Recent Advances in Solution-Processable Small-Molecule Cathode Interfacial Layers for Organic Solar Cells

Organic solar cells (OSCs) are a promising sustainable energy technology, where cathode interfacial layers (CILs) play a vital role in improving device efficiency and stability. This review highlights recent progress in solution-processable small-molecule cathode interfacial materials (CIMs), which offer advantages such as well-defined molecular structures, high purity, excellent reproducibility, and tunable optoelectronic properties. The review systematically discusses both nonconjugated and conjugated small-molecule CIMs, with a focus on classical systems derived from perylene diimide (PDI) and naphthalene diimide (NDI), as well as emerging materials featuring novel conjugated backbones. The review establishes clear relationships between structure, property, and application by examining how molecular design strategies, such as backbone engineering, side-chain functionalization, and self-doping, influence interfacial properties and device performance. Furthermore, the review evaluates the commercial potential of small-molecule CIMs in terms of synthetic scalability, cost-effectiveness, thickness insensitivity, and long-term stability. By connecting fundamental research with practical processing requirements, this work provides a comprehensive perspective on small-molecule CIMs, aiming to bridge laboratory innovation and industrial application for high-performance OSCs.