Dipole assisted exciton dissociation at conjugated polymer/fullerene photovoltaic interfaces: A molecular study using density functional theory calculations

Using density functional theory calculations we probe the electronic structure of the P3HT/fullerene supramolecule. Our simulations indicates that the mix of the polymer and fullerene wave functions give rise to a set of intragap charge transfer (CT) states at the polymer/fullerene interface. This overlap of wavefuctions produces a charge transfer in the ground state. The interaction between this effect and the permanent dipole in the polymer creates a dipole across the supramolecule pointing from the fullerene to the polymer. We find that an efficient exciton dissociation is possible because (i) the CF states are energetically favorable intermediary levels for the electron transfer from the polymer to the fullerene: (ii) there is a potential energy barrier height that blocks the hole transfer to the P3HT chain of the complex, keeping the charge carrier confined in the chain next to the P3HT/fullerene system. This barrier is created by the potential energy of the supramolecule's dipole moment immersed in the electric field of the electron-hole pair. Finally, using a simple analytical model we show that the dipole's induced barrier height depends critically on the orientation of the dipole vector relative to the polymer backbone. From this analysis we anticipate the main features of a conjugated polymer/fullerene complex with a high efficiency on exciton dissociation. (C) 2010 Elsevier B.V. All rights reserved.