Role of Excess FAI in Formation of High-Efficiency FAPbI₃-Based Light-Emitting Diodes

Introducing extensively excess ammonium halides when forming perovskites has recently been demonstrated as an effective approach to improve the performance of perovskite light-emitting diodes (PeLEDs). Here, Cs_0.17FA_0.83PbI_2.5Br_0.5 is used as a model system to elucidate the impact of introducing excess formamidinium iodide (FAI) on the crystallization process of the perovskite film and operation of the corresponding PeLED. The excess FAI ratio is varied from 0 to 120 mol% and the crystallization process of the perovskite through in situ absorbance, in situ photoluminescence, and ex situ X-ray diffraction measurements is systematically monitored. The results suggest that excess FAI triggers formation of a compact wide-bandgap intermediate phase in the as-deposited film, which then transforms to isolated and highly crystalline perovskite grains upon annealing. Using excitation correlation photoluminescence spectroscopy it is found that excess FAI results in a lower density of deep trap states and therefore a reduction of nonradiative losses in the material. This leads to a greatly enhanced maximum external quantum efficiency (EQE) from 0.25% (stoichiometric) to 12.7% (90 mol% excess). Furthermore, the FAI-excess perovskite film is optimized with Pb(SCN)₂ and 5-ammonium valeric acid iodide additives and achieve a record radiance of 965 W Sr⁻¹ m⁻² for near-infrared PeLEDs and a high EQE of 17.4%. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim