Professor Yang Yang and a team of researchers at UCLA Henry Samueli School of Engineering and Applied Science and UCLA's California Nanosystems Institute (CNSI) have announced they have fabricated an organic photovoltaic based on a novel tandem architecture that combines multiple cells with different absorption bands. The resultant device achieved a certified power conversion efficiency of 8.62%.

The findings where then published in Nature Photonics, however after the paper had been accepted for publication the research team fabricated more devices with a new infra-red absorbing polymer materials – supplied by Sumitomo Chemical – the resultant devices where then certified by NREL as having a power conversion efficiency of 10.6% - this is a world record for organic photovoltaic devices. The frabricated devices had an area of 12mm²

This study is expected to open a new direction for polymer chemists to pursue designs of new materials for tandem polymer solar cells. It is considered an important step towards the commercialisation of polymer based solar cells.

To use solar radiation more effectively, the research team stacked, in series, multiple photoactive layers with complementary absorption spectra to construct a tandem polymer solar cell. Their tandem structure consists of a front cell with a larger (or high) band gap material and a rear cell with a smaller (or low) band gap polymer, connected by a designed interlayer.

When compared to a single-layer device, the tandem device is more efficient in utilising solar energy, particularly by minimising other energy losses. By using more than one absorption material, each capturing a different part of the solar spectrum, the tandem cell is able to maintain the current and increase the output voltage. These factors enable the increase in efficiency, the researchers said.

Shuji Doi, research group manager for Sumitomo Chemical, said, “The solar spectra is very broad and covers the visible as well as the invisible, the infrared and the UV.” Shuji added, “We are very excited that Sumitomo's low–band gap polymer has contributed to the new record efficiency.”

Gang Li, a member of the research faculty at UCLA Engineering and a co-author of the Nature Photonics paper, said, “We have been doing research in tandem solar cells for a much shorter length of time than in the single-junction devices.” Gang added, “For us to achieve such success in improving the efficiency in this short time period truly demonstrates the great potential of tandem solar cell technology.”

Professor Yang Yang said his team hopes to reach 15 percent efficiency in the next few years.

www.engineer.ucla.edu www.sumitomo-chem.co.jp