Efficient Electron Injection from Acyloin-Anchored Semisquarylium Dyes into Colloidal TiO2 Films for Organic Dye-Sensitized Solar Cells

Semisquarylium dyes use a novel acyloin anchor group to strongly bind to TiO2 semiconductors. Efficient acyloin anchor mediated electron injection into nanocrystalline TiO2 is demonstrated, allowing highly efficient dye-sensitized solar cells with IPCEs > 80%. The acyloin anchor can thus be viewed as a true alternative to the standard carboxylic acid anchor group. The opto-electronic and electron injection properties of the most basic semisquarylium dye SY404 are compared to the modified semisquarylium dye DD1 and the carboxylic acid anchored indoline dye D131 using a combination of ultrafast and photoemission spectroscopy. For SY404, ultrafast injection times of ∼50 fs are found despite a small energetic driving force between dye excited states and TiO2 conduction band minimum. This is possible due to the strong electronic coupling of the semisquarylium dyes to the TiO2 surface mediated by the acyloin anchor. For a better overlap with the solar spectrum, the semisquarylium dyes are modified by substitution with a larger donor moiety (DD1). While for DD1 the overall absorption increases, the injection process slightly slows down; however, it still proves fast enough for very efficient injection. Compared to the carboxylic acid anchored indoline dye D131, the SY404 dye injects more than seven times faster despite a ∼150 meV smaller driving force.