Tandem polymer solar cells set energy-conversion record

UCLA scientists say they’ve significantly enhanced polymer solar cells’ performance by building a new ‘tandem’ device that combines multiple cells with different absorption bands.

The US Department of Energy’s National Renewable Energy Laboratory has certified with a power-conversion efficiency of  10.6 percent — a new record.

By using cells with different absorption bands, tandem solar cells are an effective way to harvest a broader spectrum of solar radiation. But the efficiency doesn’t automatically increase by simply combining two cells – the materials for the tandem cells have to be compatible with each other.

Until now, this has meant thaT the performance of tandem devices has lagged behind single-layer solar cells.

Now, though, the UCLA team has demonstrated highly efficient single-layer and tandem polymer solar cells featuring a low-band-gap–conjugated polymer specially designed for the tandem structure. The band gap determines the portion of the solar spectrum a polymer absorbs.

“Envision a double-decker bus,” says professor Yang Yang. “The bus can carry a certain number of passengers on one deck, but if you were to add a second deck, you could hold many more people for the same amount of space. That’s what we’ve done here with the tandem polymer solar cell.”

The team stacked multiple photoactive layers with complementary absorption spectra. Their tandem structure consists of a front cell with a high band gap material and a rear cell with a low band gap polymer, connected by an interlayer.

It’s more efficient than a single-layer device, they say, particularly by minimizing other energy losses. By using more than one absorption material, each capturing a different part of the solar spectrum, the tandem cell can maintain the current and increase the output voltage, bringing the increase in efficiency.

“Everything is done by a very low-cost wet-coating process,” says Yang said. “As this process is compatible with current manufacturing, I anticipate this technology will become commercially viable in the near future.”