**2.2 Optical absorption measurements**

The optical absorption properties of nanostructured TiO2 electrodes adsorbed with combined CdS/CdSe QDs were investigated using PA spectroscopy. The scattering effects in the optical absorption of nanostructured TiO2 electrodes adsorbed with combined CdS/CdSe QDs can be minimized by employing the PA method. Figure 1 shows the schematic diagram of a photoacoustic spectrometer. Typical gas-microphone method was applied in the PA spectroscopic investigation (Rosencwaig & Gersho, 1977). The PA cell was composed of an aluminum cylinder with a small channel at the periphery in which a microphone (electret condenser type) was inserted (Shen &Toyoda, 2004). The inside volume of the cell was approximately 0.5 cm3. The cell was suspended by four rubber bands

Fig. 1. Schematic diagram of a photoacoustic spectrometer, composed with light source, monochrometer, mechanical chopper, cell, preamplifier, lock-in amplifier, etc.

M Zn(CH2COO) and 0.1 M Na2S aqueous solution for 1 min for each dip (Yang et al., 2002; Shen et al., 2008). In reference (Shen et al., 2008), we showed that the short-circuit-current density (Jsc), open-circuit voltage (Voc), and photovoltaic conversion efficiency (η) were enhanced by the ZnS coating, except for fill factor (FF). Although we applied the scanning electron microscopy (SEM) observation for visual investigation, we could not observe the difference of the morphologies with and without the ZnS coating up to 50,000 magnifications, indicating that the ZnS is coated with several atomic layers. In the future, we are going to observe the morphology of the ZnS coating layer by applying the transmission electron microscopy (TEM). To our knowledge, there are few reports in which ZnS coating has been applied to CdSe QD-sensitized solar cells, although ZnS-capped CdSe QDs dispersed in solution have been used for strong photoluminescence applications (Hines &

The optical absorption properties of nanostructured TiO2 electrodes adsorbed with combined CdS/CdSe QDs were investigated using PA spectroscopy. The scattering effects in the optical absorption of nanostructured TiO2 electrodes adsorbed with combined CdS/CdSe QDs can be minimized by employing the PA method. Figure 1 shows the schematic diagram of a photoacoustic spectrometer. Typical gas-microphone method was applied in the PA spectroscopic investigation (Rosencwaig & Gersho, 1977). The PA cell was composed of an aluminum cylinder with a small channel at the periphery in which a microphone (electret condenser type) was inserted (Shen &Toyoda, 2004). The inside volume of the cell was approximately 0.5 cm3. The cell was suspended by four rubber bands

**Chopper** 

Fig. 1. Schematic diagram of a photoacoustic spectrometer, composed with light source,

monochrometer, mechanical chopper, cell, preamplifier, lock-in amplifier, etc.

**Xe-Lamp Sample** 

**Lock-in Amplifier** 

**Microphone** 

**Pre-Amplifier**

**Data**

**Computer** 

**Optical Lens PA Cell** 

Sionnet, 1996).

**2.2 Optical absorption measurements** 

**Monochromator** 

to prevent interference from external vibration. The cell window made of quartz was highly transparent throughout the observed wavelength range, and the sample holder could be easily removed from the cell to maintain the optical configuration. A 300 W xenon lamp was used as the light source. Monochromatic light through a monochromator was modulated at 33 Hz using a mechanical chopper and was focused within the PA cell. Light was focused on the sample over an impinging area of 0.20 cm3. Modulation frequency of 33 Hz was determined to exclude the saturation effect of the spectrum. In this case (modulation frequency: 33 Hz), the optical absorption length is longer than the thermal diffusion length, indicating that the PA signal intensity is proportional to the optical absorption coefficient (no saturation effect) (Rosencwaig & Gersho, 1977). The PA signal was monitored by first passing the microphone output through a preamplifier and then into a lock-in amplifier. The data were averaged to improve the signal-to-noise ratio (S/N). The spectra were taken at room temperature in the wavelength range of 250 - 800 nm. The PA spectra were obtained by the normalization to the PA signal intensity of carbon black sheet that was proportional to the light intensity only. A UV cut filter was used for the measurements in the wavelength range of 600 - 800 nm to avoid the mixing of second harmonic light. The conditions for all the PA measurements (optical configuration, path-length, irradiation area, excitation light intensity etc.) were fixed
