Photo-Induced Electron Transfer from Dye or Quantum Dot to TiO2 Nanoparticles at Single Molecule Level

Dye-sensitized solar cell (DSSC) has attracted wide attention for the potential application to convert sunlight into electricity. Organic dyes blended with TiO2 nanoparticles (NPs) have been recognized as important light harvesting materials especially in the visible spectral range (Hara et al., 2002; Bisquert et al., 2002; Gratzel, 2001; Ferrere & Gregg, 2001; Hagfeldt & Gratzel, 2000; Cahen et al., 2000). The functional materials are assembled in a sandwiched type to undergo photon-induced current process. Following photoexcitation, the embedded dye molecules may lead to electron transfer (ET) to the TiO2 conduction band. The injected electron flows through the semiconductor network and the external load to the counter electrode. At the counter electrode, the oxidized dye is reduced by electron donation from an electrolyte, and then the circuit becomes complete. The electron transfer kinetics in most dye/TiO2 systems is as rapid as in the time regime of femtosecond to several hundred picoseconds. The injected electrons are localized in either subband or surface states of TiO2 semiconductor. A fraction of the electrons, detrapped thermally from the reduced semiconductor, may possibly undergo recombination with the oxidized dye molecules. Such a back ET process takes place slowly from subnanoseconds to several milliseconds. An efficient solar cell deign should control lowering the rate of back electron transfer to prolong the lifetimes of charge-separated states. Therefore, characterizing kinetics of the forward and backward ET may be conducive to facilitating the working efficiency of a solar cell design. Among a variety of DSSC designs, Grätzel and coworkers have applied ruthenium-based dyes adsorbed on the TiO2 thin film, thereby achieving a very high power-conversion efficiency >11% (Gratzel, 2003, 2005). Despite a much lower efficiency in comparison, quantum dots (QDs) adopted recently to substitute for dyes have been popularly investigated including PbS (Plass et al., 2002; Ju et al., 2010), InAs (Yu et al., 2006), CdSe (Lee & Lo, 2009; Fan et al., 2010), CdS (Baker & Kamat, 2009; Lee & Lo, 2009), and PbSe (Luther et al., 2008; Choi et al., 2009). QDs have potential to be an alternative as electron donors (Robel et al., 2006; Kamat, 2008), for their unique properties such as size-dependent tunable energy gap (Yu et al., 2003; Kamat, 2008), a broad absorption band with large absorption cross sections (Yu et al., 2003), and multiple exciton generation (Yu et al., 2003; Luther et al., 2007; Kim et al., 2008; Sambur et al., 2010). When QDs absorb a photon to form an electron-hole