Quantum Dot Solar Cells

Michael LoCascio, "Application of Semiconductor Nanocrystals to Photovoltaic Energy Conversion Devices," (Sept. 2002), www.evidentech.com

Titanium dioxide is a wide bandgap semiconductor able to efficiently conduct charge if it is injected into the conduction band. The pores of the films are saturated with a monolayer of adsorbed dye molecules and the pores are further filled with a liquid electrolyte containing iodide/triiodide redox couple in a non-aqueous electrolyte. A transparent counter electrode is placed over the titanium dioxide and the edges of the cell are sealed. Upon photoexcitation of the cell, the excited dye molecules inject electrons efficiently into the titanium dioxide conduction band. The injected electrons traverse the titanium dioxide with little loss and are collected at the conducting glass substrate. After passing through an external circuit and delivering power to a load, the electrons re-enter the cell at the counter electrode that reduces the triioidide into iodide, which subsequently reduces the photo-oxidized dye back to its original state. Thus, the Gratzel cell effectively separates light collection and charge separation functions with dye and titanium dioxide respectively and therefore eliminates the inherent tradeoffs associated with conventional solar cells that perform both functions in a single junction. For the nanocrystal-sensitized cell, nanocrystals are substituted for the dye molecules. Nanocrystals are far superior to conventional organic dyes in several respects and offer the potential of even higher efficiencies as well as a longer operational lifespan.