Solar Cell Electron Transfer Dynamics Measured By Nist Scientists - National Institute of Standards and Technology - Brief Article

Journal of Research of the National Institute of Standards and Technology, March-April, 2001

Inexpensive solar-to-electric energy converters based on organometallic dyes impregnated on nanoparticle substrates of titanium dioxide ([TiO.sub.2]) and other related semiconductors are being explored as replacements for silicon-based solar cells. These new solar cells could lead to low cost (less than $ 11W) photo-voltaic alternatives. Applications of the dye-sensitized photo-electrochemical cells include integrated power systems for consumer electronics, smart cards, electrochromatic windows, mirrors, and eyewear. The benefits of these devices include those derived from renewable energy sources--decreased emission of greenhouse gases and pollutants and the economic benefits of the lower cost energy sources.

Determining detailed mechanisms and underlying materials properties of such devices is key to understanding their function and improving solar collection and current-generating efficiencies. NIST researchers were the first to apply time-resolved infrared spectroscopy to unambiguously reveal that electron injection from excited electronic states of the dye molecules (e.g., different complexes of Ruthenium) to the [TiO.sub.2] occurs on the femtosecond ([less than][10.sup.-14] s) timescale. Groups worldwide now accept their technique and use their findings in related studies. The researchers have also examined detailed back electron transfer and electrolyte quenching dynamics in working cells by applying nanosecond ultraviolet and visible transient absorption spectroscopy. Their findings indicate that subtle changes in dye molecular structure affect the electron injection yields and overall recombination rates. One exciting discovery is that substituting tin oxide ([SnO.sub.2]) for [TiO.sub.2] sufficiently chang es the acceptor levels and electronic coupling efficiencies to produce cells with absorbed photon-to-current efficiencies approaching 40 %, constant across the visible spectrum. These findings suggest that minor modifications to the adsorbed dye and substrate properties could lead to cells with efficiencies approaching theoretical conversion limits.