Charge Separation in Organic Solar Cells: Energy Bending versus Energy Disorder

Charge Separation in Organic Solar Cells: Energy Bending versus Energy DisorderGjergji SiniChemistry Departement, University of Cergy-Pontoise, Cergy-Pontoise, (France)Abstract:In the domain of organic solar cells (OSC), the long-lasting dominance of fullerene-based acceptors has been recently overcome. Different mechanisms that could explain this enormous boost in solar cell efficiency have been suggested. Recent theoretical studies highlight the dominant role that the energy disorder and the entropy play on the charge separation efficiency at the donor-acceptor (D-A) interface. [1-2] However, implicit contributions from driving forces other than the energy disorder make unclear the individual impacts of different factors. Comparative studies between the impacts of each mechanism are thus still missing but highly desirable for the design of new materials.Aiming to obtain deeper insights on the mechanisms impacting the charge separation efficiency at the D-A interface, here we focus on the relative impact of two driving forces: the energy bending (EB) and the energy disorder. We firstly discuss results obtained by means of density functional theory (DFT) methods, allowing to (i) demonstrate the presence of a substantial driving force (EB) between the donor-acceptor interface and the pure donor- and acceptor domains; (ii) identify the reasons for this driving force (geometric deformations and direct-contact intermolecular polarization). [3] In a second time, we compare the impact of the EB and of the energy disorder on the charge separation efficiency by means of Kinetic Monte-Carlo (KMC) simulations. Our results suggest large impact of the EB on the charge separation efficiency, which is much stronger than the effect of the energy disorder. [4] References:[1] D. A. Vithanage, et al. Nature Communications, 4, 2334, (2013)[2] Hood, S. et al. J. Phys. Chem. Lett. 7, 4495, (2016)[3] Sini, G., et al., Adv. Energy Mater. 1702232, (2018)[4] L. Sousa, V. Coropceanu, D. A. da Silva Filho, G. Sini. Manuscript in preparation