Molecular Packing in the Active Layers of Organic Solar Cells Based on Non-Fullerene Acceptors: Impact of Isomerization on Charge Transport, Exciton Dissociation, and Nonradiative Recombination

The nature of the molecular packing within the active layer has a significant impact on the performance of organic solar cells. In pure phases, the packing configurations modulate the electronic properties and energy differences between states that govern charge-transport rates. At donor and acceptor interfaces, the molecular configurations control the electronic couplings and charge-transfer state energies associated with exciton dissociation and charge recombination. Here, we determine the impact of isomerization in the nonfullerene acceptors FNIC1 and FNIC2, using a combination of density functional theory calculations and molecular dynamic simulations. We compare the molecular packings found in the bulk heterojunctions with the polymer donor PTB7-Th (PTB7-Th:FNIC1 and PTB7-Th:FNIC2) and correlate their configurations with the electronic properties and essential rate-dependent processes. We also consider a fullerene-based system with the same polymer donor (PTB7-Th:PC71BM) to assess the fundamental differences between fullerene and nonfullerene acceptors in terms of these properties. We demonstrate several factors that contribute to the experimentally observed power conversion efficiencies in these systems. Importantly, the power conversion efficiencies in some of these blends could be enhanced via better control of the packing configurations.