While organic solar cells overtook the long-sought power conversion efficiency threshold of 10%, the mechanism of free charge carriers generation upon sunlight absorption remains obscure and robust structure-property relationships are far from being set. In this talk, I will present a perspective on our recent work on the modelling of donor-acceptor interfaces for organic photovoltaic applications, for which we propose a multiscale approach based on a synergistic combination of atomistic molecular dynamics, quantum chemistry and microelectrostatic calculations [1,2]. By considering the prototypical P3HT:PCBM heterojunction [3] and an homeotropically oriented interface between discotic liquid crystals [4] as case studies, I will discuss the crucial role of microscopic electric fields, environment polarization, structural and energetic disorder, on the energetics of electron-hole separation. The approximation of localized carriers, mandatory in microelectrostatic models, will be then relaxed to allow for charge delocalization and mixing between Frenkel and charge transfer excitons, obtaining a realistic effective Hamiltonian for the excited states at the P3HT:PCBM interface. While delocalization has a minimal impact on our estimates for the electron-hole capture radius (~4 nm) and separation energy barrier (~0.2 eV), the mixing between Frenkel and charge transfer excitons has important implications on the optical properties of the interface and on the mechanism of free charges generation. Our results provide insights on the driving force for photocarriers generation in different systems, and their generalization suggests rational rules for performance optimization in organic solar cells.
[1] J. Cornil, et al., Accounts of Chemical Research, 46, 434 (2013). [2] F. Castet, G. D’Avino, L. Muccioli, J. Cornil and D. Beljonne, Physical Chemistry Chemical Physics, 16, 20279 (2014). [3] G. D’Avino et al., J. Phys. Chem. C, 117, 12981 (2013). [4] J. Idé et al., J. Am. Chem. Soc, 136, 2911 (2014).