Research On Light-emitting Diodes Based On All Inorganic Perovskite Materials

In the field of display devices,the industrialization of organic light-emitting diodes(OLEDs)is relatively mature,yet OLEDs are not the most ideal photoelectric materials,people are committed to finding more excellent photoelectric materials,the discovery of perovskite attracts attention of people.Perovskite is a kind of crystal materials with type general formula of ABX₃,which is one of the most common minerals in the world named after the natural mineral Ca Ti O₃.Among them,halide perovskite materials(A in ABX₃ is monovalent organic or inorganic cation,usually formamidine,methylammonium or cesium,B is lead,tin or bismuth cation,and X is halogen anion)have excellent photoelectric properties,such as strong light absorption characteristics,long carrier life,considerable exciton diffusion length,which are ideal photoelectric materials and often used in light-emitting diodes,solar cells,laser and other photoelectric devices.All inorganic cesium lead halide perovskite materials(such as Cs Pb Br₃)surf an exciting research wave in perovskite light-emitting diodes(Pe LEDs)due to their high color purity(the full width at half maximum is less than 20 nm),low cost,high thermal stability,and high photoluminescence quantum yield(PLQY up to about90%).After several years,quite rapid progress has been appeared as the maximum external quantum efficiency(EQE)of green perovskite LED based on Cs Pb Br₃ has exceeded 20%.However,the luminescence efficiency and stability of Pe LEDs have not reached the basic standard of commercialization.The main possible reasons are as following.One is(1)the low internal quantum efficiency of perovskite light-emitting diodes and low exciton utilization reduce the luminescence efficiency of the Pe LEDs.The other is(2)a large amount of nonradiative recombination paths in the interface layer.PEDOT:PSS is often used as a hole transport layer adjacent to the perovskite layer to provide hole injection and transport in Pe LEDs.However,most of the excitons may be quenched when the perovskite film directly contacts the PEDOT:PSS layer due to the metal property of PEDOT:PSS,resulting in nonradiation recombination of the charge and reducing the device efficiency.At the same time,there is a large hole injection barrier(0.65 e V)between PEDOT:PSS and perovskite light-emitting layer,which limits the carrier balance in the light-emitting layer.In view of the two possible reasons above,this paper has been studied based on the improvement of the internal quantum efficiency of perovskite and the reduction of the luminescent quenching of the interface layer.Some feasible methods proposed to improve the luminescent performance of Pe LEDs have been proved through experimental verification,data and principle analysis,which provides new ideas for further improving the performance of Pe LEDs in the future.The main research contents are as follows:Firstly,in the way of improving the internal quantum efficiency and exciton utilization of perovskite,it has been reported that traditional fluorescent materials or phosphorescent materials are used as dopants to transfer energy to the excited states of perovskite emitters.Nevertheless,traditional fluorescent materials can merely collect 25%singlet excitons,hence the maximum internal quantum efficiency of traditional fluorescent materials can not exceed 25%,a lot of excitons have been wasted.Although the phosphorescent materials based on Ir or Pt and Re have achieved 100%internal quantum efficiency,the high cost of Ir or Pt and Re,as rare metal elements in the earth,limits the large-scale commercial production of Pe LEDs.Based on this,this paper proposes to utilize the thermal activation delayed fluorescence(TADF)material 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene(2Cz PN),which can realize 100%internal quantum efficiency,as an ultra-thin buffer layer to transfer the excitons energy on 2Cz PN to the excited states of Cs Pb Br₃ through an efficient F?rster energy transfer.100%internal quantum efficiency and improved luminescence performance of Pe LEDs have been achieved without using heavy metal elements.The optimized Pe LED based on Cs Pb Br₃ with ultra-thin 2Cz PN buffer layer showed enhanced electroluminescence(EL)performance,lower turn on voltage of 3.6V,the maximum luminance of 7660 cd/m²,the maximum current efficiency of 2.83cd/A,the maximum luminance and maximum current efficiency of Pe LED with ultra-thin 2Cz PN buffer layer are 7 times and 5.5 times compared with that of the traditional Pe LED without ultra-thin 2Cz PN buffer layer,respectively.Next,in the aspect of reducing the luminescence quenching of the interface layer,several works have been discussed and reported in order to overcome the large hole injection barrier and exciton quenching at the interface between the hole injection layer and emission layer(HIL/EML),however few attentions have been paid to the physical mechanism of exciton loss,especially the possible exciton loss at the HIL/EML interface.This paper introduces a method to improve the electroluminescence efficiency of Pe LEDs by introducing a hole transport layer m CP between PEDOT:PSS and Cs Pb Br₃ emission layer to raise the hole injection and transport efficiency of devices and reduce the exciton quenching at HIL/EML interface.The experimental results exhibited that the electroluminescence performance of Pe LEDs are significantly improved after adding the m CP layer.The turn on voltage of the optimized Pe LED based on m CP is?3.0 V,the maximum luminescence is?14247 cd/m²,and the maximum current efficiency is?4.86 cd/A,which are?3.73 times and?4.23 times compared to that of the original one,respectively.This work improves the luminescence efficiency of Pe LEDs efficiently by simple method of interface engineering compared with traditional Pe LEDs.