Well-aligned arrays of vertically oriented ZnO nanowires electrodeposited on ITO-coated glass and their integration in dye sensitized solar cells

Abstract

We report on the effects of post-growth hydrothermal treatment and thermal annealing on properties of vertically aligned ZnO nanowires arrays (NWs). The samples were electrochemically deposited (ECD) on indium–tin oxide (ITO)-coated glass substrates and subjected to post-growth hydrothermal treatment (HT) at 150 °C and, for the purpose of comparison, to conventional thermal annealing (CTA) in a furnace at 150, 400, and 600 °C in air. Sample characterization was realized using X-ray diffraction (XRD), SEM, TEM, selected-area electron diffraction (SAED) and photoluminescence (PL). Thermal annealing does not induce significant changes of morphology, but influences the structural and optical properties. At the same time we found that the HT induces more significant improvement of properties of ZnO nanowires arrays (ZnO NWs) on ITO. The results show that the ECD ZnO NWs are single-crystalline with hexagonal structure and c-axis perpendicular to ITO substrate. Only one peak at about 379 nm was observed in the photoluminescence spectra at room temperature which showed an intensity increase after hydrothermal treatment. This corresponds to the increase of the optical quality of ZnO NWs. The best optical quality for ZnO NWs was found after the hydrothermal treatments at 150 °C in our experiment. The high-quality electrodeposited NW layers have been used, after sensitization with the highly absorbing D149 dye, as a photoanode in dye sensitized solar cells (DSCs) and the impact of post-growth treatment of the nanowires on DSCs performances has been evaluated. The photocurrent of the solar cells increased significantly after HT or CTA at 150 °C leading to a maximum overall photovoltaic conversion efficiency (PCE) of 0.66% at 100 mW/cm 2, based on short-circuit photocurrent density, open-circuit voltage and fill factor of 3.283 mA/cm 2, 0.606 V and 33.3%, respectively. The obtained results are interesting in view of the low layer roughness and pave the way for implementation of high-quality electrodeposited ZnO NW arrays in DSCs fabrication.