Singlet Fission in a Binary System: Effect of Structural Changes and Temperature

Singlet fission is a promising mechanism to boost the performance of solar cells. In order to establish efficient singlet fission, it is essential to understand and control competing decay channels such as the occupation of trap states and the formation of excimers, for which the local ordering of the molecules plays an essential role. To systematically explore effects of molecular packing, a promising strategy is to blend the material of interest with another molecular compound. In the present study, we investigate binary thin films of anthradithiophene (ADT) and the high bandgap molecule hexaphenylbenzene with different compound ratios. By utilizing X-ray diffraction techniques and steady state optical spectroscopy, we identify a phase separating regime where the structural quality can be systematically tuned. Combining these subtle structural modifications with time-resolved and temperature-dependent photoluminescence spectroscopy allows us to independently manipulate the energetic landscape and thermal activation of kinetic processes. Eventually, our findings enable us to refine current models of the excited-state photophysics of ADT, suggesting that singlet fission critically depends on the number of neighboring ADT molecules, whereas excimer formation is not significantly affected by mesostructural changes. We propose that control of mesostructure and temperature can be a pathway toward controlling the balance between singlet fission and other competing channels.