Variable-Range Hopping Conduction in the Kesterite and Wurtzstannite Cu2ZnGeS4 Single Crystals

Abstract

Progress in the solar cell science and technology, based on quaternary compounds, recently has been focused on the materials with a partial or full cation substitution of Ge for Sn. In particular, this stimulated an interest to the Cu₂ZnGeS₄ (CZGeS) compound. However, for its effective utilization, deeper investigations of CZGeS transport properties are required. In the present manuscript, the resistivity, ρ(T), of the p-type CZGeS single crystals with the kesterite (KS) and wurtzstannite (WS) structures is investigated in the temperature range of ~50–300 K. The dependence of ρ(T) is described by the Mott type of the variable-range hopping (VRH) charge transfer mechanism within a broad temperature interval of ~60–150 K for the KS samples and of ~100–260 K for the WS samples. A quantitative analysis of the resistivity data in the VRH conduction regime permitted determination or estimation of a series of important microscopic parameters. These include the localization radius of acceptors, their concentration and density of the acceptor states, as well as positions of the mobility threshold and of the Fermi level in the acceptor band.