Influence of spin relaxation induced by molecular vibration on thermally activated delayed fluorescence

Abstract

Abstract Thermally activated delayed fluorescence (TADF), an effective mechanism to break the 25% statistic limit of organic light-emitting diodes (OLEDs) internal quantum efficiency, has become an active topic recently. The key to germinate TADF is the achievement of efficient reverse intersystem crossing from triplet spin state to singlet state by thermal activation, which is obviously a temperature dependence process. The direct way of thermal activation is the absorption of phonon energy, in which the transition rate from triplet state to singlet state has the Boltzmann distribution function dependence of the temperature. Nevertheless, the molecular vibration could engender spin relaxation of excitons, giving rise to different temperature dependence. This could be regarded as an indirect way of thermal activation. Here, we investigate the effect of spin relaxation caused by molecular vibration on TADF and analyze the change principles of the efficiency of TADF processes versus temperature. It is found that the experimental dependence could be well explained when the spin relaxation induced by molecular vibration is considered. Therefore, the consideration of this process helps us to understand TADF more comprehensively.