A Team of Oregon BEST Researchers Have New Take on Improving Solar Cells

The holy grail of solar energy research is finding a way to boost the efficiency of solar cells.

A multi-university team of Oregon BEST researchers believes attaining higher efficiency might be possible via a little-explored theory that “amplifies” the photocurrent to convert more energy from sunlight into electricity, instead of losing it as waste heat in the solar cell.

“We’re trying to improve the conversion efficiency of sunlight in a photovoltaic device by taking the electrons produced by high-energy photons and converting them into additional electrons inside the PV device,” says University of Oregon physics professor Steven Kevan, who is collaborating on the project with fellow UO physics professor David Cohen and five other researchers.

The fundamentals of the idea have been around for a long time and all the physics are more or less known, say the researchers, who are using existing materials and making subtle modifications to attain a better conversion efficiency.

“This is basically all about amplifying the charge before it ever comes out of the device,” says Cohen. “In PV cells, excess energy is currently lost as heat, so we’re asking how can we retain that energy as an extra current in the form of more electrons.”

The functioning of a solar cell involves two steps: absorption of sunlight to produce free charged carriers, and the transport of those carriers out of the device to serve as a source of electrical energy. The efficiency of a solar cell depends on 1) how many free carriers are generated inside the device for a given amount of sunlight, and 2) the fidelity with which the carriers are transmitted through the device and into the external world.

“Our primary goal is to capture this energy by a process called impact ionization, wherein the excess energy of those carriers is transformed in a concerted fashion to produce more free carriers,” says Kevan (on right in photo). “This is a sort of amplification or gain that is designed into the material. If all the extra carriers produced in this fashion can be collected, the efficiency of the solar cell will be correspondingly increased.”

The key material ingredient of this approach is the junction between the harvesters and the host, so a wide range of techniques is used to synthesize, optimize, measure, analyze, and model the interfaces in these complex, composite PV materials.

Funding from Oregon BEST enabled the research team to purchase a piece of lab equipment that dramatically speeds the testing process.

“This lab apparatus is absolutely key to proving proof of principle,” Cohen says.

The research could benefit the current PV industry, as well as next-generation solar cell technology. If successful, the results could easily be incorporated into current PV manufacturing technologies, where even a small boost in efficiency would transform the industry.

“Our near term goal is to optimize heterojunction-assisted impact ionization (HAII) to amplify the conversion efficiency in existing PV materials,” says Cohen. “At the same time, we will use the knowledge gained through this effort to design entirely new classes of PV materials and structures in which optimizing the HAII process is a key ingredient.”

The research project combines expertise from three universities in thin film synthesis, analysis, and mathematical and computational modeling. The team—which also includes Oregon State University physics professors Janet Tate and Guenter Schneider, OSU mathematics professor Malgorzata Peszynska, UO chemistry professor Geri Richmond, and Angus Rockett at the University of Illinois—is seeking funding from the National Science Foundation, as well as industry partners in order to advance the research.

“If we could show a bump in efficiency, we’ll have companies beating on our doors,” says Cohen.