Nonvolatile Solid Additive with a Simple Structure for Nonfullerene Organic Solar Cells

In organic solar cells (OSCs), precise control over the bulk heterojunction (BHJ) morphology is critical for optimizing device performance. A widely adopted strategy for regulating BHJ morphology involves the incorporation of small amounts of additives. However, employing volatile solid additives necessitates precise volatilization control, which remains challenging. While nonvolatile solid additives with high boiling points have been explored, most exhibit large and structurally complex architectures, raising concerns regarding their role in OSCs and their potential impact on charge transport. Thus, a nonvolatile solid additive with both a high boiling point and a simple molecular structure is highly desirable. In this study, 4‐bromobiphenyl (BBP), a nonvolatile solid additive with a high boiling point and a simple chemical structure, was incorporated into OSCs to modulate BHJ morphology. The addition of BBP enhanced donor–acceptor miscibility, suppressed excessive aggregation, and promoted a uniform morphology with improved crystallinity. Consequently, the power conversion efficiency significantly increased to 18.11%, surpassing that of OSCs incorporating volatile solid additives while also enhancing long‐term stability. This work provides a promising strategy for achieving stable BHJ morphology and improving OSC performance through nonvolatile solid additives.