Reducing intersystem crossing rates of boron emitters for high-efficiency and long-lifetime deep-blue OLEDs

Abstract

Blue organic light-emitting diodes (OLEDs) using boron emitters with narrow emission bandwidths hold great promise for next-generation high-resolution display applications. However, the challenge of designing deep-blue boron emitters that can achieve both excellent efficiency and superior operational stability persists. Herein, by integrating a tetrahydroquinoline type donor, a boron core and another pairing donor, a series of deep-blue emitters with asymmetric structures (B-N-S-1/2/3) were designed and synthesized. Their photophysical properties reveal that the donor units have a significant influence on the luminescent properties, including the emission wavelength, full-width at half-maximum and, especially, excited state kinetic constants. Notably, B-N-S-1 and B-N-S-2 exhibit typical thermally activated delayed fluorescence (TADF) characteristics and large intersystem crossing rate (k_ISC) constants, while B-N-S-3 demonstrates only prompt fluorescence emission with a small k_ISC of 0.38 × 10⁸ s⁻¹. In deep-blue OLEDs with an anthracene-based host, singlet (S₁) excitons of the emitter could populate triplet (T₁) excitons by intersystem crossing (ISC), which is then quenched by the host material bearing lower T₁ energy, resulting in energy loss. In addition, the populated T₁ excitons possess high emission energy, accelerating material decomposition. Hence, the deep-blue OLED based on B-N-S-3 with a reduced k_ISC shows a maximum external quantum efficiency of 6.7%, Commission Internationale de L’Eclairage color coordinates of (0.128, 0.119), and a notably extended operational lifetime (LT₉₅) of 136 h, indicating that reducing the ISC rate of the emitters is a viable approach for enhancing the operational stability of blue fluorescent OLEDs.