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Sydney Uni scientists catch low-energy photons

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A joint project between Sydney University and Germany’s Helmholtz Centre for Materials and Energy claims to offer a low-cost boost to solar cell efficiency.

The efficiency question for solar cell developers is how to capture more energy from more photons, since “wasted” photons merely get turned into heat.

That’s harder than it looks, since the photo-electric cells that turn light into power respond to a narrow range of wavelengths. The problem becomes more acute in lower cost thin film cells; while cheaper than bulk silicon crystal cells, they respond to a narrower set of wavelengths.

Sydney University’s Professor Tim Schmidt and Dr Klaus Lips from Helmholtz aims to capture “lost” photons. Their results, published in the journal Energy & Environmental Science (abstract here), say their “upconversion” technique boosts the theoretical maximum efficiency of solar cells to beyond 40 percent.

The process uses organic dye molecules to capture two longer-wavelength red photons (wavelength around 650-700nm) and emit a single photon in the 550-600nm band.

In a phenomenon called “triplet-triplet annihilation”, the energy absorbed by two triplet molecules from the red photons is combined to release a single yellow photon.

"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," Professor Schmidt said.

"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."

Schmidt also says the upconversion technique is relatively low cost. Current research work focused on developing new materials and tuning device optics is costly because it demands that both devices and manufacturing are re-engineered.

On the other hand, the upconverter approach yields an efficiency gain of two percent when used with a standard amorphous silicon solar cell and an “unoptimised” upconversion unit.

The yield could be even higher. Solar cells’ inefficient use of the solar spectrum sets an upper limit of 33.7 percent to their efficiency, known as the Shockley-Queisser limit. Schmidt believes the theoretical efficiency limit using upconversion could be as high as 43 percent.

Current cells haven’t yet reached the Shockley-Queisser limit – even record-breaking cells in the labs operate at around 25 percent – but even adding two percent to one of today’s commercial devices (operating at around 22 percent efficiency) is a considerable boost. ®

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