Article pubs.acs.org/JPCC
Modeling Diffusion of Tin into the Mesoporous Titanium Dioxide Layer of a Dye-Sensitized Solar Cell Photoanode Julienne Kabre and Robert J. LeSuer* Department of Chemistry and Physics, Chicago State University, Chicago, Illinois 60628, United States S Supporting Information *
ABSTRACT: 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.
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INTRODUCTION Dye-sensitized solar cells (DSSC) have received a lot of attention since the initial introduction of a mesoporous semiconductor as a high surface area photoanode.1 The typical DSSC consists of a conductive glass substrate coated with a thin film of titanium dioxide nanoparticles.2 The sol−gel thin film of titanium dioxide can be formed through a variety of methods,3 including low-temperature procedures,4 from either commercially supplied or synthesized nanoparticles.5 However, one of the more frequently used techniques is doctor blading followed by a sintering step. Films that are ∼10 μm thick can be fabricated with relative ease using this method; however, it has been noted that more structured films may provide improved photophysical properties.6 The morphology of doctor-bladed films is generally altered via two processes. Thermal pretreatment of titanium dioxide nanoparticles has been shown to increase the amount of dye that can be adsorbed onto the surface.7 Also, inclusion of poly(ethylene glycol) in the TiO2 slurry used for doctor blading can alter the film porosity in a systematic manner.8 Much of the theoretical framework built for DSSCs has focused on transport of the redox electrolyte through the DSSC9 and photoinduced electron transport through the titanium dioxide nanoparticle network.10,11 While little work has focused on the semiconductor/ conductive glass interface of a DSSC, it is known that Sn4+ © 2012 American Chemical Society
doping of TiO2 leads to alteration of the electronic and structural properties of the semiconductor.12,13 Rutile SnO2 is known to promote anatase-to-rutile converstion of TiO214 which can be problematic in a DSSC given that anatase, the less thermodynamically stable polymorph, is preferred for DSSC applications.15 A better understanding of this interface is critical as DSSC technology is applied to flexible or other novel substrates. Recently, it was shown that tin from the conductive coating of TEC-8 glass appears to diffuse into the titanium dioxide film.16 Energy dispersion spectroscopy (EDS) of 7 μm TiO2 films on conductive glass that were subjected to focused ion beam (FIB) milling showed the tin signal expanding 3−4 μm into the TiO2 layer. It was suggested that mass transport of the tin is due in part to the high temperature used in the sintering step, and it may influence the overall performance of the photoanode. Quantifying the effect of tin diffusion on the overall performance of a DSSC is complicated by the concerted influence temperature has on the structure of the titanium dioxide film and the soda lime glass substrate. In particular, for optimal photoanode performance, the titanium dioxide nanoReceived: June 21, 2012 Revised: July 26, 2012 Published: August 15, 2012 18327
dx.doi.org/10.1021/jp3061366 | J. Phys. Chem. C 2012, 116, 18327−18333
The Journal of Physical Chemistry C
Article
particles should be in the anatase form. At temperatures above 800 °C the structure of the nanoparticles begins to transform into the rutile phase.17,18 At low (
