Modeling Diffusion of Tin into the Mesoporous Titanium Dioxide Layer of a Dye-Sensitized Solar Cell Photoanode

Dye-sensitized solar cells (DSSC) utilize a photoanode consisting of a mesoporous semiconducting thin film coated onto a conductive substrate. Typically, the semiconductor is composed of titanium dioxide nanoparticles and the conductive substrate is a thin layer of fluorine-doped tin oxide on glass. Scanning electron microscopy coupled with energy dispersion spectroscopy (SEM/EDS) has been used to investigate mass transport of tin from the conductive layer into the mesoporous semiconductor. EDS maps of tin distribution through the photoanode cross section have been modeled using Fick’s second law of diffusion. Photoanodes fabricated using a doctor-blading method and sintering at temperatures ranging from 450 to 600 °C exhibit tin distributions in the TiO₂ layer corresponding to tin diffusion coefficients between 3.2 × 10^–5 and 59 × 10^–5 μm² s^–1. Diffusion of tin into the glass substrate is also observed, but at lower rates. The magnitude of the tin in TiO₂ diffusion coefficient is consistent with diffusion through grain boundaries.