Modified PEDOT:PSS With S-PSS Hole Injection Layer For High-efficiency Perovskite Light-emitting Devices:enhanced Hole Injection And Reduced Luminescence Quenching

Organic and inorganic hybrid perovskite materials possess the advantages of both organic and inorganic luminescent materials such as solution-processability,high and balanced charge carrier mobility,tunable bandgap,and good color purity.Due to these favorable properties,hybrid organic and inorganic halide perovskites are promising for light-emitting diodes.Both conventional and inverted perovskite light-emitting diodes employing the respective metal oxide and organic charge injection layer have been reported.Inverted devices comprising ITO/hole injection layer?HIL?/perovskite emission layer/electron transport layer/low work-function metal cathode easily prepare and can be built on the flexible substrates comprising with the devices excluded a metal oxide layer such as TiO₂ or ZnO layer processed at high temperatures.Due to the advantages of planarization of ITO substrates to reduce the leakage current and relatively high work function to promote hole injection,PEDOT: PSS is one of the most commonly used hole injection layers in such devices.Nevertheless,the work-function of PEDOT: PSS is not high enough to match the valence band maximum of CH₃NH₃PbBr₃ at ca.-5.7 eV,limiting hole injection into the emission layer.In addition,PEDOT:PSS severely quenches perovskite luminescence,reducing the external quantum efficiency?EQE?of the related light emitting devices.To solve the problems,we study the influences of the addition of S-PSS into PEDOT: PSS on its morphology,surface composition and work-function as well as the morphological,structural,photo-physical and electroluminescent properties of the overlying CH₃NH₃PbBr₃ layers.CH₃NH₃PbBr₃ light emitting devices with S-PSS modified PEDOT:PSS HIL show more than one order of magnitude higher EQE than the analogous devices with an intrinsic PEDOT: PSS HIL,which can be attributed to the reduction of CH₃NH₃PbBr₃ luminescence quenching and promotion of hole injection due to the formation of a highly PSS-enriched layer on the top surface of HTL.The main work includes the following aspects:?1?The morphologies of the S-PSS and PEDOT: PSS composite layers and the overlying CH₃NH₃PbBr₃ layers were studied using AFM and SEM.All HTLs show similar uniform and smooth morphologies with the root mean square?RMS?roughness value of ca.1 nm.A compact CH₃NH₃PbBr₃ layer with full surface coverage and similar closely packed grains is formed atop of HILs with or without the S-PSS incorporation.The XRD diffractrograms of CH₃NH₃PbBr₃ films on top of various HILs show the characteristic diffraction peaks of CH₃NH₃PbBr₃ crystallites with similar crystal orientation and crystallinity.The morphology,composition and crystal structure of CH₃NH₃PbBr₃ emission layers are nearly unaltered upon using HILs with various S-PSS contents,which allows to establish a clear correlation between interface modifications and device performance.?2?We proceed to study the surface composition and work function of the S-PSS: PEDOT: PSS composite HILs and photoluminescence?PL?property of the overlying CH₃NH₃PbBr₃ emission layers.It is commonly recognized that PEDOT: PSS films show vertical phase segregation,leading to the formation of a PSS-enriched top surface.X-ray photoelectron spectroscopy measurements indicate the ratio of the PEDOT to PSS content decreases rapidly as the volume ratio of S-PSS increases,indicating the formation of a highly PSS-enriched layer on the top surface of HIL.The work function values of the samples with the S-PSS: PEDOT: PSS ratios of 5 and 7.5 are determined to be ca.5.2 eV by ultraviolet photoelectron spectroscopy,0.3 eV higher than that of an intrinsic PEDOT: PSS layer?4.9 eV?,which can be attributed to the reduction of filled state densities near the Fermi level caused by the PSS enrichment on the top surface.The increase of workfunction of HILs is expected to reduce hole injection barrier between HIL and emission layer,resulting in enhanced hole injection.This is confirmed by the current densityvoltage measurement of the hole-dominated devices employing HILs with various S-PSS ratios.The current density of hole-dominated devices first increases with increasing the volume ratio of S-PSS from 0.5 to 5,then starts to decrease with further increase the volume ratio of S-PSS to7.5,probably as a result of the formation of a thick insulting SPSS layer.The steady-state PL intensities of CH₃NH₃PbBr₃ EML concomitantly increase with increasing S-PSS ratio.Furthermore,the time-resolved luminescence decay measurements indicate CH₃NH₃PbBr₃ film on top of modified PEDOT: PSS with S-PSS ratio of 5 shows larger life-time compared with the counterpart on top of an intrinsic PEDOT: PSS,indicating alleviated PL quenching caused by the formation of a highly PSS-enriched top layer.?3?CH₃NH₃PbBr₃ light-emitting diodes using modified PEDOT: PSS HILs with various S-PSS ratios are prepared and measured.The EQE,PE,current density and luminance of devices increase sharply with increasing S-PSS volume ratios from 0 to 5.CH₃NH₃PbBr₃-based light-emitting diodes with S-PSS volume ratio of 5 show the maximum EQE of 7.2% and PE of 19 lm W^-1,which are 14-20 times those of the analogous devices using a pristine PEDOT: PSS layer and among the best reported values for the light emitting devices using a neat MAPbBr3 layer.Further increase of S-PSS ratio to 7.5 reduces the EQE and PE,probably due to the formation of a thick insulting S-PSS layer at the surface of HIL,which impedes hole injection.The significant enhancement of device efficiency is attributed to the reduction of luminescence quenching and promotion of hole injection due to the formation of a highly PSS-enriched layer on the top surface of hole injection layers.