Design And Study On Organic Light-emitting Devices Of Donor-acceptor Type Fluorescent Molecules Based On Benzo [c][1,2,5] Thiadiazole

As a new generation of flat-panel display technology,OLEDs(Organic Light Emitting Diodes)have many advantages,such as wide angle,fast response,light weight and the whole curing active luminous.It has occupied a large market share,and has the good prospects for development.Organic electroluminescent materials are the core of electroluminescent devices.The first generation of fluorescent material to(8-hydroxyquinoline)-aluminum(Alq3)as the representative,because only 25% singlet excitons and three singlet exciton 75% completely wasted,so the light emitting efficiency is low;the second generation is a phosphorescent material with iridium complexes as the representative,although the theory can be realized in the the quantum efficiency of 100%,but the use of phosphorescent materials in precious metals(such as iridium,platinum)with limited resources,the price is expensive,and the high efficiency blue phosphorescent materials is still lacking,which limits its further development.Considering the low cost of pure organic fluorescent material,the researchers pay more attention to it again.They promote the rate of reverse intersystem crossing(RISC)by designing new structure to improve the efficiency of fluorescent materials,which is based on thermally activated delayed fluorescent(TADF),triplet-triplet annealing(TTA)and hybrid state-“hot exciton” machanism.Among them,the hybrid state-“hot exciton” marerials is potential to a new material system with independent intellectual property rights for its advantages,such as without doping,stable efficiency,faster response and more sutible for realizing blue light.In this paper,we construct D-A type electroluminescent materials based on Benzo[c] [1,2,5] thiadiazole(BZ),and study the design strategy of device structure systematicly.Benzo[c] [1,2,5] thiadiazole(BZ)is not only a excellent acceptor,but also has a larger energy level difference of T2-T1,which can help to blocking the internal conversion.All of this provides a structure basis for the reverse intersystem crossing(RISC)to singlet exciton.When connecting the BZ to carbazole(Cz),the materals show high exciton utilization,and don't have delayed luminescent component.Their electroluminescent spectra are 560 nm,570 nm and 580 nm respectively,belong to orange light-emitting.The main research work of the paper is preparation and optimization of devices.On the one hand,we use the reasonable device structure to explore the relationship between molecular structure and electroluminescent properties,such as the different connection ways,different proportion between the donor and acceptor(the ratio of donor and acceptor is 1:2 or 2:1)and ? conjugate area(increase the benzene ring between the donor and acceptor or not).On the other hand,by considering the efficiency and stability(including trun-on voltage,the efficiency and the efficiency roll-off),we acknowledge the general strategy of device structure design of this D-A type materials.The main conclusions of the paper are as follows:(1)In the two systems,the materials with higher ? conjugate(CzP-BZP?DCzP-BZ and DBZP-Cz P)shows better electroluminescence performance.It is because their higher photoluminescence efficiency,and the other reason is their reasonable combinations.The CT components contribute more to the use of electroluminescent excitons.(2)The stability of bilateral replaced materials is better than unilateral replaced materials.Upon this,we can indicate that the increase of donors and conjugate degree can help to inhibition efficiency roll-off.In addition,although the relatively large distortion of Cz-BZP,causing by the relatively separation between the donor and acceptor,is beneficial to the carrier transition.It also increases the possibility of the carrier-exciton quenching,which will lead to the efficiency roll-off.(3)Doping is not necessary for hybridized materials to improved device performance.The doped devices efficiency of bilateral replaced BZ(DBZP-CzP)is twice as high as those of non-doped devices.But on the other hand,the replaced of carbazole benzene(CzP-3-BZP and DCzP – BZ)is more conducive to carrier injection and transport.They exhibit excellent nondoped device efficiency(EQE = 1.93%,2.71%),and the efficiency doesn't have obvious improvement in doped device.It indicates that we cannot consider the doping device by avoiding aggregating and improving carrier transport simply.Meanwhile,this series of experiments further prove that the hybridized materials have the advantage of nondoped.(4)For D-A type materials,we should consider the the effect of exciplex when design multilayer devices.It is easy for Cz-BZP to form an exciplex with common hole transport layer,and the insertion of appropriate exciton barrier layer can help to avoid this problem.