Highly fluorescent solid-state benzothiadiazole derivatives as saturated red emitters for efficient solution-processed non-doped electroluminescent devices

Abstract

Four donor–π–acceptor type saturated red emitters, namely BTZn, employing two isomeric bis(n-hexylthiophen-2-yl)-benzothiadiazoles as the π–acceptor core and a triphenylamine donor as multiple substituents, were designed and synthesized. The highly twisted conformation between n-hexylthiophene and benzothiadiazole or the steric configuration in the molecules efficiently interrupts molecular π-conjugation and inhibits intermolecular π–π interactions, affecting the optical, thermal and electrical properties of the material and resulting in high solid-state emission. In particular, BTZ2 and BTZ4 bearing four triphenylamines substituted on the thiophene rings displayed strong fluorescence in the film state with quantum yields of >54%. All four compounds as non-doped emitters in solution processed hole-transport layer free OLEDs showed saturated red color emission with electroluminescence peaks over 648 nm. Especially, the BTZ2-based non-doped device exhibited excellent device performance with a maximum external quantum efficiency (EQE_max) of 4.15%, maximum luminance of 6842 cd m⁻² and CIE coordinates of (0.65, 0.36), which is among the best electroluminescence performances for simple solution-processable non-doped red OLEDs with a similar color.