Ultrafast Electron and Hole Dynamics in CdSe Quantum Dot Sensitized Solar Cells

Abstract

A potential candidate for next-generation solar cells is dye-sensitized solar cells (DSSCs). Much attention has been directed toward DSSCs employing nanostructured TiO2 electrodes and organic-ruthenium dye molecules as the light-harvesting media. The high porosity of nanostructured TiO2 film enables a large concentration of the sensitizing dye molecules to be adsorbed. The attached dye molecules absorb light and inject electrons into the TiO2 conduction band upon excitation. The electrons are then collected at a back conducting electrode, generating a photocurrent. DSSCs exhibit high photovoltaic conversion efficiencies of about 11% and good long-term stability. In addition, they are relatively simple to assemble and are low-cost (O’Regan & Gratzel, 1991; Gratzel, 2003; Chiba et al., 2006). However, in order to replace conventional Si-based solar cells in practical applications, further effort is needed to improve the efficiency of DSSCs. A great amount of work has been done on controlling the morphology of the TiO2 electrodes by employing ordered arrays of nanotubes, nanowires, nanorods and inverse opal structures (Adachi et al., 2003; Paulose et al., 2006; Law et al., 2005; Song et al., 2005; Nishimura et al., 2003) in order to improve the electron transport and collection throughout the device. Another important factor in improving the performance of DSSCs is the design of the photosensitizer. The ideal dye photosensitizer for DSSCs should be highly absorbing across the entire solar light spectrum, bind strongly to the TiO2 surface and inject photoexcited electrons into the TiO2 conduction band efficiently. Many different dye compounds have been designed and synthesized to fulfill the above requirements. It is likely that the ideal photosensitizer for DSSCs will only be realized by co-adsorption of a few different dyes, for absorption of visible light, near infrared (NIR) light, and/or infrared (IR) light (Polo et al., 2004; Park et al., 2011). However, attempts to sensitize electrodes with multiple dyes have achieved only limited success to date. Narrow-band-gap semiconductor quantum dots (QDs), such as CdS, CdSe, PbS, and InAs, have also been the subject of considerable interest as promising candidates for replacing the sensitizer dyes in DSSCs (Vogel et al., 1990, 1994; Toyoda et al., 1999, 2003; Peter et al., 2002; Plass et al., 2002; Shen et al., 2004a, 2004b, 2006a, 2006b, 2008a, 2008b, 2010a, 2010b; Yu et al., 2006; Robel et al., 2006; Niitsoo et al., 2006; Diguna, et al., 2007a , 2007b; Kamat, 2008, 2010; Gimenez et. al., 2009; Mora-Sero et al., 2009, 2010). These devices are called QD-sensitized solar cells (QDSCs) (Nozik, 2002, 2008; Kamat, 2008). The use of semiconductor QDs as sensitizers