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Energy-Gap Law for Photocurrent Generation in Fullerene-Based Organic Solar Cells: The Case of Low-Donor-Content Blends

Collado-Fregoso, Elisa, Pugliese, Silvina N., Wojcik, Mariusz, Benduhn, Johannes, Bar-Or, Eyal, Perdigón Toro, Lorena, Hörmann, Ulrich, Spoltore, Donato, Vandewal, Koen, Hodgkiss, Justin M., Neher, Dieter
Journal of the American Chemical Society 2019 v.141 no.6 pp. 2329-2341
dissociation, electric current, energy, field experimentation, fullerene, solar cells, spectroscopy
The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C₆₀ and small amounts of organic donor molecules. We find a pronounced decline of all photovoltaic parameters with decreasing CT state energy. Using a combination of steady-state photocurrent measurements and time-delayed collection field experiments, we demonstrate that the power conversion efficiency, and more specifically, the fill factor of these devices, is mainly determined by the bias dependence of photocurrent generation. By combining these findings with the results from ultrafast transient absorption spectroscopy, we show that blends with small CT energies perform poorly because of an increased nonradiative CT state decay rate and that this decay obeys an energy-gap law. Our work challenges the common view that a large energy offset at the heterojunction and/or the presence of fullerene clusters guarantee efficient CT dissociation and rather indicates that charge generation benefits from high CT state energies through a slower decay to the ground state.