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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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Cation Mutation

The octet rule

ZnS -> CuGaS2 -> Cu2ZnGeS4

Cu2GaInS4 -> Cu2ZnSnS4

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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.

Thank you for your attention!

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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.