Dye-sensitized solar cells based on hierarchical flower-like metal oxide semiconductors.

摘要

Dye-sensitized solar cells (DSSCs) are widely recognized as one of the most promising of several alternative, cost-effective concepts for solar-to-electric energy conversion that has been offered to challenge conventional Si solar cells over the past two decades. The major components of a DSSC include an n-type semiconductor (e.g., TiO2), a sensitized (i.e., dye), and a redox electrolyte. A sensitizer is chemically tethered to the semiconductor surface by functional anchoring moieties (usually carboxyl group) to harvest a broad range of spectrally distributed light and transfer energy from absorbed photons to excite electrons. TiO2 is one of the most widely used n-type large band gap semiconductor with an energy band gap of 3.2 eV. However, the cell efficiency is limited due to the high charge recombination rate and the low electron mobility characteristics of TiO2.;In this study, we utilized ZnO as the n-type semiconductor to fabricate dye-sensitized flower-like ZnO solar cells as ZnO possesses a wide band gap similar to TiO2 and much higher electron mobility than TiO2. Moreover, ZnO carries advantages of being easy crystallization and anisotropic, making the fabrication process viable. Two methods were employed to craft ZnO nanostructures: hydrothermal and chemical bath deposition, from which hierarchically structured ZnO flowers were yielded. Such hierarchical structures combined the advantages of 1D nanostructures which have direct pathway for electron transport, and nanoparticles which offer a large surface area and thus increased dye loading. In addition, the hierarchical structures also facilitate the light trapping.;The hydrothermal process was systematically explored by varying precursor concentration, alkaline condition, reaction time, and reaction temperature. The precursor of chemical bath deposition approach was different from that of hydrothermal process and thus leads to a different mechanism. All nanostructures were made on fluorine-doped tin oxide (FTO) glass and the morphology was examined by SEM and the composition and crystallinity was verified by XRD. Subsequently, all samples were fabricated into DSSCs. The cell performance was different from different nanostructures and the optimized nanostructures with the highest efficiency were then converted to TiO2. The TiO2 structures were prepared by one-step synthesis (i.e., liquid phase deposition) using the as-prepared ZnO as template. During the deposition process, the dissolution of ZnO and the precipitation of TiO2 occurred simultaneously. Therefore the morphology of TiO2 depended on ZnO nanostructures, yielding a hollow nanotubular structures. After conversion, the TiO2 hierarchical structures on the FTO glass were fabricated into devices and the cell performance was evaluated.