**2. Experimental procedure**

476 Solar Cells – New Aspects and Solutions

dyes, and larger intrinsic dipole moments leading to rapid charge separation (Underwood et al., 2001). The demonstration of multiple exciton generation (MEG) by impact ionization has fostered an interest in colloidal semiconductor QDs (Schaller et al, 2006; Trinh et al., 2008). One of the most attractive configurations to exploit these fascinating properties of semiconductor QDs is the quantum dot-sensitized solar cell (QDSC) (Nozik, 2002; Klimov, 2006). The efficient formation of more than one photoinduced electron-hole pair (exciton) upon the absorption of a single photon is a process not only of a great current scientific interest but is potentially important for optoelectronic devices that directly convert solar radiant energy into electricity. The demonstration of MEG by impact ionization in colloidal semiconductor QDs could push the thermodynamic photovoltaic conversion efficiency limit of solar cells up to 44% (Klimov, 2006) from the current 31% of the Shockley-Queisser detailed balance limit (Shockley & Queisser, 1961). The optimization of QDSCs can benefit from the intensive effort carried out with DSC. Although the photovoltaic conversion efficiencies of QDSCs lag behind those of DSCs and the use of semiconductor QDs as light absorbers requires the development of new strategies in order to push the performance of QDSCs, QDSCs have attracted significant attention among researchers as promising third-

In this chapter, we describe the performance of QDSCs based on CdSe QD sensitizer on nanostructured TiO2 electrode with a pre-adsorbed layer of CdS QDs (termed combined CdS/CdSe QDs) proposed by Niitsoo et al (Niitsoo et al, 2006) and developed by other groups (Lee & Lo, 2009; Sudhagar et al., 2009). They showed that a pre-adsorbed layer of CdS prior to CdSe adsorption improved the QDSC's performance. Hence it is interesting and useful to investigate the detailed function of combined CdS/CdSe QDs sensitizer on performance of QDSCs, together with the basic studies of optical absorption and photocurrent characteristics. Information regarding the optical absorption properties is initially necessary in order to investigate the electronic states of combined CdS/CdSe QDs for future photovoltaic cell applications. However, few accurate studies of the optical absorption properties of combined CdS/CdSe QDs adsorbed on nanostructured TiO2 electrodes have been carried out. The main reason for this is the difficulty in using the conventional transmission method because of strong light scattering by the highly porous structure of the nanostructured TiO2 electrodes. However, scattering effects can be minimized by employing the photothermal (PT) technique. In general, an optically excited solid relaxes to thermal equilibrium by the emission of photons (radiative processes) or phonons (nonradiative processes). In the PT technique, the signal detected is directly proportional to the thermal energy (heat production) induced by the absorbed photons through nonradiative processes (emission of phonons). Heat production by nonradiative processes has been detected by several methods (Tam, 1986). The PT signal is less sensitive to light scattering effects than conventional spectroscopy signals, and the ability of the PT technique to produce optical absorption spectra from strongly scattering media has been demonstrated, in particular using photoacoustic (PA) method which is a PT technique (Inoue et al., 2006; Toyoda et al., 2009). Thus, the PT technique is a useful technique for studying the optical absorption spectra of the strongly scattering and/or opaque samples with which we are dealing in our investigations. PA method detects the acoustic energy produced by heat generation through nonradiative processes in materials (Rosencwaig & Gersho, 1977). The PA cell, which is a small gas-tight enclosure with a sensitive acoustic microphone built in one wall, monitors the temperature changes in the sample produced by absorbed photons through nonradiative processes. Periodic temperature changes in the

generation photovoltaic devices.

Experimental sections are divided into three sections, 1) sample preparations on TiO2 electrodes and CdS/CdSe quantum dots, 2) optical absorption measurements by photoacoustic technique, and 3) photoelectrochemical current (incident photon to current conversion efficiency) and photovoltaic measurements.
