Quantum dots (QDs) possess exceptional optoelectronic properties resulting from size-dependent spatial confinement of their electronic wavefunctions. One of the most interesting properties of QDs is the ability to form multiple interacting exction pairs, known as ‘multiexctions’. Multiexcitons can be generated in QDs either via simultaneous absorption of multiple photons or by the multiexciton generation process, where the QDs are excited with high energy photons of at least twice the band-gap. Multiexcitons are of high interest for applications such as nanocrystal laser, photovoltaics, and photocatalysis. For example, multiexcitons hold the potential to enhance the efficiency of a photocatalytic reaction requiring multiple charge carriers. In such QD-based applications, multiexciton lifetime plays a key role. Exemplarily in photocatalysis, a sufficiently long-lived multiexciton species will be required for an efficient carrier transfer to the catalytic center. Unfortunately, multiexcitons in QDs undergo fast non-radiative Auger recombination (up to a few 100s of ps) due to the strong spatial overlap of electron and hole wavefunctions. However, it has been demonstrated that by tuning parameters such as QD size, shape, and surface properties, multiexciton lifetimes can be prolonged. Therefore, understanding the relation between structural parameters of QDs and multiexciton lifetime is necessary for QD-based applications exploiting multiexciton species. 

In this contribution we demonstrate how broadband transient absorption data can support the identification of multiexciton species via characterization of spectral signatures. The approach enables us to gain detailed insight into multiexciton relaxation and to determine multiexciton binding energies. We analyze excitation intensity-dependent transient absorption data sets globally by applying a Markov Chain Monte Carlo sampling target analysis capable of simultaneously modeling spectrally resolved data sets at varying excitation intensities. Exemplarily, results for the comparative investigation of colloidal CdSe QDs of different sizes and varying surface functionalization are presented. We compare the properties of trioctylphosphine oxide (TOPO) capped and sulfide (S²⁻) capped QDs. The S²⁻ ligand introduces additional surface traps resulting in potentially strong hole localization at the QD surface. With this approach, we are able to detect multiexcitons up to tetraexcitons (i.e., four bound excitons). The comparative analysis of the intensity-dependent transient absorption data reveals that the multiexciton lifetime is majorly dependent on the extent of carrier wavefunction overlap. The reduced carrier wavefunction overlap, with increasing QD size, and with carrier localization at QD surface (as introduced by S²⁻ ligands) leads to prolonged multiexciton lifetime. Additionally, the reduced carrier wavefunction overlapping results in reduced multiexciton binding energies.