Effect of Ge content on diode parameters of superstrate type Cu₂ZnSn_1−xGe_xS₄/CdS heterostructures prepared by all-solution process

The present research aims to study the effect of Ge concentration on diode parameters of Cu₂ZnSn_1−xGe_xS₄/CdS solar cells using Ge/Sn ratio in the range of 0–0.25 mol%. A stable molecular ink was formulated to prepare Ge-alloyed Cu₂ZnSnS₄ (CZTS) thin films and integrated into the superstrate configuration of glass/FTO/TiO₂/CdS/CZTGS/graphite heterostructure. Ge incorporation in CZTS lattice was confirmed by XRD analysis and Raman scattering analysis. A detailed inspection of Raman results suggests an increment in Cu–Zn disorder in CZTS films with the increase in Ge. The absorbance measurements revealed that the band gap of Cu₂ZnSn_1−xGe_xS₄ (CZTGS) was increased systematically from 1.46 to 1.61 eV by increasing the Ge concentration. The dark current–voltage (J–V) characteristics of the heterostructures were analyzed by thermionic emission model using Rhoderick, Cheung–Cheung and Norde methods. By increasing Ge amount, the ideality factor was decreased significantly from 5 to 2.28, while the series resistance was reduced from 980 to 75 Ωcm². The J–V measurements under illumination indicate that the controlled Ge incorporation in CZTGS/CdS heterostructures improved the photovoltaic parameters such as open circuit voltage, fill factor (FF) and the device efficiency. Stability study of the devices revealed that the Ge incorporated solar cells are more stable than the one without Ge. The present research may provide a promising way for the design of efficient superstrate solar cells or other optoelectronic devices.