Two 8？μm thick TiO2 photoelectrodes have been sensitized separately by N719 dye molecules and CdS quantum dots for a comparison study. Photoinduced absorption (PIA) spectroscopy was employed to investigate the mechanistic properties of electrons under illumination conditions comparable to sunlight. The PIA spectrum of both electrodes (in the presence of electrolyte) is due to electrons in TiO2 and iodine radicals in the electrolyte. In the absence of redox electrolyte, both electrodes show long-lived photoinduced charge-separation with lifetime in a millisecond range (8.5？ms for Q-dot-sensitized TiO2 and 11.5？ms for dye-sensitized TiO2). 1. Introduction Nanostructured solar cells sensitized by organic dyes (DSSCs) [1–6] or by inorganic short bandgap semiconductors (also called quantum dots, QDs) [7–10] have attracted a great deal of interest. They are capable to obtain efficient conversion of solar energy to electricity at a low cost comparative to conventional semiconductor photovoltaic devices [11, 12]. The approach of using semiconductor colloids for the design of optically transparent thin semiconductor films is considered as a unique and an alternative for the amorphous silicon solar cells. Using this approach, dye-sensitized solar cells based on bi- and polypyridyl ruthenium complexes have achieved solar-to-electrical energy conversion efficiencies of 10-11% under AM 1.5 irradiation [1–3]. On the other hand, wide bandgap semiconductors have also been sensitized by short bandgap quantum dots (CdSe/TiO2 [6], CdS/TiO2-SnO2 [10]) as alternative to dye sensitization. Vogel and coworkers [8] have investigated the sensitization of nanoporous TiO2, ZnO, and so forth by Q-sized CdS. Photocurrent quantum yields up to 80% and open-circuit voltages up to 1 V range were obtained. In contrast with the dye-sensitized solar cells, fundamental understanding of factors controlling the interfacial electron transfer reactions in QD sensitized solar cells is limited. In this paper, we report photoinduced absorption spectroscopy of an organic (N719 dye)-sensitized and inorganic (Q-dot CdS) semiconductor-sensitized TiO2 (8？μm) photoelectrodes under illumination conditions comparable to sunlight in order to compare the mechanistic properties of electrons. 2. Experimental 2.1. Preparation of Nanostructured TiO2 Films We take care more about similarity in TiO2 thicknesses in order to insure the same length of e-transfer from different sensitizer to external circuit. Before coating conducting glass ITO with TiO2 nanoparticles, first we coated a blocking layer by immersing
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