Enhancing The Performance Of Mixed-halide Perovskite Based Light-emitting Devices By Organic Additive Inclusion

In the past few years,mixed halide perovskite,featured by large absorption,tunable bandgap,facile solution processing,and low defect density,has attracted large attention due to its use in optoelectronic devices such as solar cells,organic light-emitting diodes,photodetectors and laser.This thesis mainly studies the properties of mixed Br and I perovskites and their application in light-emitting diodes.Despite its bandgap tenability in the range of 1.52-2.38 eV,MAPbBr_x I_1-x-x shows relatively low PLQE,which restricts the EQEs of light-emitting devices.In addition,when MAPbBrxI1-x is subjected to optical and electrical excitation,halide segregation occurs,leading to the formation of a I-rich phase,which compromises PL and EL spectrum stability.This thesis aims to solve the problems through the addition of SPPO13 and TPBI in the antisolvent during the film formation process.The influence of TPBI and SPPO13 addition on the morphology andcrystal size of MAPb(Br_0.5I_0.5)₃ is studied.It is found that with the increasing concentration of the additive solutions,the crystals size on decreases.However,when the concentration of SPPO13 solution has been increased to 2 mg/ml,there was net-structured SPPO13 layer on the surface.In the XRD diffractograms,two diffraction peaks occurred at 14.4°and 29.2°respectively,originating from the?100?and?200?crystal faces.With increasing SPPO13 concentration,the full-width at half maximum of the?100?peak increases,indicating a decreased crystallinity,which is consistent with the results of the SEM measurement.After the CB-produced sample was exposed to 405nm UV light,the full-width at half maximum of the?100?peak increases,indicating the formation of I-rich phase.By comparison,the changes of the full-width at half maximum were much less for the SPPO13 crystallized sample,which shows that the organic additives can effectively suppress the halide phase separation.PL measurements of CB,SPPO13 and TPBI processed CH₃NH₃Pb(Br_0.5I_0.5)₃ samplesindicate that addition of SPPO13 and TPBI can significantly increase the PL intensity of MAPb(Br_0.5I_0.5)₃,e.g.PL intensity of the sample processed with 1.0 mg/ml SPPO13 processed device is approximately 5.8 times that of the CB-processed analogs.The PLQE of CB-crystallized sample is 0.2-0.3%,while that of the SPPO13-crystallized sample is 0.8-1%.The PL maximum of the CB-crystallized sample red shifted from 645 nm to 653 nm,and 665nm,respectively,after two and four minexposure to UV laser.In contrast,the PL maximum of the SPPO13-crystallized sample remained the same and red shifted 2 nm after two min and 4 min laser irradiation.From the above results,it can be concluded that SPPO13 can passivate defects on the surface or grain boundary in the sample and inhibit halogen phase separation to some degree.Similar enhancement in PL intensity and PL spectrum stability is observed in the sample processed with TPBI.Light-emitting devices with the structure of ITO/PEDOT:PSS?70 nm?/CH₃NH₃Pb(Br_0.5I_0.5)₃?150 nm?/TmPyPB?60 nm?/CsF?1 nm?/Al?100 nm?are fabricated to investigate the influences of organic additives on the electroluminescent properties of MAPb(Br_0.5I_0.5)₃.The maximum luminance,luminance efficiency and EQE are 20 cd/m²,0.05 cd/A,and 0.06%,respectively.Whereas,the devices processed with SPPO13 solution show the maximum luminance,luminance efficiency and EQE of 210 cd/m²,0.32 cd/A,and 0.42%,respectively.Addition of SPPO13 increases the luminance efficiency by a factor of six,which is largely attributed to defect passivation effect of SPPO-13.The electroluminescence spectrum of CB-crystallized device red shifted from 645 nm to 658 nm when driven under 60 mA/cm² and 80mA/cm²,meanwhile an enlarged FWHM,and the increased spectral weight at 700 nm were observed.By comparison,EL maximum of the SPPO13 crystallized devices remained at 645 nm,and FWHM of EL spectra increases only slightly,which reflected that the SPPO13 can to a large degree depress the halogen phase separation.Enhanced luminance efficiency and EL spectrum stability have also been found in TPBI-processed devices,which reflected that addition of a Lewis base is a general method for suppression of halide segregation.The undercoordinated lead atoms interact with P=O moieties in SPPO13,which compensates halogen vacancies and decreases the possible migrating paths of halide ions,thereby suppressing halide separation.