TADF additive pushes polymer solar cells above 20% efficiency

Researchers in China report a layer-by-layer polymer solar cell design that uses a thermally activated delayed fluorescence additive to improve film formation and cut energy losses. The device reached 20.18% efficiency, one of the strongest results reported for binary layer-by-layer organic photovoltaics.

Why it matters: - Polymer solar cells could offer lightweight, flexible, and scalable power generation. - The new approach clears a key hurdle by improving both nanoscale morphology and non-radiative recombination at the same time. - The result points to a more reproducible route for high-performance organic photovoltaics.

What happened: - Researchers from Zhejiang University and the Zhejiang University-Hangzhou Global Scientific and Technological Innovation Center reported a thermally activated delayed fluorescence, or TADF, additive-assisted layer-by-layer strategy for polymer solar cells. - The study appeared in Chinese Journal of Polymer Science in 2026. - The team blended the TADF molecule 4CzIPN into the acceptor layer solution. - The devices used a conventional ITO/2PACz/active layer/PDINN/Ag structure. - The optimized device reached 20.18% power conversion efficiency. - The same device delivered 0.915 V open-circuit voltage, 27.05 mA/cm² short-circuit current density, and 81.53% fill factor.

The details: - 4CzIPN has a small singlet-triplet energy gap of 0.083 eV. - The additive forms a favorable Type I energy alignment with the acceptor L8-BO. - In situ UV-Vis and photoluminescence measurements showed faster acceptor crystallization during film formation. - The same measurements showed less fluorescence quenching. - Atomic force microscopy showed a smoother and more uniform nanofibrillar network. - Root-mean-square roughness dropped to 2.086 nm from 2.494 nm. - Space-charge-limited current measurements showed higher and more balanced mobilities. - Hole mobility reached 6.52×10-4 cm²/(V·s), and electron mobility reached 7.08×10-4 cm²/(V·s). - Time-resolved photoluminescence showed exciton lifetime increased to 1.36 ns from 1.23 ns. - Light-intensity-dependent measurements indicated reduced bimolecular and trap-assisted recombination. - Photoluminescence quantum yield rose from 9.46% to 11.09% for acceptor films. - Photoluminescence quantum yield rose from 0.20% to 0.33% for blend films. - The authors said the TADF additive played a dual role by refining the donor-acceptor network and reducing non-radiative voltage losses. - The authors also said the small singlet-triplet gap supports reverse intersystem crossing, which extends exciton lifetime and improves charge generation. - The reported efficiency places the device among the highest for binary layer-by-layer processed polymer solar cells.

Between the lines: - The result suggests that TADF materials can do more than boost light emission in other contexts; they can also help tune morphology and charge behavior in solar cells. - The work matters because layer-by-layer processing gives better control than blend casting, but it has had fewer examples of strong performance than bulk heterojunction designs. - The use of a non-volatile solid additive may make the process more compatible with large-area coating. - The approach could also be extended to ternary or tandem architectures.

What's next: - The researchers say the strategy could be adapted to more complex solar cell designs. - The combination of improved morphology and reduced losses may support future work on scalable, commercially relevant organic solar cells. - The DOI for the study is 10.1007/s10118-026-3653-2.