Asymmetric side chain design boosts thick film organic solar cell efficiency

by Riko Seibo

Tokyo, Japan (SPX) Dec 14, 2025

Researchers from Beijing Normal University and Qingdao University have developed a nonfused ring electron acceptor (NFREA) called TT-Ph-C6 that improves the performance of thick-film organic solar cells. The material targets limitations in film-thickness tolerance and fill factor that have held back NFREAs compared with fused-ring acceptors.

The team, led by Prof. Zhishan Bo, Prof. Cuihong Li, Prof. Yahui Liu and Prof. Hao Lu, used an asymmetric side-group engineering strategy on TT-Ph-C6. This design led to devices with a fill factor of 80.1 percent, reported as the highest value so far for NFREA-based organic solar cells.

TT-Ph-C6-based devices reached a power conversion efficiency (PCE) of 18.01 percent while maintaining performance at increased active-layer thickness. The cells delivered a PCE of 15.18 percent at 200 nanometers and 14.64 percent at 300 nanometers, placing these results among the strongest efficiencies reported for non-fused acceptors at such thicknesses.

Structurally, TT-Ph-C6 incorporates asymmetric phenylalkylamino side chains that increase solubility and promote compact three-dimensional molecular stacking. The crystal structure shows strong p-p interactions with a stacking distance of 3.21 angstroms and multiple S—O/N noncovalent locks that support a dense packing network.

These structural features support balanced charge transport in the active layer. Electron mobility reaches 2.48 + 10^-4 cm2 V^-1 s^-1, and the ratio of electron to hole mobility (ue/uh) is close to 1 in thick films, helping to preserve short-circuit current and fill factor.

The researchers also report extended exciton diffusion and efficient charge transfer in TT-Ph-C6 blends. The exciton diffusion length is 17.2 nanometers, compared with 13.4 nanometers for a symmetric reference material, and the hole transfer efficiency reaches 91.2 percent, which helps suppress recombination losses.

Device fabrication used o-xylene as a processing solvent, avoiding halogenated additives. The team is now working on scaling wide-strip coating processes toward meter-scale flexible modules suitable for roll-to-roll manufacturing.

Future work will expand the library of asymmetric side chains to target tandem architectures and lower bandgap systems. The results highlight side-chain design as a practical route to cost-effective, high-efficiency thick-film organic solar cell technologies.

Research Report:Asymmetric Side-Group Engineering of Nonfused Ring Electron Acceptors for High-Efficiency Thick-Film Organic Solar Cells