Spectroscopic insights into charge-carrier dynamics modulated by donor-acceptor molecular distribution in organic photovoltaic materials

Understanding the relationship between molecular distribution and charge-carrier dynamics is crucial for optimizing organic photovoltaic (OPV) performance. In this study, we employed advanced spectroscopic techniques, including flash-photolysis time-resolved microwave conductivity (FP-TRMC) to probe free-charge dynamics. Utilizing a model electron donor–acceptor dyad (P3HT–NDI), where poly(3-hexylthiophene) (P3HT) is covalently linked to naphthalene diimide (NDI), and a blended system (P3HT/NDI), we investigated the impact of acceptor distribution uniformity on the charge-carrier dynamics. Electronic absorption spectroscopy revealed that P3HT–NDI thin films achieve uniform NDI distribution, suppressing aggregation, while P3HT/NDI blends exhibit heterogeneous distribution with significant NDI clustering even under a similar NDI loading. FP-TRMC measurements upon photoexcitation of P3HT domains showed comparable free charge generation efficiencies (∼3.40 % for P3HT–NDI and ∼ 3.09 % for P3HT/NDI). However, the P3HT/NDI blend exhibited prolonged charge lifetimes (∼570 ns) than the dyad system (∼435 ns), attributed to NDI clustering, which mitigates charge recombination. By correlating spectroscopic and morphological data, we demonstrate the critical role of molecular distribution in governing charge dynamics, offering insights into designing high-performance OPVs with controlled interfacial properties.