Inside a nanocrystal-based photodiode using photoemission microscopy

As nanocrystal-based devices gain maturity, a comprehensive understanding of their electronic structure is necessary for further optimization. Most spectroscopic techniques typically examine pristine materials and disregard the coupling of the active material to its actual environment, the influence of an applied electric field, and possible illumination effects. Therefore, it is critical to develop tools that can probe device in situ and operando. Here, we explore photoemission microscopy as a tool to unveil the energy landscape of a HgTe NC-based photodiode. We propose a planar diode stack to facilitate surface-sensitive photoemission measurements. We demonstrate that the method gives direct quantification of the diode's built-in voltage. Furthermore, we discuss how it is affected by particle size and illumination. We show that combining SnO₂ and Ag₂Te as electron and hole transport layers is better suited for extended-short-wave infrared materials than materials with larger bandgaps. We also identify the effect of photodoping over the SnO₂ layer and propose a strategy to overcome it. Given its simplicity, the method appears to be of utmost interest for screening diode design strategies.