZnSnS4,
The Cu+Zn Disorder in KS Structure
KS
Cu+Zn
disordered
KS
The experimentally
observed ST structure for
Cu2ZnGeS4 and
Cu2ZnSnSe4 et al. should
be partially disordered KS.
Schorr, et al. , Eur. J. Mineral. 19, 65 (2007).
Parasyuk, et al., J. Alloys Compd. 397, 85 (2005).
Neutron diffraction
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Defect physics: Does Cu2ZnSnS4 behave as CuInSe2?
How about in Cu2ZnSnS4?
Is Cu vacancy still dominant?
Will defect complexes also lead
to benign characters and help
charge separation? [S.-H. Wei and S. B. Zhang, J. Phys. Chem. Solids 66, 1994 (2005)]
Defect VCu VIn CuIn InCu
∆Hf 0.60 3.04 1.54 3.34
Level 0.03
(p)
0.17
(p)
0.29
(p)
0.20
(n)
CuInSe2:
Cu vacancy is the dominant
intrinsic defect and has a shallow
acceptor level.
Defect complexes such as
(2VCu-+InCu
2+) have particularly low
formation energies, contributing to
the electrically benign character.