Study On Preparation Of Solution-processed MoO₃ Hole Transport Layer And Its Application To Electroluminescent Devices

In recent years,metal halide perovskite materials have received extensive attention,due to their excellent photoelectric properties,and have shown good prospects for applications in solar cells,photodetectors and light-emitting diodes（LED）.Among them,LED with perovskite as the emission layer has attracted much attention due to its excellent color purity and low cost,considered as a strong candidate for the next generation of flat panel display technology.Since the first report of perovskite light emitting diodes（Pe LEDs）with external quantum efficiency（EQE）of 0.1%,the device performance has made great progress,and the EQE of near-infrared and green devices has exceeded 20%,reaching the standard for commercial applications.Although Pe LED has met the practical application in terms of performance,including operational stability and environmental stability,remains a huge challenge.Many teams have adopted various strategies to improve equipment performance.Generally speaking,people mainly improve the stability of devices from the aspects of optimizing the quality of perovskite films and adjusting the device structure.From the perspective of optimizing the quality of the light-emitting layer,the quantum yield of perovskite films is mainly improved by using perovskite materials with different structures,such as multi-quantum well quasi-two-dimensional structures or quantum dot structures,or by regulating the surface topography of perovskite films and passivating internal defects（such as introducing ligands or additives）.In terms of device structure,researchers mainly use modification layers such as alumina to solve the problem of suppressing exciton quenching and charge balance inside the device.In addition,the charge transport layer in Pe LEDs also greatly affects device performance.The use of a suitable hole transport layer（HTL）can make carrier injection more balanced and conducive to the deposition of perovskite films.At present,poly（3,4-vinyldioxythiophene）:poly（4-phenylene sulfonate）（PEDOT:PSS）is often selected as the HTL due to its excellent hole injection and transport capability.However,the acidic and hygroscopic properties of PEDOT:PSS will lead to the erosion of ITO substrate and even adjacent functional layers.That will induce metal elements（tin/indium）can diffuse into PEDOT:PSS and even into the perovskite layer,forming a non radiation trap position,which extinguishes the luminescence and ultimately affects the performance and stability of the device.In order to eliminate the corrosion and heavy metal doping caused by PEDOT:PSS,we introduce a stable inorganic p-type semiconductor material MoO₃ to replace PEDOT:PSS as a HTL to prepare perovskite light emitting diodes.Compared with PEDOT:PSS,MoO₃ has excellent properties such such as good air stability,high carrier mobility,high transparency and good conductivity.In this paper,The MoO₃ HTLs were fabricated from the solution of phosphomolybdic acid in acetonitrile by solution-processing spin-coating technology.And compare and explore the effect of MoO₃ film as HTL on the crystallization process and device performance of perovskite MoO₃ films with different surface morphology by changing the concentration of phosphomolybdic acid solution and the time of oxygen plasma treatment of the films.On this basis,we continue to modify the MoO₃film with an ultra-thin Li F layer to further improve the performance of Pe LED.The main research contents are as follows:（1）MoO₃ films were prepared using different concentrations of phosphomolybdic acid solutions and different oxygen plasma treatment times,and perovskite films were prepared on them.Through test characterization such as contact angle,Atomic Force Microscope（AFM）and Scanning Electron Microscope（SEM）,we found that the MoO₃film obtained by spin-coating a phosphomolybdic acid solution followed by 10 min oxygen plasma treatment at a concentration of 60 mg/ml had the best wettability and minimal surface roughness.And on it,the best crystalline,smooth and dense perovskite film can be prepared.（2）On the basis of the above research,PeLED was prepared using MoO₃ films treated in different ways as HTL,and the effect of MoO₃films on the performance of electroluminescent devices was studied.We found that when the MoO₃ film obtained by 60 mg/ml phosphomolybdic acid solution and 10 min oxygen plasma treatment was used as HTL,the device current efficiency was the best,which was 16.7 cd/A.In addition,by testing the Space Charge Limited Current（SCLC）,it was found that the MoO₃ film with the best efficiency as HTL had the lowest trap density.Fewer defect states reduce non-radiative recombination within the device,further improving device performance.Through comparison,it is found that the performance of MoO₃-based HTL devices is comparable to that of Pe LEDs with PEDOT:PSS as HTL（17.5 cd/A）,but its working stability is much higher than that of PEDOT:PSS devices.Therefore,we believe that MoO₃ can completely replace PEDOT:PSS as HTL to prepare high-stability Pe LED.（3）The treatment of MoO₃ films with oxygen plasma is replaced by introducing an ultra-thin Li F layer modification layer between the MoO₃ hole transport layer and the perovskite layer.It is found that the surface wettability and roughness of MoO₃ film modified by Li F layer with a thickness of 0.5 nm are improved,and the device performance is significantly improved,compared with the unmodified device,the performance is improved by 18%.