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

Nature Energy volume 1, Article number: 16089 (2016) Cite this article

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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.

Bulk heterojunction (BHJ) OSCs are promising for the realization of low-cost solar energy conversion due to their attractive properties of light weight, mechanical flexibility and roll-to-roll processability method1,2,3,4,5,6,7. One of the most important factors that limit the efficiency of OSCs is the relatively large voltage loss from the bandgap Egap of the absorber to the open-circuit voltage (Voc) of the cell8. For example, the most efficient OSC reported to date has a Voc of 0.77 V for an Egap of 1.65 eV, indicating a voltage loss of 0.88 V (ref. 6). In contrast, state-of-the-art c-Si or perovskite cells have voltage losses in the range of 0.40–0.55 V (ref. 8). The large voltage loss in high-efficiency OSCs is due to two main factors. One is the relatively large non-radiative recombination loss in OSCs, evidenced by extremely low electroluminescence quantum efficiency (EQEEL) of OSC blends (typically in the range of 10−6–10−8)9. The other is the existence of a significant offset between the bandgap of the donor/acceptor materials and the energy of the CT state (Egap − ECT)10,11,12,13,14. The energy offset is the driving force of charge separation in OSC. Historically, it was believed that a significant driving force is necessary to achieve fast and efficient charge separation in OSCs. There have been several attempts to reduce the driving force and thus the voltage loss in state-of-the-art OSCs15,16,17,18,19,20,21,22,23. Most of these attempts resulted in significantly reduced external quantum efficiency (EQE) and power conversion efficiency (PCE) for the solar cell devices. Among these attempts, the best result was achieved by a polymer named PNOz4T that yield a Voc of 0.96 V and a high efficiency of 8.9%23. In that case, the charge separation kinetics was relatively slow (100 ps) and the photoluminescence (PL) quenching was about 66%, which was attributed to large domain size. The existence of a significant driving force in state-of-the-art OSCs creates a problematic trade-off between the Voc and short-circuit current density (Jsc) of the OSCs, limiting the maximum achievable efficiency for OSCs.

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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.

Author information

Author notes

  1. Jing Liu and Shangshang Chen: These authors contributed equally to this work.

Authors and Affiliations

  1. Department of Chemistry and Energy Institute, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong

    Jing Liu, Shangshang Chen, Guofang Yang, Jingbo Zhao & He Yan

  2. Department of Physics Chemistry and Biology (IFM), Linköping University, Linköping SE-581 83, Sweden

    Deping Qian, Jonas Bergqvist, Fengling Zhang, Olle Inganäs & Feng Gao

  3. Department of Physics and Organic and Carbon Electronics Laboratory, North Carolina State University, Raleigh, North Carolina 27695, USA

    Bhoj Gautam, Harald Ade & Kenan Gundogdu

  4. State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China

    Guofang Yang & Wei Ma

  5. The Hong Kong University of Science and Technology-Shenzhen Research Institute No. 9, Yuexing 1st Road, Hi-tech Park, Nanshan Shenzhen 518057, China

    He Yan

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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, Feng Gao or He Yan.

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The authors declare no competing financial interests.

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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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