Doping InP Quantum Dots with Cu+ slows down Hot Electron Cooling

Abstract

Nonresonant excitation of colloidal quantum dots (QDs) creates hot carriers that subsequently cool down to the band edges or are trapped in localized states. Carrier cooling and trapping typically happens on timescales from femtoseconds to picoseconds, orders of magnitude faster than the nanosecond to microsecond timescales of radiative recombination. Understanding cooling and trapping is relevant for (hot) carrier extraction in photovoltaics and to increase the luminesence output of QDs used as phosphor. We investigate carrier cooling and trapping in InP QDs with a ZnSe1-xSx shell. Undoped QDs are compared to Cu+-doped QDs, where the Cu+ ion serves as a designed hole trap. Using pump probe transient absorption spectroscopy with femtosecond time resolution, we are able to monitor the population of electrons in the conduction band. Our comparative study shows that hot electron cooling is almost an order of magnitude slower in the Cu+-doped QDs than in the undoped QDs. We ascribe this to rapid hole trapping on the Cu+ ion. This confirms the model in which hot electron cooling goes via an Auger-like process, where the hot electron transfers its excess energy to the hole which subsequently relaxes by phonon coupling. In our Cu+-doped QDs the hole is trapped on a Cu+ ion on sub-picosecond timescales, so the Auger cooling pathway is unavailable to the hot electron. Instead it must cool down via another, slower, pathway, most likely by coupling to high-energy vibrations at the surface of the QDs. This must also mean that hole trapping on the Cu+ ion is faster than the Auger cooling timescale of the undoped QDs, which is of the order of 300 fs. Our results provide insight in the behaviour of hot electrons and holes in the short time period after excitation of both Cu+-doped and undoped InP QDs.