Molecular Design And Excited State Lifetime Tuning For Efficient Thermally Activated Delayed Fluorescent Organic Electroluminescence Application

Since the invention of organic light-emitting diodes?OLEDs?by Dr.C.W.Tang in1987,the development of materials and device architectures for OLED application have been studied for over 30 years.At present,compared with other display technologies,a strategy of the commercial display application is based on a combination of conventional blue fluorescent dyes and red/green phosphorescent materials containing noble heavy-metal atoms,which had led to a rapid progress of commercialization of OLEDs.Nowadays,high quality display products based on OLED technology have been emergingly available in market.However,according to the spin-statistic rule,the traditional fluorescent emitters can utilize 25%electric-generated excitons only.Even for materials with triplet-triplet annihilation?TTA?effect,only 62.5%electric-generated excitons can be utilized,which is not conducive to highly efficient device application.For phosphorescent materials,though the internal quantum efficiency?IQE?could reach 100%under the condition of electroluminescence,the materials normally contain precious metal atoms,limited crustal reserves and it remains a big challenge to recycle all of them.Facing these issues and to the fulfillment of a 100%IQE in electroluminescent device,materials exhibiting thermally activated delayed fluorescent?TADF?mechanism are used to address these challenges.Owing to the very small singlet-triplet splitting energy(?E_ST),the triplet excitons could be efficiently up-converted to the radiative singlet state,yielding very efficient TADF.Nevertheless,for the large-scale application of TADF materials,there are still lots of issues remain unaddressed.Works on material design,inner photo-physical process investigation and device architecture optimization should be conducted.Analogous to the problems phosphorescent materials are facing,owing to the millisecond to microsecond range excited state lifetime,some critical issues remained unaddressed concerning TADF materials.For instance,the challenge of the realization of deep-blue TADF emission and a severe efficiency roll-off occurs at high brightness in TADF materials based OLEDs.The exploitation on how to suppress severe bimolecular interaction and to improve the efficiency of up-conversion process is urgently needed,which would do favor to increase the device performance for a coming age of using TADF materials in practical display and illumination devices.In this thesis,we focus on the excited state tuning of TADF materials,combined with theoretical calculation,photophysical analysis and device performance characterization.It is hoped to explore the microscopic images in the working process of TADF mechanism and provide potential new solutions to the difficulties faced by realizing efficient TADF.There are three key issues we stress on:1.First,an attempt is made to simplify the molecular design strategy by exploring the"rate-limited"factor in tuning the excited state lifetime of TADF materials.The excitondynamic process is selectively regulated by means of molecular design,such asalternation of dihedral angles,molecular rigidity and transition properties of chemicalunits.Then,the photo-physical and electroluminescent properties are corelated to themolecular structure.Through this analysis,the key factor in tuning excited state lifetimecould be found.2.Based on the understanding of the first chapter,further attempts are made to resolve thebasic contradiction of realizing a large radiative rate and a small?E_ST simultaneously.Bymeans of multi-radiative channel design and“solid state solvation”effect,designstrategies to resolve the contradiction are put forward.As a result,correspondingTADF-OLED achieved a high performance at high brightness with very small efficiencyroll-off.3.Further,attempts to solve the basic contradiction of simultaneously achieving weakintramolecular charge transfer state and a small?E_ST through molecular energy levelregulation are carried out.By introducing chemical groups with n-?*and?-?*transitioncharacteristics and planarizing the molecular skeleton,deep-blue materials with delayedfluorescent excited state lifetime are realized,together with highly efficient deep-blueTADF-OLEDs.