Mardi 11 avril à 11h00,
Salle Louis Weil, E424
Orateur : Gabriele D’AVINO
"Modelling charge transfer in organic semiconductors : from photovoltaics to molecular doping"
Charge transfer (CT) interactions between electron donor (D) and acceptor (A) molecules are central to many physical phenomena in chemistry, biology and materials science. I will report about our modelling of CT phenomena in organic photovoltaics and in doped semiconductors, for which we propose a multiscale approach based on first-principles and model Hamiltonian electronic structure calculations, and classical schemes to account for electrostatics and polarization in disordered systems.
The early steps of the charge generation in organic solar cells take place at the interface between an electron D (e.g. pi-conjugated polymers) and A (e.g. fullerene derivatives), where molecular (Frenkel) excitons split into space-separated electron-hole pairs. Our calculations for a prototypical polymer/fullerene interface show that bound localized CT states coexist with delocalized free charges that can be also reached by direct photoexcitation. This reconciles the recent experimental reports of ultrafast exciton separation (“hot” process) with the evidence that high quantum yields do not require excess electronic or vibrational energy (“cold” process). 
The conductivity of organic semiconductors can be increased by orders of magnitude by introducing dopant impurities, although mechanistic aspects are currently unclear. Our calculations for the prototypical F4TCNQ-doped pentacene crystal, based on embedded many-body perturbation techniques of recent development , show that while the acceptor level lies very deep in the gap, excitonic electron-hole attraction and polaronic effects rationalize the possibility for room-temperature dopant ionization. These findings question the applicability of the standard picture of doping in inorganic semiconductors to organics and provide a coherent framework explaining available experimental data. 
 G. D’Avino, Y. Olivier, L. Muccioli, D. Beljonne, J. Phys. Chem. Lett. 7, 536 (2016)
 J. Li, G. D’Avino, I. Duchemin, D. Beljonne, X. Blase, J. Phys. Chem. Lett. 7, 2814 (2016)
 J. Li, G. D’Avino, A. Pershin, D. Jacquemin, I. Duchemin, D. Beljonne, X. Blase, submitted (2017)