Researchers from Beijing Normal University and Qingdao University have developed a non-fused ring electron acceptor (NFREA) called TT-Ph-C6 that improves the performance of thick-film organic solar cells. The material addresses limitations in film thickness tolerance and fill factor that have held NFREAs back compared to 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 yet for NFREA-based organic solar cells.
TT-Ph-C6-based devices achieved a power conversion efficiency (PCE) of 18.01 percent, while maintaining performance with increased active layer thickness. The cells delivered PCE of 15.18 percent at 200 nanometers and 14.64 percent at 300 nanometers, making these results among the strongest efficiency improvements reported for unfused acceptors at such thicknesses.
Structurally, TT-Ph-C6 contains asymmetric phenylalkylamino side chains that increase solubility and promote compact three-dimensional molecular stacking. The crystal structure exhibits strong pp interactions with a stacking distance of 3.21 angstroms and multiple S—O/N noncovalent locks supporting a dense packing network.
These structural features support balanced charge transport in the active layer. The electron mobility reaches 2.48 + 10^-4 cm2 V^-1 s^-1, and the electron-hole mobility ratio (ue/uh) is close to 1 in thick films, which preserves the short-circuit current and filling factor.
The researchers also report extensive exciton diffusion and efficient charge transfer in TT-Ph-C6 mixtures. The exciton diffusion length is 17.2 nanometers, compared to 13.4 nanometers for a symmetric reference material, and the hole transfer efficiency reaches 91.2 percent, which helps suppress recombination losses.
In the fabrication of the device, o-xylene was used as a processing solvent, avoiding halogenated additives. The team is now working to scale up wide strip coating processes into flexible, meter-scale modules suitable for roll-to-roll production.
Future work will expand the library of asymmetric sidechains to target tandem architectures and lower bandgap systems. The results highlight sidechain design as a practical route to cost-effective, high-efficiency thick-film organic solar cell technologies.
Research report:Asymmetric side group engineering of unfused ring electron acceptors for high-efficiency thick film organic solar cells
