Organic solar cells (OSCs) are attracting increasing attention as lightweight, flexible, and potentially low-cost photovoltaic technologies. However, achieving high power conversion efficiency while maintaining long-term stability, mechanical flexibility, and affordable manufacturing remains a major challenge. Conventional perylene diimide (PDI)-based electron-transport layers (ETLs) often suffer from limited alcohol solubility, excessive molecular crystallization, and oxidative degradation, which can compromise charge transport and device durability. Researchers from Ulsan National Institute of Science and Technology (UNIST) and the Korea Advanced Institute of Science and Technology (KAIST) have developed a molecular design strategy that addresses these interconnected limitations, enabling organic solar cells with efficiencies exceeding 20%, improved bending durability, and reduced dependence on expensive silver electrodes.
Why This Organic Solar Cell Strategy Matters
The electron-transport layer plays a critical role in extracting electrons, suppressing interfacial recombination, and maintaining the operational stability of organic solar cells. However, conventional PDI-based materials face two major limitations. Their strong π–π interactions can lead to excessive crystallization and rough film surfaces, while vulnerable molecular bonds are susceptible to reactive oxygen species (ROS)-induced degradation. In addition, some materials require acidic additives to improve alcohol solubility, introducing further complications during processing. Overcoming these limitations simultaneously is essential for translating high-efficiency organic solar cells into flexible and scalable photovoltaic technologies.
Innovative Design and Mechanism
The research team established a molecular design framework based on controlling the bay positions of PDI molecules through sterically twisted substituents and polar functional groups. Using this strategy, they developed H75-DMA, incorporating bulky dimethylnaphthylamide (DMA) groups. The twisted molecular configuration disrupts excessive π–π stacking, suppressing over-crystallization and producing smoother, more uniform films. Meanwhile, the polar DMA groups improve solubility in alcohol without requiring acidic additives. Importantly, this molecular architecture reduces the susceptibility of vulnerable C–H and N–H bonds to oxidative degradation, suppressing ROS-driven chemical evolution and interfacial trap accumulation. These effects collectively stabilize interfacial energetics and facilitate efficient electron transport.
Outstanding Performance
The optimized H75-DMA-based organic solar cells achieve a power conversion efficiency of 20.07%, while retaining approximately 80% of their initial efficiency after 384 hours of air storage. The material also delivers substantial improvements in mechanical properties. Flexible devices exhibit a crack-onset strain exceeding 21% and a toughness of 6.72 MJ m -3 , maintaining approximately 83% of their initial efficiency after 2,500 bending cycles. Furthermore, flexible semitransparent devices achieve a light-utilization efficiency of approximately 3%, demonstrating the potential for applications requiring both transparency and photovoltaic performance. Notably, replacing the conventional silver cathode with copper still enables an efficiency of 18.65%, retaining approximately 97% of the performance of silver-based devices.
Applications and Future Outlook
This work demonstrates that molecular-level engineering of electron-transport layers can simultaneously address efficiency, stability, flexibility, and manufacturing cost in organic photovoltaics. The additive-free alcohol-processing capability offers a more practical approach to solution-based fabrication, while the compatibility with flexible and semitransparent architectures expands potential applications in wearable electronics, portable power sources, and building-integrated photovoltaics. More broadly, the H75 molecular framework provides transferable design principles for developing stable and efficient PDI-based interfacial materials. Further optimization of large-area processing and long-term operational stability could accelerate the transition of flexible organic solar cells toward practical applications.
Nano-Micro Letters
News article
Polar‑Twisted Electron‑Transport Layer Simultaneously Breaks Efficiency‑Flexibility‑Cost Limits in Organic Solar Cells
13-Aug-2026