MSE Seminar Series: Laurens Siebbeles
Friday, December 2, 2011
Room 2110 Chemical and Nuclear Engineering Bldg.
301 405 5240
Dynamics of Excited States and Charges in Polymers and Nanocrystals for Optoelectronics
Optoelectronic Materials Section
Department of Chemical Engineering
Delft University of Technology, The Netherlands
The seminar will report studies of the behavior of electronic excited states (excitons) and excess charge carriers in conjugated polymers and semiconductor quantum dots and nanorods. These materials have fascinating optical and electronic properties that are of interest for applications in e.g. solar cells, photodiodes, light-emitting diodes, field-effect transistors and nanoscale molecular electronics.
Thin blend films of the polymer poly(3-hexyl thiophene) (P3HT) and the electron acceptor PCBM (a C60 derivative) attract a great deal of attention as the light-absorbing and charge transporting material in plastic solar cells. We studied the mechanism of charge carrier photogeneration in these films by ultrafast optical pump-probe and terahertz spectroscopy. Photoexcitation leads to formation of free electrons and holes, rather than coulombically bound electron-hole pairs in close proximity.1,2
The absorption of the infrared part of the solar spectrum can be enhanced by adding PbS quantum dots to a P3HT:PCBM film. We studied charge transfer from photoexcited quantum dots to P3HT and PCBM. While charges undergo transfer from the quantum dots to the organic components, they do not contribute to the terahertz photoconductivity. The absence of formation of free mobile charges can be explained by coulomb interaction with charge-induced dipoles in the highly polarizable quantum dots.
The generation of two or more excited states for the absorption of a single energetic photon is of interest for development of highly efficient (up to 44%) solar cells. For PbSe quantum dots in solution this process of carrier multiplication leads to multiple excitons within a quantum dot.3,4 By contrast, in films carrier multiplication leads to generation of free mobile charges that can move freely from one quantum dot to another.5,6 The charges are readily available for use in optoelectronic devices even without employing any complex donor/acceptor architecture or electric fields.
We found from terahertz photoconductivity measurements that photoexcitation of CdS nanorods does produce charges that are freely mobile within a rod. At higher density the conductivity becomes independent on the number of charges in a rod. This unexpected behavior is attributed to effects of quantum confinement.
1) J. Piris, T.E. Dykstra, A.A. Bakulin, P.H.M. van Loosdrecht, W. Knulst, M.T. Trinh, J.M. Schins and L.D.A. Siebbeles, J. Phys. Chem. C, 113, 14500-14506 (2009)
2) W.J. Grzegorczyk, T.J. Savenije, T.E. Dykstra, J. Piris, J.M. Schins and L.D.A. Siebbeles, J. Phys. Chem. C, 114, 5182-5186 (2010)
3) M.T. Trinh, A.J. Houtepen, J.M. Schins, T. Hanrath, J. Piris, W. Knulst, A.P.L.M. Goossens and L.D.A. Siebbeles, Nano Lett., 8, 1713-1718 (2008)
4) M.T. Trinh, L. Polak, J.M. Schins, A.J. Houtepen, R. Vaxenburg, G.I. Maikov, G. Grinbom, A.G. Midgett, J.M. Luther, M.C. Beard, A.J. Nozik, M. Bonn, E. Lifshitz and L.D.A. Siebbeles, Nano Lett., 11, 1623-1629 (2011)
5) E. Talgorn, Y. Gao, M. Aerts, L.T. Kunneman, J.M. Schins, T.J. Savenije, M.A. van Huis,H.S.J. van der Zant, A.J. Houtepen and L.D.A. Siebbeles, Nature Nanotech., DOI:10.1038/nnano.2011.159 (2011)
6) M. Aerts, C.S. Suchand Sandeep, Y. Gao, T.J. Savenije, J.M. Schins, A.J. Houtepen, S. Kinge and L.D.A. Siebbeles, Nano Lett., DOI: 10.1021/nl202915p (2011)