Olive-shaped ZnO nanocrystallite aggregates as bifunctional light scattering materials in double-layer photoanodes for dye-sensitized solar cells

Abstract

Olive-shaped ZnO nanocrystallite aggregates were synthesized using an economical and environment-friendly aqueous-solution method. The submicron-sized hierarchical nanostructure is composed of highly crystalline ZnO nanoparticles about 20nm in diameter and has an overall dimension of approximately 150nm×300nm. We employed a low-temperature (150°C for 1h) thermal treatment process for the fabrication of bi-layer photoelectrode, with commercial ZnO nanoparticles (∼20nm) as the under-layer and submicron-sized structures as the light-scattering over-layer. The N719-sensitized dye-sensitized solar cells incorporating the aggregate over-layer reached a higher power conversion efficiency of 4.43% than those incorporating a solid scatterer over-layer (3.57%). 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.