Spectroscopic studies of electron injection in quantum dot sensitized mesoporous oxide films

Optimization of interfacial charge transfer in quantum dot (QD) sensitized mesoporous oxide films is crucial for the efficient design of QD sensitized solar cells (QDSSC). We employ TeraHertz time-domain spectroscopy (THz-TDS), transient absorption (TA) and time-resolved luminescence measurements, combined with transmission electron microscopy (TEM) and current-voltage measurements to study injection of electrons from PbSe QDs that are chemically linked to mesoporous oxide films. We illustrate that the interpretation of injection experiments is ambiguous for time-resolved optical measurements (TA and time-resolved luminescence) because nonradiative recombination processes at the oxide-QD interface have the same spectroscopic signature as electron injection. Complementary THz-TDS and current-voltage measurements demonstrate electron injection from the 1S_e level of PbSe QDs into SnO₂ mesoporous films, but injection into TiO₂ films is not observed, although the time-resolved optical measurements could be interpreted as indicating injection. That injection takes place into SnO₂ but not into TiO₂ can be explained by the energy alignment of the 1S_e level of the PbSe QD, which is energetically favorable and unfavorable respectively with respect to the oxide conduction band edges of SnO₂ and TiO₂. THz-TDS experiments demonstrate that electron injection from 5.5 nm PbSe QDs into SnO₂ occurs on a time scale of 125 ± 40 ps. THz-TDS experiments further reveal a subensemble of photogenerated carriers in these QD-oxide systems that recombine within 10 ps after photoexcitation. These carriers are likely located in QD clusters formed on the oxide surface that are apparent from TEM images and where efficient nonradiative recombination processes occur. Control of QD cluster formation is therefore essential in the optimization of QDSSC devices because the time scale of carrier recombination in the QD clusters is shorter than the time scale of electron injection into the mesoporous oxide film.