Solid Additives Promote Side-Chain Ordering and Molecular Packing To Enhance the Performance of Organic Solar Cells

Incorporating solid additives is an effective strategy to regulate the morphologies of active layers and optimize the performance of organic solar cells (OSCs), yet their molecular-level mechanisms remain elusive. Here, three halogenated solid additives, namely, 1,8-dichloronaphthalene, 1,8-dibromonaphthalene, and 1,8-diiodonaphthalene, are investigated for their effect on PM6:Y6 blends. All three enhance the power conversion efficiency (PCE) of the devices, with 1,8-dibromonaphthalene yielding the highest value. <i>Ex situ</i> atomic force microscopy and <i>in situ</i> photoluminescence measurements reveal that the improved performance originates from favorable phase separation driven by promoted Y6 aggregation. Grazing incidence wide-angle X-ray scattering and interface-specific sum frequency generation spectroscopy further show that this favorable morphology stems from the enhanced crystallinity and increased ordering of alkyl side chains in both PM6 and Y6. Our work advances the fundamental understanding of the working mechanism of OSCs mediated by solid additives and offers guidance for the rational design of new solid additives for OSCs.