Kinetics of Prompt Fluorescence Decay in Rubrene Films. Manifestations of the Migration of T Excitons

Abstract

The kinetics of fast fluorescence decay in organic semiconductors due to the splitting of the excited singlet state S1 into a pair of triplet (T) excitons is significantly influenced by the process of reverse TT annihilation. It is shown that a correct interpretation of this effect requires taking into account the stochastic migration of T excitons. A two-state model (TSM) is proposed for describing the effect of migration on the kinetics of TT annihilation and, thus, on the kinetics of the fission of the S1 state. In the TSM, the migration effect is interpreted in terms of transitions between the [TT] state of the interacting excitons (at small T−T distances) and the [T + T] state of freely diffusing excitons (at large T−T distances). Within the framework of the TSM, an analytical expression for the fluorescence decay kinetics (FDK) $$I_{S_1 } (t)$$ from the S1 state is derived. This expression is used to describe the FDK measured in amorphous rubrene films in the absence of the magnetic field (B = 0) and in the magnetic field B = 8.1 kG. Adjusting the parameters of the model makes it possible to reproduce the experimentally measured FDK with good accuracy. An analysis of the theoretical FDK obtained revealed a significant contribution of T migration to $$I_{S_1 } (t)$$ , which manifests itself, in particular, in the characteristic dependence $$I_{S_1 } (t) \sim t^{ - 3/2}$$ at long times.