Rethinking Charge Transport and Recombination in Donor-Diluted Organic Solar Cells

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Wang, Chen ; Wöpke, Christopher ; Seiler, Toni ; Faisst, Jared ; List, Mathias ; Kuhn, Meike ; Joseph, Bekcy ; Ehm, Alexander ; Zahn, Dietrich R. T. ; Vaynzof, Yana ; Herzig, Eva M. ; Mackenzie, Roderick C. I. ; Würfel, Uli ; Saladina, Maria ; Deibel, Carsten:
Rethinking Charge Transport and Recombination in Donor-Diluted Organic Solar Cells.
In: Advanced Materials. (2026) . - e23681.
ISSN 1521-4095
DOI: https://doi.org/10.1002/adma.202523681

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Abstract

We systematically investigate PM6:Y12 bulk-heterojunction solar cells with donor fractions ranging from 1% to 45%, linking morphology, charge transport, and recombination to device performance. Complementary structural and spectroscopic methods reveal that a percolating PM6 network forms even at below 5% donor content, with lamellar stacking and vertical composition gradients that do not hinder the charge extraction. The reduction of the effective active layer conductivity toward low donor fractions obeys a three-dimensional percolation model, indicating that charge transport is governed by network topology rather without a pronounced percolation threshold. A transition from nongeminate Langevin recombination to a dispersive Smoluchowski-type loss occurs below 5% donor fraction. The latter regime is also nongeminate, i.e., pertains to recombination of the total charge carrier density. Correspondingly, we observe that the Langevin reduction in the higher donor fractions – mostly dominated by redissociation of electron–hole pairs after encounter – changes toward low donor fractions: in these cases, the nongeminate loss rate exceeds the prediction of the Langevin model. This regime coincides with increasing transport resistance due to topology-limited hole conduction, leading to reduced fill factors despite a high retained charge-generation efficiency. Our results demonstrate that strong donor dilution preserves photogeneration if a continuous donor network is maintained, and unveil how topology-controlled transport and non-Langevin recombination jointly define the performance limits of donor-diluted organic solar blends.