Time-dependent density functional theory for electron-nuclear dynamics is applied to study ultrafast photoinduced dynamics in dye-sensitized TiO2, quantum dots and carbon nanotubes in real-time and at the atomistic level of detail.

The dye/semiconductor systems derive from the Gratzel solar cell and address the general problems of the organic/inorganic interface that commonly arise in photovoltaics, photochemistry and molecular electronics. The simulation resolves the controversy regarding the origin of the ultrafast ET by showing it is possible with both adiabatic and non-adiabatic (NA) mechanisms. The alizarin-TiO2 system constitutes a particularly interesting and novel case, where efficient electron injection into the edge of the TiO2 conduction band (CB) occurs without energy loss. The simulation indicates that the electron is injected from alizarin into a localized surface state within 8 fs and spreads into the bulk on a 100fs or longer timescale. Vibrational motions of the chromophore are particularly important in generating a nonuniform distribution of photoexcited states and driving the ET process.

Following a photoexcitation between the second van Hove singularities, the electrons and holes in the (7,0) zig-zag carbon nanotube decay to the Fermi level on characteristic subpicosecond timescales. Surprisingly, despite a lower density of states, electrons relax faster than holes. The relaxation is primarily mediated by the C-C stretching G-phonons. Hole dynamics are more complex than electron dynamics. In addition to G-phonons, holes couple to lower frequency breathing modes and decay over multiple time-scales.

The electron and hole charge carriers in a PbSe quantum dot show slow nearly symmetric relaxation through multiple intermediate states. The relaxation is non-exponential, in agreement with the observed strongly non-Lorentzian spectral line-shapes. Both electrons and holes interact with low frequency phonons. Holes decay only slightly faster than electrons rationalizing the highly efficient carrier multiplication in PbSe nanocrystals reported recently in relation to improved solar power conversion.