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

TL;DR

This study investigates how donor dilution affects morphology, charge transport, and recombination in organic solar cells, revealing that a continuous donor network is crucial for maintaining efficiency despite reduced conductivity.

Contribution

It provides a detailed understanding of the interplay between morphology, charge transport, and recombination in donor-diluted organic solar cells, highlighting the importance of network topology.

Findings

Percolating donor networks form below 5% donor content.

Charge transport follows a 3D percolation model without a sharp threshold.

Recombination transitions from Langevin to dispersive regimes at low donor fractions.

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 towards 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 towards 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.