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

Аннотация

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.