AmericanChemicalSociety.com

The solar conversion efficiency of dye-sensitized solar cells (DSSCs) is dependent on the short circuit current and the open circuit voltage. The short circuit current of a DSSC is dependent on the photoactive surface area of the cell. DSSCs have enormous surface areas, which lend to their relatively high efficiencies. However, efficiencies of DSSCs are kinetically limited by competition between recombination and transport of charge carriers within a wide bandgap semiconductor. These two factors can both be expressed by the (i) electron diffusion length of the system, which is a function of electron diffusion coefficients, and (ii) lifetime. The absence of an inherent electric field within the nanoparticles affects charge carrier transport and back reactions, resulting in diffusion coefficients and lifetimes that react in opposition. As a result, the electron diffusion length remains constant. Transport and recombination are the slowest two processes, both occurring on a millisecond timescale. These restrictions place an upper limit on the overall thickness of the nanoparticle film and thus inhibit maximum light absorption. However, device performance and diffusion length can be improved with better coverage of titania surfaces with dye because of a reduction in recombination events.

The impregnation of the titania nanocrystalline film is typically done by soaking the matrix in a solution of the dye. Previously, researchers showed that supercritical fluid (SCF) impregnation yielded efficiencies nearly 20% better than conventional dip coating processes. This process was significantly faster than dip coating, achieving maximum adsorption in minutes rather than hours, and the devices had improved electron diffusion lengths. This suggests that the thickness of the titania film could be increased if SCF impregnation were used. The goal of this research program is to further understand the effect of film thickness, porosity and pore diameter on the effectiveness of SCFs to improve dye impregnation and device performance. Therefore, two objectives are required to achieve this goal: (1) control the morphology and thickness of titania nanoparticle films and (2) determine the optimum conditions for dye impregnation.

Titania film preparation: Titania films are typically 10 microns thick in DSSC devices. The standard procedure for preparing these films is to spread a titania paste or spin-coat a titania suspension of nanoparticles on a transparent conductive oxide, such as indium doped tin oxide (ITO). These films are then sintered to improve particle inner-connectivity. Mechanical and stability problems occur when thicker semiconductor films are prepared, resulting in film cracking and delamination. We have developed a new process to generate thicker titania films on both ITO and fluorine doped tin oxide (FTO). These films are up to 25 microns thick in a single layer. Film delamination and cracking are significantly reduced by careful control over the processing parameters. We are also investigating the formation of titania films with different architectures to control porosity and improve electron transport in DSSCs.

Dye impregnation in sc-CO₂: Wet chemical impregnation approaches have significant problems with pore penetration at diameters of 1 – 100 nm because of the surface tension of the liquid. However, SCFs allow deep pore penetration (e.g. interstices between nanoparticles) because of the reduced surface tension of the solvent medium, reducing impregnation problems. The dye N-719 is often used as a photosensitizer in DSSCs. The solubility of N-719 dye in various ethanol/CO₂ mixtures was measured by introducing a known amount of dye into a high-pressure, variable-volume view cell equipped with a sapphire window for visual observation. Absorbance spectroscopy was used to measure the concentration as a function of temperature (35 – 60 °C) and pressure (100 – 400 bar). This data is important in defining the optimum conditions for dye impregnation into the titania films.

Future work will focus on measuring the dye impregnation as a function of titania thickness and morphology. The impregnation results will be correlated to device performance. The aim will be to determine (i) the film parameters where SCFs are necessary to impregnate the dye and (ii) if the thicker films enabled by SCF impregnation can further enhance the performance of DSSCs.