CsPbBr_xI_3-x thin films with multiple ammonium ligands for low turn-on pure-red perovskite light-emitting diodes

All-inorganic α-CsPbBr_xI_3-x perovskites featuring nano-sized crystallites show great potential for pure-red light-emitting diode (LED) applications. Currently, the CsPbBr_xI_3-x LEDs based on nano-sized α-CsPbBr_xI_3-x crystallites have been fabricated mainly via the classical colloidal route including a tedious procedure of nanocrystal synthesis, purification, ligand or anion exchange, film casting, etc. With the usually adopted conventional LED device structure, only high turn-on voltages (> 2.7) have been achieved for CsPbBr_xI_3-x LEDs. Moreover, this mix-halide system may suffer from severe spectra-shift under bias. In this report, CsPbBr_xI_3-x thin films featuring nano-sized crystallites are prepared by incorporating multiple ammonium ligands in a one-step spin-coating route. The multiple ammonium ligands constrain the growth of CsPbBr_xI_3-x nanograins. Such CsPbBr_xI_3-x thin films benefit from quantum confinement. The corresponding CsPbBr_xI_3-x LEDs, adopting a conventional LED structure of indium-doped tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/CsPbBr_xI_3-x/[6,6]-phenyl C61 butyric acid methyl ester (PCBM)/ bathocuproine (BCP)/Al, emit pure-red color at Commission Internationale de l'éclairage (CIE) coordinates of (0.709, 0.290), (0.711, 0.289), etc., which represent the highest color-purity for reported pure-red perovskite LEDs and meet the Rec. 2020 requirement at CIE (0.708, 0.292) very well. The CsPbBr_xI_3-x LED shows a low turn-on voltage of 1.6 V, maximum external quantum efficiency of 8.94%, high luminance of 2,859 cd·m^-2, and good color stability under bias.