Photovoltaic Performance of TiO2 Dye Sensitized Solae Cells Coated with ZnO
Authors: Murali Ramakrishna 
Published: February 2020

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 VOC and Jsc 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 JSC values. For further characterization of TiO2/ZnO DSSC, the incident photo-to-current conversion efficiency (IPCE) or externalquantum efficiency (EQE) test was performed.