Germanium Incorporation in Cu2ZnSnS4 and Formation of a Sn-Ge Gradient

Alloying of Cu2ZnSnS4 (CZTS) with Ge can potentially promote grain growth and suppress the formation of Sn-related defects. Herein, a two-step fabrication route based on compound co-sputtering and sulfurization at a high temperature is used to prepare Ge-incorporated CZTS (Cu2ZnGexSn1 - xS4 [CZGTS]). For Cu2ZnGeS4 (CZGS), films deposited using elemental Ge and binary GeS targets are compared. The recrystallization is shown to be promoted for the absorbers deposited using Ge target, possibly due to lower sulfur content in the precursor suppressing the formation of wurtzite-like phases during sputtering. The grain growth and crystallinity in CZGTS are slightly improved for x = 0.2 but not for higher concentration of the incorporated Ge. Owing to the composition-dependent electronic properties, compositionally graded CZGTS films may be beneficial for reducing recombination towards the back contact. Hence, herein, the successful formation of a steep concentration gradient with Ge and Sn is demonstrated by the deposition of a CZGS/CZTS precursor stack followed by sulfurization with varying time periods.