Dye-sensitized solar cells (DSSCs) are one of the most promising alternative renewable energy sources, which have attracted much attention due to their high conversion efficiency, simple manufacturing process, low production cost, and non-toxicity. TiO₂ nanostructures have been mostly used in DSSCs because of their unique photovoltaic and photochemical properties. Mainly, they can provide high surface area for efficient dye adsorption and diffusion pathway for charge transfer processes. Recently, many efforts have been done to improve the conversion efficiency of nanostructured TiO₂ photoanodes. One of the promising approaches to enhance the performance of TiO₂ -based DSSCs is the suppression of recombination reactions at the TiO₂ /dye/electrolyte interfaces. Due to a wide direct band gap (3.37 eV) and the large excitation binding energy (60 meV) of ZnO nanostructures, they have been supposed to be an appropriate inherent barrier layer between the photoelectrode and electrolyte in TiO₂ -based DSSCs. In the present work, the sol gel dip coating method was adopted for the deposition of ZnO films on TiO₂ NPs. This is a very versatile and simple technique, that does not need any vacuum system of sophisticated instrumentation.

F-doped SnO₂ conducting glass (FTO) was used as substrates. They were _cleaned under ultrasonic treatment (5 min in acetone, 5 min in alcohol), rinsed with double distilled water, and finally treated for 2 min in a 45% nitric acid solution prior to drying in nitrogen atmosphere, and used to make both the working and counter electrodes. A 2M aqueous TiCl₄ solution was obtained by dilution of concentrated TiCl₄ (Across chemicals) with pre-cooled distilled water solvent in an ice bath and kept in refrigerator. Doctor Blade technique was acquired to deposit TiO₂ paste on FTO layers. The coated electrodes were annealed gradually under airflow in programmed heating steps. ZnO films were deposited on the TiO₂ NPs by the following procedure: 1 mM of zinc nitrate (AR, Emerck) was dissolved in ethanol (solution A). 1 ml of this solution A was diluted diluted 10 times to obtain 1x 10^-4 M solution (solution B), 1 ml of this solution B was durther diluted ten times to obtain 1 x 10^-5 M solution (solution C), 1 ml of solution C was diluted ten times to obtain 1 x 10^-6 M solution. (solution D). 30 ml of above solutions were separately taken in a 50 ml beaker kept at 75°C, prior to dipping the TiO₂ NPs, the pH of the four solutions was adjusted to 9 by NH₄OH. Four separate coated TiO₂ NPs were dipped individually in the above four Zn²⁺ solutions. The dipping time was kept constant at 30s. For Dynamic light scattering experiments (DLS), ZnO films were coated under identical conditions on clean glass substrates with the above solutions. The films were withdrawn from the solutions after 30 min and washed with ethanol and dried at room temperature. These four films were further heated in air at 350°C for ZnO formation.

Dynamic light scattering (DLS) method was used to study the particle size distribution of the ZnO nanostrures on TiO₂. Fig.2 shows the distribution, It is observed that the particle size increases with increase of Zn²⁺ concentration from 10^-5 M to 10^-3 M.

The slight improvement of V_OC and J_sc values obtained after ZnO coating, indicates the effect of ZnO. The effect of ZnO NPs shifts the Fermi level of TiO₂ NPs [16] provides extra traps on TiO₂ nanoporous structure to capture the carriers and reduce the rate of recombination reactions. In fact, the deposition of the appropriate amount of ZnO NPs could act as charge trapping to capture the carriers, which resulted in suppressing the recombination reactions and subsequently enhancing the J_SC values. For further characterization of TiO₂/ZnO DSSC, the incident photo-to-current conversion efficiency (IPCE) or externalquantum efficiency (EQE) test was performed.