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

Abstract

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.