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.

Automated summary

As nanocrystal-based devices mature, understanding their electronic structure becomes crucial. Most spectroscopic techniques overlook the effects of the device's environment, electric field, and illumination. Thus, it is essential to develop tools for in situ and operando device analysis. In this study, photoemission microscopy is used to examine the energy landscape of a HgTe nanocrystal-based photodiode. The method allows for quantification of the diode's built-in voltage and provides insights into the effects of particle size and illumination. The combination of SnO2 and Ag2Te is identified as a more suitable choice for extended-short-wave infrared materials.