Fabrication And Mechanism Research Of Highly Efficient And Stable Perovskite Light-Emitting Diodes

Since the first light-emitting diode（LED）based on metal halide perovskites（MHPs）was reported in 2014,MHPs has attracted extensive attention due to the advantages of continuous spectral tunability,high color purity,solution processing and low cost.Through continuous efforts of researchers,the external quantum efficiency（EQE）of perovskite LEDs（Pe LEDs）based on MHPs have been rapidly increased from less than 1%at the beginning to more than 20%.However,the operating lifetime of Pe LED is much lower than that of organic light-emitting diodes.Thus how to achieve both high EQE and long operating lifetime is still an urgent problem to be solved.At present,the main methods commonly used to improve Pe LEDs performance include process optimization,quasi-two-dimensional（quasi-2D）perovskite structure and additive passivation,etc.Among them,the process optimization methods reported in the literatures are relatively complicated and difficult to achieve commercial application.In addition,although quasi-2D perovskite is the most commonly used perovskite structure for Pe LED preparation at present,its wide distribution of multiple dimensions will cause energy loss during the transfer process,reduce the energy transfer rate,and too many low-dimensional perovskite components will also increase the internal Joule heat of Pe LED,thus affecting the device operating lifetime.Therefore,it is debatable whether the quasi-2D perovskite structure can achieve efficient and stable Pe LED.On the other hand,most of the additives reported in the literatures are polar small molecule materials,which is difficult to improve the water and oxygen resistance of perovskite films.Based on the above ideas,this paper mainly adopts process optimization,additive strategy,cation component regulation,and ligand dimension regulation to carry out research on the performance optimization of Pe LEDs in green light and deep-red light,and tries to find the optimal perovskite structure and the simplest preparation method to achieve high efficiency and stability of Pe LED.Finally,the EQE of green and deep-red Pe LEDs reached 28.1%and 21.6%,respectively,which are the highest records reported in the literatures（until July 2022）.This paper provides a good research idea for achieving Pe LEDs with high efficiency and high stability.The specific research contents are as follows:（1）The performance of Pe LEDs based on 3D perovskite was optimized by using recrystallization process.Methylammonium bromide（MABr）was dissolved in N,N-dimethylformamide（DMF）and ethanol（Et OH）mixed solvents.Then the MABr solution was dropped on the surface of perovskite film for spin coating,resulting in the effectively improved crystal quality of perovskite and a self-formed interface layer with the improved carrier transport balance.Since DMF is a good solvent for MHPs,it will cause the re-dissolution of perovskite film.MABr dissolved in DMF will re-coordinate with pristine perovskite and thus new perovskite with smaller and more uniform grain size will form.At the same time,MABr dissolved in Et OH will form an interface layer attached to the upper surface of perovskite film,which efficiently balances carrier transport.The maximum luminance of green Pe LEDs prepared based on this method is 27692.2 cd m^-2,which is more than 11 times higher than the control device.（2）High efficiency and high stability green Pe LEDs based on a synergetic dual-additive strategy.18-crown-6（crown）and poly（ethylene glycol）methyl ether（MPEG-MAA）were introduced into quasi-2D perovskite to eliminate defects and improve the water and oxygen resistance of the film.The dual additives containing C-O-C bonds can not only effectively reduce the perovskite defects but also destroy the self-aggregation of organic ligands,inducing the formation of perovskite nanocrystals with quasi-core/shell structure.On the other hand,the C=C bond at the end of MPEG-MAA can be polymerized to obtain comb-like polymer during annealing,thus improving the water and oxygen resistance of perovskite films.Finally,the state-of-the-art green Pe LEDs with a normal EQE of 25.2%and a maximum EQE of 28.1%are achieved,which is the highest record reported in literature.In addition,the operating lifetime of the device in air environment is increased over ten times,providing a novel and effective strategy to make Pe LEDs with high efficiency and long operating lifetime.（3）High efficiency deep-red Pe LEDs were prepared by mixed cation strategy.MA⁺and Cs⁺were mixed to prepare perovskite films to overcome the problems of poor stability of water and oxygen resistance of pure MA-based perovskite and instability ofαphase of Cs-based perovskite.By adjusting the ratio of MA and Cs cations,the perovskite crystallization rate is effectively improved and the perovskite film with high radiation recombination rate is obtained.Meanwhile,the low-dimensional phase in quasi-2D perovskite can be effectively suppressed by mixed cation strategy.Based on this method,the spectral stability of deep red Pe LED was significantly improved and the EQE reached 18.48%.（4）High performance deep-red Pe LED is achieved by self-passivation and dimension control of ligand.This work is targeted to select two organic ligands of phenethylammonium iodide（PEAI）and 2-phenoxyethylamine iodide（POEAI）with similar structure.They were combined with 1-naphthylmethylammonium iodide（NMAI）to prepare quasi-2D perovskite.The results show that oxygen atoms in POEAI can not only effectively passivate the defects of perovskite but also destroy the self-aggregation of organic ligands,thus inhibiting the generation of low-dimensional phase in perovskite.However,PEAI with strong aromatic stacking increases the energy transfer process and reduces the carrier recombination rate due to the enhancement of low-dimensional phase.Eventually,the deep-red Pe LEDs based on NMAI:POEAI=4:1（N4PO1）with a maximum EQE of 21.6%are achieved,which reaches the highest report up to now.At the same time,the operating lifetime of the device is obviously improved due to the low dimensional phase is effectively suppressed.This work provides a good reference for the realization of simple and efficient Pe LEDs production without additives.