Research On Quasi 2D Perovskite Light-Emitting Diodes Assisted By Organic Small Molecules

Metal halide perovskite perovskites have attracted great attention and are widely used in photoelectric devices like perovskite light emitting diodes(PeLEDs)in recent years owing to their excellent photoelectric properties,such as adjustable band gap,high color purity,high photoluminescence quantum yield,strong defect tolerance,high carrier mobility and solution preparation.Quasi two dimensional(quasi-2D)perovskites with quantum confinement are regarded as one of the most potential emission layers due to its good thermal stability and excellent exciton binding energy,however,the excitons capture and utilization rate is still very low,resulting in low excitons radiation recombination efficiency.In addition,the defects of perovskite film and imbalanced carrier in quasi-2D PeLEDs also limit its performance and stability.Therefore,in order to solve these three problems,three small organic molecules are used as additives or buffer layer to improve the radiation recombination efficiency of excitons,resulting in enhancing the performance and stability of quasi-2D PeLEDs.Firstly,in order to improve the excitons radiative recombination efficiency in PeLEDs,an encapsulated thermally activated delayed fluorescence(Encapsulated TADF)was introduced into quasi-2D perovskite.The energy of the triplet excitons were transferred to the singlets via reverse intersystem crossing(RISC)in Encapsulated TADF,and then the energy on the singlets of encapsulated TADF is transferred to quasi-2D perovskite through F?rster energy transfer.Furthermore,the excitons radiative recombination efficiency in PeLEDs is improved under the energy funneling of quasi-2D perovskite.In addition,the introduction of Encapsulated TADF into quasi-2D perovskite can help to obtain the perovskite emission layer with good compactness,less pinholes and high coverage.Finally,compared to the control PeLEDs,the luminance and current efficiency of the quasi-2D PeLEDs based on Encapsulated TADF are increased by 2.43 times and 5.32 times,respectively,reaching the maximum luminance of 19517.76 cd/m2 and the maximum current efficiency of 38.04 cd/A.Meanwhile,the operational stability of quasi-2D PeLEDs is also increased by 2.98 times under the help of the Encapsulated TADF.Secondly,for reducing the defects of quasi-2D perovskite film,we introduced Benzyltriphenylphosphonium chloride(BTPPC)into the perovskite precursor solution.The X-ray diffraction(XRD)patterns showed that the introduction of BTPPC can improve the crystallization of perovskite,and the scanning electron microscope(SEM)images exhibited that BTPPC can help to form smooth,uniform and flat quasi-2D perovskite films.Meanwhile,Fourier transform infrared spectroscopy(FTIR)showed that there is interaction between BTPPC and quasi-2D perovskite.SEM and FTIR indicated BTPPC can passivates the defects of quasi-2D perovskite films.Therefore,the photoluminescence(PL)intensity and photoluminescence quantum yield(PLQY)of the film are enhanced.Compared with the control devices,the electroluminescent(EL)performance and stability of quasi-2D PeLEDs assisted by BTPPC has been greatly improved,with the current efficiency increasing from 9.0 cd/A to 36.7 cd/A,and the halflifetime of quasi-2D PeLEDs prolonging from 27 min to 8.7 h.Finally,aiming at the problem of excitons quenching caused by carrier imbalance,we introduced 2,2’-(5-(9,9’-spirobi[fluoren]-2-yl)-1,3-phenylene)bis(1-phenyl-1Hbenzo[d]imidazole)(SF-PBI)between quasi-2D perovskite emission layer and electron transport layer..It can be found that SF-PBI can block electrons injection,balance the carrier concentration because the LUMO of SF-PBI is higher than that of 2,2’,2"-(1,3,5Benzinetriyl)-tris(1-phenyl-l-H-benzimidazole)(TPBi),and the mobility is lower than that of TPBi,leading to reducing excitons quenching.According to the time-resolved PL spectrum,the films evaporated SF-PBI exhibits better performance with stronger PL intensity,and longer the transient lifetime,compared with the direct evaporation of TPBi in perovskite emission layer.Finally,the brightness of quasi-2D PeLEDs is 36603 cd/m2 and the maximum current efficiency is 52.05 cd/A,which is 2.29 times and 4.09 times higher than that of the control device,respectively.To sum up,the exciton utilization in quasi-2D perovskite is improved by introducing Encapsulated TADF,the defects are passivated by small molecular chloride,resulting in high-quality perovskite emission layer film,and the carrier concentration is balanced with the help of bipolar interface layer,therefore,the performance and stability of quasi-2D PeLEDs are enhanced.It provides a simple and effective method for preparing highperformance quasi-2D PeLEDs.