Photoconductive PbSe thin films and the role of potassium iodide

Abstract

The PbSe thin films are sensitized by oxidation and iodization process to improve the infrared photoresponse enabling higher performing photodetectors and photovoltaic devices. Iodine is widely used for PbSe sensitization, and it influences the thin film morphology and also forms a PbI2 passivation layer. Potassium Iodide (KI) has been used as an additive in chemical bath deposited films of PbSe since 1959 and continues to this day. This work aims to better understand the role of KI in PbSe films as it improves MWIR photoresponse by as much as 300% compared to samples without KI. In this research paper, we show that low levels of KI (180 mMol) densify the PbSe film, increase grain size, reduce film stress/cracking, and improve IR absorption (proportional to dose). The transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies show thermal annealed PbSe films with KI convert round equiaxed grains to highly faceted grains coated with PbI2 ranging from 3 to 10 nm, throughout the film. The dense PbSe grains encapsulated in PbI2 lead to a more uniform PbSeO3 layer following thermal oxidation. The electrical resistance versus temperature measurements shows a strong phase transition occurs at 220 °C only for samples with KI. X-ray photoelectron spectroscopy studies confirm that PbSe films with KI annealed in an argon atmosphere at 460 °C have a surface layer of PbI2, and no potassium was detected, suggesting desorption or diffusion of potassium into the PbSe grains. PbSe films with KI showed 30x higher electrical resistance (RD) under dark conditions, leading to reduced carrier concentration (lower dark current) and improved photoresponse without impacting electron mobility. In summary, we have clarified the role of KI in PbSe thin films and how it can be used to improve the infrared photoresponse.