Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange

Abstract

The electronic properties of colloidal quantum dots (QDs) are critically dependent on both QD size and surface chemistry. Modification of quantum confinement provides control of the QD bandgap, while ligand-induced surface dipoles present a hitherto underutilized means of control over the absolute energy levels of QDs within electronic devices. Here, we show that the energy levels of lead sulfide QDs, measured by ultraviolet photoelectron spectroscopy, shift by up to 0.9 eV between different chemical ligand treatments. The directions of these energy shifts match the results of atomistic density functional theory simulations and scale with the ligand dipole moment. Trends in the performance of photovoltaic devices employing ligand-modified QD films are consistent with the measured energy level shifts. These results identify surface-chemistry-mediated energy level shifts as a means of predictably controlling the electronic properties of colloidal QD films and as a versatile adjustable parameter in the performance optimization of QD optoelectronic devices.

Efficient Water Splitting Cascade Photoanodes with Ligand‐Engineered MnO Cocatalysts

“…In the sulfite oxidation with extremely fast oxidation kinetics, in other words, Na 2 SO 3 removing the injection barrier without affection the charge separation, surface recombination can be negligible. Therefore, most of the previously reported results related to BiVO 4 ‐based photoanodes13, 14, 16, 19, 47, 51, 52, 53 were measured in sulfite oxidation condition to show photo‐electrochemical properties of BiVO 4 ‐based electrodes independently of its poor water oxidation kinetics, as shown in Table S2 (Supporting Information). The photo‐electrochemical current densities of the BiVO 4 ‐based photoanodes4, 13, 14, 16, 17, 19, 20, 21, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 64, 65, 66, 67, 68, 69, 77 were plotted as a function of potential versus RHE.…”

“…The shifts of the band edge energies of MnO can still be tuned over a wide range by controlling the intrinsic dipole moment of the ligand. Since the orientation and coverage of the ligands on the surface, and the contribution of the effective dipole moment are weakly coupled, we can predict the change in the energy shift from the controlled change in the intrinsic dipole moment of the ligand 50, 51, 52, 53, 54…”

The role of surface passivation for efficient and photostable PbS quantum dot solar cells

“…While improvement in PCE of QD solar cells has been thoroughly sought after over the years, the photostability of this technology still remains an open challenge to further increase its technology readiness level, albeit initial promising results on low-performance PbS QD solar cells 11 . The Achilles' heel of this photovoltaic technology is the surface of QDs, which remarkably impacts QDs' energy bands 12 , mid-gap states 13 , carrier transport 14 and stability 15 .…”

Single-step colloidal quantum dot films for infrared solar harvesting

“…Strategies that photonically trap IR light in the active layer can leverage the high dielectric constant of the IR-bandgap CQD solids. 22 It is also important to address the band alignment between the CQD active layer and the electron-accepting electrode: halide treatment of CQDs, while beneficial for optoelectronic properties, results in a deeper conduction band; 23 this makes injection into shallow work-function electron accepting electrodes, such as unmodified TiO 2 or ZnO, inefficient, as evidenced by PL quenching measurements ( Figure S2). As a result, new deep work function electrode materials with sufficient electron mobility and the capability of being doped to heavily n-type will be required for further progress.…”