Dispersion-Mediated Active-Layer Interfacial Regulation Using Halogenated Thioanisole Additives Achieves High-Efficiency Organic Solar Cells

Abstract

Precise control of noncovalent interactions between donor and acceptor using additives is essential for high-efficiency bulkheterojunction (BHJ) organic solar cells (OSCs). However, the role of processing additives in regulating interfacial interactions and morphology evolution remains unclear. Here, a halogen series of volatile thioanisole solid additives, 4-fluorothioanisole (4-FTA), 4-chlorothioanisole (4-CTA), 4-bromothioanisole (4-BTA), and 4-iodothioanisole (4-ITA), is used to elucidate how halogen substitution regulates intermolecular interactions and morphology evolution in PM6:L8-BO blends. From F to I, the additives exhibit increasing polarizability and quadrupole moments. This trend strengthens additive-mediated donoracceptor interactions. It also enhances molecular packing and phase compatibility. Energy decomposition analysis reveals dispersion-dominated interactions across all systems, while Hirshfeld surface and fingerprint analyses show that 4-BTA achieves an optimal balance between interaction strength and structural order, featuring enriched C-C and S-C short-range contacts that promote cooperative slipped π-π stacking. This balanced interaction landscape enables precise morphology control. The 4-BTA-treated film shows a smooth surface (1.20 nm), enhanced π-π stacking (3.65 Å). It also exhibits balanced charge transport and ultrafast hole transfer (0.32 ps). As a result, PM6:L8-BO devices processed with 4-BTA deliver a power conversion efficiency of 19.32%. Applying the same strategy to D18:L8-BO further increases the efficiency to 20.21%. These results demonstrate that halogen engineering of volatile solid additives provides a general and effective design strategy for translating dispersion-mediated interfacial regulation into high-performance OSCs.