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THERE is greater potential to widen the band width of light available for conversion by photovoltaic cells into electrical energy, according to new Australian research.

University of Sydney researchers,led by Tim Schmidt, report in the journal Physical Chemistry Chemical Physics that they have overturned a general scientific consensus that an 11.1% upper limit exists on up-conversion of low energy photons. Schmidt says the work is “fundamental breakthrough which will open up a lot of opportunities for applied R&D in third generation photovoltaics.”

Laboratory tests using laser light achieved efficiencies conservatively estimated at 25%, and the team speculates this could be pushed as high as 40%. Photovoltaic cells generally utilise material with a single energy threshold which produces voltage by promoting electrons above this threshold upon absorption of light. Photons with energy below this threshold cannot be harvested and their energy is converted to heat which lowers the cell’s efficiency.

The Sydney team’s outcomes were achieved by mixing a sensitiser, tetrakisquinoxalinoporphyrin palladium(II) (PQ4Pd,) with rubrene in a toluene solution, and exposing the mix to a range of laser frequencies.

“It does work under incoherent [solar] radiation but the purposes of the study were to prove the non-existence of this upper limit so we used a laser,” Schmidt says.

The discovery opens a pathway to boosting the efficiency of photovoltaic conversion of solar energy to electrical current from current upper limits around 35% to as high as 50% and even higher when cells are exposed to concentrated sunlight.

One approach would be to use bifacial cells which are sensitive to light on both sides.

“The idea is to take a bifacial cell and put the up-converting material on the dark side, so the light transmitted through the cell which is below the band gap gets up-converted and shone back into the cell at a useful colour,” Schmidt says.