In bulk heterojunction (BHJ) organic solar cells (OSCs) both the electron affinity (EA) and ionization energy (IE) offsets at the donor–acceptor interface should equally control exciton dissociation. Here, we demonstrate that in low-bandgap non-fullerene acceptor (NFA) BHJs ultrafast donor-to-acceptor energy transfer precedes hole transfer from the acceptor to the donor and thus renders the EA offset virtually unimportant. Moreover, sizeable bulk IE offsets of about 0.5 eV are needed for efficient charge transfer and high internal quantum efficiencies, since energy level bending at the donor–NFA interface caused by the acceptors’ quadrupole moments prevents efficient exciton-to-charge-transfer state conversion at low IE offsets. The same bending, however, is the origin of the barrier-less charge transfer state to free charge conversion. Our results provide a comprehensive picture of the photophysics of NFA-based blends, and show that sizeable bulk IE offsets are essential to design efficient BHJ OSCs based on low-bandgap NFAs.
|Original language||English (US)|
|State||Published - Oct 26 2020|
Bibliographical noteKAUST Repository Item: Exported on 2020-10-29
Acknowledged KAUST grant number(s): CRG, OSR-2018-CARF/CCF-3079
Acknowledgements: This publication is based on work supported by the KAUST Office of Sponsored Research (OSR) under award nos. OSR-2018-CARF/CCF-3079 and OSR-CRG2018-3746. D.A. acknowledges funding from the BMBF grant InterPhase and MESOMERIE (grant nos. FKZ 13N13661, FKZ 13N13656) and the European Union Horizon 2020 research and innovation program ‘Widening materials models’ under grant agreement no. 646259 (MOSTOPHOS). D.A. also acknowledges the KAUST PSE Division for hosting his sabbatical in the framework of the Division’s Visiting Faculty program. A.M. acknowledges funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. 844655 (SMOLAC). We thank L. Sinatra of KAUST and Quantum Solutions LLC for assisting with the PLQY measurements. G.T.H acknowledges K. Graham and A. Amassian (and previous group members including M. Tietze and G.O.N. Ndjawa) for having designed and installed and worked on the IPES setup. In particular, G.T.H. acknowledges K. Graham’s kind assistance during the reconfiguration and optimization of the IPES setup, as well as U. Sharif for technical assistance.