Phonon-Assisted Auger Process Enables Ultrafast Charge Transfer in CdSe Quantum Dot/Organic Molecule

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. We achieved a one-to-one comparison between ab initio nonadiabatic molecular dynamics calculations and transient absorption spectroscopy experiments, which allowed 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. Our calculation shows a relatively slow (τ = 516 fs, in good agreement with our measured τ = 500 fs) charge transfer in a 1.7 nm diameter CdSe quantum dot/dicationic methyl viologen system that we explain by the existence of a large energy detuning and weak vibronic coupling. We predict that this charge transfer becomes ultrafast (τ = 20 fs) for smaller quantum dots (1.2 nm diameter in our calculations) due to an efficient, phonon-assisted Auger process triggered by a strong electron-hole coupling.