The upside of upconversion

After more than a year in lab, University of Sydney’s researcher Tim Schmidt is seeing results, and they come in a dazzling array of colors.

Schmidt and Professor Max Crossley, in the university’s school of chemistry, are devising a low-cost device to harvest low energy photons, with the potential of boosting the efficiency of conventional solar cells using a process called upconversion.

“Upconversion is the conversion of light of a given photon energy to light of a higher photon energy,” Schmidt told the Cleantech Group.

The increase in upconversion efficiency enables an efficiency increase in single threshold solar cells of about one-third. Schmidt said this is done by harvesting the part of the solar spectrum currently unused by solar cells.

“What we have shown is this form of upconversion is not fundamentally limited by 11 percent efficiency,” said Schmidt, who is a molecular spectroscopist, or someone who studies how molecules interact with light.

See a photograph of the upconversion process »

The team is synthesizing unique sensitizer and emitter molecules to bring about tailor-made devices to boost solar energy conversion efficiencies in two solar cell types: amorphous silicon and crystalline silicon.

The sensitizer absorbs the light and the emitter emits the light, he said.

However, the team is working to find more robust emitter molecules. The ones they are using are sensitive to oxygen and couldn’t be deployed outside the lab, Schmidt said.

“We want to find one that is intrinsically efficient but will be robust in the presence of oxygen,” he said.

They are targeting amorphous silicon because it lets light in the red part of the spectrum, which works better with the upconversion process.

The findings are expected to pave the way to boosting the efficiency limits to more than 50 percent, under the standard solar spectrum, and up to 63 percent under 100-fold solar concentration.

“This will be the start of high efficiencies,” he said.

Schmidt said the results to date have been achieved using a special laser.

“We dumped all the light into the sample all at once, so all the molecules storing energy were all storing energy at the same time and converting light all at the same time,” he said, adding that they also figured out what makes the process inefficient.

In the end, he said the goal is to produce material that would be placed behind a solar cell to improve its efficiency.

Upconversion efficiency itself has been achieved in the lab, but sticking it into solar cells and continuing to achieve high efficiencies in the field is still being pursued.

With one breakthrough under its belt, the University of Sydney plans to continue its research efforts with connections at the University of New South Wales (UNSW), Australia.

UNSW’s school of photovoltaic and renewable energy engineering is internationally recognized for its research in photovoltaics, most of which is now conducted under the ARC Centre of Excellence. Although UNSW has been working on upconversion, Schmidt said the school has not achieved the efficiencies attained by University of Sydney researchers.

Schmidt hopes to leverage UNSW’s established relationships with companies such as China’s Suntech Power Holdings, U.S.-based Spectrolab, Germany’s Roth & Rau, and Australia's CSG Solar, a spinout from UNSW (see Spectrolab solar cell breaks 40% efficiency barrier, Watch for falling solar margins, says report and Q-Cells fond of CIGS). 

CSG gets its name from its technology, crystalline silicon on glass, where a thin layer of silicon is deposited directly onto a glass sheet, and is suitable for mounting outdoors.

In September, Suntech (NYSE:STP) beat its own world record in conversion efficiency for its multi-crystalline silicon photovoltaic module. The photovoltaic module manufacturer said its commercial grade module reached 16.5 percent conversion efficiency (see Suntech surpasses its own module efficiency record).

Martin Green, research director of the ARC Centre of Excellence, said in a news release that the conversion level set a new benchmark for high performing multi-crystalline modules.

Cleantech developments making news in the past 24 hours

• Nexamp closes Series A round, brings on new CEO
• Water innovation beyond desal: the new opportunities?
• Areva to launch solar business with Ausra takeover

View the last week of cleantech developments here | Subscribe to full feed