Abstract
Cu-Zn disorder is unavoidable but plays an important role in high-efficiency Cu2ZnSnS4 and Cu2ZnSnSe4 solar cells. Using the cluster expansion method along with Monte Carlo (MC) simulations, we study the Cu-Zn disorder, considering cases both with and without vacancies. We find that the 2a, 2c, and 2d Wyckoff sites all show order-disorder transitions for both cases, in agreement with recent experiments supporting disorder at all 2a, 2c and 2d sites, but, in contrast to early experiments, supporting Cu-Zn disorder only at 2c and 2d sites. Below the transition temperature in non-stoichiometric cases, we find that excess Zn prefers to occupy 2c over 2a sites due to the greater similarity of 2c sites to 2d sites. Such site preferences indicate that Cu-Zn occupations exhibit some new kind of ordering rather than randomly distributed at 2a and 2c sites. We find that while Cu-Zn disorder reduces the band gap, the site preferences in non-stoichiometric samples increase the band gaps by suppressing Cu-Zn disorder. Generally, lowering annealing temperatures, while increasing Zn and vacancies, will lead to larger band gaps.
Highlights
In the development of thin-film photovoltaic technology, Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) with multi-cations have been considered to be promising absorber materials, containing only non-toxic and earth-abundant elements
In this letter, using the cluster expansion method22 and Monte Carlo (MC) simulations,23 we investigate the Cu-Zn disorder at Cu-Zn sites (2a, 2c and 2d) in both stoichiometric and non-stoichiometric CZTS and CZTSe
Our present study shows that the picture of completely random occupations at 2a, 2c and 2d sites is not accurate and indicates an intermediate configuration of Cu-Zn disorder in CZTS, which is closer to recent experiments supporting Cu-Zn disorder at all 2a, 2c and 2d sites,15,16 rather than early experimental suggestions that Cu-Zn disorder occurs only in the Cu-Zn plane
Summary
In the development of thin-film photovoltaic technology, Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) with multi-cations have been considered to be promising absorber materials, containing only non-toxic and earth-abundant elements. One main reason for the low efficiency is the large open-circuit voltage deficit.. An orderdisorder transition with a critical temperature of around 533 K in CZTS thin films was revealed using near-resonant Raman scattering.. An orderdisorder transition with a critical temperature of around 533 K in CZTS thin films was revealed using near-resonant Raman scattering.13 Such a transition has been further confirmed by other experimental tools such as neutron scattering in CZTS, photoluminescence in CZTSe5 and electroreflectance in Cu2ZnSn(S,Se)4.6 While these previous experiments indicated Cu-Zn disorder just in the An orderdisorder transition with a critical temperature of around 533 K in CZTS thin films was revealed using near-resonant Raman scattering. Such a transition has been further confirmed by other experimental tools such as neutron scattering in CZTS, photoluminescence in CZTSe5 and electroreflectance in Cu2ZnSn(S,Se)4.6 While these previous experiments indicated Cu-Zn disorder just in the
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