title: Dipolar Molecular Capping in Quantum Dot-Sensitized Oxides: Fermi Level Pinning Precludes Tuning Donor-Acceptor Energetics.
abstract: Reducing the donor-acceptor excess energy (ΔGET) 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 ΔGETlosses 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/SnO2-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 ΔGET) 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.
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, D-55128 Mainz, Germany.|Graduate School of Material Science in Mainz, University of Mainz , Staudingerweg 9, 55128 Mainz, Germany.
authors.forename: Hai I
authors.identifier: 0000-0003-0940-3984
authors.initials: HI
authors.lastname: Wang
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, D-55128 Mainz, Germany.
authors.forename: Hao
authors.initials: H
authors.lastname: Lu
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, D-55128 Mainz, Germany.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Nagata
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, D-55128 Mainz, Germany.
authors.forename: Mischa
authors.identifier: 0000-0001-6851-8453
authors.initials: M
authors.lastname: Bonn
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, D-55128 Mainz, Germany.
authors.forename: Enrique
authors.identifier: 0000-0003-1021-4929
authors.initials: E
authors.lastname: Cánovas
keywords: Fermi level pinning; PbS quantum dots; THz spectroscopy; interfacial electron transfer; molecular dipole; quantum dot-sensitized solar cells; sensitized system