Carrier multiplication and cooling in semiconductor quantum dots

Abstract

In semiconductor quantum dots (QDs), charge carrier cooling is in direct competition with carrier multiplication (CM), a process in which one absorbed photon excites two or more electrons that may improve the light conversion efficiency of photovoltaic devices. CM by an initially hot charge carrier occurs in competition with cooling, with the respective rates determining the CM efficiency. Until now, the factors that determine the onset energy and efficiency of CM have not been convincingly explained. Most research on cooling involves low photoexcitation energies close to the band gap, while the competition between CM and cooling takes place at higher energies where an electron or hole has an excess energy that is at least equal to the band gap. Moreover, CM rates have only been calculated theoretically, while experimental studies of CM have focused mostly on proving its occurrence in various materials. Understanding charge carrier cooling at high excess energy and comparing this to experimental CM rates is therefore of great interest. Chapters 2 and 3 of this thesis are aimed at understanding charge carrier cooling, while Chapters 4 and 5 relate this to the onset energy and efficiency of CM. The presented results are a large step forward in understanding cooling and CM and allow for a screening of materials with an onset of CM close to twice the band gap energy. Such materials are of great interest for development of highly efficient photovoltaic devices.