Effect of reaction temperature on film quality and cell performance: Comparative study of single and mixed cation/halide perovskites

Reaction temperature has been demonstrated to play a critical role in the crystallization and quality control of MAPbI 3 perovskite films (single cation/halide perovskite; MA: methylamine, CH 3 NH 3 ), which should also be true for mixed cation/halide perovskite systems. However, there is a lack of direct comparative studies on the effect of reaction temperature on the film quality and cell performance of various perovskite systems. Herein we have systematically investigated the effect of reaction temperature (controlled by changing the temperature of ammonium salt solutions in this work) on film quality and cell performance of single and mixed cation/halide perovskite systems, namely (FAPbI 3 ) 1-x (MAPbBr 3 ) x , FA x MA 1-x PbI 3 and MAPbI 3 (FA: formamidine, CH(NH 2 ) 2 ) by means of absorption/photoluminescence spectroscopy, X-ray diffraction, scanning electron microscopy and various electrical measurements. The results show that an appropriate increase in the reaction temperature is favorable for obtaining high-quality perovskite films and improving cell performance, while an excessively high reaction temperature can cause the generation of photovoltaic non-active phase and the reduction of cell performance. In addition, the mixed-cation/halide perovskite systems are more susceptible to reaction temperature relative to the single cation/halide perovskite with respect to phase purity and grain size. The optimal ammonium salt solution temperatures for preparing (FAPbI 3 ) 1-x (MAPbBr 3 ) x , FA x MA 1-x PbI 3 and MAPbI 3 films and corresponding solar cells were found to be around 30, 30 and 55 °C, respectively, in our two-step deposition process. The (FAPbI 3 ) 1-x (MAPbBr 3 ) x based device prepared from the ammonium precursor solution at 30 °C delivered the highest efficiency of 18.09%, which can be attributed to the reduced defect density in the perovskite film and the accelerated charge extraction at interfaces. This work highlights the distinct optimal reaction temperatures for different perovskite systems in the two-step deposition process and provides a simple thermodynamic regulation method to improve cell performance. • Effects of reaction temperature (RT) on the film quality and cell performance of various perovskite systems were studied. • An appropriate increase in (RT) is favorable for obtaining high-quality perovskite films and improving cell performance. • Mixed-cation/halide perovskite systems are more susceptible to (RT) than MAPbI 3 in terms of phase purity and grain size. • The optimal ammonium salt solution temperatures for preparing single and mixed cation/halide perovskites are 55 and 30 °C. • The (FAPbI 3 ) 1-x (MAPbBr 3 ) x based device prepared at 30 °C delivered the highest efficiency of 18.09%.