Colloquium: Laurens Siebbeles



Q105, Vrije Universiteit, De Boelelaan 1081, Amsterdam

Dynamics of excitons and charges in organic materials and semiconductor quantum dots

Prof.dr. Laurens Siebbeles, TU Delft

Faculty of Sciences




Laurens D.A. Siebbeles

Optoelectronic Materials Section, Dept. of Chemical Engineering, Delft University of Technology

Themptye seminar will report studies of the behavior of electronic excited states (excitons) and excess charge carriers in conjugated polymers, covalent organic frameworks and semiconductor nanoparticles. These materials have fascinating optical and electronic properties that are little understood and are of interest for applications in e.g. solar cells, photodiodes, light-emitting diodes, field-effect transistors and nanoscale molecular electronics. The mechanism of charge carrier photogeneration in thin films of the polymer P3HT and the electron acceptor PCBM (C60 derivative) was studied by ultrafast optical pump-probe and terahertz spectroscopy. It is inferred that photoexcitation leads to formation of free electrons and holes, rather than Coulombically bound electron-hole pairs in close proximity.

We studied the mobility of charges in covalent organic frameworks consisting of phthalocyanine units that are strongly coupled by pi-pi stacking in an eclipsed configuration.  Charges were generated by irradiation with 3 MeV electrons from Van de Graaff accelerator, and were probed by microwave conductivity measurements. The virtually temperature independent charge mobility suggest a band-like mechanism of charge motion, rather than hopping via localized states. According to quantum mechanical simulations, eclipsed stacking of the phthalocyanine units can lead to a high charge mobility of ~100 cm2/Vs, which largely exceeds that for conventional organic semiconductors.

The generation of two excited states for the absorption of a single photon in semiconductor quantum dots was studied. We established that this process of multiple exciton generation (MEG) occurs in PbSe quantum dots. Adding a PbS shell around a PbSe core quantum dot leads to delocalization of electron and hole wave functions into the PbS shell. Surprisingly, this does not affect the MEG efficiency. Possible causes for the anomalous independence of MEG on the presence of a PbS shell will be discussed. Using a combination of ultrafast terahertz photoconductivity and optical absorption measurements, it was found that photoexcitation of a film of strongly coupled PbSe quantum dots leads directly to formation of free charge carriers rather than excitons.