The most striking property of QDs is the massive changes in electronic structure as a function of size. As the size decreases, the electronic excitations shift to higher energy, and the oscillator strength is concentrated into just a few transitions (Murray et al., 1993). Therefore, controlling quantum size confinement in monodisperse QDs is the most obvious method not only to extend the range of the QDs absorbance from the visible to near infrared range but also to align the energy levels with respect to the wide-bandgap nanostructure. Herein, the CdSe QDs and TiO2 system as a model is introduced for the direction of the optimization. The driving force for the electron separation and transfer is dictated by the energy difference between the conduction band energies. The conduction band of TiC2 is at – 0.5 V vs NHE. If we assume the larger CdSe particles have band energy close to the reported value of -0.8 V vs NHE, we can use the increase in bandgap as the increase in driving force for the electron transfer. Since the shift in the conduction band energy is significantly greater than the shift in valence band energy for quantized particles (Norris & Bawendi, 1996), we can expect the conduction band of CdSe QDs to become more negative (on NHE scale) with decreasing particle size. As the particle size decreases from 7.5 to 2.4 nm, the first excitonic peak shifts from 645 nm to 509 nm and the conduction band shifts from -0.8 V vs NHE to –
1. 31 V, the electron transfer rate improve by nearly 3 orders of magnitude (Robel et al., 2007). PbS nanocrystals have similar properties with the light absorption range extending from visible to near infrared (Hyun et al., 2008).
Another method that can broaden the spectral absorption range is the use of nanocomposite absorbers (Lee & Lo, 2009). Semiconductor QDs are excellent building blocks for more sophisticated nanocomposite absorbers, which the QDs combined with each other with different size or type. The well-known example of the nanocomposite is the combination of CdS and CdSe QDs. The combination can be used as co-sensitizers to provide enhanced performance compared to the use of each individual semiconductor QDs. When CdSe QDs are assembled on a TiO2/CdS electrode, the co-sensitized electrode (TiO2/CdS/CdSe) has an absorption edge close to that of TiO2/CdSe electrode but its absorbance is higher than those of TiO2/CdS and TiO2/CdSe electrodes both in the short wavelength region (<550 nm) where both CdS and CdSe are photoactive and long wavelength region (ca. 550-700 nm) which belong to the CdSe due to the complementary effect of the composite sensitizers. When CdS is located between CdSe and TiO2, both the conduction and valence bands edges of the three materials increase in the order: TiO2<CdS<CdSe, which is advantageous to the electron injection and hole recovery of CdS and CdSe. This clearly shows that nanocomposite absorbers can improve these systems through two different beneficial effects. Cn the one hand, the spectral absorption range can be broadened. Cn the other, the re-organization of energy levels between CdS and CdSe forms a stepwise structure of band – edge levels (Lee & Lo, 2009).