Effect of Optical Interference on External Quantum Efficiency of PbS Quantum Dot Photodiode

Sensors that detect light in the shortwave infrared (SWIR) region are increasingly being used in various automation systems because they are harmless to the human optic nerve. In this study, we developed a sensor for SWIR using colloidal quantum dots (CQDs) and assessed the external quantum efficiency and light absorption characteristics of the device in relation to the thickness of the CQD film, which functions as the active layer. To fabricate photodiodes (PDs), the conductivity of the CQD thin film was increased using a ligand exchange process. This conductive CQD film had a high refractive index, which led to optical interference in the PDs. Consequently, the light absorption in the PD depended on the thickness of the CQD film. We confirmed this phenomenon using the transfer matrix method after analyzing the optical properties of the conductive CQD films using ellipsometry. Our findings indicated that to effectively detect light in regions such as SWIR, the optical interference and tunable absorption must be considered in PD designs by tuning the CQD size.