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

Abstract

We quantify the rate and efficiency of picosecond electron transfer (ET) from PbS nanocrystals, grown by successive ionic layer adsorption and reaction (SILAR), into a mesoporous SnO2 support. Successive SILAR deposition steps allow for stoichiometry- and size-variation of the QDs, characterized using transmission electron microscopy. Whereas for sulfur-rich (p-type) QD surfaces substantial electron trapping at the QD surface occurs, for lead-rich (n-type) QD surfaces, the QD trapping channel is suppressed and the ET efficiency is boosted. The ET efficiency increase achieved by lead-rich QD surfaces is found to be QD-size dependent, increasing linearly with QD surface area. On the other hand, ET rates are found to be independent of both QD size and surface stoichiometry, suggesting that the donor-acceptor energetics (constituting the driving force for ET) are fixed due to Fermi level pinning at the QD/oxide interface. Implications of our results for QD-sensitized solar cell design are discussed.

Increased Quantum Dot Loading by pH Control Reduces Interfacial Recombination in Quantum-Dot-Sensitized Solar Cells

“…This proposed mechanism is further supported by previous literature experiments showing a decrease in the yield of hole transfer from PbS QDs to spiro-OMeTAD for PbS QDs larger than 2.5 nm; the authors also observe a faster decay of the spiroOMeTAD cation with increasing QD size, leading them to conclude that the hole can be back-transferred from the spiro-OMeTAD cation to the QD if the QD valence band shifts to high enough energy levels. 36 Our observed result of shorter recombination lifetimes with increased SILAR cycles runs contrary to previous reports on recombination in CdSe 51,72 and PbS 73 QDSSCs. In the report on PbS QDs on SnO 2 substrates, Cánovas and colleagues found that recombination lifetimes increased from 2.6 to 8.1 ns from 1 to 3 SILAR cycles, 73 using optical pumpÀterahertz probe spectroscopy that focuses on the short-range recombination processes, which the authors attribute to recombination to oxidized QDs.…”

“…The self-saturation in QD growth could be due to pore-blocking, as discussed above, or due to epitaxial growth of the QDs, where the nanocrystal size limit is ultimately determined by any lattice mismatch between the TiO 2 substrate and the QD. 73 Another possible explanation for the saturation of QD growth is that once some QDs are nucleated in the first cycle or two, additional Pb 2þ and S 2À adsorption ARTICLE occurs on the already nucleated QDs, which may be the case if PbS strongly prefers to grow on itself over TiO 2 . Overall, the largest gains in QD coverage of the TiO 2 surface with base-assisted growth were seen initially at 2 SILAR cycles, and each of the three bases investigated produced similar gains in the number and size of QDs.…”

Composition‐Dependent Passivation Efficiency at the CdS/CuIn1-xGaxSe2 Interface

“…The data reveal that τ slow (linked to CIGS bulk recombination) is barely affected by fluence over the analyzed range of excitations. However, τ fast (linked to surface recombination at the air/CIGS interface) becomes smaller in amplitude and that at elevated photon fluxes, increasingly slower trapping timescales are obtained (consistent with the notion that that trapping rate is directly proportional to trap density) . Also, we observe that the carrier dynamics of the low Ga content for ZnO/CdS/CIGS samples at low fluences (open green dots in Figure a) correlate very well with those obtained for air/CIGS samples at high photon fluences, suggesting that photodoping reaches almost the same passivating effect as the CdS‐capping.…”

Boosting Power Conversion Efficiencies of Quantum-Dot-Sensitized Solar Cells Beyond 8% by Recombination Control

“…electrons in the oxide photogenerated from QDs 51,52. This selectivity allows us to directly quantify electron transfer and recombination dynamics, and obtain information about electron transfer yields as a function of post-inorganic treatments 53. Carrier dynamics in the oxide electrode were found to be invariant towards pump fluence up to 160 µJ/cm 2 (seeFigure S10a), indicating that electronelectron interactions in the oxide are negligible (even for irradiances well exceeding 1 sun power density).…”