The potential of quantum dot (QD) films for optoelectronic devices such as light emitting diodes, solar cells, and photodetectors has been verified with the success of ligand-exchange process, which can tune the electronic properties of QDs. Accompanied with the insight into this tunability of electronic properties, large efforts have been made for decades to investigate the band structure of QD films. Although photoelectron spectroscopy (PES) has been regarded as the most acceptable tool to measure the band edge position of QD films, the PES measurement of QD films with non-conducting surface-ligands has been hardly studied and even the critical charge-up problem has been frequently neglected. Here, we employed photoelectron yield spectroscopy (PYS) to explore the band edge position of QD films with long insulating ligands. PYS measurement was not affected by not only the underlying substrates (from metals to semiconductors with large band-gap), but also thickness of QD films. Through PYS measurement, we successfully investigated band edge shifts under partial ligand-exchange which could maintain dispersibility of colloidal QDs and hydrophobicity of QD films, simultaneously. This understanding on band structure of QD films with partial ligand-exchange can give a guide to optimize band alignment of QD films without detriment to fabrication process which are important factors to deposit multilayer films for optoelectronic devices.

Photoelectron yield spectroscopic studies on tunable band edge position of hydrophobic quantum dot films

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