Binary organic solar cells with efficiency over 20% enabled by solid additives with side-chain halogenation

Abstract

Halogenated solid additives have been demonstrated to selectively interact with active layer components, thereby optimizing the performance of organic solar cells (OSCs). However, the selection strategy for halogenated additives and their complex intermolecular interactions with the active layer remain poorly understood. This work proposes a side-chain halogenation strategy for solid additives and introduces three side-chain halogenated solid additives, 2-(fluoromethyl)naphthalene (2-FMN), 2-(chloromethyl)naphthalene (2-CMN) and 2-(bromomethyl)naphthalene (2-BMN). Compared with 2-methylnaphthalene (2-MN) without side-chain halogenation, side-chain halogenated additives exhibit stronger non-covalent interactions with BTP-eC9. They selectively adsorb onto the acceptor surface and act as a “temporary molecular bridge” to optimize molecular packing, charge transport and recombination dynamics. As a result, the D18:BTP-eC9 binary device processed with 2-BMN achieves a remarkable power conversion efficiency (PCE) of 20.02%, representing one of the highest values reported for binary devices. Furthermore, an improved PCE of 20.22% is achieved in the D18:BTP-eC9:ZY-4Cl ternary device processed with 2-BMN. This study validates the side-chain halogenation strategy as an effective approach for realizing high performance OSCs.