Identification of Two Regimes of Carrier Thermalization in PbS Nanocrystal Assemblies

TL;DR

This study reveals two distinct regimes of carrier thermalization in PbS nanocrystal assemblies, using a phenomenological model to analyze photoluminescence and photocurrent data, highlighting the effects of assembly density and ligand length.

Contribution

It introduces a phenomenological model based on Kirchhoff's law to identify and quantify thermalized photocarriers and their temperatures in PbS quantum dot assemblies.

Findings

Highly compact assemblies exhibit wide energy distribution of thermalized carriers.

Ligand length influences whether thermalization involves only excitons or a broader carrier distribution.

The model accurately explains the observed Stokes shift and thermalization behavior.

Abstract

We bring fresh insight into the ensemble properties of PbS colloidal quantum dots with a critical review of the literature on semiconductors followed by systematic comparisons between steady-state photocurrent and photoluminescence measurements. Our experiments, performed with sufficiently low powers to neglect nonlinear effects, indicate that the photoluminescence spectra have no other noticeable contribution beside the radiative recombination of thermalized photocarriers (i.e. photocarriers in thermodynamic quasi-equilibrium). A phenomenological model based on the local Kirchhoff law is proposed that makes it possible to identify the nature of the thermalized photocarriers and to extract their temperatures from the measurements. Two regimes are observed: for highly compact assemblies of PbS quantum dots stripped from organic ligands, the thermalization concerns photocarriers distributed over a wide energy range. With PbS quantum dots cross-linked with 1,2-ethanedithiol or longer organic ligand chains, the thermalization concerns solely the fundamental exciton and can quantitatively explain all the observations, including the precise Stokes shift between the absorbance and luminescence maxima.