Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers

Zhong, Yufei; Causa', Martina; Moore, Gareth John; Krauspe, Philipp; Xiao, Bo; Günther, Florian; Kublitski, Jonas; Shivhare, Rishi; Benduhn, Johannes; BarOr, Eyal; Mukherjee, Subhrangsu; Yallum, Kaila M.; Réhault, Julien; Mannsfeld, Stefan C. B.; Neher, Dieter; Richter, Lee J.; DeLongchamp, Dean M.; Ortmann, Frank; Vandewal, Koen; Zhou, Erjun; ... (2020). Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers. Nature Communications, 11(1) Springer Nature 10.1038/s41467-020-14549-w

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Organic photovoltaics based on non-fullerene acceptors (NFAs) show record efficiency of 16 to 17% and increased photovoltage owing to the low driving force for interfacial charge- transfer. However, the low driving force potentially slows down charge generation, leading to a tradeoff between voltage and current. Here, we disentangle the intrinsic charge-transfer rates from morphology-dependent exciton diffusion for a series of polymer:NFA systems. Moreover, we establish the influence of the interfacial energetics on the electron and hole transfer rates separately. We demonstrate that charge-transfer timescales remain at a few hundred femtoseconds even at near-zero driving force, which is consistent with the rates predicted by Marcus theory in the normal region, at moderate electronic coupling and at low re-organization energy. Thus, in the design of highly efficient devices, the energy offset at the donor:acceptor interface can be minimized without jeopardizing the charge-transfer rate and without concerns about a current-voltage tradeoff.

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