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ABSTRACT Hybrid perovskite solar cells (PSCs) under normal operation will reach a temperature above ∼ 60 °C, across the tetragonal-cubic structural phase transition of methylammonium lead iodide (MAPbI 3 ). Whether the structural phase transition could result in dramatic changes of ionic, electrical and optical properties that may further impact the PSC performances should be studied. Herein, we report a structural phase transition temperature of MAPbI 3 thin film at ∼ 55 °C, but a striking contrast occurred at ∼ 45 °C in the ionic and electrical properties of MAPbI 3 due to a change of the ion activation energy from 0.7 eV to 0.5 eV. The optical properties exhibited no sharp transition except for the steady increase of the bandgap with temperature. It was also observed that the activation energy for ionic migration steadily increased with increased grain sizes, and reduction of the grain boundary density reduced the ionic migration. 

In a mesoporous hybrid perovskite solar cell (PSC), the mesoporous scaffold plays key roles in controlling the crystallization of the perovskite material and in the charge carrier transport, and hence is critical for developing highly efficient PSCs. Here we report a study on blending micrometer-long TiO 2 nanorods (NRs) into the commonly used nanoparticles (NPs) to optimize the mesoporous structure, with the aim of enhancing the perovskite material loading and connectivity as well as light harvesting. It was found that with 5–10% of NR incorporation, a uniform scaffold can be spin-coated and the PSC performance was improved. In comparison to the pure NP-based device, the power conversion efficiency was increased by about 27% when 10% of the NRs were incorporated, due to enhanced light harvesting and charge collection. However, with more NR blending, a homogeneous scaffold cannot be formed, resulting in PSC performance degradation. These findings contribute to a better design of mesoporous scaffolds for high-performance PSCs.