CAREER: Transparent, passivating, and carrier-selective heterojunction contacts for silicon and cadmium telluride solar cells

Arizona State University, Scottsdale AZ

Investigators

Abstract

Nontechnical: This CAREER project aims to increase the efficiency of conversion of sunlight into electricity by solar cells. The project will explore the materials science and physics of electrical contacts to solar cells. In so doing, it will advance the understanding of how these contacts limit the operation of the cells. Every solar cell must perform two processes: convert light into excited electrons and extract those electrons in the form of current. The former process is performed by a semiconductor absorber material; the latter by two electrical contacts between which the absorber is sandwiched. Although solar cells are mass manufactured and now generate 1% of the electricity consumed in the U.S., there are still fundamental questions that are unanswered. The research community does not yet understand which properties determine if a material will make an excellent electrical contact and how to quickly find and make materials with the desired properties. This project will provide answers to these questions with a combination of measurements and simulations applied to existing model systems, as well as experiments to use the resulting understanding to develop new contacts. Success in the project will lay the groundwork for solar cells that are 10-15% more efficient than today's cells, reducing the cost of solar electricity generation to below 3 cents per kilowatt-hour. This, in turn, is predicted to accelerate the deployment of solar energy, resulting in 17% of U.S. electricity being generated by solar in 2030 instead of the 5% projected in a business-as-usual scenario. In addition to these scientific, environmental, and societal impacts, this project will also train community college, undergraduate, and graduate students for the 260,000 solar jobs presently in the US through a week-long, hands-on "Solar Cell 101" course in which they make cells and modules from start to finish. Technical: Many photovoltaic (PV) technologies have arrived at absorber materials that have low non-radiative recombination rates and thus could support cell voltages approaching the detailed-balance limit, but no technology has developed comparatively ideal electrical contacts that are transparent, that passivate the absorber surface, and that selectively extract electrons or holes from the absorber. This project targets key questions that inhibit rapid progress in heterojunction contact understanding and technology using (1) a characterization suite that links together the properties of carrier-selective layers, contacts containing those layers, and PV cells containing those contacts, and (2) a new deposition technique using dry-cluster spraying to enable intimate control of the stoichiometry of contact layers without sputter damage. These two platforms will be applied to crystalline silicon and cadmium telluride PV cells as model systems, as these technologies have readily available high-quality absorbers (which makes interpretation of the function of the contacts more apparent), a collection of previously developed contacts for these materials is available for analysis but ideal contacts have yet to be discovered, and any advances in understanding and technology resulting from the project will have large research and commercial impact. After studying the underlying physics of state-of-the-art existing contacts, the project will progress to depositing and testing heretofore unexplored carrier-selective layers, including wide-bandgap, heavily doped, amorphous or polycrystalline III-V materials. This research will address the critical impediment--excellent contacts--to 27%-efficient silicon PV cells and 25%-efficient cadmium telluride cells. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.