High triplet energy bipolar host materials with the combination of dibenzofuran and benziimidazobenzoimidazole moieties for blue thermally activated delayed fluorescence emitter

摘要

Two bipolar host materials were designed and synthesized for blue thermally activated delayed fluorescence (TADF) OLEDs. Both bipolar materials, 5-(4-(dibenzo[b,d]furan-2-yl)-pyridin-2-yl)-5H-benzo[d]benzo-[4,5]imidazo[1,2-a]imidazole (4-DBFBI) and 5-(5-(dibenzo[b,d]furan-2-yl)-pyridin-2-yl)-5H-benzo[d]benzo-[4,5]imidazo[1,2-a]imidazole (5-DBFBI), were constructed with a hole-transporting type of benzimidazole moiety and an electron-transporting type of dibenzofuran moiety, and these derivatives exhibit high triplet energy over 3.10 eV. To achieve a high triplet energy level, a pyridine linker was placed between the benzimidazole and dibenzofuran moieties. As a result, 4-DBFBI and 5-DBFBI showed a high triplet energy level of 3.06 eV and 2.96 eV, respectively. Further, blue TADF devices were fabricated with our synthesized bipolar host materials. For our current study, we adopted 5,10-diphenyl-15-(10-(2,4,6-triisopropylphenyl)-10H-dibenzo[b,e][1,4]oxaborinin-3-yl)-10,15-dihydro-5H-diindolo[3,2-a:3 ',2 '-c]carbazole (PXB-DI) as the blue TADF dopant. The maximum external quantum efficiencies of the 20 wt% blue TADF devices were 31.8 and 32.5% for 4-DBFBI and 5-DBFBI, respectively. Moreover, the 4-DBFBI based host device exhibited a very low efficiency roll-off of 0.93 up to 1000 cd m(-2), which is attributed to its well-balanced charge transporting ability.