High Fill Factor Perovskite Solar Cells by Temperature Control in Single and Mixed Halide Precursor Solution and Films

Hybrid organic/inorganic perovskite solar cells have attracted much attention as an alternative to conventional silicon solar cells due to their solution processability and low production cost. They have higher power conversion efficiency than that of organic solar cells and potential to overcome the efficiency of inorganic solar cells. Different parameters such as annealing temperature, solvent, and composition of precursor play significant role to achieve efficient perovskite solar cells. Thermal annealing and precursor composition play critical roles in crystallinity and morphology of perovskite thin films for efficient charge transport, thus improving the fill factor (FF) of the cells. In this work, the role of annealing temperature on the crystallinity of perovskite (CHNH3PbI3/ CH3NH3PbI3-xClx) thin films casted from single (without PbCl2) and mixed (with PbCl2) halide precursors were studied. Increasing annealing temperature of perovskite films from 80 oC to 120 oC at the step of 10 oC led to agglomeration of perovskite crystals in mixed halide perovskite films, thus decreasing the efficiency of the cells from 11.49% to ~ 6%. However, morphology and crystal size of single halide perovskite films did not change with increase in annealing temperature, resulting in almost equal efficiency (~11.5%) of the cells. Morphology and crystallinity of the xi perovskite films were studied using atomic force microscopy (AFM), and x-ray diffraction (XRD) respectively. Moreover, the morphology of films casted using 60 oC and 150 oC annealed precursor solutions of single and mixed halide were studied. The color of precursor solutions of single and mixed halide changed from yellow to light brown when annealed from 60 oC to 150 oC. Annealing precursor solutions of both single and mixed halide at 150 oC resulted in dense film morphology, thus significantly improving the FF. A FF of 78% and efficiency of 13.2 % were achieved in single halide precursor based perovskite cells, whereas a FF of 81% and efficiency of 10.56 % were achieved in mixed halide precursor based perovskite solar cells