Realizing highly efficient blue electrofluorescence by optimized hybridized local and charge transfer state and balanced carrier mobilities

Abstract

Carrier mobilities of emitter layer materials are crucial for the efficiency and efficiency stability of non-doped organic light-emitting diodes (OLEDs). However, realizing high and balanced carrier mobilities, particularly electron mobility for organic materials, remains challenging. Herein, we design four blue-emissive quasi-equivalent hybridized local and charge transfer (HLCT) benzoxazole-phenylcarbazole derivatives, and discover the influence of three molecular designs (meta-linkage, phenyl-increasing and large group substitution) on OLED performance. Phenyl group insertion can enhance π-conjugation and regulate the locally excited (LE) component of the HLCT state, which not only contributes to high photoluminescent quantum yield (PLQY) but also maintains the charge-transfer (CT) property for the triplet utilization in OLED. More importantly, π-conjugation enhancement contributes to more regular aggregation, increasing carrier mobilities. Meanwhile, a large group (e.g., tert-butyl) substitution can hinder aggregation and afford bad carrier mobilities between adjacent molecules. As a result, we realize balanced and boosted carrier mobilities (hole: 2.64 × 10−6 cm2 V−1 s−1; electron: 2.40 × 10−5 cm2 V−1 s−1) in the non-doped OLED of 2-(4′-(9-phenyl-9H-carbazol-3-yl)-[1,1′-biphenyl]-4-yl)benzo[d]oxazole (3-PCZPBO), corresponding to the highest external quantum efficiency (EQEmax) of 7.1% with low efficiency rolling-off. This study provides a valuable strategy for the design of efficient blue emitters.