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Accueil > Séminaires > 2012

Energy landscape for charge separation in model organic/organic heterojunctions

Frédéric Castet, ISM (Bordeaux)

Séminaire LCPQ

Salle de séminaire IRSAMC

Among the successive steps describing the light-to-electricity conversion process in organic photovoltaic cells, the splitting of the Coulomb bound electron-hole pairs at the donor/acceptor interface into free charge carriers remains an open question. In that respect, the so-called electric-field assisted mechanism assumes that interfacial electric fields, arising from local interfacial dipoles, can favor the charge separation. On the other hand, the phonon-assisted mechanism describes the long-range charge separation using a “hot CT state†, whose excess energy compensates for the Coulomb binding [1,2]. In order to gain insight into the mechanism that prevails for charge separation in organic-based solar cells, model organic/organic interfaces of increasing complexity were investigated by means of quantum chemical calculations. Our investigations demonstrate that the discontinuity of the electrostatic field at interfaces involving fullerene molecules translates into the appearance of induced dipoles that reshuffle the energy landscape explored by the charges as they move from the interface to the bulk. Depending on the relative orientation of the donor and acceptor molecules, these local electrical fields provide the driving force for splitting the charge-transfer states into free charges [3,4]. We also show that tuning the chemical structure of the interacting molecules allows controlling these electrostatic effects.

References :
[1] Arkhipov et al. Appl. Phys. Lett. 2003, 82, 4605.
[2] Lee et al. J. Am. Chem. Soc. 2010, 132, 11878.
[3] Linares et al. 
J. Phys. Chem. C 2010, 114, 3215.
[4] Verlaak et al.
 Adv. Funct. Mat. 2009, 19, 3809.