Overcoming the 5% EQE Ceiling in Deep-Blue Fluorescent OLEDs with Hybridized Local and Charge Transfer Featured Phenanthroimidazole-Carbazole Emitters

Abstract

The design and development of deep-blue fluorescent organic emitters with Commission Internationale de l’Eclairage (CIEy) < 0.06 and external quantum efficiency (EQE) surpassing the theoretical limit of ~5.0% remains an enduring focus in organic light-emitting diodes (OLEDs). Herein, we report two new deep-blue organic emitters based on phenanthroimidazole (PI) frameworks, functionalized at the N1 and C2 positions. Specifically, carbazole was introduced at the N1 position via its C3 site to enhance hole-transporting ability and thermal stability, while either biphenyl (PICz-BP) or cyanophenyl (PICz-CN) was attached at the C2 position to extend π-conjugation and finely tune the photophysical properties. Photophysical studies revealed that, both emitters exhibit deep-blue emission, with emission maxima (λem) ~ 409 nm for PICz-BP and 419 nm for PICz-CN. The slightly red-shifted emission observed for PICz-CN is attributed to enhanced charge-transfer character induced by the electron-withdrawing cyano group. Further, solvatochromic studies and computational analysis revealed that both compounds exhibit hybridized local and charge-transfer (HLCT) excited states, a feature known to enhance radiative decay while maintaining high color purity. Both emitters displayed high photoluminescence quantum yields (PLQYs) and good thermal stability, making them promising candidates for organic light emitting diodes (OLEDs). Their performance was evaluated in both doped and non-doped OLED devices. Notably, the doped device employing PICz-BP achieved excellent performance, delivering a maximum EQE of 6.1% with deep-blue emission (CIEy ≈ 0.06). Remarkably, the non-doped device based on PICz-BP also retained comparable color purity and exhibited an EQE of 4.4%, underscoring the intrinsic emissive strength and stability of the material without the need for a host matrix. Overall, the successful design strategy leveraging HLCT character enabled both emitters to exceed the theoretical EQE limit of 5%, highlighting their potential for high-performance deep-blue OLEDs.