The ZnO sol-gel spin-coating method is a combination of two techniques applied to produce thin nanostructured ZnO films: The sol-gel technique consists of the prepara-tion, under defined conditions, of a ZnO solution from the mixture of a ZnO precursor [Zn(CH3COO)2·2H2O: zinc acetate dehydrate] and a solvent (2-methoxyethanol) with the addition of a stabilizing substance (ethanola-mine). The clear transparent colloidal solution thus ob-tained, containing suspended ZnO nanoparticles, is de-posited on the surface of a substrate by the spin-coating technique [1-4]. Nanostructured ZnO films prepared by this method have been used as photo-electrodes in DSSC fabrication [5,6], but low efficiencies were recorded compared to TiO2-based DSSCs. However, the deposi-tion of a ZnO layer on the surface of a nanoporous TiO2 film to improve the DSSC photovoltaic properties is an active issue in the context related to surface treatment of TiO2 photo-electrodes by thin metal oxide films [7-11]. Kao et al. [12] employed the sol-gel spin-coating method to prepare TiO2 and ZnO spin-coated TiO2 electrodes for DSSCs. Multilayer coating was applied to TiO2 to reach the required thickness and a monolayer of ZnO was ap-plied to spin-coat the TiO2 film. All the photovoltaic pa-rameters were increased upon ZnO spin-coating, result-ing in the promotion of the power conversion efficiency η from 2.5% to 3.25%. In this paper, we use the sol-gel technique to prepare ZnO solutions of 5 different pre-cursor concentrations (0.1, 0.2, 0.3 and 0.4 M). An amount of 5 drops from each solution is used to deposit a ZnO layer on the TiO2 photo-electrode surface by spin- coating. The measured efficiency of the associated DSSC shows a variation with the ZnO precursor concentration, exhibiting a maximum at 0.1 M, beyond which it shows a sharp decrease to 0%. It is explained that this optimal amount of the deposited ZnO forms the thinnest layer that can create the required energy barrier to reduce the rate of back-recombination of electrons to dye molecules and electrolyte species, enhancing the photocurrent with- out affecting the dye-adsorption efficiency of the TiO2 film.