Exciton lifetime broadening and distribution profiles of PbS colloidal quantum Dot thin films using frequency-and temperature-scanned photocarrier radiometry

Photocarrier radiometry (PCR) has been applied to PbS colloidal quantum dot (CQD) thin films for the absolute measurement of emission lifetimes. Continuous and discrete models describing lifetime distribution have been developed. The former yields homogeneous and inhomogeneous Voigt lifetime broadening. The latter comprises two models: one is based on direct ad hoc fits to PCR amplitude and phase data, reconstructing a superposition of multiple independent lifetime line spectra; the other is a variational method developed to invert simultaneous integral equations in the lifetime distribution function (spectrum) and involves similar fits to the PCR in-phase and quadrature signal channels to reconstruct the lifetime spectrum. A PCR theory with a rate equation model taking into account the lowest 1S-1S exciton state splitting and multiphonon-assisted carrier trapping to states outside a PbS QD was also developed. For noninteracting (uncoupled) CQDs, longitudinal optical (LO) phonon-exciton interactions limit the radiative lifetime and luminescence photon flux throughout the entire 100-300 K temperature range. For interacting (coupled) CQDs, variational and ad hoc lifetime line spectra exhibit similar mechanisms to uncoupled CQDs with the important addition of direct exciton-to-exciton transitions in the form of efficient transport (hopping, tunneling, or FRET) at all temperatures, which result in de-excitation dynamics dominated by radiative emission channels.