Compositional dependence of structural and electronic properties of Cu2ZnSn(S,Se)4alloys for thin film solar cells

Abstract

A thin-film solar cell based on Cu2ZnSn(S,Se)4 (CZTSSe) alloy was recently found to exhibit a light to electricity conversion efficiency of 10%, making it competitive with the more mature Cu(In,Ga)Se2 based technologies. We study the compositional dependence of the physical properties of CZTSSe alloys through first-principles calculations and find that, these mixed-anion alloys are highly miscible with low enthalpies of formation, and the cations maintain the same ordering preferences as the parent compounds Cu2ZnSnS4 and Cu2ZnSnSe4. The band gap of the CZTSSe alloy decreases with the Se content almost linearly, and the band alignment between Cu2ZnSnS4 and Cu2ZnSnSe4 is of type-I, which allows for more facile n-type and p-type doping for alloys with high Se content. Based on these results we analyze the influence of composition on the efficiency of CZTSSe solar cells and explain the high efficiency of the cells with high Se content.

Formation of nanometer-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films and their effect on solar cell efficiency

“…This assumption is supported by the finding that the composition of single CZTSe grains of the Cu-rich precursor is Cu-poor [31]. Hence, the formation energy of V Cu vacancies is decreased compared to stoichiometric CZTSe leading to a high population of these defects [6,17,32] and the diffusion of Cu can take place via V Cu vacancies. In parallel, excess Cu and Se may cover the CZTSe surface by a few monolayer thin film of Cu-Se, similar to the work of Niki et al [26].…”

Why are kesterite solar cells not 20% efficient?

“…However, the former one depends on the achievement of flat-band conditions under illumination, which might not be the case and would strongly influence the results. There are also two theoretical studies: one finds a spike like configuration [16], the other one a cliff for the sulphide kesterite [17] and a spike for selenides [18]. The observed differences in the recombination path between pure selenide absorbers and S-containing ones strongly support a cliff at the interface of sulphur containing absorbers and a spike at the interface of pure selenide absorbers.…”

Towards the growth of Cu 2 ZnSn 1−x Ge x S 4 thin films by a single-stage process: Effect of substrate temperature and composition

“…This is because this earth-abundant material presents p-type conductivity, a high absorption coefficient and direct band gap energy from 1.0 to 1.5 eV depending on the anions' ratio [1]. Nowadays, a top efficiency of 12.6 % has been achieved [2].…”