(Upper left) Red image showing the 3D structure of the CsSnI3 panel, absorption is at 1.3eV (via Nature)
As seen on element 14, there are a wide array of solar cells providing different ideas for solar energy harvesting. The Gratzel cell, first developed by Michael Gratzel and Brian O’Regan, was unique in that it used an organic liquid as an electrolyte that evenly covered the entire area of the solar cell. This would provide a lot of surface area for photovoltaic reactions, but fundamental problems kept it from advancing. Since the electrolyte is a liquid, there was the risk of leaking and this liquid was capable of corroding solar panel if it did. Not only that, but the lifespan for a Gratzel cell was only 18 months.
Nanotechnology experts and researchers from Northwestern University, lead by Robert P. H. Chang, decided to tackle on these issues with the Gratzel cell to produce a new version. This version is not technically a Gratzel cell because of some fundamental differences but builds on similar concepts and promises to provide dramatic improvement in cost, longevity and efficiency. Their studies were supported by the NSF, U.S. DoE, and the Initiative for Energy and Sustainability at Northwestern.
To start, the corrosive liquid has been switched out by a thin-film, p-type semiconductor compound, CsSnI3, which can be poured in the cell as a liquid and then solidifies. This new cell also uses titanium dioxide spheres as the n-type semiconductor and a monolayer dye molecule that connects the two. The thin-film, CsSnI3 compound is a natural light absorber, but each cell is coated with additional light absorbing dye. Chang says the solid electrolyte will allow for more efficient, more stable and longer lasting cells.
Each cell measures 1 cm by 1 cm by 10 microns thick. Each is composed of millions and millions of 20 nanometer nanoparticles. Chang calculated that this size maximized the amount of spheres that could fit, while optimizing the space between them to allow for conduction.
Preliminary tests at Northwestern resulted in an efficiency of approximately 10.2%, the highest for any solid-state solar cell with dye sensitizer. This value is very close to the highest Gratzel efficiency obtained which was 11-12% but still below the conventional silicon efficiency of about 20%.
Chang is confident that the low cost and small size of the cells will make them competitive. He adds that these solar cells are perfect candidates for automated manufacture, which would reduce costs even more.
Of course, there are still many improvements to be made to increase the energy conversion efficiency. This concept of solid-state electrolyte could even be applied to other types of solar cells so as Chang said, “This is only the beginning… there is a lot of room to grow”. A paper titled, “All-Solid-State Dye-Sensitized Solar Cells With High Efficiency” was published by the Northwestern team on May 24 in the journal Nature.