Influence of Electrolyte Composition on Ultrafast Interfacial Electron Transfer in Fe-Sensitized TiO2-Based Solar Cells

TiO2-based dye-sensitized solar cells employing Fe(2,2′-bipyridine-4,4′-dicarboxylic acid)2(CN)2(F2CA) have been studied by spectroscopic, electrochemical, photoelectrochemical, time-resolved spectroscopic, and computational methods in an effort to assess how intercalating and passivating electrolyte additives influence photovoltaic performance in this class of devices. Ru(2,2′-bipyridine-4,4′-dicarboxylic acid)2(NCS)2(N3)-sensitized titanium dioxide (TiO2) devices were also studied by similar methods to provide a comparative standard. Ground-state spectroscopic and electrochemical measurements show that the dye electronic structure was influenced in a nonsystematic way by the presence of various electrolyte components such as lithium (Li+), tetrabutylammonium (TBA+), and pyridyl additives, whereas the conduction band edge of TiO2was impacted in a systematic way. An order-of-magnitude increase in short-circuit current (Jsc) and cell efficiency (η) was observed for F2CA-sensitized devices with our selected electrolytes as compared to electrolyte compositions that are typically employed. These substantial gains in F2CA-sensitized device performance correlated with conduction band edge stabilization attributed to electrolyte cation intercalation. Time-resolved spectroscopic and computational studies explain how stabilization of the conduction band edge, which allows for better dye-conduction band overlap, increases the injection yield for F2CA–TiO2assemblies. These studies serve to elevate our understanding of how electrolyte composition may affect injection and photovoltaic performance in sensitizer assemblies with short-lived charge-separated excited states.