Abstract :
Dye-sensitized solar cells (DSSCs) convert solar energy to electricity employing dye molecules attached to a semiconductor surface. Some of the most efficient DSSCs use Ru-based chromophores. Fe-based dyes represent a cheaper and more environmentally friendly alternative to these expensive and toxic dyes. The photoactive state of Fe-based chromophores responsible for charge-separation at the dye-semiconductor interface is, however, deactivated on a sub-picosecond time scale via the intersystem crossing (ISC) into a low-lying photo-inactive quintet state. Therefore, development of Fe-based dyes capable of fast interfacial electron transfer (IET) leading to efficient charge separation on a time scale competitive with the ISC events is important. This research investigates how linker groups anchoring Fe(bpy)2(CN)2, (bpy = 2,2’-bipyridine), a prototypical Fe-based dye, onto the TiO2 semiconductor surface, influence the IET rates in the dye/semiconductor assemblies. Linkers groups investigated include carboxylic acid, phosphonic acid, hydroxamate, and catechol. We employ time-dependent density functional theory (TDDFT) to obtain absorption spectra of Fe(bpy)2(CN)2 with each linker, and quantum dynamics simulations to investigate the IET rates between the dye and the (101) TiO2 anatase surface. For all attachments, TDDFT calculations show similar absorption spectra with two main bands corresponding to the metal-to-ligand charge transfer transitions. Hydroxamate linker couples the dye to the semiconductor most efficiently, leading to the fastest IET rates, the phosphonic acid linker exhibits the slowest IET rates. Utilizing the hydroxamate linker will lead to the most efficient IET and photon-to-current conversion efficiencies of the Fe(bpy)2(CN)2 – sensitized solar cells.
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