Structural, optical and electrical properties of evaporated kesterite films with different off-stoichiometric type

Abstract

Kesterite thin films with four different off-stoichiometry have been synthetized by sequential evaporation of ternary (Cu2SnS3 or Cu3SnS4) and binary (ZnS) layers, following sulfurization (with elemental S) at 450 ºC or 500 ºC. Zn-poor compositions (Zn/Sn < 1.0) are used to study the effect of substitutional SnZn defects, together with VZn defects when Cu/Sn ~ 2.0 (sample CZTS1) or with CuZn defects for Cu/Sn > 2.0 (sample CZTS2). Besides, the effect of cationic disorder is also studied for Zn/Sn ~ 1.0, producing ZnCu and SnCu defects when Cu/Sn ~ 2.0 (sample CZTS3) or CuZn and CuSn for Cu/Sn > 2.0 (sample CZTS4). The crystalline structure, morphology, optical and electrical properties of the different samples have been analyzed comparatively by X-ray diffraction (XRD), scanning electron microscopy (SEM), spectrophotometry and coplanar electrical measurements. The highest crystallinity was achieved by the reaction of Cu3SnS4 and ZnS (sample CZTS4) at 450 ºC, with a wide bandgap and low resistivity, which remain unchanged when increasing the heating temperature to 500 ºC.