Temporal response of optically pumped organic semiconductor lasers and its implication for reaching threshold under electrical excitation

We study the step response of optically pumped organic semiconductor lasers and find that lasing is sustained for < 150 ns following pump turn on due to annihilation quenching of guest singlet with host triplet excitons. We derive a general criterion for reaching threshold that critically depends on the pump rise time, the singlet-triplet quenching rate, and the spin statistics of exciton formation. These results imply that singlet-triplet annihilation alone fundamentally restricts lasing by electrical excitation to conditions that are not practically feasible in organic semiconductors.