Efficient And Stable Perovskite Light-Emitting Diodes:Material Design And Device Optimization

Metal halide perovskites have become the most attractive semiconductor material due to their remarkable optical and electrical properties such as high absorption coefficient,high electron/hole mobility,long carrier diffusion length,low defect density,etc.At the same time,perovskites also have the advantages of continuously tunable emission wavelength,high photoluminescence quantum yield,narrow emission full width at half-maximum and low-temperature solution processing,which provides an opportunity for its future application in display and lighting fields.Since perovskite electroluminescent devices at room temperature were first successfully fabricated in 2014,the device efficiency of perovskite light-emitting diodes(LEDs)has developed rapidly.Up to now,the peak external quantum efficiency(EQE)of red/near-infrared(NIR),green and blue perovskite LED have reached 23.2%,28.1%and 13.8%,respectively.Compared to the rapid improvement in the device efficiency of perovskite LED,the progress in stability is still relatively lagging behind.So far,the best operational lifetime of perovskite LED is less than 1000 hours,which is far shorter than the requirements for commercialization.The operational instability of perovskite LED is mainly affected by two factors:instability arising from material intrinsic structure,such as perovskite material undergo significant changes in structural and optoelectronic properties under phase transitions,thermal stress,air exposure,and illumination etc.The other instability suffers from perovskite LED device operation,such as electricfield-induced ion migration,degradation of the charge transport layer resulting from joule heat generated during device working,a significant luminescence quenching caused by the diffusion of metallic species from electrodes.In conclusion,the stability of perovskite materials and electroluminescent devices,need further research and improvement.Based on the above research background,we prepared new perovskite materials to improve the stability and optoelectronic properties,studied the effective suppression methods of ion migration,optimized the performance of perovskite LED and then obtain efficient and stable perovskite LED.The specific research work is divided into the following parts:1.The intrinsic soft lattice nature of halide perovskites makes them very tolerant to defects,however,the soft lattice also results in low stability towards external stresses such as heating and humidity,as well as high density of phonons and strong electronphonon coupling.Here,the polymerized perovskites were prepared by solid state polymerization of unsaturated 4-vinylbenzylammonium as organoammonium cations without damaging the perovskite structure and its tolerance to defects.The polymerized perovskites show enhanced stability and flexibility compared to regular threedimensional and two-dimensional(2D)perovskites.Furthermore,the polymerized 4vinylbenzylammonium group improves perovskite lattice rigidity substantially,resulting in reduced electron-phonon coupling.2.Based on the study of crystal rigidity and electron-phonon coupling of the polymerized perovskites,we further researched the optoelectronic performance of the polymerized perovskites.By measuring temperature-dependent photoluminescence quantum yield,transient photoluminescence,and electron/hole mobility,we found that the reduced electron-phonon coupling in the polymerized perovskites suppress nonradiative recombination and enhanced carrier mobility.Based on the above improved optoelectronic performance,we prepared the polymerized perovskite LED.Due to the suppression of electron-phonon coupling,the device of two-dimensional(2D)polymerized perovskite as a light-emitting layer has bright green luminescence at room temperature.We further studied the quasi 2D polymerized perovskite LED,the highest efficiency of the optimized device can reach 23.2%,and the operational stability has also been significantly improved.3.We found that no ion migration or phase segregation in Sn-based perovskites under long time illumination and strong electric field.The results of photoluminescence and X-ray diffraction before and after illumination,as well as fluorescence microscopy and energy spectrum of element distribution before and after electric polarization all prove that Sn based perovskite is more stable than Pb-based perovskite.Theoretical calculations and temperature-dependent conductivity results demonstrate that the absence of ion migration in Sn based perovskites is attributed to stronger Sn-halide bond and higher mobile ions activation energy(Ea).The operational stability and spectral stability of Sn based perovskite LED are obviously improved compared to Pb based perovskite LED.