Synthesis And Electro-optical Property Investigation Of Exciplex Host Materials Used For Solution-processed OLEDs

The possibility of fabrication of excellent performance in organic light-emitting devices（OLEDs）in which the intramolecular spin up-conversion from nonradiative triplet state（T₁）to the radiative singlet state（S₁）（thermally activated delayed fluorescence（TADF））was recently demonstrated.The success of TADF emitters relies on the small energy difference(ΔE_ST)between the singlet（S₁）and triplet（T₁）excited states,resulting in an efficient reverse intersystem crossing（RISC）（i.e.,from T₁ back to S₁）.Therefore,the electro-generated nonemissive triplet manifold can be relaxed back to the S₁ state to give a high IQE.But the electrondonating groups and electron-accepting groups of TADF materials are in a molecule,which determines that the highest occupied molecular orbital（HOMO）should be distributed at the electron-donating units,whereas the lowest unoccupied molecular orbital（LUMO）should be distributed at the electron-accepting moieties.To get smallerΔE_ST,HOMO and LUMO need to be separated as far as possible.This requirement is in conflict with gaining high fluorescent properties,bringing about an enormous challenge in design of the single molecule.Fortunately,exciplex-based emitters are a kind of materials with TADF properties,which form between electrondonating（D）and electron-accepting（A）molecules,exhibiting an intrinsically smallΔE_ST（ca.0-50 meV）due to small orbital overlap between the excited state and the ground state.Moreover,exciplex materials in design are not as complicated as TADF materials.The exciplex state can be formed by properly selecting the donor and acceptor materials.However,solution-processable donor molecules are limited by conventional materials,such as TCTA,TAPC and m-MTDATA.And solution-processable acceptor materials are deficient seriously.To solve these problems,the main content of this paper is as follows:1.From the point of view of the electron materials:we designed and synthesized two new electron-transporting materials of BMZ-TB and BMZ-TZ as a system to research,which could efficiently form exciplexes with the conventional hole-transporting materials of the star-shaped compounds m-MTDATA,TAPC and TCTA in the 1:1 molar blend film through the spin-coating technique.The thermal,photophysical and electrochemical properties of BMZ-TB and BMZ-TZ were investigated systematically.Especially,TCTA:BMZ-TZ-based device had high performance,showing a very low efficiency roll-off with the current efficiency of 24.9 cd A^-11 at 1000 cd m^-2,23.7 cd A^-11 at 5000 cd m^-2,and 22.0 cd A^-11 at 10000cd m^-2.Additionally,studies of the exciplex-type host materials reveal that provided a definite acceptor molecule,the triplet energy of the exciplex host can be controlled by changing the HOMO level of the donor component and the energy level of exciplex is higher than that of its component or lower than that of its guest,which inevitably leads to poor device performance2.From the point of view of the donor materials:we designed and synthesized a Ph-O-TCTA molecule.This molecule connected Benzene-1,3,5-triol to TCTA by non-conjugated alkyl chains.Then,we systematically studied the thermal,photophysical and electrochemical properties of Ph-O-TCTA and TCTA.The results reveal that this design strategies can improve solubility of TCTA by means of alkyl chains and retain the intrinsic properties of TCTA,such as high triplet energy.Ph-O-TCTA mixed with PhPO could form efficiently an exciplex state.Ph-O-TCTA:PhPO as a host for blue phosphorescent（FIrpic）solution-processed OLED（POLED）exhibits an excellent electroluminescence performance with the maximum current efficiency（CE）33.6 cd A^-1.Moreover,Ph-O-TCTA:PhPO as a host for blue fluorescent（5TCzBN）solution-processed OLED（TADF-OLED）also achieves an outstanding efficiency with the CE of 35.3 cd A^-1.To the best of our knowledge,these performances are among the highest efficiencies of blue OLEDs achieved by solution-process.Furthermore,this is the first report that Ph-O-TCTA:PhPO-based exciplex can be a host of both blue phosphorescent and thermally activated delayed fluorescent solution-processed OLEDs.We believe that this work will provide fundamental insight for the development of exciplexes applied in solution-processed blue OLEDs.