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

TLDR

Colloidal quantum dot electronic properties depend on size and surface chemistry, and ligand‑induced surface dipoles offer a previously underexplored route to tune absolute energy levels. This work investigates whether ligand‑induced surface dipoles can predictably shift the energy levels of lead sulfide QDs for use in electronic devices. Ultraviolet photoelectron spectroscopy shows ligand treatments shift lead sulfide QD energy levels by up to 0.9 eV, with the direction and magnitude matching density‑functional theory predictions and correlating with ligand dipole moments, and photovoltaic device performance trends confirm that surface‑chemistry‑mediated energy level shifts can be used to optimize QD optoelectronic devices.

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.