Efficient Perovskite Solar Cells by Temperature Control in Single and Mixed Halide Precursor Solutions and Films

Thermal annealing and precursor composition play critical roles in crystallinity control and morphology formation of perovskite thin films for achieving higher photovoltaic performance. In this study we have systematically studied the role of annealing temperature on the crystallinity of perovskite (CHNH₃PbI₃) thin films cast from single (without PbCl₂) and mixed (with PbCl₂) halide precursors. Higher annealing temperature leads to agglomeration of perovskite crystals. The effects of annealing temperature on the performance of perovskite solar cells are different in single and mixed halide processed films. It is observed that the perovskite crystallinity and film formation can be altered with the addition of lead chloride in the precursor solution. We report that single halide perovskite solar cells show no change in morphology and crystal size with increase in annealing temperature, which was confirmed by UV-vis absorption spectroscopy, X-ray diffraction (XRD), and atomic force microscopy (AFM). However, mixed halide perovskite (CH₃NH₃PbI_3-xCl_x) solar cells show significant change in crystal formation in the active layer when increasing annealing temperature. In addition, heating perovskite precursor solutions at 150 °C can lead to enhancement in solar cell efficiency for both single and mixed halide systems. Perovskite solar cells fabricated using heated precursor solutions form dense film morphology and thus significantly improved fill factor up to 80% with power conversion efficiency exceeding 13% under AM 1.5 condition.