Cooling Mixed\nA‑Site Halide Perovskites: Impact\nof Temperature on Optical and Structural Properties

The widespread utilization of perovskite-based photovoltaics\nrequires\nprobing both the structural and optical properties under extreme operating\nconditions to gain a holistic understanding of the material behavior\nunder stressors. Here, we investigate the temperature-dependent behavior\nof mixed A-site cation lead triiodide perovskite thin films (85% methylammonium\nand 15% formamidinium) in the range from 300 to 20 K. Through a combination\nof optical and structural techniques, we find that the tetragonal-to-orthorhombic\nphase transition occurs at ∼110 K for this perovskite composition,\nas indicated by the change in the diffraction pattern. With decreasing\ntemperature, the quantum yield increases with a concurrent elongation\nof the carrier lifetimes, indicating suppression of nonradiative recombination\npathways. Interestingly, in contrast to single A-site cation perovskites,\nan additional optical transition appears in the absorption spectrum\nwhen the phase transition is approached, which is also reflected in\nthe emission spectrum. We propose that the splitting of the optical\nabsorption and emission is due to local segregation of the mixed cation\nperovskite during the phase transition.