Characterization of anisotropic charge carrier mobility in PM6:Y6 blend film using time-resolved terahertz spectroscopy

In organic photovoltaic (OPV) devices, charge transport from donor–acceptor interfaces to electrodes is crucial for efficient photon-to-current conversion. In state-of-the-art OPV systems featuring non-fullerene acceptors (NFAs), the face-on packing of planar NFAs can result in anisotropic charge transport mobilities in the in-plane and out-of-plane directions. Nevertheless, compared to the extensive research focusing on in-plane charge mobility using time-resolved terahertz spectroscopy (TRTS), the more critical out-of-plane charge mobility has rarely been explored due to technical limitations. Here, we utilized a high-sensitivity TRTS detection system that varies the angle of incident light to examine the mobility differences in different directions within a model system of a PM6:Y6 film. Our findings indicate that at an optimized annealing temperature of 110 °C, the out-of-plane mobility reaches 2.4 cm2 V−1 s−1, approximately six times that of the in-plane mobility, which is 0.41 cm2 V−1 s−1. This substantial anisotropy necessitates a combination of molecular-level π–π stacking and optimal domain morphology. These results provide key evidence for optimizing molecular orientation to establish favorable charge transport pathways for better-performing OPV devices.