Air-stable formamidinium/methylammonium mixed lead iodide perovskite integral microcrystals with low trap density and high photo-responsivity

Abstract

Here based on integral microcrystal (IMC) thin films of halide perovskites containing formamidinium (FA)/methylammonium (MA) mixed cations, afforded by a facile approach combining an anti-solvent and inverse temperature crystallization, we investigate the impact of the addition of MAPbBr₃ on the phase, thermal and environmental stabilities as well as the opto-electronic properties in FA-based IMC films. By single-crystal XRD, FA based IMCs have been found to possess a perfect cubic structure showing a slight lattice contraction compared to pristine FAPbI₃ crystals. In conjunction with optical and electrical analyses, the essential role of the introduced MA and Br ions in stabilizing the black phase in FA-based IMCs has been clarified, which explains the observed enhancement of photoluminescence and reduced trap densities. We also achieve stable pure FAPbI₃ crystals that do not exhibit a yellow-phase transition after one month in air. By utilizing (FAPbI₃)_1−x(MAPbBr₃)_x IMCs as the photo-absorber, we realize highly photo-responsive photodiodes with a satisfactory stability in air and thermal stability upon heating. Of interest, the best photoresponsivity exceeding 300 A W⁻¹ is achieved upon appropriate air-exposure, which is among the highest values reported for FA-based perovskite photodetectors. The air-modified optoelectronic behaviour can be related to the trap passivation through the surface physisorption of the environmental O₂, leading to reduced trap densities and resultant harmful SRH recombination.