While the overall efficiency of conventional silicon solar cells has continued to improve in recent years, the technology faces a natural theoretical limit at around 33%. This is because the laws of physics prevent the cells from absorbing photons below a certain energy level, meaning that this low-energy light cannot be converted into electricity and is simply lost. Now researchers have found a way join two energy-poor red photons to form a single energy-rich yellow photon, allowing the harvesting of this part of the spectrum currently unused by single p-n junction crystalline silicon solar cells, and potentially enabling a record-breaking efficiency of 40%.

The technique, called “photochemical upconversion,” relies on two different types of molecules that are placed behind the solar cell in a solution to combine two low-energy photons into a single high-energy photon. The first type of molecule absorbs the energy-poor red photons, preventing them from escaping and storing them in a persistent state. This persistent state lasts long enough so that the energy can be transferred to a second, organic molecule when they encounter each other in the solution.

When two of these excited organic molecules then encounter each other, one returns to its base state and the other assumes a higher energy state. This higher-energy state is extremely short-lived, as the molecule then sends off a single yellow photon that is if a high enough energy to be absorbed by the solar cell.

Red light from a laser pointer is converted into higher-energy yellow light as it passes through the liquid photochemical upconverter (Photo: University of Sydney)

"We are able to boost efficiency by forcing two energy-poor red photons in the cell to join and make one energy-rich yellow photon that can capture light, which is then turned into electricity," says University of Sydney Associate Professor Schmidt who developed the so-called “turbo for solar cells” with partners at Helmholtz Centre Berlin for Materials and Energy. “We now have a benchmark for the performance of an upconverting solar cell. We need to improve this several times, but the pathway is now clear."

The photochemical upconversion technique was detailed in the journal Energy & Environmental Science, earlier this year.

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