Reduced Singlet–Triplet Annihilation for Low Threshold Amplified Spontaneous Emission from a Blue Polyfluorene Electroluminescent Organic Semiconductor

A blue light-emitting conjugated polymer, namely, aryl-polyfluorene (aryl-F8), having a high neat-film photoluminescence quantum yield (73%) and a radiative decay rate (2 × 10 9 s –1 ) is reported. Excimer emission from the polymer is significantly reduced in its neat film, unlike many other wide band gap blue emitters achieved as a result of the bulky aryl groups attached to the polymer chain. Amplified spontaneous emission (ASE) under optical excitation is observed in a pristine film at an excitation fluence, E th ∼ 3 μJ cm –2, which is one of the lowest among polymer-based optical gain media in this spectral range reported so far. The well-separated spectra of stimulated emission and long-lived triplet absorption and very low triplet yields explain the achievement of a low ASE threshold. Under electrical excitation, no singlet–triplet annihilation is detected in the light-emitting diode fabricated with this polymer in a hybrid structure. Thus, aryl-F8 emerges as an attractive optical gain medium for lasing applications.