Dye sensitization of low-index atomically flat TiO₂ surfaces

Abstract

Dye-sensitization of large band-gap semiconductors has been known for over a century since this is the process used in silver halide photography. There has been a renewed interest in this topic following Gratzel's invention of an efficient dye sensitized solar cell. A solar-to-electric energy conversion efficiency of over 10% has been obtained for this system. However, due to the large surface area and irregular structure of the nanocrystalline titanium dioxide (TiO2), there is still little fundamental information about the interface between the semiconductor and the adsorbed dye layer. We use atomically flat single crystal surfaces to help understand the binding of dye molecules to various TiO2 surfaces. Atomically flat surfaces of anatase (101), (001) and rutile (100), (001) single crystals were successfully prepared and their surface flatness verified with atomic force microscopy (AFM). Dye sensitization with the ruthenium complex dye N3 (cis-di(thiocyanato)-bis(2,2'-bipyridyl-4,4'- dicarboxylate) ruthenium(II)) was studied on these surfaces. The adsorption isotherms were measured on all four surfaces. Rutile (100) and anatase (101) gave higher incident photon to current conversion efficiency (IPCE) values than the other two surfaces. Study of adsorption kinetics revealed a two-step adsorption process and the slow adsorption step was fit with a Langmuir kinetic model. The adsorption and desorption rate constants for each surface were derived by fitting the adsorption data. Differences in IPCE were explained based on the structure of N3 and the geometry and reactivity of the binding sites on the four surfaces. A structurally similar series of carboxylated thiacyanine dyes were also employed in exploring the fundamentals of dye sensitized solar cells as an attempt to understand the interaction of organic dye molecules and TiO2 surfaces. A simple method for measuring the surface coverage of these adsorbed dyes onto TiO2 surfaces was developed. Quantum yields for sensitized photocurrent generation were studied and dimer and extended H-aggregates formation of some dye molecules were resolved. We attempt to understand these experimental results with structural models of the TiO2 surfaces.