Two-Dimensional Exciton Diffusion in an HJ-Aggregate of Naphthobisoxadiazole-Based Copolymer Films

Understanding exciton diffusion properties is essential for organic optoelectronics such as organic solar cells (OSCs) because excitons in semiconducting polymer solids need to migrate as quasiparticles to a donor/acceptor interface to dissociate into charges in OSCs. Previous studies mainly focused on the diffusion length of excitons. In addition, the dimensionality of exciton diffusion is an important property because OSCs usually consist of semicrystalline conjugated polymers wherein anisotropic exciton diffusion is expected. Herein we investigate the dimensionality of exciton diffusion in a naphthobisoxadiazole-based low-bandgap polymer PNOz(4)T through transient absorption spectroscopy. We track the time evolution of the excitation intensity dependent transient signals caused by singletsinglet exciton annihilation. By analyzing the time dependence of the annihilation rate coefficient, we find that excitons generated in PNOz(4)T crystalline domains show anisotropic two-dimensional diffusion along the backbone and p-stacking directions, which can be rationalized by comparable intraand interchain exciton couplings.