Spectroscopic Studies of Electron Injection in Quantum Dot Sensitized Mesoporous Oxide Films

Abstract

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 SnO 2 mesoporous films, but injection into TiO 2 films is not observed, although the time-resolved optical measurements could be interpreted as indicating injection. That injection takes place into SnO 2 but not into TiO 2 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 SnO 2 and TiO 2 . THz-TDS experiments demonstrate that electron injection from 5.5 nm PbSe QDs into SnO 2 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.

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

“…It is clear that, given our current state of understanding of THz spectroscopy to these types of systems, future applications of time-resolved terahertz spectroscopy can focus on systems of increasing complexity, such as those encountered in photovoltaic devices where different semiconductor phases are intermixed on nanometer lengths scales (Esenturk et al, 2009). Such complex systems may include, for instance, semiconductor nanostructures (quantum dots or nanorods) embedded in an organic polymer semiconductor phase and quantum dot or dye-sensitized transition metal oxide systems (Tiwana et al, 2009;Nemec, Rochford et al, 2010;Pijpers et al, 2010Pijpers et al, , 2011. The ability to monitor exciton and carrier dynamics on ultrafast time scales, across a wide frequency range, without the necessity to apply contacts to the material makes time-resolved terahertz spectroscopy a rather unique tool for studying carrier dynamics, for instance, allowing us to monitor the conversion of excitons into free charge carriers and vice versa.…”

“…By monitoring the hole polarizability as a function of time after excitation, one can evaluate population dynamics of the hole states. This principle has been exploited by Pijpers et al, (2007Pijpers et al, ( , 2008 to study carrier multiplication in highly excited, small gap, InAs QDs. Moreover, coupling femtosecond optical pump-THz probe measurements with ultrafast optical spectroscopies (such as time-resolved luminescence or transient absorption) can therefore help separate hole from electron dynamics, which is generally challenging.…”

Interplay Between Structure, Stoichiometry, and Electron Transfer Dynamics in SILAR-based Quantum Dot-Sensitized Oxides

“…Hence, following optical excitation of the QD, the time-dependent pump-induced absorption of the THz probe in a QD-sensitized system can be directly correlated with ET from the QDs to the oxide. 14,33,35,36 Note that not only the ET rate but also the efficiency of the ET process can be obtained from OPTP measurements, through the amplitude of the photoinduced THz absorption (more details are given in the Supporting Information Figures S4 and S5). …”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…Depending on the band alignments, fast electron injection from the NC into the mesoporous film will occur, making it possible to use such superstructures as QDsensitized solar cells, akin to the well-known dye-sensitized Grätzel solar cells [144][145][146][147].…”