**2.1 Sample preparation**

The method for the preparation of nanostructured TiO2 electrodes has been reported in a previous paper (Shen & Toyoda, 2003). A TiO2 paste was prepared by mixing 15 nm TiO2 nanocrystalline particles (Super Titanai, Showa Denko; anatase type structure) and polyethylene glycol (molecular weight: 500,000) in pure water. The resultant paste was then deposited onto transparent conducting substrates [F-doped SnO2 (FTO), sheet resistance: 10 μΩ/sq]. The TiO2 electrodes were then sintered in air at 450 ºC for 30 min to obtain good necking and to sublimate polyethylene glycol. The highly porous nanostructure of the films (the pore sizes were on the order of a few tens of nanometers) was confirmed from scanning electron microscopy (SEM) images. The thicknesses of the films were measured and found to be ~ 5 μm by examining the cross sectional SEM images.

At first, CdS QDs were adsorbed onto nanostructured TiO2 electrodes (pre-adsorbed layer) from the common NH3 bath with a solution composition of 20 mM CdCl2, 66 mM NH4Cl, 140 mM thiourea, and 0.23 M ammonia to obtain a final pH ~9.5 (Niitsoo et al., 2006; Jayakrishnan et al., 1996). The TiO2 electrodes were immersed in a container filled with the final solution. The adsorption was carried out at room temperature in the dark for 40 min.

The CdSe QDs were prepared by using a chemical bath deposition (CBD) technique (Shen & Toyoda, 2004; Shen et al., 2004; Gorer & Hodes, 1994). First, for the Se source, an 80 mM sodium selenosulphate (Na2SeSO3) solution was prepared by dissolving elemental Se powder in a 200 mM Na2SO3 solution. Second, an 80 mM CdSO4 and 120 mM of a trisodium salt of nitrilotriacetic acid [N(CH2COONa)3] were mixed with the 80 mM Na2SeSO3 solution in a volume ratio of 1: 1: 1. TiO2 electrodes adsorbed with CdS QDs were placed in a glass container filled with the final solution at 10 ºC in the dark for various times (from 2 to 24 h) to promote CdSe QDs adsorption. To investigate the role of pre-adsorbed layer of CdS QDs, the CdSe QDs only were adsorbed directly on nanostructured TiO2 electrodes with the same sample preparation conditions as mentioned above.

After the adsorption of CdSe QDs, the samples were coated with ZnS for surface passivation of the QDs by successive ionic layer adsorption and reaction (SILAR) for three times in 0.1

Optical Absorption and Photocurrent Spectra of CdSe Quantum

as far as possible to compare each of the PA signals and spectra directly.

**2.3 Photoelectrochemical current and photovoltaic measurements** 

Photocurrent measurements were performed in a sandwich structure cell (i.e., in the twoelectrode configuration) with Cu2S film on brass as the counter electrode (termed the Cu2S counterelectrode). The applied electrolyte was polysulfide solution (1 M Na2S + 1 M S). It is well known that the electrocatalytic activity of Pt with a polysulfide electrolyte is not satisfactory for photovoltaic cell applications and alternative counter electrode materials with higher activity such as Cu2S and CoS have been reported (Hodes et al., 1980). The higher electro-catalytic activities of these materials are due to a reduction in the charge transfer resistance between the redox couple and the counterelectrode (Giménez et al. 2009). The Cu2S counterelectrodes were prepared by immersing brass in HCl solution at 70ºC for 5 min and subsequently dipping it into polysulfide solution for 10 min, resulting in a porous Cu2S electrode (Hodes et al., 1980). The cells were prepared by sealing the Cu2S counter-electrode and the nanostructured TiO2 electrode adsorbed with combined CdS/CdSe QDs, using a silicone spacer (~ 50 μm) after the introduction of polysulfide electrolyte. The IPCE value was evaluated from the short-circuit photocurrent with a zero-shunt meter using the same apparatus and conditions as those used for the PA measurements. The incident light intensity was measured by an optical power-meter. The spectra were taken at room temperature in the wavelength of 250 – 800 nm. The conditions for all the measurements (optical configuration, path-length, irradiation area, excitation light intensity etc.) were fixed as far as possible to compare the IPCE values and spectra directly. Photovoltaic properties were characterized under a one sun illumination (AM 1.5: 100 mW/cm2) using a solar simulator by the

measurements of photocurrent versus photovoltage to investigate Jsc, Voc, FF, and η.

Figure 2 shows the PA spectra of the nanostructured TiO2 electrodes adsorbed with combined CdS/CdSe QDs for different adsorption times, together with that adsorbed with CdS QDs only. The pre-adsorption times for CdS QDs were fixed at 40 min (average diameter: ~ 4.2 nm).

**3. Results and discussion** 

Dots Adsorbed on Nanocrystalline TiO2 Electrode Together with Photovoltaic Properties 479

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

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 & Sionnet, 1996).
