Influence of TiO2/Perylene Interface Modifications on Electron Injection and Recombination Dynamics

It is well-known that the efficiency of dye sensitized solar cells can be improved by controlling the interface energetics using molecular interface modifiers. Whereas this leads to a beneficial band level shifting, it also affects the interfacial electron injection and recombination dynamics. Here we demonstrate a significant retardation of the injection process and a loss of ultrafast recombination components by coadsorbing inert gases and solvents with increasing dipole moments on a TiO2/perylene interface. Model perylene dyes with different electronic couplings to the colloidal TiO2 films were subject to precisely defined chemical environments, and the electron transfer dynamics was investigated with femtosecond transient absorption spectroscopy. The coadsorption of N-2 and Ar doubled the injection times compared to the ultrafast sub-60 fs electron injection in vacuum. The introduction of solvents led to injection retardations by Up to 2 orders of magnitude. This slow-down correlates well with the degree of polarity of the chemical species and is consistent with it calculated electronic shift of the oxide conduction band relative to the injecting molecular level. The ultrafast component of the nonexponential back electron transfer was significantly reduced with coadsorbent polarity.