The effect of Ag alloying of Cu2(Zn,Cd)SnS4 on the monograin powder properties and solar cell performance

Abstract

Cation substitution in Cu₂ZnSn(S,Se)₄ offers a path to improved solar cell device performance through the modification of the structural, optical and electrical properties of the absorber material. In the present study, the effect of partial substitution of copper with silver in Cu_1.85(Zn_0.8Cd_0.2)_1.1SnS₄ monograin powders was investigated. (Cu_1−xAg_x)_1.85(Zn_0.8Cd_0.2)_1.1SnS₄ (ACZCTS) monograin powders with different ratios of [Ag]/([Cu] + [Ag]) were synthesized from CuS, Ag₂S, ZnS, CdS, SnS and elemental sulfur in the liquid phase of KI as the flux material in evacuated quartz ampoules at 740 °C. EDX analysis of the as-grown materials in the entire series revealed that the Ag concentration was different in the synthesized materials than in the corresponding precursor mixtures. Furthermore, the Ag concentration at the surface of crystals was found to be higher than in the bulk of crystals. By increasing the input Ag content, the shape of grown crystals changed from sharp edged tetragonal to a more rounded form. Raman spectra showed a shift in the A peak position from 336 to 334 cm⁻¹ as the Ag content increased in (Cu_1−xAg_x)_1.85(Zn_0.8Cd_0.2)_1.1SnS₄. The changes in the recombination processes were studied by temperature dependent photoluminescence spectroscopy and current–voltage characteristics analysis. It was found that the acceptor concentration in the absorber can be modified by varying the Ag content in ACZCTS. The incorporation of Ag into the Cu_1.85(Zn_0.8Cd_0.2)_1.1SnS₄ monograin absorber material improved the efficiency of monograin layer solar cells from 6.62% (x = 0) to 8.73% (x = 0.01).