An Organic Semiconductor Lasing Crystal Featuring Triplet-Triplet Annihilation

Abstract

Organic semiconductor lasers are attractive for low thresholds and cost, but triplet accumulation hampers their electrically pumped development. Compared to existing organic lasing materials, triplet-triplet annihilation (TTA) systems are capable of tolerating high triplet concentrations and may facilitate stable laser emission under electrical pumping. To avoid energy losses in doped multicomponent TTA systems, herein, we report an organic semiconductor lasing material BH001 with TTA properties, which combines concurrent triplet harvesting and lasing within a single molecular framework. Dislocations between π-conjugated planes reduce π-π stacking-induced fluorescence quenching, yielding high photoluminescence quantum yield (PLQY) in the crystal. The TTA process in BH001 can be observed through a color change from red to blue by the sensitization of PtOEP. Given that nanosecond/femtosecond transient absorption (ns-TA and fs-TA) spectroscopy has demonstrated the appreciable ability of BH001 to generate triplet states, TTA-delayed fluorescence of pure BH001 crystal was directly detected using a streak camera. A laser constructed from this TTA crystal achieved low-threshold blue emission at 440 nm (P_th = 15.4 µJ/cm²), which is increased in an oxygen atmosphere, suggesting the involvement of triplets. Upon excitation with nanosecond laser pulses that are more prone to cause triplet stacking, the BH001 crystal exhibits stimulated emission behavior. This study demonstrates a lasing molecule with TTA properties, highlighting its potential in continuous wave (CW) pumped and ultimately electrically pumped systems.

Keywords

lasing organic single crystals / singlet fission / time-resolved spectroscopy / triplet-triplet annihilation (TTA)

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