High-Photostability, Low-Threshold, Nearly Quasi-CW Amplified Spontaneous Emission Based on Deep Blue Molecules with a Hybridized Local and Charge-Transfer Excited State

Organic semiconductors have been widely used to fabricate optoelectronic devices due to their low-cost processing, high mechanical flexibility, high tunable emission wavelength, etc. However, the development of a deep blue organic laser remains constrained, and continuous-wave (CW) operation faces significant challenges due to issues such as singlet-triplet annihilation (STA) and triplet accumulation. In this study, we use two synthesized blue organic molecules, 2-(4-(7-(dibenzo[b,d]furan-3-yl)-9,9-dipropyl-9H-fluoren-2-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (PWO1) and 2-(4-(7-(dibenzo[b,d]furan-4-yl)-9,9-dipropyl-9H-fluoren-2-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (PWO2), with a hybridized local and charge-transfer (HLCT) mechanism to study their lasing characteristics. We found that the incorporation of fluorene bridges into molecules significantly enhances the oscillator strength, thus greatly reducing the amplified spontaneous emission (ASE) threshold, which reaches values as low as 1.15 and 0.60 μJ cm⁻² for PWO1 and PWO2, respectively, and a long operational lifetime of 10⁵ pump pulses under ambient conditions is observed for PWO2. At a high repetition frequency of 200 kHz, which approximates quasi-continuous-wave (qCW) operation, PWO2 exhibits an invariable ASE threshold, indicating its good photostability. The superior lasing performance is attributed to the negligible overlap between the triplet excited-state absorption and ASE spectra. Furthermore, the external quantum efficiency (EQE) values of the fabricated deep blue organic light-emitting diodes based on PWO1 and PWO2 reach values of 8.22% and 7.26%, respectively, with extremely low efficiency roll-off, which is particularly favorable for the high-current injection required for electrically pumped organic lasers.