Asymmetric side‑group engineering of nonfused ring electron acceptors for high‑efficiency thick‑film organic solar cells

As demand for low-cost, printable photovoltaics grows, nonfused ring electron acceptors (NFREAs) are attractive because they skip lengthy ring-closure steps, but their film-thickness tolerance and fill factors still lag behind fused-ring counterparts. Now, researchers from Beijing Normal University and Qingdao University, led by Prof. Zhishan Bo, Prof. Cuihong Li, Prof. Yahui Liu and Prof. Hao Lu, present an asymmetric side-group strategy that delivers a record FF of 80.1 % and maintains >14 % efficiency even at 300 nm active-layer thickness—setting new performance benchmarks for thick-film NFREA devices.

Why TT-Ph-C6 Matters

• Highest Fill Factor: 80.1 % is the highest FF reported for NFREA-based OSCs.
• Exceptional Thick-Film Tolerance: 15.18 % PCE at 200 nm and 14.64 % at 300 nm—among the highest efficiencies for non-fused acceptors.
• Scalable Synthesis: Five-step, high-yield route avoids costly ring-closure reactions.

Innovative Design & Features

• Asymmetric Phenylalkylamino Side Chains: Enhance solubility and promote compact 3-D π-stacking (dπ–π = 3.21 Å) with multiple S···O/N locks.
• Balanced Charge Transport: Electron mobility reaches 2.48 × 10^-4 cm² V^-1 s^-1; μe/μh ≈ 1 in thick films preserves Jsc and FF.
• Extended Exciton Diffusion: 17.2 nm diffusion length (versus 13.4 nm for symmetric control) and 91.2 % hole-transfer efficiency suppress recombination.

Applications & Future Outlook

• Green Processing: Devices fabricated from o-xylene without halogenated additives.
• Roll-to-Roll Potential: Team is scaling wide-strip coating toward metre-length flexible modules.
• Next Steps: Asymmetric side-chain library will be expanded for tandem and low-energy-gap applications.

This molecular design strategy offers a cost-effective pathway to high-efficiency, thick-film organic solar cells and underscores the importance of side-chain engineering in next-generation photovoltaics. Stay tuned for more breakthroughs from Prof. Bo, Prof. Li, Prof. Liu and Prof. Lu’s groups!