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Electron transfer at a donor-acceptor quantum dot-metal oxide interface is a process fundamentally relevant to solar energy conversion architectures as, e.g., sensitized solar cells and solar fuels schemes. As kinetic competition at these technologically relevant interfaces largely determines device performance, this Review surveys several aspects linking electron transfer dynamics and device efficiency; this correlation is done for systems aiming for efficiencies up to and above the ∼33% efficiency limit set by Shockley and Queisser for single gap devices. Furthermore, we critically comment on common pitfalls associated with the interpretation of kinetic data obtained from current methodologies and experimental approaches, and finally, we highlight works that, to our judgment, have contributed to a better understanding of the fundamentals governing electron transfer at quantum dot-metal oxide interfaces., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

The long-term stability of dye-sensitized solar cells (DSSCs) is determined to a large extent by the photodegradation of their sensitizers. Understanding the mechanism of light-induced decomposition of dyes sensitizing a mesoporous oxide matrix may therefore contribute to solutions to increase the life span of DSSCs. Here, we investigate, using ultrafast terahertz photoconductivity measurements, the evolution of interfacial electron-transfer (ET) dynamics in Ru(4,4'-dicarboxylic acid-2,2'-bipyridine) 2 (NCS) 2 (N3) dye-sensitized mesoporous TiO 2 electrodes upon dye photodegradation. Under inert environment, interfacial ET dynamics do not change over time, indicating that the dye is stable and photodegradation is absent; the associated ET dynamics are characterized by a sub-100 fs rise of the photoconductivity, followed by long-lived (≫1 ns) electrons in the oxide electrode. When the N3-TiO 2 sample is exposed to air under identical illumination conditions, dye photodegradation is evident from the disappearance of the optical absorption associated with the dye. Remarkably, approximately half of the sub-100 fs ET is observed to still occur but is followed by very rapid (∼10 ps) electron-hole recombination. Laser desorption/ionization mass spectrometry, attenuated total reflection-Fourier transform infrared, and terahertz photoconductivity analyses reveal that the photodegraded ET signal originates from the N3 dye photodegradation product as bi-isonicotinic acid (4,4'-dicarboxylic acid-2,2'-bipyridine), which remains bonded to the TiO 2 surface via either bidentate chelation or bridging-type geometry.

Harvesting multiexcitons from semiconductor quantum dots (QDs) has been proposed as a path toward photovoltaic efficiencies beyond the Shockley-Queisser limit. Although multiexciton generation efficiencies have been quantified extensively in QD structures, the challenge of actually collecting multiple excitons at electrodes-a prerequisite for high-efficiency solar cell devices-has received less attention. Here, we demonstrate that multiexciton collection (MEC) at the PbS QD/mesoporous SnO 2 interface can be boosted 5-fold from ∼15 to reach ∼80% quantum yield, by partial localization of holes in a QD molecular capping shell. The resulting weakened Coulombic interactions give rise to reduced Auger recombination rates within the molecularly capped QDs, so that biexciton Auger relaxation, competing with MEC, is strongly suppressed. These results not only highlight the importance of surface chemistry and energetics at QD/ligand interfaces for multiexciton extraction but also provide clear design principles for realizing the benefits of MEG in sensitized systems exploited in solar cells and fuel geometries.

Reducing the donor-acceptor excess energy (ΔG ET ) associated with electron transfer (ET) across quantum dot (QD)/oxide interfaces can boost photoconversion efficiencies in sensitized solar cell and fuel architectures. One proposed path for engineering ΔG ET losses at interfaces refers to the tuning of sensitizer workfunction by exploiting QD dipolar molecular capping treatments. However, the change in workfunction per debye in QD solids has been reported to be ∼20-fold larger when compared to the effect achieved in QD-sensitized architectures. The origin behind the modest workfunction tunability in QD-sensitized oxides remains unclear. Here, we investigate the interplay between QD dipolar molecular capping, interfacial QD-oxide ET rates, and QD workfunction in PbS QD/SnO 2 -sensitized interfaces. We find that interfacial QD-to-oxide ET is invariant to both the nature and strength of the specific QD dipolar capping treatment. Photoelectron spectroscopy reveals that the resolved invariance in ET rates is the result of a lack of QD workfunction (and hence ΔG ET ) tuning, despite effective molecular dipolar capping. We therefore conclude that Fermi level pinning precludes tuning donor-acceptor energetics by dipolar molecular capping in strongly coupled quantum dot-sensitized oxides.

The nature of the photoconductivity in solution-processed films of methylammonium lead iodide perovskite is investigated by determining the variation of the photoconductive response with temperature. Ultrabroadband terahertz (THz) photoconductivity spectra in the 0.3-10 THz range can be reproduced well by a simple Drude-like response at room temperature, where free charge carrier motion is characterized by an average scattering time. The scattering time determined from Drude fits in the 0.3-2THz region increases from ∼4 fs at 300 K (tetragonal phase; mobility of ∼27 cm(2) V(-1) s(-1)) to almost ∼25 fs at 77 K (orthorhombic phase, mobility of ∼150 cm(2) V(-1) s(-1)). For the tetragonal phase (temperature range 150 < T < 300 K) the scattering time shows a ∼T(-3/2) dependence, approaching the theoretical limit for pure acoustic phonon (deformation potential) scattering. Hence, electron-phonon, rather than impurity scattering, sets the upper limit on free charge transport for this perovskite.

At present, quantum-dot-sensitized solar cells (QDSCs) still exhibit moderate power conversion efficiency (with record efficiency of 6-7%), limited primarily by charge recombination. Therefore, suppressing recombination processes is a mandatory requirement to boost the performance of QDSCs. Herein, we demonstrate the ability of a novel sequential inorganic ZnS/SiO2 double layer treatment onto the QD-sensitized photoanode for strongly inhibiting interfacial recombination processes in QDSCs while providing improved cell stability. Theoretical modeling and impedance spectroscopy reveal that the combined ZnS/SiO2 treatment reduces interfacial recombination and increases charge collection efficiency when compared with conventional ZnS treatment alone. In line with those results, subpicosecond THz spectroscopy demonstrates that while QD to TiO2 electron-transfer rates and yields are insensitive to inorganic photoanode overcoating, back recombination at the oxide surface is strongly suppressed by subsequent inorganic treatments. By exploiting this approach, CdSe(x)Te(1-x) QDSCs exhibit a certified record efficiency of 8.21% (8.55% for a champion cell), an improvement of 20% over the previous record high efficiency of 6.8%, together with an additional beneficial effect of improved cell stability.

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.

Photoinduced electron transfer processes from semiconductor quantum dots (QDs) molecularly bridged to a mesoporous oxide phase are quantitatively surveyed using optical pump-terahertz probe spectroscopy. We control electron transfer rates in donor-bridge-acceptor systems by tuning the electronic coupling strength through the use of n-methylene (SH-[CH2]n-COOH) and n-phenylene (SH-[C6H4](n)-COOH) molecular bridges. Our results show that electron transfer occurs as a nonresonant quantum tunneling process with characteristic decay rates of β(n) = 0.94 ± 0.08 and β(n) = 1.25 per methylene and phenylene group, respectively, in quantitative agreement with reported conductance measurements through single molecules and self-assembled monolayers. For a given QD donor-oxide acceptor separation distance, the aromatic n-phenylene based bridges allow faster electron transfer processes when compared with n-methylene based ones. Implications of these results for QD sensitized solar cell design are discussed.

We report the direct and unambiguous determination of electron transfer rates and efficiencies from PbSe quantum dots (QDs) to mesoporous SnO2 films. We monitor the time-dependent electron density within the oxide with picosecond time resolution using Terahertz spectroscopy, following optical excitation of the QDs using a femtosecond laser pulse. QD-oxide electron transfer occurs with efficiencies of ∼2% in our samples under 800 nm pumping with a marked dependence on QD size, ranging from ∼100 ps injection times for the smallest, ∼2 nm diameter QDs, to ∼1 ns time scale for ∼7 nm QDs. The size-dependent electron transfer rates are modeled within the framework of Marcus theory and the implications of the results for device design are discussed.