Intrinsic role of alkyl side chains in disorder, aggregates, and carrier mobility of nonfullerene acceptors for organic solar cells: A multiscale theoretical study

Abstract

Modifications to the alkyl side chains of Y6-type nonfullerene acceptors (NFAs) continuously break through the organic solar cells (OSCs) efficiency by enhancing electron mobility. However, the role of side chains in molecular aggregation and charge transport across different aggregates remains unclear. By employing a multiscale approach in combination with density functional theory (DFT), molecular dynamics (MD) simulations, and kinetic Monte Carlo (KMC), we addressed the issue of how side chains impact molecular aggregation, energy disorder, and the formation of near-macroscopic (∼0.3 µm) conductive network, which are critical for boosting electron mobility. Specifically, the side-chain structure greatly influences the un-conjugated enveloping effect on backbones within aggregates. The effect diminishes with longer linear side chains and is further minimized by using branched side chains. Though static energy disorder increased, the improved connectivity of the conductive network led to a notable increase in electron mobility (from 2.4 × 10⁻⁴ to 3.9 × 10⁻⁴ cm²·V⁻¹·s⁻¹). The findings offer insight into controlling molecular aggregation via alkyl side chains, which helps to further unlock the potential of Y6-type NFAs.