Study On All Inorganic Halide Perovskite Quantum Dot Light Emitting Diodes

The importance of information technology and information industry is increasing in the era of electronic information,when the science and technology is developing rapidly.Display instruments have become the most important information carrier in daily life and scientific fields and are used in diverse media,including televisions,mobile phones,and other panels.With the development of semiconductor technology in recent decades and the growing needs for display quality,the International Telecommunications Union has introduced a new display standard Rec.2020,which requires the resolution to be increased from 4 K to 8 K,the color resolution to be increased from 8 bits to 12 bits,and the color gamut is doubled.Perovskite quantum dots have the advantages of low cost and easy preparation,and have unique defect tolerance characteristics,which can achieve high photoluminescence quantum yields without delicate shell passivation.In addition,light emission from perovskites shows narrow linewidths(<100 me V),resulting in high color purity.The color gamut of displays made by perovskite quantum dots can cover up to 140%of the National Television System Committee standard and is close to 100%of the new Rec.2020standard,making them promising for ultra-high-definition displays.However,at the beginning of this research,perovskite quantum dot LEDs are still in the development stage,and there are still some problems,including poor material stability,unclear structure-efficiency relationship,easy to be damaged by polar solvents,large dependence on thermal evaporation equipment in the process of device preparation,and lower device efficiency than traditional quantum dot LEDs.In view of the above problems,the following research contents are carried out from the aspects of material chemistry and device physics.In order to improve the phase stability,Zn I₂ was introduced as a Zn²⁺source during the synthesis process to prepare Zn²⁺alloyed CsPbI₃ perovskite quantum dots with different proportions of Zn²⁺instead of Pb²⁺.Zn²⁺alloyed perovskite quantum dots not only do not change its cubic crystal phase,but also increase the effective Goldschmidt tolerance factors and the phase stability of the material.The Zn²⁺alloyed perovskite quantum dots still maintain a photoluminescence quantum yield of 77%after storing in ambient atmospheres for 70 days.Simultaneously,the photoluminescence quantum yield of alloyed CsPbI₃ perovskite quantum dots with substitution ratio of 0.36 is as high as 98.5%.The reason is that the increase of exciton binding energy promotes the radiative decay rate,while the degradation of the defect state density significantly reduces nonradiative decay rate.This revealed the relationship between the structure and efficiency of perovskite quantum dots.In addition,the nanocrystals switched from n-type for CsPbI₃ to nearly ambipolar for the alloyed quantum dots.The hole injection barrier of LEDs was effectively eliminated by using alloyed quantum dots,and the maximum luminance and maximum external quantum efficiency of the device has increased from unalloyed 500 cd m^-2,7%to 2202 cd m^-2,15.1%for alloyed ones.On the basis of the above-mentioned devices,optimizing the carrier injection balance in the device is undoubtedly an effective method to improve the device performance.Here,bismuth(Bi)was introduced into perovskite quantum dots LEDs to form a silver-bismuth(Ag-Bi)bilayer anode.Ag diffuses into defective 2-nm thick Bi layer easily to form an alloy-like state.The device with a double-layer metal anode has lower contact resistance and charge transport resistance,which promotes the injection of holes in the device,and makes the injection of holes and electrons in the light-emitting layer reach a more balanced state.As a result,the turn on voltage and luminescence changed from 2.4 V and 2200 cd m^-2 for Cs Pb_1-xZn_xI₃based LEDs with Ag monolayer anode to 2.2 V and 3714 cd m^-2 for devices with Ag-Bi bilayer anode.In addition,the performance of CsPbI₃ and Cs Pb(Br/I)₃ perovskite quantum dot LEDs has also been effectively improved by using Ag-Bi bilayer anode.Compared with the vacuum evaporation method,the all-solution process is cost-effective and easy to manufacture in large area.However,the development of solution-processed perovskite quantum dot LEDs has been very slow.This is mainly resulted from the fact that only few types of charge transport layers can be employed for subsequent deposition steps,thus leading to injection barriers,charge mobility mismatch and charge injection imbalance inside these LEDs.Herein,4,4?-bis-(carbazole-9-yl)biphenyl(CBP)is introduced as dopant into poly(9,9-dioctylfluorene-co-N-(4-(3-methylpropyl))diphenylamine)(TFB)hole transport layer,which efficiently modulated the hole mobility to make it closer to the electron mobility of electron transport layer.At the same time,the introduction of CBP adjusted the energy level of hole transport layer,resulting in the barrier free injection of the charge carrier in the as-fabricated solution-processed perovskite quantum dot LEDs.Consequently,the luminance of red LEDs reached 2990 cd m^-2 and the external quantum efficiency achieved 8.1%,which is the optimal performance for the solution-processed perovskite quantum dot LEDs to date.Additionally,the turn-on voltage and roll-off has also been improved by the more balanced charge injection.In order to prepare red perovskite quantum dot LEDs that are more in line with Rec.2020 display standards.A facile in-situ passivation strategy was utilized to prepare highly efficient Cs Pb(Br/I)₃ perovskite quantum dots,where calcium bromide was used as the bromine precursor for the hot injection synthesis.The radiative recombination rate was increased due to the effective exciton confinement as well as the nonradiative recombination rate was reduced by decreasing the surface defects.Besides,the conductivity of the perovskite quantum dot film has been significantly improved.Consequently,LEDs based on the passivated CsPb(Br/I)₃ perovskite quantum dots with the color coordinate(0.704,0.292)exhibited an excellent peak EQE value of 13.2%and a maximum luminance of 11 233 cd m^-2,which is a record value for pure red PQD LEDs.Additionally,MnBr₂,CoBr₂ and ZnBr₂ could play similar roles as Ca Br₂ do,leading to the improvement in both the material properties and the device performances.