TY  - JOUR
A1  - Collado-Fregoso, Elisa
A1  - Pugliese, Silvina N.
A1  - Wojcik, Mariusz
A1  - Benduhn, Johannes
A1  - Bar-Or, Eyal
A1  - Perdigon-Toro, Lorena
A1  - Hörmann, Ulrich
A1  - Spoltore, Donato
A1  - Vandewal, Koen
A1  - Hodgkiss, Justin M.
A1  - Neher, Dieter
T1  - Energy-gap law for photocurrent generation in fullerene-based organic solar cells
BT  - the case of low-donor-content blends
JF  - Journal of the American Chemical Society
N2  - 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 C-60 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.
Y1  - 2019
U6  - https://doi.org/10.1021/jacs.8b09820
SN  - 0002-7863
VL  - 141
IS  - 6
SP  - 2329
EP  - 2341
PB  - American Chemical Society
CY  - Washington
ER  -