Unveiling additive effects on molecular packing and charge transfer in organic solar cells: an AIMD and DFT study

Additive assisted strategies play a crucial role in optimizing the morphology and improving the performance of organic solar cells (OSCs), yet the molecular-level mechanisms remain unclear. Here, we employ ab initio molecular dynamics (AIMD) and density functional theory (DFT) to elucidate the influence of typical additives of 1,8-diiodooctane (DIO) and 3,5-dichlorobromobenzene (DCBB) on molecular packing, electronic structures, and charge transport. It can be observed that both additives can enhance the stacking properties of the donor and acceptor materials, yet they have different effects on the local electrostatic environment. In excited-state electronic structures, DCBB induces more localized and directional charge-transfer pathways, whereas DIO facilitates delocalized transitions. In addition, DCBB with a larger HOMO-LUMO gap exerts stronger regulation of interfacial electronic interactions. Moreover, as the additive approaches the donor and acceptor, the charge transfer rate increases significantly, especially for the DCBB, benefiting from hydrogen bonding between DCBB and the acceptor. As a result, DIO reduces recombination energy through improved packing, whereas DCBB stabilizes excited states and enhances intramolecular charge transfer, resulting in higher open circuit voltages (V_OC). This work provides fundamental insights into additive-material interactions, offering theoretical guidance for the rational design of high-performance additives in OSCs.