Fast charge separation in a non-fullerene organic solar cell with a small driving force

Abstract

Fast and efficient charge separation is essential to achieve high power conversion efficiency in organic solar cells (OSCs). In state-of-the-art OSCs, this is usually achieved by a significant driving force, defined as the offset between the bandgap (Egap) of the donor/acceptor materials and the energy of the charge transfer (CT) state (ECT), which is typically greater than 0.3 eV. The large driving force causes a relatively large voltage loss that hinders performance. Here, we report non-fullerene OSCs that exhibit ultrafast and efficient charge separation despite a negligible driving force, as ECT is nearly identical to Egap. Moreover, the small driving force is found to have minimal detrimental effects on charge transfer dynamics of the OSCs. We demonstrate a non-fullerene OSC with 9.5% efficiency and nearly 90% internal quantum efficiency despite a low voltage loss of 0.61 V. This creates a path towards highly efficient OSCs with a low voltage loss.

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Figure 1: Chemical structures and photovoltaic performance.
Figure 2: Optical and electrical characterizations.
Figure 3: Transient absorption data.
Figure 4: EQEEL spectra.

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Acknowledgements

The work described in this paper was partially supported by the National Basic Research Program of China (973 Program; 2013CB834701 and 2014CB643501), the Hong Kong Research Grants Council (T23–407/13 N, N_HKUST623/13, and 606012), HK JEBN Limited, and the National Science Foundation of China (nos 21374090 and 51361165301). X-ray characterization by NCSU was supported by the Office of Naval Research (award nos N000141410531 and N000141512322). Ultrafast spectroscopy work at NCSU was supported by Office of Naval Research (award no N000141310526 P00002). X-ray data were acquired at beamline 7.3.3 and 11.0.1.2 at the Advanced Light Source, which was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02–05CH11231. The research at Linköping is financially supported by the Swedish Research Councils (VR, grant no. 330-2014-6433 and FORMAS, grant no. 942-2015-1253), the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (faculty grant SFO-Mat-LiU # 2009-00971), the European Commission Marie Skłodowska-Curie actions (grant nos 691210 and INCA 600398), and a Wallenberg Scholar grant to O.I.

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Contributions

J.L. and S.C. contributed equally to this work. J.L. synthesized the polymer and carried out the cyclic voltammetry, atomic force microscopy, transmission electron microscopy and PL characterizations; S.C. fabricated the solar cell devices and carried out ultraviolet measurements; D.Q. performed the FTPS and EL experiments supervised by F.G.; G.Y. analysed the GIWAXS and R-SoXS data supervised by W.M.; B.G. performed the TAS experiments supervised by K.G.; J.B. carried out the IQE measurement; J.Z., F.Z., H.A. and O.I. helped in the data interpretation. H.Y., F.G., J.L., S.C. and J.Z. wrote the manuscript; H.Y. conceived and directed the project; all authors discussed the results and commented on the final manuscript.

Corresponding authors

Correspondence to Kenan Gundogdu or Feng Gao or He Yan.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

Supplementary Note 1, Supplementary Figures 1–35, Supplementary Tables 1–3, Supplementary Methods, Supplementary References. (PDF 2882 kb)

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Liu, J., Chen, S., Qian, D. et al. Fast charge separation in a non-fullerene organic solar cell with a small driving force. Nat Energy 1, 16089 (2016). https://doi.org/10.1038/nenergy.2016.89

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