Synthesis And Electro-optical Property Investigation Of The Novel Exciplex Host Materials Used For Solution-processed OLED

So far,in order to obtain the organic light emitting devices?OLEDs?with outstanding performance,most of the emitting material layers are adopted with host-guest doping system,thus it can be seen that the host substances play a key part in OLEDs.Based on its own bipolar transmission characteristic,minimal single-triplet energy gap,low opening voltage and simple design method,the exciplex host materials stand out among all kinds of host materials.At present,most of the exciplex host materials are used in vacuum evaporation devices,in contrast,the work of applying them to high performance devices fabricated by solution process is not too much.However,solution process has the superiorities of lesser cost,more simplifies process,more accurate control of mixing conditions,higher material utilization ratio and larger film-forming area,which can not be matched by the evaporation process.All of these are beneficial to the commercial production of solution-processed OLEDs?s-OLEDs?,so that the s-OLEDs have a wider market prospect.Therefore,it is imperative and important to develop more exciplex host materials which can be used in s-OLEDs.In the way of studying the exciplex,our research group found that the donor and acceptor molecule mixed with each other will form an extra complex produced by the donor-donor interaction,especially in s-OLEDs.It is called excimer,which usually causes energy loss in devices and affects the performance of devices.Therefore,to restrain the generation of excimer and investigate the formation mechanism of exciplex is the main research contents of this paper.The work of this article is carried out in the following:1.This is the first work of the alkyl chain spirodifluorene group connected to the periphery of the classical donor molecule 4,4',4''-Tris?carbazol-9-yl?-triphenylamine?TCTA?.A novel donor molecule TCTA-O-SP which is suit for wet process was designed and synthesized,and its thermal stability and photo-electric properties were also tested.The external spirodifluorene molecules have a large steric hindrance effect,which can effectively weaken the interaction between donor molecules,and inhibit the production of excimer.The results show that the introduction of alkyl chain groups can maintain the original properties of TCTA,thus TCTA-O-SP still has a thermal decomposition temperature of 403?and a high triplet state energy level of3.20 eV.The photoluminescence spectra?PL?shows that TCTA-O-SP:PhPO can successfully form the exciplex with a new emission peak in 409 nm;in comparison with the electroluminescence spectrum?EL?of TCTA:PhPO and TCTA-O-SP:PhPO,the results show that the excimer emission peak of TCTA-O-SP:PhPO is much weaker than that of TCTA:PhPO,which indicates that this kind of molecular design is effective.The EL emission peak of TCTA:PhPO and TCTA-O-SP:PhPO have a red shift of 64 and 66 nm than that of PL emission peak,indicating that the exciplex of local state is formed under the light-excitation condition,while the exciplex of charge transfer state is formed under the electro-excitation condition.Using TCTA-O-SP:PhPO as well as TCTA:PhPO as the host,we prepared OLEDs by solution process with the emitter of FIrpic,and the maximum current efficiencies were 19.2 cd A^-11 and 24.6 cd A^-1,respectively,which are all belong to the desired performance range of solution-processed blue PhOLED.2.In order to further explore the formation mechanism of local state and charge transfer state exciplex,by changing the number of alkyl chain carbazole groups,we have designed two different novel donor molecules TCTA-O-CZ and TCTA-O-2CZ,and their thermal stability and photo-electric properties were also tested.The increase of the peripheral alkyl chain carbazole groups can not only effectively inhibit the formation of excimer,but also regulate the distance between donor molecules and acceptor molecules,thus it is an effective means to study and verify the formation mechanism of exciplex.The PL emission peak of the exciplex formed by TCTA-O-CZ:PhPO is at 450 nm and the EL emission peak is at 480nm,which indicate that the exciplex of TCTA-O-CZ:PhPO is in local state under the light-excitation condition,and in the charge transfer state under the electro-excitation condition.There is no new emission peak in the PL spectrum of TCTA-O-2CZ:PhPO,which indicates that there is no local state exciplex formed.However,TCTA-O-2CZ:PhPO produced a new emission peak at 480 nm in the EL spectrum,which indicated that the charge transfer state exciplex was formed,thus the interaction distance between the donor and acceptors can cause effect on the formation mechanism of the exciplex was verified.OLEDs were fabricated by wet processing with TCTA-O-CZ:PhPO and TCTA-O-2CZ:PhPO as the host and FIrpic as the guest,and the turn-on voltages were 4.2 V and 5.1 V,respectively,which suggested that the introduction of alkyl chains may inhibit effective charge transfer and affect carrier transport balance.