Temperature- and Size-Dependent Exciton Dynamics in PbS Colloidal Quantum Dot Thin Films Using Combined Photoluminescence Spectroscopy and Photocarrier Radiometry

Abstract

Lead salt colloidal quantum dots (CQDs) are promising active components for applications in electronic and optoelectronic devices such as photodetectors, light-emitting diodes, and solar cells. Detailed understanding of exciton dynamics in these nanocrystalline films is crucial for their practical applications. Photocarrier radiometry (PCR) is a dynamic spectrally integrated frequency-domain photoluminescence (PL) method, the spectral bandwidth of which is gated to eliminate thermal infrared photons due to nonradiative recombination. On the other hand, PL spectroscopy is a steady-state method that offers complementary spectrally resolved information. Combined PL temperature spectra and PCR temperature and frequency measurements were applied to PbS CQD thin films capped with oleic acid. Enhanced PL intensities at high temperatures originating in thermally activated exciton transfer from trap to exciton states were found in two samples consisting of two different sizes of quantum dots. A theoretical excitonic diffusion-wave PCR model was developed to extract exciton recombination lifetimes, hopping diffusivity, and trapping rates. It was found that the smaller size (3 nm) quantum dots exhibit considerably improved excitonic transport properties compared to the larger quantum dots (4 nm), including longer effective lifetime, higher degree of localization (diffusion length), and smaller trapping and thermal emission rates. Therefore, small-size quantum dots are more suitable for optoelectronic device applications.