title: Two-photon photoemission study of competing Auger and surface-mediated relaxation of hot electrons in CdSe quantum dot solids.
abstract: Solids composed of colloidal quantum dots hold promise for third generation highly efficient thin-film photovoltaic cells. The presence of well-separated conduction electron states opens the possibility for an energy-selective collection of hot and equilibrated carriers, pushing the efficiency above the one-band gap limit. However, in order to reach this goal the decay of hot carriers within a band must be better understood and prevented, eventually. Here, we present a two-photon photoemission study of the 1Pe→1Se intraband relaxation dynamics in a CdSe quantum dot solid that mimics the active layer in a photovoltaic cell. We observe fast hot electron relaxation from the 1Pe to the 1Se state on a femtosecond-scale by Auger-type energy donation to the hole. However, if the oleic acid capping is exchanged for hexanedithiol capping, fast deep hole trapping competes efficiently with this relaxation pathway, blocking the Auger-type electron-hole energy exchange. A slower decay becomes then visible; we provide evidence that this is a multistep process involving the surface.
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, E-IF: Solar Fuels, Berlin, Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Sippel
authors.forename: Wiebke
authors.initials: W
authors.lastname: Albrecht
authors.forename: Dariusz
authors.initials: D
authors.lastname: Mitoraj
authors.forename: Rainer
authors.initials: R
authors.lastname: Eichberger
authors.forename: Thomas
authors.initials: T
authors.lastname: Hannappel
authors.forename: Daniel
authors.initials: D
authors.lastname: Vanmaekelbergh
mesh_terms: D019187:Cadmium Compounds / Q000737:chemistry*; D004583:Electrons; D006358:Hot Temperature; D036103:Nanotechnology; D056951:Photoelectron Spectroscopy; D017785:Photons; D045663:Quantum Dots / Q000737:chemistry*; D018036:Selenium Compounds / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D019187:Cadmium Compounds; D018036:Selenium Compounds; C058667:cadmium selenide