Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

The role of exciton lifetime for charge generation in organic solar cells at negligible energy-level offsets

Nature Energy volume 5, pages 711–719 (2020)Cite this article

Subjects

Abstract

Organic solar cells utilize an energy-level offset to generate free charge carriers. Although a very small energy-level offset increases the open-circuit voltage, it remains unclear how exactly charge generation is affected. Here we investigate organic solar cell blends with highest occupied molecular orbital energy-level offsets (∆EHOMO) between the donor and acceptor that range from 0 to 300 meV. We demonstrate that exciton quenching at a negligible ∆EHOMO takes place on timescales that approach the exciton lifetime of the pristine materials, which drastically limits the external quantum efficiency. We quantitatively describe this finding via the Boltzmann stationary-state equilibrium between charge-transfer states and excitons and further reveal a long exciton lifetime to be decisive in maintaining an efficient charge generation at a negligible ∆EHOMO. Moreover, the Boltzmann equilibrium quantitatively describes the major reduction in non-radiative voltage losses at a very small ∆EHOMO. Ultimately, highly luminescent near-infrared emitters with very long exciton lifetimes are suggested to enable highly efficient organic solar cells.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Rent or buy this article

Get just this article for as long as you need it

$39.95

Learn more

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Donor and acceptor materials and their energy-level alignment.
Fig. 2: Exciton dynamics of the donor and acceptor materials and their blends.
Fig. 3: Exciton splitting efficiency and its correlation with external and internal quantum efficiencies.
Fig. 4: J–V characteristics, FTPS and EL of all the solar cells.
Fig. 5: Voltage loss analysis as a function of Eoffset.
Fig. 6: Experimental and analytic analysis of ΔVOC,non-rad.

Data availability

All data generated or analysed during this study are included in the published article and its Supplementary Information data files. Source data are provided with this paper.

References

  1. Green, M. A. et al. Solar cell efficiency tables (Version 53). Prog. Photovolt. 2, 3–12 (2019).

    Article  Google Scholar 

  2. Xu, X. et al. Single‐junction polymer solar cells with 16.35% efficiency enabled by a platinum(ii) complexation strategy. Adv. Mater. 31, 1901872 (2019).

    Article  Google Scholar 

  3. Liu, Z. et al. Open-circuit voltages exceeding 1.26 V in planar methylammonium lead iodide perovskite solar cells. ACS Energy Lett. 1, 110–117 (2018).

    Google Scholar 

  4. Polman, A., Knight, M., Garnett, E. C., Ehrler, B. & Sinke, W. C. Photovoltaic materials: present efficiencies and future challenges. Science 352, aad4424 (2016).

    Article  Google Scholar 

  5. Geisz, J. F., Steiner, M. A., García, I., Kurtz, S. R. & Friedman, D. J. Enhanced external radiative efficiency for 20.8% efficient single-junction GaInP solar cells. Appl. Phys. Lett. 103, 0–5 (2013).

    Article  Google Scholar 

  6. Miller, O. D., Yablonovitch, E. & Kurtz, S. R. Intense internal and external fluorescence as solar cell approach the SQ efficiency limit. IEEE J. Photovolt. 2, 1–27 (2012).

    Article  Google Scholar 

  7. Deibel, C. & Dyakonov, V. Polymer–fullerene bulk heterojunction solar cells. Rep. Prog. Phys. 73, 96401 (2010).

    Article  Google Scholar 

  8. Green, M. A. et al. Solar cell efficiency tables (version 50). Prog. Photovolt. Res. Appl. 25, 668–676 (2017).

    Article  Google Scholar 

  9. Zheng, Z. et al. Efficient charge transfer and fine-tuned energy level alignment in a THF-processed fullerene-free organic solar cell with 11.3% efficiency. Adv. Mater. 29, 1604241 (2017).

    Article  Google Scholar 

  10. Qian, D. et al. Design rules for minimizing voltage losses in high-efficiency organic solar cells. Nat. Mater. 17, 703–709 (2018).

    Article  Google Scholar 

  11. Eisner, F. et al. Hybridization of local exciton and charge-transfer states reduces non-radiative voltage losses in organic solar cells. J. Am. Chem. Soc. 141, 6362–6374 (2019).

    Article  Google Scholar 

  12. Holliday, S. et al. High-efficiency and air-stable P3HT-based polymer solar cells with a new non-fullerene acceptor. Nat. Commun. 7, 11585 (2016).

    Article  Google Scholar 

  13. Lin, Y. et al. An electron acceptor challenging fullerenes for efficient polymer solar cells. Adv. Mater. 27, 1170–1174 (2015).

    Article  Google Scholar 

  14. Yuan, J. et al. Single-junction organic solar cell with over 15% efficiency using fused-ring acceptor with electron-deficient core. Joule 3, 1–12 (2019).

    Article  Google Scholar 

  15. Fan, B. et al. Achieving over 16% efficiency for single-junction organic solar cells. Sci. China Chem. https://doi.org/10.1007/s11426-019-9457-5 (2019).

  16. Sweetnam, S. et al. Characterization of the polymer energy landscape in polymer:fullerene bulk heterojunctions with pure and mixed phases. J. Am. Chem. Soc. 136, 14078–14088 (2014).

    Article  Google Scholar 

  17. Dong, Y. et al. The binding energy and dynamics of charge-transfer states in organic photovoltaics with low driving force for charge separation. J. Chem. Phys. 150, 104704 (2019).

    Article  Google Scholar 

  18. Xie, Y. et al. Assessing the energy offset at the electron donor/acceptor interface in organic solar cells through radiative efficiency measurements. Energy Environ. Sci. 12, 3556–3566 (2019).

    Article  Google Scholar 

  19. Falke, S. M. et al. Coherent ultrafast charge transfer in an organic photovoltaic blend. Science 344, 1001–1005 (2014).

    Article  Google Scholar 

  20. Howard, I. A., Mauer, R., Meister, M. & Laquai, F. Effect of morphology on ultrafast free carrier generation in polythiophene:fullerene organic solar cells. J. Am. Chem. Soc. 132, 14866–14876 (2010).

    Article  Google Scholar 

  21. Ziffer, M. E. et al. Long-lived, non-geminate, radiative recombination of photogenerated charges in a polymer/small-molecule acceptor photovoltaic blend. J. Am. Chem. Soc. 140, 9996–10008 (2018).

    Article  Google Scholar 

  22. Gélinas, S. et al. Ultrafast long-range charge separation in organic semiconductor photovoltaic diodes. Science 343, 512–516 (2014).

    Article  Google Scholar 

  23. Vandewal, K., Benduhn, J. & Nikolis, V. C. How to determine optical gaps and voltage losses in organic photovoltaic materials. Sustain. Energy Fuels 2, 538–544 (2018).

    Article  Google Scholar 

  24. Rau, U., Blank, B., Müller, T. C. M. & Kirchartz, T. Efficiency potential of photovoltaic materials and devices unveiled by detailed-balance analysis. Phys. Rev. Appl. 7, 1–9 (2017).

    Article  Google Scholar 

  25. Classen, A. et al. Absence of charge transfer state enables very low VOC losses in SWCNT:fullerene solar cells. Adv. Energy Mater. 9, 1801913 (2018).

    Article  Google Scholar 

  26. Kirchartz, T. & Rau, U. Detailed balance and reciprocity in solar cells. Phys. Satus Solidi Mater. Sci. 205, 2737–2751 (2008).

    Article  Google Scholar 

  27. Rau, U. Reciprocity relation between photovoltaic quantum efficiency and electroluminescent emission of solar cells. Phys. Rev. B 76, 085303 (2007).

    Article  Google Scholar 

  28. Benduhn, J. et al. Intrinsic non-radiative voltage losses in fullerene-based organic solar cells. Nat. Energy 2, 17053 (2017).

    Article  Google Scholar 

  29. Heiber, M. C., Baumbach, C., Dyakonov, V. & Deibel, C. Encounter-limited charge-carrier recombination in phase-separated organic semiconductor blends. Phys. Rev. Lett. 114, 136602 (2015).

    Article  Google Scholar 

  30. Lopez, S. A., Sanchez-Lengeling, B., de Goes Soares, J. & Aspuru-Guzik, A. Design principles and top non-fullerene acceptor candidates for organic photovoltaics. Joule 1, 857–870 (2017).

    Article  Google Scholar 

  31. Burkhard, G. F., Hoke, E. T. & McGehee, M. D. Accounting for interference, scattering, and electrode absorption to make accurate internal quantum efficiency measurements in organic and other thin solar cells. Adv. Mater. 22, 3293–3297 (2010).

    Article  Google Scholar 

  32. Bronstein, H. et al. Indacenodithiophene-co-benzothiadiazole copolymers for high performance solar cells or transistors via alkyl chain optimization. Macromolecules 44, 6649–6652 (2011).

    Article  Google Scholar 

  33. Biniek, L. et al. Optimization of the side-chain density to improve the charge transport and photovoltaic performances of a low band gap copolymer. Org. Electron. 13, 114–120 (2012).

    Article  Google Scholar 

  34. Biniek, L. et al. 3,6-Dialkylthieno[3,2-b]thiophene moiety as a soluble and electron donating unit preserving the coplanarity of photovoltaic low band gap copolymers. J. Polym. Sci. A 50, 1861–1868 (2012).

    Article  Google Scholar 

  35. Risko, C., McGehee, M. D. & Brédas, J. L. A quantum-chemical perspective into low optical-gap polymers for highly-efficient organic solar cells. Chem. Sci. 2, 1200–1218 (2011).

    Article  Google Scholar 

  36. Ku, J., Lansac, Y. & Jang, Y. H. Time-dependent density functional theory study on benzothiadiazole-based low-band-gap fused-ring copolymers for organic solar cell applications. J. Phys. Chem. C 115, 21508–21516 (2011).

    Article  Google Scholar 

  37. Chochos, C. L. et al. Experimental and theoretical investigations on the optical and electrochemical properties of π-conjugated donor–acceptor–donor (DAD) compounds toward a universal model. J. Chem. Phys. 149, 124902 (2018).

    Article  Google Scholar 

Download references

Acknowledgements

A.C. and C.J.B. gratefully acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), Project no. 182849149-SFB 953. C.J.B. gratefully acknowledges financial support through the ‘Aufbruch Bayern’ initiative of the state of Bavaria (EnCN and SFF) and the Bavarian Initiative ‘Solar Technologies go Hybrid’ (SolTech) and funding from DFG project DFG INST 90/917. C.C.L. thanks the European Union for the financial support. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreements no. 761112 (PRESTIGE) and no. 820789 (OLEDSOLAR).

Author information

Authors and Affiliations

  1. Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nürnberg, Erlangen, Germany

    Andrej Classen, Larry Lüer, Jonas Wortmann, Andres Osvet, Karen Forberich, Thomas Heumüller & Christoph J. Brabec

  2. Advent Technologies SA, Platani, Greece

    Christos L. Chochos & Vasilis G. Gregoriou

  3. Institute of Chemical Biology, National Hellenic Research Foundation (NHRF), Athens, Greece

    Christos L. Chochos

  4. National Hellenic Research Foundation (NHRF), Athens, Greece

    Vasilis G. Gregoriou

  5. Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford, UK

    Iain McCulloch

  6. KAUST Solar Center (KSC), King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia

    Iain McCulloch

  7. Helmholtz-Institute Erlangen-Nürnberg (HI ERN), Erlangen, Germany

    Thomas Heumüller & Christoph J. Brabec

  8. Bavarian Center for Applied Energy Research (ZAE Bayern), Erlangen, Germany

    Christoph J. Brabec

Authors
  1. Andrej Classen
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Christos L. Chochos
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Larry Lüer
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. Vasilis G. Gregoriou
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. Jonas Wortmann
    View author publications

    You can also search for this author in PubMed Google Scholar

  6. Andres Osvet
    View author publications

    You can also search for this author in PubMed Google Scholar

  7. Karen Forberich
    View author publications

    You can also search for this author in PubMed Google Scholar

  8. Iain McCulloch
    View author publications

    You can also search for this author in PubMed Google Scholar

  9. Thomas Heumüller
    View author publications

    You can also search for this author in PubMed Google Scholar

  10. Christoph J. Brabec
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

T.H. and C.J.B. conceived the idea for this study and supervised the work. A.C. fabricated all the measured devices and samples, performed J–V characterizations, EQE measurements, FTPS spectroscopy, CV measurements and absorbance measurements. C.L.C. and V.G.G. synthesized the polymers used in this work and performed the DFT calculations. A.O. and A.C. performed the TRPL measurements. J.W. and A.C. performed the steady-state PL and EL measurements. K.F. performed the optical transfer-matrix calculation of IQE. L.L. developed the effective two-states model and utilized this theoretical framework to fit the experimental data. T.H., I.M., C.J.B. and A.C. evaluated and interpreted the experimental findings. The manuscript was written and commented on by all the authors.

Corresponding authors

Correspondence to Larry Lüer, Thomas Heumüller or Christoph J. Brabec.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Figs. 1–38, Tables 1–5, Notes 1–4 and refs. 1–9.

Reporting Summary

Supplementary Data 1

Supplementary Data for Supplementary Table 3

Source data

Source Data Fig. 2

Source Data Fig. 2.

Source Data Fig. 3

Source Data Fig. 3.

Source Data Fig. 4

Source Data Fig. 4.

Source Data Fig. 5

Source Data Fig. 5.

Source Data Fig. 6

Source Data Fig. 6.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Classen, A., Chochos, C.L., Lüer, L. et al. The role of exciton lifetime for charge generation in organic solar cells at negligible energy-level offsets. Nat Energy 5, 711–719 (2020). https://doi.org/10.1038/s41560-020-00684-7

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41560-020-00684-7

This article is cited by

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing