Design And Device Application Of Highefficiency Thermally Activated Delayed Fluorescence Materials

Thermally activated delayed fluorescence(TADF)provides a great potential for the realization of efficient and stable organic light-emitting diodes(OLEDs).However,there are still some urgent issues regarding TADF materials and their devices.This work focuses on solving the challenges of TADF materials sensitive to doping concentration,low efficiency and large roll-off of solution-processed TADF-OLEDs,and few highperformance chiral TADF emitters.Based on this,twelve novel TADF emitters are designed and synthesised,and the following results are achieved after systematic investigation of their optoelectronic properties.1.It is still challenging for blue TADF emitters to simultaneously achieve high efficiency,high brightness and low CIE y value.Here,the design and synthesis of two new benzonitrile-based TADF emitters(namely 2PhCz2CzBn and 2tCz2CzBn)with a symmetrical and rigid hetero-donor configuration are reported.The TADF-OLEDs doped with both emitters can achieve a high external quantum efficiency(EQE)over 20%and narrowband blue emission of 464 nm with a CIE y<0.2.Moreover,the incorporation of a terminal tert-butyl group can weaken the intermolecular π-π stacking in non-doped TADF emitter,and thus significantly suppress the self-aggregationcaused emission quenching for the enhanced delayed fluorescence.A peak EQE of 21.6%is realized in the 2tCz2CzBn-based non-doped device with an extremely low turn-on voltage of 2.7 V,a high brightness over 20,000 cd m-2,a narrow full-width at halfmaximum of 70 nm,and CIE color coordinate of(0.167,0.248).2.The development of highly efficient solution-processed OLEDs has been lagging behind and there is an urgent need to develop non-doped TADF materials with both high efficiency and excellent compatibility to the wet methods.Based on the previous chapter,two pairs of blue TADF isomers are designed and synthesized with a hetero-donor configuration for the realization of high photoluminescent quantum yield(PLQY).The incorporation of four tert-butyl groups in the molecules can effectively increase the molecular solubility and reduce the aggregation-caused self-quenching of excitons in neat films by inhibiting the intramolecular vibrational relaxation and the intermolecular π-π stacking.All four TADF compounds exhibit insensitivity to doping concentration.The 2C2TBn exhibits a maximum absolute quantum yield of 78%in non-doped films.The vacuum-processed non-doped OLED based on sky-blue 2C2TBn as emitter achieves a maximum EQE of over 20%.Solution-processed non-doped OLED is achieved with 2C2TBn exhibiting the record-high EQE of 25.8%.3.The OLEDs with ultra-thick light-emitting layers and simple structures are very attractive for large-area applications.We demonstrate an efficient and stable organic electroluminescence based on a neat and ultrathick single emitter,which is consisted of a beyond 0.2 μm-thick TADF molecule(3T2CBN)with multiple hetero carbazole donor units.The distorted molecular conformation and the encapsulation of the peripheral tert-butyl groups lead to high PLQY and excellent solubility.The vacuumprocessed non-doped submicron TADF-OLED based on this ultrathick emitter exhibits a maximum EQE of 20.7%and low efficiency roll-off(EQE=18.9%)at a higher brightness of 1,000 cd m-2.In particularly,a recorded maximum EQE of 23.9%with a very flat efficiency roll-off(18.4%)at 1000 cd m-2 in solution-processed non-doped ultrathick OLED(EML thick=0.23 μm),which is comparable to the state-of-the-art solution-processed TADF-OLEDs.These findings make some contribution to the development of large area OLEDs and represent an important advancement in the industrialization of OLEDs,as well as future trends in ultra-thick undoped light emitting layers.4.To fabricate efficient orange-red TADF-OLEDs,a pair of TADF isomers 3,6,11triAC-BPQ and 3,6,12-triAC-BPQ are designed and synthesized based on rigid dibenzo[f,h]pyrido[2,3-b]quinoxaline(BPQ)acceptor and 9,9-dimethyl-9,10dihydroacridine(Ac)donors.Both materials exhibit red emission in neat film and small singlet-triplet energy gap 0.10 and 0.03 eV,respectively.Compared with the stable intramolecular charge transfer(ICT)state of 3,6,12-triAC-BPQ,3,6,11-triAC-BPQ shows hybridized local and charge transfer(HLCT)character due to the H-bond between the 11-position Ac and 10-position N atom.As a result,the doped film of 3,6,11-triAC-BPQ shows a high PLQY value and the related TADF-OLED achieves a high EQE of 22.0%in orange-red region.5.The circularly polarized organic light-emitting diodes(CP-OLEDs)that enable circularly polarized luminescence(CPL)are promising for 3D display and photonic applications.However,the device efficiency and CPL character of CP-OLEDs still lag behind the practical requirements.Here,two pairs of axially chiral emitting enantiomers,(R/S)-ODQPXZ and(R/S)-ODPPXZ,are reported by fusing(R/S)-octahydrobinaphthol chiral source,diphenyl quinoxaline(DQ)/dibenzo[a,c]phenazine(DP)acceptors and phenoxazine(PXZ)donors.The twisted donor-acceptor configuration endows them TADF properties with small AEST of 0.16 and 0.07 eV,high PLQY of 92%and 89%in doped films,and obvious mirror-image CPL characteristics,respectively.The CP-OLEDs based on these TADF enantiomers not only show a maximum EQE of 28.3%and 20.3%for(R/S)-ODQPXZ and(R/S)-ODPPXZ,but also display the CPEL with dissymmetry factors(gEL)of 6.0 × 10-4 and 2.4× 10-3,respectively.The high efficiency and obvious CPL of(R/S)-ODPPXZ arise from a synergetic interplay of the rigid TADF skeleton and the chiral unit.