The Binding Energy and Dynamics of Charge Transfer States in Non-fullerene Organic Solar Cells
Yifan Dong a, Hyojung Cha a, Jiangbin Zhang b, Ernest Pastor a, Pabitra Shakya Tuladhar a, Artem Bakulin a, James Durrant a
a Department of Chemistry and Centre for Processable Electronics, Imperial College London, White City Campus, London, W12 0BZ, UK, United Kingdom
b Optoelectronics Group, Cavendish Laboratory, University of Cambridge, UK., J.J. Thomson Avenue, Cambridge, United Kingdom
Proceedings of Online International Conference on Hybrid and Organic Photovoltaics (OnlineHOPV20)
Online, Spain, 2020 May 26th - 29th
Organizers: Tracey Clarke, James Durrant, Annamaria Petrozza and Trystan Watson
Poster, Yifan Dong, 125
Publication date: 22nd May 2020
ePoster: 

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, I will present 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 the kinetics of the donor and acceptor excitons, respectively. The extracted kinetics suggest that slow (15 ps) generation of charge-transfer states (CTS) is followed by significant geminate recombination. 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 additional activation energy of 14 ± 7 meV, which is assign to the de-trapping of mobile carriers. These results provide a comprehensive picture of photophysics in a system representing the new generation of OPV blends with a small driving force for charge separation.

I would like to thank the Optoelectronics Group at the University of Cambridge for sharing the Global analysis codes. 

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