Enhanced Performance of Dye-sensitized Solar Cells Aided by Olive-shaped ZnO Nanocrystallite Aggregates as the Light-scattering Layer

Olive-shaped ZnO nanocrystallite aggregates were synthesized for dye-sensitized solar cells (DSSCs). The submicron-sized hierarchical nanostructure is composed of highly crystalline ZnO nanoparticles about 20 nm in diameter and has an overall dimension of approximately 150 × 300 nm. An economical and environment-friendly aqueous solution method was developed to synthesize the olive-like aggregate. This template-free self-assembly method involved the mixing of zinc nitrate and sodium hydroxide aqueous solutions at a low temperature (80 °C) and aging the mixture for a particular length of time. We employed a low-temperature (150 °C for 1 h) thermal treatment process for the fabrication of bilayer photoelectrode, with commercial ZnO nanoparticles (~ 20 nm) as the underlayer and submicron-sized structures as the light-scattering overlayer. The N719-sensitized DSSCs containing the aggregate overlayer reached a power conversion efficiency of 4.4 %, 33 % higher than that attained by DSSCs incorporating large solid particles (200–500 nm) as the scattering layer. The enhanced overall conversion efficiency of aggregate-based cells was correlated with a prominent increase in the short-circuit current density. Optical and dye-loading investigations show that this improvement can be attributed to the dual functionality of the olive-shaped nanocrystallite aggregates, which have excellent light-scattering ability to enhance photon capture while providing a large surface area for sufficient dye adsorption.