Influence of TiO 2 /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 TiO 2 /perylene interface. Model perylene dyes with different electronic couplings to the colloidal TiO 2 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 a 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.