Precise active-layer morphology regulation in organic solar cells via novel volatile solid additives that drive acceptor aggregation

Abstract

Two novel volatile solid additives, 2-bromo-1,3,5-trichlorobenzene (TCBB-1) and 3,4,5-trichlorobromobenzene (TCBB-2), are introduced into PM6:L8-BO non-fullerene systems to regulate film nano-morphology, which can be controlled by strong, electrostatically driven interactions. DFT-derived electrostatic surface potential (ESP) maps reveal pronounced negative lobes on the halogen atoms of both additives; these regions are perfectly matched to the electron-deficient cores of L8-BO. The resulting coulombic attraction seeds ordered acceptor domains, increases acceptor crystallinity, and generates a dense, fibrillar network that increases electron mobility by more than an order of magnitude compared with the additive-free film. As a result, binary devices treated with TCBB-1 and TCBB-2 achieve power conversion efficiencies (PCEs) of 18.93% and 18.01%, respectively, surpassing the 16.09% of the control devices. Remarkably, the OSC treated with TCBB-1 based on PM6:L8-BO:BTP-eC9 further increased the PCE to 19.40%. The same ESP-guided interactions that refine lateral packing also orchestrate a favorable vertical stratification: the additives migrate toward the air interface during spin-coating and leave behind a donor-rich bottom layer that suppresses interfacial recombination and retards morphological evolution. Notably, these additives also impart excellent device stability, with TCBB-1-treated devices retaining over 95% of their initial PCE after 15 days. This work underscores the potential of volatile solid additives as a dual-function strategy to simultaneously boost efficiency and long-term stability in OSCs.