Indication of cooperative light amplification in 5,5″-bis(4-biphenylyl)−2,2′:5,2″-terthiophene single crystals at room temperature

Among the large number of π-conjugated compounds, thiophene/phenylene co-oligomers have been one of the promising candidates for both optically pumped light amplification media and light emission media in electroluminescence devices owing to their high thermal stability, high photoluminescence (PL) quantum yield and high carrier mobility. Of these, the single-crystal state of 5,5″-bis(4-biphenylyl)−2,2′:5′,2″-terthiophene (BP3T), which has been frequently used as an active medium in organic light-emitting transistor devices, indicates an unusual light amplification process in its PL spectra. Herein, we explain two insights about the indication of a cooperative light amplification process in BP3T single crystals under optical pumping. The first one is excitation beam area dependence for the nonlinear amplification threshold in PL spectra at room temperature, which has previously been carried out at 12 K. The second one is time-delayed amplified emission in various excitation stripe lengths.