Two-Step Crystallization for Low-Oxidation Tin-Based Perovskite Light-Emitting Diodes

Metal halide perovskites attract significant attention because of their excellent optoelectronic and semiconducting properties. However, there are environmental concerns related to the toxicity of the lead metal that is mainly used in these perovskites. PEA2SnI4 perovskite is a potential candidate for lead-free perovskites because of its pure red emission. Although, undesired Sn4+ oxidation results in the deterioration of PEA2SnI4 perovskite. We demonstrate the two-step crystallization of PEA2SnI4 through the (i) reprecipitation and (ii) recrystallization processes. A film prepared using this method exhibits narrowed emission, with a full width at half-maximum from 30.0 to 26.1 nm, because of its homogeneous emission. Moreover, the Sn4+ content of two-step-crystallized PEA2SnI4 films is five times lower than that of a control film. Diffusion-ordered spectroscopy analysis indicates that the two-step precursor exhibits a smaller hydrodynamic radius crystal seed, which enhances crystallization during spin coating. The resulting two-step crystallized PEA2SnI4-based light-emitting diode (LED) exhibits a maximum external quantum efficiency (EQE) of 0.4% with an average of 0.2%, which is two times greater than that of the control device. This two-step approach may be generalized to synthesize other lead-free materials.