Solvent-assisted microstructural evolution and enhanced performance of porous ZnO films for plastic dye-sensitized solar cells

A low-temperature process for fabricating porous ZnO films on plastic, indium tin oxide-coated polyethylene naphthalate substrates is developed for their use in dye-sensitized solar cells. A special attention is paid to modification of microscopic morphologies for enhancing interparticle connection. ZnO films having two kinds of macroscopic morphologies (flower-like particles and densely packed nanoparticles) are fabricated at temperatures below the heatproof temperature of the substrate, and subsequently immersed in mixed solvents composed of water and ethanol at 90 °C. The immersion leads to the growth of constituting ZnO particles and also the evolution of interparticle connection, depending on solvent compositions. The cell performance is largely improved especially in a short-circuit current density and a power conversion efficiency. The immersion effect is more remarkable for the cell using the densely packed ZnO film, with a 62% increase in the current density and an 84% increase in the conversion efficiency. In consequence, our plastic N719-sensitized ZnO cell shows the conversion efficiency as high as 4.1%.