Electron injection from photoexcited chemisorbed dyes into zinc oxide is known to proceed in a stepwise manner, yet the origin of the injection retardation remains controversial. Here we present a complementary time-resolved spectroscopy study on the electron injection dynamics from organic dyes into ZnO using model perylene derivatives with systematically lengthened bridge units to clarify the influence of the positively charged cation on the escape of the injected electron. The combination of transient absorption, opticalpump terahertz-probe, and time-resolved two-photon photoemission spectroscopy reveals that the delayed release of charges into ZnO is independent of Coulomb attraction between the dye cation and the injected electron. Rather, following dye photoexcitation the primary acceptor states of electron transfer into ZnO are interface states formed between the dye and the ZnO surface, which retard the formation of free charges.

Sample Preparation. ZnO nanorods were grown on 1.5 mm thick [0001] oriented quartz substrates by chemical bath deposition (0.4 M NaOH and 0.01 M ZnNO 3 ) at 80 °C. A 40 nm thick ZnO seed layer was deposited on the substrates prior to nanorod

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.

A low molecular weight purely organic Forster resonance energy transfer (FRET) sensitizer system combining multiple chromophores into a single molecule via covalent attachment is designed to adjust the ratio and relative position of the fluorol donor and coumarin acceptor units. A phenyl based scaffold accommodates both FRET partners and is connected to a carboxylic anchor group for adsorption on a semiconductor surface. The functionality of the complete ensemble is demonstrated by UV−vis and luminescence lifetime measurements. The energy and charge transfer dynamics of the FRET assemblies in solution and adsorbed on ZnO nanorods are measured by femtosecond transient absorption spectroscopy. Optical pump THz probe (OPTP) spectroscopy is applied to detect the injected electrons in the ZnO electrode, confirming the full electronic pathway from FRET light harvesting to subsequent charge carrier injection.

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.