Modelling interfacial diffusion processes for CZTS thin film photovoltaics

Abstract

Inexpensive and nontoxic Cu2ZnSnS4 (CZTS) based thin-film solar cells have features which make it well placed to replace silicon, binary (CdTe) and ternary Cu(In,Ga)S semiconductors in photovoltaic cells, and shows great promise for commercial photovoltaic applications. As-grown CZTS shows p-type conductivity, most probably due to the shallow acceptor levels of the intrinsic defects, CuZn and VCu. CZTS, which provides the function of an absorber layer, is complicated by the multiplicity of chemical elements, resulting in many possible primary native defects as well as secondary phases to consider, including vacancies, antisites and ZnS precipitates. Furthermore, CZTS is thermodynamically stable in only a relatively narrow range of the atomic chemical potentials, with practical, efficient devices obtained in relatively Cu-poor and Zn-rich conditions; it is under these growth conditions that ZnS is thought to constitute a significant competing secondary phase. Such secondary phases affect the performance of photovoltaic devices by reducing the volume of the absorber layer and decreasing the short circuit current. In addition to the absorber layer, CZTS-based PV devices require a buffer layer that provides the n-type portion of the junction. CdS has been commonly used as a partner in the heterojunction, but ZnS is a potential alternative buffer layer to replace CdS due to the toxicity of cadmium. Density functional theory has been used to investigate the geometric and electronic properties of CZTS and its secondary phases. Under given growth conditions, the phase stability and the formation energy of the point defects, impurities and secondary phases are explained. It is found that CuZn has the lowest formation energy compared to the other native defects under normal growth conditions, and VCu has a shallow accepter level. Redistribution of native defects and impurities, as well as interdiffusion of elements at the heterojunction, is mediated by cation vacancies. In order to identify the most stable defects inside CZTS, and the influence of these defects on p-type conductivity, formation energies and diffusion barriers have been calculated. The Cu vacancy has the lowest diffusion barrier and formation energy compared with other native cation vacancies defects, and is therefore of importance for both doping and the intermixing of phases.