Solid Additive Strategies for High-Performance and Eco-friendly Organic Solar Cells: A Comprehensive Review

Organic solar cells (OSCs), particularly those based on bulk heterojunction (BHJ) architectures, have garnered significant attention as sustainable alternatives for next-generation solar energy technologies, owing to their lightweight nature, flexibility, and compatibility with low-cost fabrication methods. Among the various strategies to enhance device performance, solid additives have emerged as a promising means to precisely control the crystallinity and surface morphology of photoactive and interfacial layers, thereby improving charge generation, transport, and collection. This review provides a comprehensive overview of solid additive materials and strategies to enhance OSCs' power conversion efficiency (PCE) and operational stability, with a particular emphasis on eco-friendly alternatives. The mechanisms by which solid additives affect the crystallinity, phase separation, molecular ordering, and charge carrier mobility are discussed, alongside recent advances in green solid additives that enable environmentally benign processing. Key challenges and future directions are also explored to support the development of high-performance, sustainable solid additive systems that can accelerate the commercial viability of OSCs.