Effects of multilayer coating and calcination procedures on the morphology of dye-sensitized solar cell semiconductor photoelectrodes

The thickness of the semiconductor film of the photoelectrode is a crucial parameter for the performance of a dye-sensitized solar cell (DSC). The simplest fabrication method of this mesoporous film is by the doctor blade technique. In order to increase the thickness, successive layers are spread and sintered, which provides higher quality films than the direct application of a single thicker layer. However, this multilayer deposition method can produce unexpected effects in the photoelectrode morphology, but these have been scarcely reported in the bibliography. In this work, the morphology and thickness of multilayered photoelectrodes consisting of TiO2 nanoparticles are studied using different microscopy techniques and gas adsorption isotherms. The thickness is varied from 1 to 6 layers (4 to 40μm) and two different sintering methods are compared. Concomitant DSCs are fabricated with a natural dye and a liquid electrolyte, and their energy conversion efficiencies are determined. The highest efficiency is obtained when 3 layers are deposited using the multi-calcination sintering method, yielding a total thickness of around 24μm. In contrast to the findings of previous papers, no longitudinal gaps between different coating layers have been detected. Nevertheless, multilayer films have cracks, both internal and open to the surface, which could enhance electrolyte diffusion and increase dye-adsorption sites. In fact, the 3-layer film, which makes the most efficient DSC, has the highest crack percentage. In contrast, for multicalcined 4-layer films the structure collapses, indicated by a negligible increase in thickness and a decrease in film porosity and specific surface area. Moreover, its cell's efficiency falls 50% relative to the 3-layer film cell.