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Lecture R.J. Janssen


20 May 2009 FWN-Building 5116.0116, Nijenborgh 4, 9747 AG, Groningen
Speaker: Prof. Dr. R.J. Janssen
Affiliation: Molecular Materials and Nanosystems, Eindhoven University of Technology
Title: The role of ground and excited charge transfer states in polymer: Fullerene solar cells
Date: Wed May 20, 2009
Start: 11.00
Location: FWN-Building 5116.0116
Host: M.A. Loi
Telephone: +31 50 363 4119


The lecture will address various aspects of charge-transfer (CT) states in polymer:fullerene solar cells. It is generally assumed that there is negligible interaction between the two components in the ground state, but in fact we find that a large fraction (> 50%) of fullerenes can actually be involved in a CT complex with the polymer. This rationalizes the high quantum efficiency of charge generation for a material with short exciton diffusion length. Further a general method is given to predict the energy of intermolecular CT states in electron donor-acceptor (D-A) blends used in organic bulk-heterojunction solar cells. It is shown that charge separation can occur when the excited singlet state is only 0.1 eV above the CT state. The open-circuit voltage of the cells is ~0.5 V below the CT energy. Hence, the maximum attainable open-circuit voltage is 0.6 V below the optical band gap. The excited CT state in PFTBT:PCBM blends has a short decay time (~4 ns) that is reduced by the application of an external electric field or with increasing PCBM content. The field induced quenching can be explained quantitatively with the Onsager-Braun model when including high electron mobility in nanocrystalline PCBM clusters. The presence of nanocrystalline domains with high local carrier mobility of at least one of the two components in an organic heterojunction may explain efficient dissociation of CT states into free charge carriers.

Last modified:22 October 2012 2.31 p.m.