Explaining the Fill-Factor and Photocurrent Losses of Nonfullerene Acceptor-Based Solar Cells by Probing the Long-Range Charge Carrier Diffusion and Drift Lengths

Nurlan Tokmoldin, Joachim Vollbrecht, Seyed Mehrdad Hosseini, Bowen Sun, Lorena Perdigón-Toro, Han Young Woo, Yingping Zou, Dieter Neher, Safa Shoaee

Research output: Contribution to journalArticlepeer-review

24 Citations (Scopus)


Organic solar cells (OSC) nowadays match their inorganic competitors in terms of current production but lag behind with regards to their open-circuit voltage loss and fill-factor, with state-of-the-art OSCs rarely displaying fill-factor of 80% and above. The fill-factor of transport-limited solar cells, including organic photovoltaic devices, is affected by material and device-specific parameters, whose combination is represented in terms of the established figures of merit, such as θ and α. Herein, it is demonstrated that these figures of merit are closely related to the long-range carrier drift and diffusion lengths. Further, a simple approach is presented to devise these characteristic lengths using steady-state photoconductance measurements. This yields a straightforward way of determining θ and α in complete cells and under operating conditions. This approach is applied to a variety of photovoltaic devices—including the high efficiency nonfullerene acceptor blends—and show that the diffusion length of the free carriers provides a good correlation with the fill-factor. It is, finally, concluded that most state-of-the-art organic solar cells exhibit a sufficiently large drift length to guarantee efficient charge extraction at short circuit, but that they still suffer from too small diffusion lengths of photogenerated carriers limiting their fill factor.

Original languageEnglish
Article number2100804
JournalAdvanced Energy Materials
Issue number22
Publication statusPublished - 2021 Jun 10

Bibliographical note

Funding Information:
The authors thank Jan Anton Koster for sharing the original drift‐diffusion simulation results and providing invaluable support in the preparation of the manuscript. The authors express the gratitude to the Alexander von Humboldt Foundation for funding. The authors also acknowledge financial support by the German Research Foundation, project numbers SH 1660/1‐1 and NE 410/20‐1. Y.Z. acknowledges the support from the National Natural Science Foundation of China (21875286). H.Y.W. acknowledges the support from the National Research Foundation (NRF) of Korea (NRF2020M3H4A3081814 and 2019R1A6A1A11044070).

Publisher Copyright:
© 2021 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH


  • diffusion length
  • drift length
  • figure of merit
  • lifetime-mobility product
  • steady-state photoconductance

ASJC Scopus subject areas

  • Renewable Energy, Sustainability and the Environment
  • General Materials Science


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