Fabrication And Performance Investigation Of Solution-Processed Organic Light-Emitting Diodes Based On Interfacial Exciplex Host

Solution-processed organic light-emitting diodes(OLEDs)fabricated by printing methods,such as inkjet printing and roll-to-roll printing,are considered to be the key technology owing to the outstanding advantages of low-cost and large-scale.However,the device performance of solution-processed OLEDs has always been lower than that of vacuum-deposited ones.And the key issue is that it is difficult to control the carrier transport and manage the triplet excitons.The exciplexes formed between the donor and acceptor possess the merits of narrow band gap,bipolar transport and thermally activated delayed fluorescence(TADF)effect,which are widely used as the hosts to reduce the carrier injection barrier,balance the carrier transporting and improve exciton utilization,thereby reducing the driving voltage and improving efficiency of OLEDs.However,due to the limitation of solution processing technology,there are few exciplex hosts suitable for solution-processed OLEDs,especially the lack of exciplex hosts with high triplet energy level for blue OLEDs.In view of this,this paper aims to improve the efficiencies of solution-processed OLEDs,and two types of dendritic and steric hindrance interfacial exciplex hosts,suitable for solution-processed OLEDs,are developed.The monochrome and white solution-processed OLEDs are fabricated with record power efficiencies,and the key problem of the serious roll-off for narrow-band OLEDs is solved.And the detailed research results are as follows:(1)Two kinds of interfacial exciplex hosts formed between a dendritic oligocarbazole donor(H2)and two pyridine-containing isomeric acceptors(B4PyMPM and B3PyMPM)are developed for solution-processed OLEDs.The problem of large hole injection barrier,commonly existing in solution-processed OLEDs,is solved by using the dendritic material H2 with high highest occupied molecular orbital(HOMO);the balanced carrier transporting is achieved by selecting acceptor with different electron mobility,thereby achieving a breakthrough of power efficiency.A low turn-on voltage of 2.5 V and a maximum power efficiency of 95.0 lm W-1 are obtained for solution-processed OLEDs based on H2/B3PyMPM interfacial exciplex host and tBuCzDBA emitter,which was the highest power efficiency among solution-processed TADF OLEDs reported at that time.Furthermore,the H2/B3PyMPM interfacial exciplex is used as both the blue emitter and the host for the yellow emitter(dmACDBA).The solution-processed white OLEDs are fabricated by tuning the doping concentration of dmACDBA,with a maximum external quantum efficiency of 14.9.%,power efficiency of 36.5 lm W-1,and CIE of(0.37,0.46).(2)A high triplet energy level interfacial exciplex is formed between carbazolebased donor(mCP)and acceptor(mSiTRZ)with triphenylsilicon steric hindrance unit.The steric hindrance of triphenylsilicon unit and the weak electron donating property of mCP weaken the interaction between the donor and the acceptor,which increase the emission energy of the exciplex host,making a high triplet energy level of 3.15 eV.The solution processed blue,yellow TADF devices and yellow phosphorescent devices based on mCP/mSiTRZ interfacial exciplex host reach high maximum external quantum efficiencies of 28.1%,25.8%and 27.5%,respectively.Furthermore,all-TADF and TADF/phosphorescence hybrid white OLEDs are fabricated by co-doping blue TADF emitter and yellow TADF or yellow phosphorescent emitter into the lightemitting layer.The high triplet energy level interfacial exciplex host ensures the energy transfer process of triplet excitons from the host to the blue emitter,and then rapidly converts to singlet ones on the TADF blue emitter with a high reverse intersystem crossing rate,avoiding the triplet-triplet annihilation.Simultaneously,the yellow emitter is sensitized through the efficient F(?)rster energy transfer from the blue emitter to the yellow emitter.The solution-processed white OLEDs achieves a low driving voltage and high power efficiencies by using a high triplet-level interfacial exciplex host to manage the triplet excitons.The solution-processed hybrid white OLEDs exhibit low driving voltages of 2.7 V,and high maximum external quantum efficiency of 3 1.1%,maximum power efficiency of 93.5 lm W-1,which are the record efficiencies for solution-processed white OLEDs,even comparable to those of most vacuum-deposited counterparts.(3)The high efficiency and low efficiency roll-off solution-processed narrow-band OLEDs are fabricated using the high triplet energy level interfacial exciplex mCP/mSiTRZ as the host,and a bulky TADF sensitizer 5tBuCzTRZ.The novel sensitizer 5tBuCzTRZ consisting of five di-tert-butylcarbazoles and one triazine has a fast reverse intersystem crossing rate,which can rapidly convert the triplet excitons to singlet ones,suppressing triplet-triplet annihilation.The Dexter energy transfer from sensitizer to multiple resonance thermally activated delayed fluorescent(MR-TADF)emitters is blocked by the increased intermolecular distance induced by the steric hindrance of the tertiary butyl groups.In addition,the singlet excitons can be transferred to the MR-TADF emitter through long-range Forster energy transfer to achieve the narrow-band emission.The maximum external quantum efficiency of solutionprocessed blue narrow-band device assembled using DtBuCzB as the blue MR-TADF emitter is up to 23.9%,and maintains 21.5%at 1000 cd m-2,with an extremely low efficiency roll-off of 10.0%.To the best of our knowledge,this is the best result for solution-processed blue MR-TADF OLEDs.