“For the last couple of years, efficiency for organic photo-voltaics was stuck around 11 to 12%,” said Michigan physicist Xiaozhou Che.

The top solution-processed non-fullerene-acceptor cell absorbs infra-red up to 950nm, and the bottom fullerene-based cell absorbs visible light starting at 350nm.

“By themselves, the cells achieve 10 to 11% efficiency. When we stack them together, we increase light absorption and efficiency improves to 15% with an anti-reflection coating,” said Che, predicting: “We can improve light absorption to increase electric current, and minimise the energy loss to increase voltage. Based on calculations, an 18% efficiency is expected in the near future for this type of multi-junction device.”

Key to the stacking process is preventing the application of the top cell from dissolving the bottom one, solved by finding a stack of interconnecting layers that prevent damage to the first cell while allowing light and electrical carriers through.

Fabrication yield has been 95% over 130 devices.

The tandem organic solar cell is the subject of the paper ‘High fabrication yield organic tandem photovoltaics combining vacuum- and solution-processed subcells with 15% efficiency‘ published in Nature Energy.