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We have achieved high-efficiency polycrystalline perovskite light-emitting diodes (PeLEDs) based on formamidinium (FA) and cesium (Cs) mixed cations without quantum dot synthesis. Uniform single-phase FA1–xCsxPbBr3 polycrystalline films were fabricated by one-step formation with various FA:Cs molar proportions; then the influences of chemical composition on film morphology, crystal structure, photoluminescence (PL), and electroluminescence (EL) were systematically investigated. Incorporation of Cs+ cations in FAPbBr3 significantly reduced the average grain size (to 199 nm for FA:Cs = 90:10) and trap density; these changes consequently increased PL quantum efficiency (PLQE) and PL lifetime of FA1–xCsxPbBr3 films and current efficiency (CE) of PeLEDs. Further increase in Cs molar proportion from 10 mol % decreased crystallinity and purity, increased trap density, and correspondingly decreased PLQE, PL lifetime, and CE. Incorporation of Cs also increased photostability of FA1–xCsxPbBr3 films, possibly due to suppressed formation of light-induced metastable states. FA1–xCsxPbBr3 PeLEDs show the maximum CE = 14.5 cd A–1 at FA:Cs = 90:10 with very narrow EL spectral width (21–24 nm); this is the highest CE among FA-Cs-based PeLEDs reported to date. This work provides an understanding of the influences of Cs incorporation on the chemical, structural, and luminescent properties of FAPbBr3 polycrystalline films and a breakthrough to increase the efficiency of FA1–xCsxPbBr3 PeLEDs.
Cho et al. (Thu,) studied this question.