Entropy-Driven Charge Separation: A Potential Explanation for the Low Energy Loss Found in OPVs with Non-fullerene Acceptors

Abstract

The recent development of non-fullerene acceptors (NFA) has led to an abrupt increase in the organic photovoltaic (OPV) efficiency from ∼12% to ∼20%. NFA OPVs can generate photocurrent efficiently without much energy loss, which contrasts with the large energy loss often needed for generating free charges from excitons in fullerene OPVs. Here, we argue that entropydriven charge separation, which allows excitons to spontaneously gain heat from the environment, is responsible for the exceptionally low energy loss found in NFA OPVs. The entropic driving force is maximized when the delocalized electron and hole wave functions in the CT exciton intersect each other via pointlike junctions. These pointlike junctions are abundant in the polymer/NFA bulk heterojunction which consists of intertwined, stringlike donor and acceptor domains in which electron delocalizes anisotropically. Such microstructure favors entropy-driven charge separation, which can enable charge separation with a minimal energy loss.