Properties Of High-concentration Doped Devices Based On Bipolar Phosphorescent Complexes And TADF/Phosphor Hybrid White Device

Recently,organic light-emitting diode（OLED）has shared the considerable market in flat panel displays and also has broad application potentials in the field of energy-saving and environment-friendly solid-state lighting in future.OLEDs have attacted more and more attention of scientific researchers and commercial companies,and been rapidly developed also.Material is the very key factor in determining the OLEDs’performance.Based on the efficient utilization of both singlet and triplet excitons of phosphorescent materials,OLED can theoretically achieve 100%internal quantum efficiency（IQE）.Therefore,phosphorescent materials have become the most widely used material system in this field.However,due to the typically poor carrier mobility and luminescence self-quenching of phosphorescent materials,high-performance phosphorescent OLED usually achieved by requiring not only some specialty additional materials to confine triplet excitons within EML,but also the precisely controlled doping concentrations,which should be a disadvantages for the preparation of reliable and reproducible commercial devices.On the other hand,organic OLED（White OLED,WOLED）has been rapidly developed over the past decade,and their performances such as efficiency,brightness,and color rendering index（CRI）have reached rather high levels.However,until now,the WOLEDs adopting either full-phosphorescent or the fluorescent-phosphorescent hybrids strategy has certain problems such as the complex structure（many luminescent components,many layers,etc.）,the moderate maximum efficiency values or the significant efficiency roll-off,and the corresponding white light with the poor CRI levels affecting the overall performance white light devices as well as their practicality.In this thesis,several phosphorescent materials with bipolar charge-transporting abilities were synthesized,which enrich the bipolar phosphorescent material system,and the corresponding phosphorescent OLEDs with high efficiencies and high doping concentration in the green to orange region were prepared.Furthermore,the position of the exciton formation zones in the emitting layers was systemically investigated also.On the other hand,a novel thermally induced delayed fluorescent（TADF）blue light emitting host material and a bipolar phosphor guest were used to construct simplifed hybrid WOLED,which realizes white-light emission with high efficiency and high color rendering quality.It is mainly divided into the following three parts:1.In chapter II,three new types of Iridium（III）complexes named dfppypg,sfppypg,and tfppypg,comparing phenylpyridine（ppy）as the first ligand and dipig（N,N′-diisoproguanidinate）as ancillary ligand were synthesized and carefully characterlized.Density functional theory（DFT）calculation demonstrated that all the three complexes possessed clear electron density-distribution separation between the HOMO and LUMO.Such HOMO and LUMO distributions provide the respective channels for the transport of holes and electrons,and the two kinds of carriers can realize free intermolecular hopping.All the three compounds showed good thermal stabilities,high PLQYs and short excited-state lifetimes.We first examined the EL performance of nondoped OLEDs adopting the neat dfppypg,sfppypg and tfppypg film as the EMLs,the maxima EQE values of 12.1%,12.2%,and 9.9%were obtained.Doped Phosphorescent OLEDs were fabricated using doped FPPCA:Phosphors（8 wt%,15 wt%and 25 wt%）films to investigate their electrophosphorescent performance,and doped devices using CBP as hosts were fabricated for comparison.high EQE values of²0%with a low roll-off could be achieved.Such high EL efficiencies over wide doping concentration range are attributed to the use of these bipolar emitters,which can help balance charge fluxes and contribute to direct exciton formation in the EMLs.2.In chapter III,doped phosphorescent OLEDs were fabricated based on the classical compound Ir（ppy）2（acac）and bipolar compound Ir（ppy）2（dipba）.When the doping concentration of Ir（ppy）₂（acac）were 7 wt%and 25 wt%,the maxima EQE values of 21.3%and 16.4%were obtained,respectively.The severe efficiency roll-offs could be assigned to the typically poor carrier mobility and luminescence self-quenching of Ir（ppy）₂（acac）.The devices using Ir（ppy）₂（dipba）emitters exhibited the maxima EQE values of 19.5%and24.0%,respectively.These high and stable EL fficiencies can also be attributed to the unique property of Ir（ppy）₂（dipba）molecule due to their bipolar charge transporting ability.A series of monitoring devices based on Ir（ppy）2（dipba）and Ir（ppy）2（acac）were fabricated to investigate the carrier recombination regions in the emitting layers.The carrier recombination regions in Ir（ppy）₂（acac）-based devices are close to the interfaces between the light-emitting layers and the electron/hole transport layers whether at low or high doping concentrations.However,the carrier recombination regions in Ir（ppy）₂（dipba）devices are throughout the entire light emitting layers.3.In chapter IV,a new butterfly shaped TADF emitting material named PHCz2BP,comprising 3,6-diphenyl-9Hcarbazole donor coupled with benzophenone acceptor,was designed,synthesized,and characterized.Here,PHCz2BP exhibited a prominent character of a typical TADF molecule featuring clear electron density-distribution separation between the HOMO and LUMO,this spatially separated frontier orbital distribution could induce a reducedΔE_ST value of PHCz2BP,suggesting that excitons may be readily harvested by efficient reverse intersystem crossing（RISC）between the T₁ to S₁ states.PHCz2BP was used not only as a neat emitting layer to construct highly efficient sky blue nondoped OLED with the very high peak EQE/power effciency values of 4.0%/6.9 lm W^-1but also as a host to sensitize high-effciency multi-color electrophosphorescence,including green,orange,and red light,with universal high EQEs of>20%.The performances of three white devices（W1,W2 and W3）based on hybrid TADF/phosphors were evaluated.For W1,although the very high peak EL efficiencies of 21.4%for EQE and 69.7 lm W^-1 were obtained,it is difficult to obtain a high CRI（50–55）due to the absence in the green region.Both high-quality white EL from W2 and W3 achieved very high forward-viewing efficiencies.Their peak EQEs and PEs were 25.6%,23.8%,and 48.3,47.4 lm W^-1,and the EQEs maintained the high level of 25.1%and 23.6%at the luminance of 1000 cd m^-2.To the best of our knowledge,such high and stable EL performance represents the best published data for a WOLED based on the effcient TADF/phosphor hybrid strategy.