Carrier transport and low temperature device properties of Cu ₂ZnSnSe₄ thin-films with different Cu and Zn/Sn ratio

Earth abundant and non-toxic quaternary semiconductor Cu ₂ZnSnSe₄ (CZTSe) is regarded as one of the alternatives for solar cell applications due to low cost and competitive photovoltaic performance. In this study, CZTSe thin-films with different Cu and Zn/Sn composition were investigated. On the other hand, for improving solar cell properties, it is important to understand from electron-hole transport to solar cell performance. The carriers in photovoltaic chalcopyrite thin-films behave differently in the vicinity of grain boundaries, which eventually effects on efficiency of the solar cell. Kelvin probe force microscopy measured local surface potential which indicates positive in the vicinity of grain boundaries in a Cu-poor CZTSe not a Cu-rich thin-film. Local current was measured by conductive atomic force microscopy under external bias reveals that carrier polarity changes due to bias direction. The electron-hole transport effects on a solar cell device which is influenced by the thin-film composition. The only stoichiometric (Cu/(Zn+Sn)=0.8, Zn/Sn=1.2) CZTSe thin-film reveals 1.5% of conversion efficiency and distinctive solar cell properties as a function of temperature. The conversion efficiency as well as J_sc and V _oc are decreased under 200 K, which is particular on the contrary to those of Cu(In,Ga)Se₂ thin-film. For improving CZTSe solar cell performance, it is essential to understand compositional effect and local electrical characteristics on CZTSe.