The solar efficiency improvement parade marches on. The latest record-buster comes from UCLA, where researchers reported that incorporating a new infrared-absorbing polymer material has boosted the efficiency of an already high-functioning tandem solar cell to 10.6 percent.
The solar efficiency of a photovoltaic (PV) cell indicates how much of the light hitting it is converted to power. Measurements are verified and tracked by the National Renewable Energy Lab (NREL) in several categories and subcategories depending on the architecture of and materials used in the cell. As you can see on the NREL’s “Best Research-Cell Efficiencies” chart, the UCLA cell comes in at less than half the efficiency of the best crystalline silicon cells. However, organic polymers have the great advantage of being cheaper to produce at high volumes. Plus, they’re light and flexible, giving them a great range of possible applications.
The UCLA work was led by Yang Yang, a professor of materials science and engineering at the university, and published online in the journal Nature Photonics. The infrared-absorbing polymer material came from Sumitomo Chemical of Japan, which is why this new record appears on the NREL chart as “UCLA-Sumitomo Chemical.”
In their paper, the Yang team report enhancing the performance of polymer solar cells by building a device with what’s called a tandem structure, where multiple cells are combined with different absorption bands. More absorption tends to equal more power, but tandem cells have their own challenge that’s held them back a bit: the materials used need to be compatible with each other in order to do the capture-and-convert dance.
According to UCLA, “To use solar radiation more effectively, Yang’s 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.”
The researchers said that 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. Thus your increase in efficiency. The Yang paper reports that efficiency at 8.6 percent. Since then, however, incorporating the Sumitomo Chemical polymer material has bumped the efficiency even higher, to a verified 10.6 percent, an indication of how quickly solar efficiencies are improving.
According to the university, the work by Yang and his team “opens up a new direction for polymer chemists to pursue designs of new materials for tandem polymer solar cells. Furthermore, it indicates an important step towards the commercialization of polymer solar cells.”
They’re thinking they could get the solar cell efficiency up to 15 percent efficiency in the next few years, so don’t be surprised to see more solar-efficiency record stories.