Effective Conductivity in Organic Solar Cells is Governed by the Harmonic Mean

Chen Wang^a, Doyoung Sun^b^,^c, Safa Shoaee^b^,^c, Maria Saladina^a, Carsten Deibel^a

^aInstitut für Physik, Technische Universität Chemnitz, 09126 Chemnitz, Germany

^bInstitute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Str.24-25, D-14476 Potsdam-Golm, Germany

^cHeterostructure Semiconductor Physics, Paul Drude Institute for Solid State Electronics, Hausvogteiplatz 5-7, 10117 Berlin, Germany

Proceedings of MATSUS Spring 2026 Conference (MATSUSSpring26)

B4 Photophysics of organic semiconductors

Barcelona, Spain, 2026 March 23rd - 27th

Organizers: Safakath Karuthedath and Jafar Khan

Oral, Chen Wang, presentation 310
Publication date: 15th December 2025

Charge transport in organic solar cells is inherently limited by the low conductivity typical for disordered semiconductors. As such, transport resistance represents a key loss mechanism, particularly for the fill factor (FF).^[1-3] While it is known that both electrons and holes contribute to this limitation, the appropriate formalism to describe their combined effect – by a harmonic or geometric mean – remains debated.
We experimentally address this issue by studying PM6:Y12 blends with systematically varied donor content and temperature. Using single-carrier devices, we extract the individual electron and hole conductivities. From light-intensity-dependent current–voltage and open-circuit voltage measurements, we independently determine the effective conductivity under operating conditions using our recently proposed method.^[4]
Our results show that the effective conductivity follows the harmonic mean of electron and hole conductivities across nearly three orders of magnitude in transport asymmetry. This trend holds across compositional and thermal variations, confirming that the slower carrier dominates charge extraction. The same harmonic relationship also applies to the effective mobility derived from space-charge limited current and resistance-dependent photovoltage measurements.
These findings provide an experimentally grounded framework for analyzing charge transport in organic semiconductors, challenging the widespread use of geometric mean models. Understanding and correctly modeling transport losses is essential for identifying the rate-limiting species and optimizing material design. The harmonic mean emerges as the correct physical descriptor for transport-limited performance in low-mobility systems.

References:

[1] Würfel, U.; Neher, D.; Spies, A.; Albrecht, S. Impact of charge transport on current–voltage characteristics and power conversion efficiency of organic solar cells. Nature Communications 2015, 6, 6951.

[2] Neher, D.; Kniepert, J.; Elimelech, A.; Koster, L. J. A. A New Figure of Merit for Organic Solar Cells with Transport-limited Photocurrents. Sci. Rep. 2016, 6, 24861.

[3] Wang, C.; MacKenzie, R. C.; Würfel, U.; Neher, D.; Kirchartz, T.; Deibel, C.; Saladina, M. Transport resistance dominates the fill factor losses in record organic solar cells. Adv. Energy Mater. 2025, 2405889.

[4] Saladina, M.; Deibel, C. Transport resistance strikes back: unveiling its impact on fill factor losses in organic solar cells. Rep. Prog. Phys. 2025, 88, 038001.

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