Phonon-assisted Auger enables ultrafast charge transfer in CdSe Quantum Dot/Organic Molecule

TL;DR

This study combines ab-initio simulations and spectroscopy to reveal how phonon-assisted Auger processes enable ultrafast charge transfer in CdSe quantum dots, with implications for optimizing nanostructure applications.

Contribution

It provides an atomistic understanding of charge transfer mechanisms in quantum dots, highlighting the role of phonon-assisted Auger processes in ultrafast transfer.

Findings

Ultrafast charge transfer (20 fs) in smaller quantum dots due to phonon-assisted Auger process.

Slower charge transfer (516 fs) in larger quantum dots caused by energy detuning and weak vibronic coupling.

Combined theoretical and experimental approach offers detailed insight into charge transfer dynamics.

Abstract

Charge transfer between photoexcited quantum dots and molecular acceptors is one of the key limiting processes in most applications of colloidal nanostructures, most prominently in photovoltaics. An atomistic detailed description of this process would open new ways to optimize existing and create new structures with targeted properties. We achieve a one-to-one comparison between ab-initio non-adiabatic molecular dynamics calculations and transient absorption spectroscopy experiments, which allows us to draw a comprehensive atomistic picture of the charge transfer process, following the time evolution of the charge carrier across the electronic landscape and identifying the thereby induced vibrations. For two quantum dot sizes we find two qualitatively different processes. For the larger structure we find a relatively slow (\tau = 516 fs) transfer process that we explain by the existence of a large energy detuning and weak vibronic coupling. For the smaller structure the process is ultrafast (\tau = 20 fs) due to an efficient, phonon-assisted Auger process triggered by a strong electron-hole coupling.