Probing Intraband Carrier Dynamics in Degenerately N-Doped PbS Quantum Dots and Their Implications in Optoelectronic Devices

Tunable intraband transitions in colloidal quantum dots (QDs) have emerged as a promising platform for electronic transitions relevant to mid- to long-wavelength infrared optoelectronic devices. However, their broad device applications have been limited by an incomplete understanding of their spectroscopic and electrical properties. In this study, we investigate both the optical and electrical characteristics of N-doped PbS QDs using ultrafast mid-infrared transient absorption spectroscopy and temperature-dependent carrier transit time measurements. Our findings reveal intraband absorption coefficients ranging from 6 × 103 to 13 × 103 cm-1, accompanied by varying carrier recombination pathways associated with different doping levels. Notably, the average carrier relaxation lifetime is 88 ps for 5.4 nm QDs, whereas highly doped 7.7 nm QDs exhibit a significantly shorter carrier lifetime of 20 ps. These results not only elucidate the intraband optical properties of PbS QDs but also provide valuable insights for comparing carrier lifetimes to carrier transit times, thereby offering guidance for the design of future mid-infrared optoelectronic devices.