Fe(II)-Polypyridines as Chromophores in Dye-Sensitized Solar Cells: A Computational Perspective.

Abstract

Over the past two decades, dye-sensitized solar cells (DSSCs) have become a viable and relatively cheap alternative to conventional crystalline silicon-based systems. At the heart of a DSSC is a wide band gap semiconductor, typically a TiO2 nanoparticle network, sensitized with a visible light absorbing chromophore. Ru(II)-polypyridines are often utilized as chromophores thanks to their chemical stability, long-lived metal-to-ligand charge transfer (MLCT) excited states, tunable redox potentials, and near perfect quantum efficiency of interfacial electron transfer (IET) into TiO2. More recently, coordination compounds based on first row transition metals, such as Fe(II)-polypyridines, gained some attention as potential sensitizers in DSSCs due to their low cost and abundance. While such complexes can in principle sensitize TiO2, they do so very inefficiently since their photoactive MLCT states undergo intersystem crossing (ISC) into low-lying metal-centered states on a subpicosecond time scale. Competition between the ultrafast ISC events and IET upon initial excitation of Fe(II)-polypyridines is the main obstacle to their utilization in DSSCs. Suitability of Fe(II)-polypyridines to serve as sensitizers could therefore be improved by adjusting relative rates of the ISC and IET processes, with the goal of making the IET more competitive with ISC. Our research program in computational inorganic chemistry utilizes a variety of tools based on density functional theory (DFT), time-dependent density functional theory (TD-DFT) and quantum dynamics to investigate structure-property relationships in Fe(II)-polypyridines, specifically focusing on their function as chromophores. One of the difficult problems is the accurate determination of energy differences between electronic states with various spin multiplicities (i.e., (1)A, (1,3)MLCT, (3)T, (5)T) in the ISC cascade. We have shown that DFT is capable of predicting the trends in the energy ordering of these electronic states in a set of structurally related complexes with the help of appropriate benchmarks, based either on experimental data or higher-level ab initio calculations. Models based on TD-DFT and quantum dynamics approaches have proven very useful in understanding IET processes in Fe(II)-polypyridine-TiO2 assemblies. For example, they helped us to elucidate the origin of "band selective" sensitization in the [Fe(bpy-dca)2(CN)2]-TiO2 assembly (bpy-dca = 2,2'-bipyridine-4,4'-dicarboxylic acid), first observed by Ferrere and Gregg [ Ferrere , S. ; Gregg , B. A. J. Am. Chem. Soc. 1998 , 120 , 843 . ]. They also shed light on the relationship between the linker group that anchors Fe(II)-polypyridines onto the TiO2 surface and the speed of IET in Fe(II)-polypyridine-TiO2 assemblies. More interestingly, our results show that the IET efficiency is strongly correlated with the amount of electron density on the linker group and that one can obtain insights into the IET in dye-semiconductor assemblies based on ground state electronic structure calculations alone. This may be useful for quick screening of a large number of complexes for use as potential sensitizers in DSSCs, especially if followed up by TD-DFT and quantum dynamics simulations for selected target compounds to confirm efficient sensitization. While our focus over the past few years has been exclusively on Fe(II)-polypyridines, the computational strategies outlined in this Account are applicable to a wide variety of sensitizers.