**2.3 Photoelectrochemical current and photovoltaic measurements**

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

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.
