Three Dimensional Organic-inorganic Hybrid Perovskite Light-emitting Diodes Fabricated Through Solution Process

As an excellent semiconductor material,perovskite material has been widely used in solar cells.In a few years,the power conversion efficiency of perovskite solar cells has increased from 3.8%to 22.7%.Surprisingly,it has excellent luminescence properties at the same time.Films prepared with a simple solution process can achieve high external quantum efficiency of luminescence and low threshold laser.In 2014,first use CH₃NH₃PbI_3-xCl_x and CH₃NH₃PbBr₃ materials to prepare NIR perovskite light emitting diodes?PeLEDs?and green PeLEDs with an external quantum efficiency and an internal quantum efficiency of 0.76%and 3.4%,0.1%and 0.4%,respectively.Recently,the current efficiency of quasi-two-dimensional green light-emitting diodes has been raised to 62.4 cd/A,and the external quantum efficiency has reached 14.36%.Due to its simple,low-cost,solution process with adjustable bandgap,PeLEDs have great application prospects in low-cost,low-energy display and lighting applications.In this dissertation,perovskite thin films were prepared by a low-cost,easy-to-operate solution process.The basic experimental conditions for the preparation of PeLEDs devices were systematically explored.The effects of perovskite films,exciton injection balance,and the physical processes at the device interface for the optoelectronic properties of PeLEDs have been studied in details.The research content mainly includes the following three parts:1.We conducted literature review before starting to prepare methylamine PeLEDs.Based on the experience of perovskite solar cells in our laboratory,PeLEDs were prepared in air and glove box,respectively.The precursor solution was prepared by adjusting the ratio of methylamine cation?CH₃NH₃Br?and inorganic lead ions?PbBr₂?,and then perovskite film was fabricated through one-step spin coating.We found that fabricating perovskite film in air is difficult,as the perovskite crystals are easily decomposed under ambient condition.Although perovskite films could also be produced,the coverage was poor.To better control the formation of perovskite film,we started to fabricate perovskite films in glove box.At the same time,we use an anti-solvent process to further improve the coverage and uniformity of the film to further improve the device performance.Subsequently,by continuously optimizing the concentration of the precursor solution and characterizing the photoelectric properties of PeLEDs,the optimal concentration and device structure of PeLEDs were obtained.2.Based on this,we focused on studying the physical processes of the interfacial exciton at the interface of hole transport layer of PEDOT:PSS and the luminescent layer of CH₃NH₃PbBr₃.We inserted an ultra-thin PFO buffer layer between the hole transport layer and the perovskite layer,and the transient fluorescence lifetime?TR-PL?,transient photovoltage lifetime?TPV?,steady-state fluorescence intensity?PL?,optoelectronic performance?J-L-V?of PeLEDs were characterized.We found that the insertion of ultra-thin PFO avoids the direct contact between PEDOT:PSS layer and CH₃NH₃PbBr₃layer,passivates the shallow defect of perovskite crystal,and slightly increases the coverage of the film.These changes thus significantly reduce exciton quenching at the interface as well as increase the probability of recombination of free charges,leading to higher probability of radiative recombination and larger brightness of the device and then improved current efficiency.3.CH?NH₂?₂PbBr₃ is called as formazan perovskite material because its organic chain is formazan.It was used in solar cell research first,and then was gradually used in lasers and light-emitting diodes due to its excellent PL performance.Because it is commonly used in quasi-two-dimensional and quantum dot devices,the preparation method is complex and time-consuming.Therefore,we studied the preparation of formazan perovskite by easy solution process.Based on the preparation of methylamine PeLEDs,we dissolved CH?NH₂?₂Br and PbBr₂ in DMF in different proportion and spin-coated on the substrate of hole-transport layer using one-step spin coating,and chlorobenzene was used to wash the film during the spin coating process.Comparing the device performance of PeLEDs with different proportion,the optimal ratio was obtained.At the same time,we also contrasted the effect of thermal annealing and vacuum annealing on device performance.Repeated experiments have found that the PeLEDs performance of thermally annealed is significantly better than that of vacuum annealed.In brief,the fundamental research on preparation of methylamine and formazan PeLEDs by solution process was studied.We found that the proportion of organic cations and inorganic lead ions,and the concentration of precursor solution had a great influence on the device performance.At the same time,the anti-solvent process modified perovskite film has a positive effect on device performance.Moreover,the physical process of exciton at the interface of PEDOT:PSS and perovskite was studied,which indicates that avoiding the direct contact of PEDOT:PSS layer and perovskite layer can reduce exciton quenching and enhance the performance of PeLEDs.This work has provided a strong experimental and theoretical basis for the research of efficient PeLEDs devices.