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.