Spectroelectrochemistry with hydrogen-doped indium oxide electrodes monitors electron and hole injection into PbS quantum dots

We combine potential-modulated absorption spectroscopy with working electrodes made of hydrogen-doped indium oxide on glass to study the electrochemically induced changes to the infrared absorption of ethanedithiol-crosslinked PbS quantum dot thin films. We demonstrate an electrochemical window of -1.5 V to 1 V vs. ferrocene/ferrocenium as well as a spectral window of 800 nm - 1900 nm, and we show how these ranges can be fine-tuned by tailoring the composition of the working electrode. We find evidence for the successful injection of electrons and holes into the quantum dots to a comparable degree. This enables us to locate the absolute positions of the band edges and the Fermi level for PbS quantum dots with different diameters, monitor how the band edge positions depend on the diameter and show a correlation between the position of the Fermi level with the appearance of in-gap states. We discuss the specific advantages of the technique presented here for monitoring the redox behaviour of IR-active quantum dots, detail the experimental challenges and devise strategies for their mitigation.