To address the issue that active layer materials in organic solar cells are susceptible to photochemical degradation under ultraviolet (UV) light, leading to performance decay, a dual-additive strategy is employed: the UV absorber 2-[2′-hydroxy-3′,5′-bis(α,α-dimethylbenzyl)phenyl]benzotriazole (UV234) and the solid additive 1,4-diiodobenzene (DIB) are introduced into the D18:L8-BO system to synergistically enhance the device photovoltaic performance and stability. Ultraviolet-visible (UV‑Vis) absorption spectroscopy was employed to investigate the effects of two additives on the optical properties of D18, L8-BO, and their blend films. UV234 can absorb part of the UV light to boost the device’s photostability, significantly influence the molecular packing behavior of the donor D18, drive its aggregation mode to shift from a mixed state to a J-aggregation-dominated mode that is more favorable for charge transport, and undergo Förster resonance energy transfer (FRET) with D18, as evidenced through photo-luminescence spectral analysis. Meanwhile, the iodine atoms of DIB interact with the nitrogen atom sites in D18 and the acceptor L8-BO molecules, effectively modulating the crystallization process and phase-separation kinetics of the blend film. The surface and bulk morphology of the blend films were investigated via atomic force microscopy (AFM) and transmission electron microscopy (TEM). Furthermore, the Flory-Huggins interaction parameters (χ) were employed to evaluate the miscibility between D18 and L8-BO. A film with an appropriate phase-separation scale, an improved interpenetrating network structure, and a more uniform surface is obtained, which effectively promotes exciton dissociation and charge transport while significantly suppressing charge recombination. To investigate the impact of different treatments on photovoltaic performance, conventional devices with the structure of ITO/2-PACz/D18:L8-BO/PNDIT-F3N/Ag were fabricated. The dual additives processed device based ultimately achieves a power conversion efficiency (PCE) of 19.55%, and its UV photostability is remarkably enhanced: after continuous irradiation under a 24 W UV lamp for 408 h, the PCE of the device co-regulated by the dual additives retains 76% of its initial efficiency, whereas the PCE of the untreated device drops to 64%.
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