Influence of Solvent on Dye‐Sensitized Solar Cell Efficiency: What is so Special About Acetonitrile?

The selection of solvent for preparing a working electrode (and to act as the electrolyte) is known to influence the efficiency of dye‐sensitized solar cells. In this topical review, results taken from a systematic study are presented from the authors’ own lab examining how protic and aprotic solvents, as well as solvent polarity, affect adsorption of carboxylic dyes on the titanium dioxide nanoparticle surface and electron injection from the dye to the semiconductor. Adsorption of dye molecules on nanoparticle surfaces is measured through second harmonic light scattering and electron injection through ultrafast transient mid‐infrared absorption. It is revealed that protic solvents do not allow direct adsorption of the dye onto the semiconductor surface, due to hydrogen bonding with the dye and competitive binding to the semiconductor surface. Aprotic solvents, on the other hand, support solvation of the dye molecules but also facilitate dye adsorption on the semiconductor nanoparticle. Among aprotic solvents, it is found that solvents with higher polarity result in larger adsorption free energy for the dye and faster electron injection. Overall, these studies reveal that aprotic solvents with high solvent polarity (such as acetonitrile) yield more efficient solar cell devices. Surface‐sensitive and ultrafast spectroscopy studies reveal that the efficiency of dye‐sensitized solar cell devices is optimized by employing aprotic solvents of high polarity. These solvents enhance adsorption of the dye onto the semiconductor surface and increase the electron injection rate. This new understanding reveals the physical basis explaining why acetonitrile is the preferred solvent of choice for fabricating such devices.