Dipolar Molecular Capping in Quantum Dot-Sensitized Oxides: Fermi Level Pinning Precludes Tuning Donor–Acceptor Energetics

Abstract

Reducing the donor-acceptor excess energy (ΔG) 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 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-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) 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.

Water-Dispersed High-Quality Graphene: A Green Solution for Efficient Energy Storage Applications

“…As a purely optical approach, THz spectroscopy can provide quantitative insight into the intrinsic charge-transport properties ( e.g., charge mobility, carrier density, etc. ) in the materials of interests in a contact-free fashion, − which is particularly powerful for accessing the electrical properties of low-dimensional materials where electrical contacts are often challenging to deposit for conventional electrical studies. In a typical OPTP measurement, the EGs deposited on the fused silica (using the Langmuir–Schaefer method) are photoexcited by a short femtosecond (∼50 fs) laser pulse with a photon energy of ∼3.1 eV (400 nm) to generate charge carriers.…”

Quantum dot-sensitized solar cells

“…Apart from the inorganic coating layer treatment, photoanode interface modification with use of organic molecules was also demonstrated to be effective for the suppression of charge recombination. [480][481][482][483][484][485] For example, Mora-Seró and coworkers showed that molecular dipoles (DT) assisted ZnS treatment can give a better control of the recombination dynamics as well as the charge injection process. 480 TiO2/QD/electrolyte interface.…”

Efficient Hot Electron Transfer in Quantum Dot-Sensitized Mesoporous Oxides at Room Temperature

“…Finally, following our previous report, all QDs were molecularly passivated by 1,4-mercaptobenzoic acid, which substantially enhances the ET efficiency from QDs to oxide. 17,18,20 To unambiguously monitor HET, we have employed optical-pump THz-probe spectroscopy, which has been used extensively to quantify electron transfer rates and efficiencies for QD−oxide systems. 10,[17][18][19]21,22 Briefly, following selective optical excitation of QDs with a femtosecond optical laser pulse, the photoconductivity of the sample is monitored by a second pulse in the THz frequency range (0.4−2 THz), with a subpicosecond time resolution.…”

“…17,18,20 To unambiguously monitor HET, we have employed optical-pump THz-probe spectroscopy, which has been used extensively to quantify electron transfer rates and efficiencies for QD−oxide systems. 10,[17][18][19]21,22 Briefly, following selective optical excitation of QDs with a femtosecond optical laser pulse, the photoconductivity of the sample is monitored by a second pulse in the THz frequency range (0.4−2 THz), with a subpicosecond time resolution. Immediately after excitation, the charge carriers are confined within the QDs, and the real conductivity is zero.…”

“…The ultrafast sub-150 fs injection time observed here for HET ensures that HET can effectively compete not only with carrier cooling but also with trapping processes in PbS QDs, especially for the ligand-passivated QDs employed here. 17,18 The absence of trapping is illustrated by experiments in which the incident photon flux was kept constant, but the excitation wavelength was varied between 800 and 400 nm (see Figure S1). This indicates that, for a given absorbed photon density, the number of the injected electrons into oxide is independent of the pump energy, i.e., ET rate.…”