Directional control of singlet exciton diffusion in crystalline polythiophene films

Abstract

Understanding and controlling the direction of singlet exciton diffusion in conjugated polymer films is crucial for organic optoelectronic devices, such as organic photovoltaics. We previously demonstrated that singlet excitons generated in poly(3-hexylthiophene) (P3HT) predominantly diffuse along the π-stacking direction due to a relatively strong H-aggregate character. In contrast, we also found that thin films of a novel naphthobisoxadiazole-based low-bandgap polymer, PNOz4T, exhibit two-dimensional (2D) exciton diffusion characteristics along both the π-stacking (interchain) and main chain (intrachain) directions. Detailed analysis revealed that 2D exciton diffusion is due to a relatively strong J-aggregate character of PNOz4T. However, it remains unclear whether this behavior is unique to PNOz4T or can be reproduced in other conjugated polymers when J-aggregate character is enhanced. Herein, we investigate how the direction of singlet exciton diffusion is controlled in PDCBT, a polythiophene with greater J-aggregate character compared to P3HT. We demonstrate that the preferential direction of the singlet exciton diffusion shifts from the π-stacking direction to the main chain direction with increasing J-aggregate character. We observe 2D exciton diffusion when these two diffusional components are balanced. This study highlights the importance of side-chain engineering to control the direction of singlet exciton diffusion and provides new insight into understanding the mechanism of exciton diffusion in conjugated polymer films.