Near-Infrared-to-Visible Upconversion Sensitized by Conductive Films of PbS Quantum Dots

Area of Science:

• Materials Science
• Photophysics
• Nanotechnology

Background:

• Photon upconversion converts low-energy photons to high-energy ones, crucial for optoelectronic applications.
• Previous solid-state upconversion relied on lead sulfide (PbS) quantum dots (QDs) with insulating ligands, limiting light absorption due to short exciton diffusion lengths.

Purpose of the Study:

• To demonstrate enhanced near-infrared-to-visible upconversion using conductive PbS QDs with short iodide ligands.
• To investigate the impact of ligand choice on upconversion efficiency and light-harvesting capabilities.

Main Methods:

• Fabrication of bilayer devices using conductive PbS QDs capped with short iodide ligands.
• Comparison of upconversion brightness and light absorption with devices using PbS QDs with native oleic acid ligands.
• Optical spectroscopy (steady-state and transient) to identify performance-limiting processes.

Main Results:

• Conductive PbS QD devices showed significantly improved upconversion brightness (30 ± 10%) compared to insulating ligand devices.
• Thick conductive devices absorbed more NIR light (9.6 ± 0.1%) while retaining brightness, a >30× improvement.
• Performance roll-off was observed beyond 1.5 monolayers, attributed to excitation back-transfer and altered organic layer morphology.

Conclusions:

• Conductive PbS QDs offer a promising pathway for efficient solid-state photon upconversion.
• Ligand engineering is critical for optimizing exciton diffusion and light absorption.
• Further architectural enhancements are needed to mitigate internal efficiency losses and realize full potential.