Cu2Sn1-xGexS3 thin film solar cells fabricated from sputtered precursors: Effects of soft-annealing process

Ge-doped Cu2SnS3 (CTGS) thin films are promising photovoltaic materials for developing low-cost second-generation solar cells. The present work reported a soft-annealing process (annealing the as-deposited precursor without chalcogenide source) for fabricating CTGS solar cells, which is an efficient method to enhance the quality of CTGS thin films and show significant reforming on both as-deposited precursor and sulfurized film. The enlarged grain size and compact microstructure provide obvious evidence of improved efficiency. Further, the reduced defect density is attributed to using a 200 °C soft-annealed precursor. A clear increase in each performance parameter such as open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) is observed as the soft-annealing temperature increased from 0 °C to 200 °C, whereas a limited enhancement was found at 300 °C. The CTGS solar cell prepared using the soft-annealing process showed an efficiency of 2.66%. In comparison, an efficiency of 1.93% has been achieved in cells fabricated without this process. The improved device performance indicates that further improvement in the CTGS solar cells could be achieved by adopting the soft-annealing precursor in an environmentally friendly manner.