Singlet exciton diffusion plays an important role in the photovoltaic conversion in organic photovoltaics (OPVs). Unlike inorganic and perovskite solar cells, free charge carriers cannot be generated directly upon light absorption in OPVs. Instead, Coulombically bound electron–hole pairs, namely, excitons are promptly generated in organic materials. As excitons cannot dissociate in themselves, they need to diffuse to a donor–acceptor (DA) hetero interface, at which they are dissociated as a result of the offset in molecular orbital energy levels. Thus, in-depth understanding of exciton diffusion dynamics is one of the most important issues to further improve device efficiency.

Here, we study diffusion dynamics of singlet excitons generated in a thin film of crystalline conjugated polymer PNOz4T, which is likely to form intrachain ordered J-aggregated crystalline structures. We measured femtosecond transient absorption under various temperatures from 294 to 78 K. Under low excitation fluences, excitons decayed monoexponentially because of radiative and non-radiative deactivations to the ground state. With increasing excitation fluence, excitons decayed faster as a result of singlet–singlet exciton annihilation (SSA). The decay of singlet exciton density including SSA is given by the following equation,

dn(t)/dt=-kn(t)-(1/2)γ(t)n²(t)

where n(t) is the exciton density at time t after the excitation, k is the intrinsic decay rate constant, and γ(t) is the rate coefficient of bimolecular deactivation due to SSA. As SSA is a diffusion-limited process, we can obtain diffusion coefficient D and the dimensionality of exciton diffusion from time-dependent bimolecular reaction rate coefficient γ(t). On the basis of these analyses, we found that two-dimensional diffusion is dominant in the crystalline domains of J-aggregated PNOz4T at room temperature while one-dimensional diffusion is dominant at low temperatures. These findings suggest that excitons in PNOz4T can diffuse in both intra- and inter-chain directions at room temperature, while the activation energies for those directions should be different. This is in sharp contrast to one-dimensional diffusion of excitons in P3HT films, where excitons preferentially diffuse to the π-stacking direction of H-aggregated P3HT crystalline domains even at room temperature because of a weak intrachain excitonic coupling [1]. In PNOz4T films at a low temperature, exciton diffusion to one of the directions would be suppressed because of larger activation energy, resulting one-dimensional diffusion. These findings suggest that the excitonic couplings and activation energy in intra- and inter-chain directions have impact on the direction and dimensionality of the singlet exciton diffusion.