The effect of Ge content on photovoltaic property of flexible Cu2ZnSn(S,Se)4 thin film solar cells

Magnetron co-sputtering method followed by selenization was used for the preparation of Cu2Zn(Sn,Ge)(S,Se)(4) (CZTGeSSe) thin film. The impact of Ge doping layer with different sputtering times on crystalline quality, surface roughness, band structure and device performance of CZTGeSSe absorber was systematically investigated. It was found that the increased Ge/(Ge+Sn) ratio could effectively promote the grain growth and improve the band mismatching of CZTGeSSe/CdS interface. The CZTGeSSe thin film with minimum roughness and increased CBO (-0.54 to -0.41eV) was obtained with the increased Ge/(Ge+Sn) ratio of 7.3%. However, oversized grain with rougher surface could result in a non-uniform coverage phenomenon of CdS layer, leading to severe interface recombination. After optimizing the Ge/(Ge+Sn) ratio, the best device performance with an efficiency of 3.19% was achieved in flexible CZTGeSSe thin film solar cells.