For more than six decades, scientists have been developing organic solar cells as a lightweight, flexible alternative to conventional silicon panels.

These carbon-based devices could power wearable electronics, curved surfaces, and solar windows. However, they have long struggled to match silicon’s efficiency because they lose too much energy while converting sunlight into electricity.

Researchers at Chiba University in Japan have now identified a way to reduce one of those losses without sacrificing the electric current generated by the cells.

The team discovered a thermodynamic boundary for photocurrent generation and found that using more rigid donor molecules could reduce radiative voltage losses. In several devices, these molecules lowered the effective charge-transfer (CT) binding energy to 0.1 electron volts (eV) or less, bringing the CT state close in energy to fully separated charges and making charge separation more efficient.

The researchers also found that photocurrent could be maintained when the donor’s optical energy gap closely matched the free-carrier energy, the energy needed to create fully separated charges. In other words, the donor material absorbed just enough energy from sunlight to free electrical charges without wasting excess energy as heat. Solar cells operating near this thermodynamic limit also tended to achieve higher power conversion efficiencies.

“In the longer term, such devices could make solar energy harvesting more practical in lightweight, flexible, and space-limited applications that are difficult for conventional silicon modules,” said Hirohiko Fukagawa, professor at the Center for Frontier Science, Chiba University, and lead author of the study.

Although laboratory organic solar cells have now surpassed 20% power conversion efficiency , researchers have continued searching for ways to reduce voltage losses while maintaining photocurrent.

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