Low-bandgap conjugated polymers enabling solution-processable tandem solar cells

The technology of polymer-based organic photovoltaic (OPV) cells has made great progress in the past decade, with the power conversion efficiency increasing from just a few per cent to around 12%, and the stability increasing from hours to years. One of the important milestones in this progress has been the invention of infrared-absorbing low-bandgap polymers, which allows the OPV cells to form effective tandem structures for harvesting near-infrared energy from the solar spectrum. In this Review, we focus on the progress in low-bandgap conjugated polymers and several tandem OPV cells enabled by these low-bandgap polymers. Specifically, we cover polymer-based tandem solar cells; hybrid tandem solar cells combining polymers with hydrogenated amorphous silicon; and unconventional solar cells. For each of these technologies, we address the challenges and offer our perspectives for future development. Low-bandgap (<1.6 eV) polymers enable polymer solar cells to form effective tandem structures for harvesting near-infrared solar energy as well as reducing thermal loss. This Review summarizes recent progress and provides a perspective on various low-bandgap polymer-containing tandem solar cells; namely, pure polymer–polymer tandem, hybrid polymer–amorphous silicon tandem and unconventional perovskite–polymer tandem solar cells.

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