Synthesis and characterization of ZnO nanowires and their integration into dye-sensitized solar cells

ZnO nanowires, grown on transparent conducting oxide substrates from aqueous solutions of methenamine and Zn(NO3)2, were integrated as the wide band gap semiconductor into dye-sensitized solar cells. ZnO nanowires and their growth mechanisms were studied using electron microscopy, x-ray diffraction and photoluminescence measurements. The solution growth method forms dense arrays of long nanowires oriented normal to the substrate surface because nanowires growing at off-normal angles are prevented from growing further when they run into neighbouring wires. Dye-sensitized solar cells with ZnO nanowires were assembled and characterized using optical and electrical measurements. Short circuit current densities of 1.3 mA cm−2, and overall power conversion efficiencies of 0.3% were achieved with 8 µm long nanowires. Photocurrent and efficiency increase with increasing nanowire length and improved light harvesting. Low surface area and a shunt that appears under light illumination limit the solar cell performance. Internal quantum efficiencies were similar for nanowires of all lengths, indicating that electron transport is not limited by the nanowire dimensions for aspect ratios less than 70.