Suppressing Aggregation through a Rigid DOBNA-Based Host for Ultralong-Lifetime Red Solution-Processed OLEDs

Abstract

To address poor operational stability of solution-processed organic light emitting diodes (s-OLEDs), a key limitation compared to vacuum-deposited device, the study proposes molecular design strategy that controls intramolecular rotational freedom for two indacene-based host materials: 5,5,8,8-tetramethyl-14-(4-(4-(triphenylsilyl)phenyl)quinazolin-2-yl)-8,14-dihydro-5H-indeno[1,2-b]indeno[2',1':4,5]thieno [3,2-g]indole (DITI-QSi) and 14-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-5,5,8,8-tetramethyl-8,14-dihydro-5H-indeno[1,2-b]indeno[2',1':4,5]thieno [3,2-g]indole (DITI-tDOBNA). The bulky tetraphenylsilyl group in DITI-QSi introduces rotational freedom to the quinazoline unit, increasing molecular flexibility and leading to severe molecular aggregation and a low photoluminescence quantum yield (PLQY). In contrast, the rigid DOBNA unit with steric hindrance from tert-butyl groups in (tDOBNA) in DITI-tDOBNA significantly suppresses such intramolecular rotation and aggregation, ensuring close host-emitter proximity and improving PLQY, which thereby enhances both Förster and Dexter energy transfer processes. Additionally, an efficient electron-withdrawing effect of DOBNA helps to improve the charge balance within the emitting layer. A red phosphorescent s-OLED incorporating DITI-tDOBNA achieves a high current efficiency of 25.1 cd/A and an external quantum efficiency (EQE) of 18.6%, with minimal efficiency roll-off. Notably, the device demonstrates outstanding operational stability, with a half-lifetime (LT50) of 1400 h.