Defect Passivation By Semiconducting P=O Molecules Toward Efficient Fapb FAPbBr₃/CsPbBr₃ Perovskites Light-Emitting Diodes

Metal halide perovskites and their light-emitting diodes（PeLEDs）have shown great potential in next-generation display and lighting technology,because of their superior luminescence properties,such as tunable wavelength,high photoluminescence quantum yield（PLQY）.color purity,etc.,and low-cost solution processability.Currently,the PeLEDs have gained breakthroughs with their external quantum efficiencies（EQEs）exceeding 20%,which benefit from the continuous development of metal halide perovskite materials and device architecture optimization.Importantly,numerous and updating studies have shown that the performance improvement of PeLEDs is seriously restricted by the generation of surface defects of perovskites.In view of this,this thesis focuses on the passivation method of surface defects of perovskite thin film.Differing from previous reports utilizing insulating passivators,we emphasize the advantages of semiconducting organic molecules with robust functional groups for not only defect passivation of perovskites,but also synergistically possessing excellent carrier transport and injection abilities,as well as perfect heterointerface energy level alignment.Based on these,high-performance PeLEDs are obtained.The thesis mainly includes the following aspects:（1）PeLED fabrication and optimization based on FAPbBr₃.In this part,triphenylphosphine oxide molecules（TPPO）with P=O functional groups are firstly utilized attempting to passivate the perovskite defects and improve the device performance.We observe that the PL intensity of the perovskite film could be significantly enhanced after antisolvent treatment comprising of abundant TPPO molecules,indicating the robust passivation effect of P=O groups,which is consistent with previous reports.However,the PeLEDs based on these films show poor device performance.This phenomenon is mainly caused by the insulating property of thick TPPO passivating layer at the surface,which inhibits the carrier transporting and injecting into the perovskite emitting layer.With this consideration,we choose 2,4,6-tris[3-（diphenylphosphino）phenyl]-1,3,5-triazine（POT2T）molecules as the top-surface modification layer,which possess not only the passivating functions by P=O groups,but also superior electron injection ability deriving from their semiconducting character.PeLEDs after thickness optimization of POT2T layer has been fabricated,which exhibits a peak EQE of 15.23%,luminance of 59,221 cd/cm² with a low turn-on voltage of 2.3 V.（2）PeLED fabrication and optimization based on CsPbBr₃.Because FAPbBr₃ needs to be crystallized quickly by the action of anti-solvent,it will inevitably lead to the introduction of human operation error and increase the difficulty of experiment repetition.Therefore,when this structure is applied to CsPbBr₃ perovskite,it does not need to introduce anti-solvent,which effectively avoids the problem of experiment repetition.The device is also constructed like FA-based perovskite,and the device performance data are obtained,and it is observed that the start-up voltage is less than the photon emission energy(V_photon),and a lower start-up voltage is obtained.After that,4,6-Bis（3,5-di（pyridin-4-yl）phenyl）-2-phenylpyrimidine（B₄PyPPm）,which is also a pyrimidine derivative,is selected instead of 4,6-Bis（3,5-di（pyridin-3-yl）phenyl）-2-methylpyrimidine（B₃PyMPm）as the electron transport layer,and the performance of light-emitting diodes are compared.In view of the experimental results,it is found that faster electron transfer/injection into the emitter is beneficial to achieve higher PeLEDs performance by enhancing upconversion electroluminescence.（3）Buried interface modification by semiconducting P=O molecules:BCPO.Combined with the previous studies,we introduced the hole transport layer with the same P=O functional group and semiconductor properties to optimize the interface defects of perovskite embedding.In this work,we selected the sandwich of semiconductor P=O molecules,namely,Bis-4-（N-carbazolyl）phenyl)phenylphosphine oxide（BCPO）.By passivating perovskite interface defects and achieving level matching,EQE is increased by 3 times（from 6.8%to 20.2%）and lower driving voltage is obtained for upconversion LEDs.