Blue Light-Emitting Diodes Based On Quasi-2D Perovskite Phase Distribution Rearrangement

Metal halide perovskites have received much attention in the field of electroluminescence(EL)due to their high color purity,high carrier mobility,tunable band gap and solution processability.Rapid progresses have been made in efficient green,red and near-infrared perovskite light-emitting diodes(Pe LEDs),and their external quantum efficiencies(EQEs)have exceeded 20%.However,the development of blue Pe LEDs,which is indispensable in display and lighting applications,is still lagging far behind.In this thesis,we developed blue Pe LEDs by regulating the phase distribution of quasi-2D perovskites,and investigated the effect of phase distribution on the performance of the blue devices.Two parts of studies were carried out as follows:(1)We incorporated sodium ions into the mixed-Cl/Br quasi-2D perovskites with phenylethyl ammonium as the organic spacer and cesium lead halide as the inorganic framework.Low-dimensional phase distribution in the quasi-2D perovskites was rearranged,and high efficiency Pe LEDs with sky-blue emission were fabricated.It was found that the sodium ions with smaller ion radius can replace some large organic cations,which leads to reducing the formation of two-dimensional phase(i.e.,n=1 phase)with strong exciton-phonon coupling and promoting the formation of other low-dimensional phases with high exciton binding energies in the quasi-2D perovskites.In this case,the excitons could be effectively utilized for light emission by cascade energy transfer,resulting in the improved efficiency of the Pe LEDs based on the resultant quasi-2D perovskites.The sky-blue devices with a stable EL peak at 488 nm showed the maximum EQE of as high as 11.7%.In addition,we appropriately increased the content of Pb²⁺in the precursor solution,and found that the agglomeration of perovskite grains was reduced and the formation of zero-dimensional perovskite was suppressed.The adjustment of Pb²⁺content improved the film morphology and further optimized the phase distribution of the quasi-2D perovskites.As a result,the reproducibility of the sky-blue Pe LEDs has been improved,and the average maximum EQEs have reached 9.6%,and the operating lifetime has been increased by 1.7 times.The rearrangement of low-dimensional phase distribution of quasi-2D perovskites by sodium ions can provide a feasible method for the preparation of high-performance blue Pe LEDs.(2)We introduced diamine ions and sodium ions to synergistically control the multiple phases in the quasi-2D perovskites for deep-blue emission.The formation of the nonradiative n=1 phase was reduced,and the bandgaps of the high-n phases were broadened to result in the emission blue-shift of the quasi-2D perovskites.Thus,the deep-blue Pe LEDs with stable EL spectra were developed.Ethylenediamine dihydrochloride(EDADCl)was firstly added into the mixed-Cl/Br quasi-2D perovskites to produce“hollow”structure in the high-n phases,leading to the increased bandgaps for deep-blue emission.Subsequently,proper addition of sodium bromide(Na Br)was used to change the low-dimensional phase distribution by suppressing the formation of n=1 phase.At the same time the synergistic effect of ethylenediamine and sodium ions led to broadening the hollowness in the high-n phases,which resulted in the further increased bandgaps and the blue-shift emission of the quasi-2D perovskites.The resulting deep-blue Pe LEDs with the stable emission peaks at 461 nm showed the maximum EQEs of 0.67%,respectively.