The binding energy and dynamics of charge-transfer states in organic photovoltaics with low driving force for charge separation

Yifan Dong, Hyojung Cha, Jiangbin Zhang, Ernest Pastor, Pabitra Shakya Tuladhar, Iain McCulloch, James R. Durrant, Artem A. Bakulin

Research output: Contribution to journalArticlepeer-review

29 Scopus citations

Abstract

Recent progress in organic photovoltaics (OPVs) has been enabled by optimization of the energetic driving force for charge separation, and thus maximization of open-circuit voltage, using non-fullerene acceptor (NFA) materials. In spite of this, the carrier dynamics and relative energies of the key states controlling the photophysics of these systems are still under debate. Herein, we report an in-depth ultrafast spectroscopic study of a representative OPV system based on a polymer donor PffBT4T-2OD and a small-molecule NFA EH-IDTBR. Global analysis of the transient absorption data reveals efficient energy transfer between donor and acceptor molecules. The extracted kinetics suggest that slow (∼15 ps) generation of charge carriers is followed by significant geminate recombination. This contrasts with the “reference” PffBT4T-2OD:PC71BM system where bimolecular recombination dominates. Using temperature-dependent pump-push-photocurrent spectroscopy, we estimate the activation energy for the dissociation of bound charge-transfer states in PffBT4T-2OD:EH-IDTBR to be 100 ± 6 meV. We also observe an additional activation energy of 14 ± 7 meV, which we assign to the de-trapping of mobile carriers. This work provides a comprehensive picture of photophysics in a system representing new generation of OPV blends with a small driving force for charge separation.
Original languageEnglish (US)
Pages (from-to)104704
JournalThe Journal of Chemical Physics
Volume150
Issue number10
DOIs
StatePublished - Mar 11 2019

Bibliographical note

KAUST Repository Item: Exported on 2020-10-01
Acknowledgements: The authors would like to thank Tom Hopper, Dr. Robert Godin, and Dr. Wenxing Yang for discussions. The authors would also like to thank Tom Hopper for proofreading the manuscript and Ahmad Alraddadi for TEM. The authors acknowledge the Optoelectronics Group in the University of Cambridge for sharing the software for global analysis. We thank the Welsh government funded Sêr Solar project and the UKRI Global Challenge Research Fund project SUNRISE (EP/P032591/1) for funding. A.A.B. is a Royal Society University Research Fellow. J.Z. thanks the China Scholarship Council for a Ph.D. scholarship (No. 201503170255).

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