Preparation And Research Of High-performance Green And Deep-blue Perovskite Light Emitting Diodes

Metal halide perovskites have become a strong contender for new light-emitting and display technologies due to their tunable band gap emission,high color purity,high photoluminescence quantum yield and excellent carrier mobility.Although the highest external quantum efficiency（EQE）of green perovskite light-emitting diodes（Pe LEDs）has exceeded 30%,the efficiency roll-off problem at high brightness still needs to be improved.The efficiency roll-off problem at high brightness is mainly attributed to the internal charge transfer imbalance,energy level barrier mismatch,severe non-radiative recombination and interface defect effects of the device.Currently,the understanding of the roll-off mechanism of Pe LEDs is not clear,and the effectiveness of the reported improvement methods and strategies is not perfect.Unlike green Pe LEDs,due to the large optical band gap,parasitic defects,and low carrier injection efficiency of deep-blue perovskites,the main challenge faced by deep-blue Pe LEDs is that their EQE is far lower than that of green,red and near-infrared Pe LEDs.Based on this,this dissertation carried out a series of researches on interface engineering of green Pe LEDs and preparation of high-performance deep-blue Pe LEDs.The main research contents are as follows:1.By introducing ethanolamine molecules into the hole transport layer,the passivation effect of ethanolamine on perovskite was utilized to effectively reduce the interface defects of the device and improve the nucleation quality of the perovskite films.The perovskite grains after interface modification were cubic in shape,with clear grain boundaries and high crystallinity,indicating that crystallinity of perovskites was effectively improved,thereby enhancing the stability of the perovskite film in air.Based on this strategy,the EQE of Pe LEDs increased from 3.46%to 7.98%,and the device brightness increased from 7400 cd/m² to 28900 cd/m².2.By using the acid-base neutralization reaction of sodium citrate and PEDOT:PSS solution,the acidity of the hole transport layer was reduced.And the formation of Na-Br ionic bond and C=O:Pb covalent bond increased the nucleation sites of perovskite,which effectively improved the coverage and crystallinity of the perovskite film,suppressed the interface defects and enhanced the hole injection ability of the hole transport layer.The experimental results showed that this improvement originated from the synergistic effect of citrate anion and sodium ion on the interface.Based on this,the green Pe LEDs achieved a maximum brightness of 80,288 cd/m² and a peak EQE of 13.02%.With the increase of device working voltage,when the device brightness was 53,709 cd/m²,its efficiency could still maintain 80%of the peak value.At an initial brightness of 100 cd/m²,the working half-life(T₅₀)of the device increased from 0.87 h to 1.4 h.3.We studied the influence of the adsorption energy difference between different organic functional ligands and halide perovskite surface on the n-value distribution of quasi-two-dimensional perovskites.By combining experiments and theoretical calculations,we confirmed that the regulation of adsorption energy can effectively control the interaction rate between ligands and perovskites,thereby achieving the purpose of precisely controlling the n-value distribution of quasi-two-dimensional perovskites,suppressing the formation of low-n phase perovskites and promoting the energy transfer between different n-value phase perovskites.Based on this regulation method,we obtained high-quality deep-blue perovskite films,and the photoluminescence quantum yield of the films reached 57%.Based on this strategy,we successfully prepared deep-blue Pe LEDs with an emission center wavelength of 457nm,and the peak EQE of the device was 4.62%.The device working half-life T₅₀ at an initial brightness of 17.2 cd/m² was 90.16 minutes,and the device had excellent electroluminescence spectral stability.In addition,this strategy was also successfully applied to cyan,sky blue,and pure blue Pe LEDs,confirming the universality of this method.4.Based on the strategy of regulating the dimensional distribution of quasi-two-dimensional perovskites by adsorption energy,we used fluorinated organic molecules to replace the original amine molecules introduced cesium trifluoroacetate into the perovskite precursor solution.A series of experiments confirmed the multivalent effect between cesium trifluoroacetate and quasi-two-dimensional perovskites:namely hydrogen bond（F···H-N）,ionic bond（F-Pb）and coordination bond（C=O:Pb）.The existence of ionic bond and coordination bond effectively reduced the defect state density of the perovskite film（the photoluminescence quantum yield of the film was close to 80%）,and improved the crystallinity of the film;the existence of hydrogen bond better immobilized the organic molecules on the perovskite surface,increased the exciton binding energy and thermal stability of the perovskite film.Molecular dynamics simulation based on 5000 fs scale further confirmed the multivalent effect on the fixation of organic molecules and perovskites.The results of semiconductor self-consistent optoelectronic simulation showed that the optimized carrier distribution increased the exciton concentration and exciton recombination rate by 1.66 and 1.64times,respectively.Based on this,high-efficiency and spectrally stable deep-blue Pe LEDs were successfully prepared,with an emission center wavelength of 459 nm,a peak EQE of 12.4%,and CIE color coordinates of（0.136,0.051）.At an initial brightness of 20 cd/m²,the device working half-life T₅₀ increased from 47 minutes to144 minutes.