Passivation-sensitive exciton finestructure produces excess Stokes shifts in colloidal quantum dots

TL;DR

This paper demonstrates that surface passivation influences the excitonic finestructure in colloidal quantum dots, causing excess Stokes shifts through coupling effects, and proposes methods to control this for improved solar cell performance.

Contribution

It reveals how passivation affects exciton energy levels and explains excess Stokes shifts without trap formation, offering strategies to optimize quantum dot solar cells.

Findings

Stokes shift correlates with surface passivation degree.

Coupling to surface states reorders excitonic transitions.

Core stoichiometry control can eliminate Stokes shift.

Abstract

The excitonic finestructure of colloidal quantum dots (CQDs) is comprised of a manifold of transitions, of which only the lowest are populated and contribute to photoluminescence. This leads to a Stokes shift in emission relative to absorption. Here we show experimentally that the Stokes shift in Pb and Cd-based chalcogenide CQDs is correlated with the degree of surface passivation, and develop a model that explains how coupling to the surface affects the core electronic states. Dark and bright transitions can reorder and split, increasing the Stokes shift even without the formation of deep traps. Our findings resolve the highly-debated topic of excess Stokes shifts in PbS nanocrystals as due to parity-forbidden transitions instead of traps. We predict that the Stokes shift in PbS can be eliminated via core stoichiometry control, a critical step towards enhancing the open circuit voltage in quantum dot solar cells.