Enhancement of performance of OLEDs using double indolo[3,2-b]indole electron-donors based emitter

Abstract

Recently, considerable efforts have been devoted to the improvement of fluorescent OLEDs because of their affordable cost and metal-free materials. Indolo[3,2-b]indole derivatives with a large planar rigid structure and great photophysical properties have attracted attentions as materials in a variety of organic electronics but their applications as OLEDs are still limited. Herein, we reported two new fluorescent OLEDs, i.e., 10,10'-(sulfonylbis(4,1-phenylene))bis(5-ethyl-5,10-dihydroindolo[3,2-b]indole) (DHID-DPS) and 2,8-bis(10-butylindolo[3,2-b]indol-5(10H)-yl)dibenzo[b,d]thiophene 5,5-dioxide (DHID-DBS), with a distorted donor-acceptor-donor structure bearing indolo[3,2-b]indole moieties as electron-donors. The DHID-DPS exhibited a smaller singlet-triplet energy gap (ΔEST = 0.18 eV) than DHID-DBS (ΔEST = 0.33 eV). In addition, DHID-DPS demonstrated the fluorescence quantum yield in doped films reaching to 32% as well as the evidence of thermally activated delayed fluorescence (TADF) properties. As a result, the optimized doped OLED by using DHID-DPS as an emitter achieved a maximum external quantum efficiency of 3.86% with a blue-green fluorescence as well as a high brightness up to 12860 cd m−2. This suggested that indolo[3,2-b]indole derivatives are potential candidates for donor moieties in OLED applications.