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.

热激活延迟荧光（TADF）是打破有机发光二极管（OLED）内部量子效率的25％统计极限的有效机制，最近已成为一个活跃的话题。发芽TADF的关键是通过热激活实现从三重态自旋态到单重态的有效逆系统间交叉，这显然是一个温度依赖性过程。热活化的直接方式是吸收声子能量，其中从三重态到单重态的转变速率具有温度的玻尔兹曼分布函数依赖性。然而，分子振动可能引起激子自旋弛豫，从而引起不同的温度依赖性。这可以被认为是热活化的间接方式。这里，我们研究了分子振动引起的自旋弛豫对TADF的影响，并分析了TADF工艺效率随温度变化的变化原理。发现当考虑分子振动引起的自旋弛豫时，可以很好地解释实验依赖性。因此，考虑此过程有助于我们更全面地了解TADF。