Heating rate tuning in structure, morphology and electricity properties of Cu2FeSnS4 thin films prepared by sulfurization of metallic precursors

•Faster heating rates induce the coexistence phase and bigger strain in thin films.•The quality of morphology has been obviously improved with lower heating rate.•Mechanisms associated with tuning in structure and optical properties were discussed.•VBM and CBM draw closer to one another gradually with the increasing heating rate.•Voc and Jsc enhanced 17.3% and 30.0%, respectively, compared to the previous work.

Cu2FeSnS4 (CFTS) thin films have been synthesized by sulfurization of the sputtered metallic layers. The disappeared coexistence phase (Cu2FeSn3S8) and reduced strain have been observed in CFTS thin films accompanying the ritardando heating rate of sulfurization, and no impurity phase is detected in the samples. It is found that the S content at the bottom of CFTS thin films turns out to be very sensitive to the heating rate. The CFTS thin film sulfurized under faster heating rate has S-poor state with a bilayer structure and lots of micro-grains occupy the bottom of the CFTS thin film, while it has been obviously improved by reducing the heating rate. The optical transmission spectrum reveals that valence band maximum and conduction band minimum of CFTS are away from one another gradually with the decreasing heating rate, corresponding to the enhanced band gap energy of CFTS from 1.42 to 1.47 eV. As for these four CFTS devices, the highest open circuit voltage is 129 mV with short circuit current of 3.25 mA/cm2, which increased by 17.3% and 30.0%, respectively, compared to the previous work.