A Nanoscopic View of Photoinduced Charge Transfer in Organic Nanocrystalline Heterojunctions

Organic photovoltaics enable cost-efficient, tunable, and flexible platforms for solar energy conversion, yet their performance and stability are still far from optimal. Here, we present a study of photoinduced charge transfer processes between electron donor and acceptor organic nanocrystals as part of a pathfinding effort to develop robust and efficient organic nanocrystalline materials for photovoltaic applications. For this purpose, we utilized nanocrystals of perylenediimides as the electron acceptors and nanocrystalline copper phthalocyanine as the electron donor. Three different configurations of donor-acceptor heterojunctions were prepared. Charge transfer in the heterojunctions was studied with Kelvin probe force microscopy under laser or white light excitation. Moreover, detailed morphology characterizations and time-resolved photoluminescence measurements were conducted to understand the differences in the photovoltaic processes of these organic nanocrystals. Our work demonstrates that excitonic properties can be tuned by controlling the crystal and interface structures in the nanocrystalline heterojunctions, leading to a minimization of photovoltaic losses.