Novel electro-fluorescent materials with hybridized local and charge-transfer (HLCT) excited state for highly efficient non-doped pure blue OLEDs

Abstract

Pure blue emitters have become a research hotspot in the field of organic light-emitting diodes (OLEDs). Achieving high-performance pure blue OLEDs is still a challenge from the perspective of molecular structure design. Herein, two molecules with 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole (PPI) as a donor and 3,4,5-triphenyl-4H-1,2,4-triazole (TAZ) as an acceptor were designed and synthesized by fine-tuning the substituent on the PPI skeleton, namely tBuPPITZ and PPITZCN, respectively. Theoretical calculation and experimental results showed that they have hybridized local and charge transfer (HLCT) characteristics, excellent thermal properties (decomposition temperature (T_d) is 481 °C for tBuPPITZ and 490 °C for PPITZCN) and pure blue emission. As a result, non-doped devices based on tBuPPITZ and PPITZCN as emitters exhibited high efficiency with the maximum external quantum efficiencies (EQE_max) of up to 6.0% and 7.5%, and the Commission Internationale de l'Eclairage coordinates (CIE) are (0.16, 0.09) and (0.16, 0.10), respectively. The outstanding device efficiency of PPITZCN can be attributed to the introduction of the cyano group, which not only improves the exciton utilization efficiency (EUE) while maintaining a relatively high photoluminescence quantum yield (PLQY), but also promotes carrier transport. Importantly, the PPITZCN-based device achieved the best performance among the pure blue HLCT-based non-doped OLEDs (0.08 ≤ CIE_y ≤ 0.10 and EL ≤ 450 nm) and is an advantageous material in the corresponding pure blue OLED field.