Spontaneous formation of potential cascade enhances charge separation in PM6-Y6 organic photovoltaics

Mechanisms that enhance charge separation at donor-acceptor interfaces are the key to material design of non-fullerene electron acceptors for high-efficiency organic photovoltaics (OPV). Here, the energetics of charge separation at the PM6-Y6 donor-acceptor interface in the state-of-the-art OPV is analyzed on the basis of quantum mechanics/molecular mechanics calculations. The electron energy level in Y6 becomes lower with increasing distance from the interface with PM6 at which the crystallinity is lower than in the bulk region. Electrostatic interactions from the multipoles of Y6 stabilize the electron in the crystalline region. The PM6-ITIC donor-acceptor interface also exhibits a similar potential cascade owing to the quadruple of ITIC. The potential cascade destabilizes charge transfer states at the PM6-Y6 interface, thereby decreasing the potential barrier for charge separation. Charge delocalization on several molecules via transfer integral further decreases the barrier for charge separation.