Revealing the role of solvent additives in morphology and energy loss in benzodifuran polymer-based non-fullerene organic solar cells

Abstract

Solvent additive (SA) treatment is the most effective strategy to obtain highly efficient non-fullerene organic solar cells (NF-OSCs). However, NF-OSCs from SA treatment usually exhibit different or even opposite effects on photovoltaic performance compared with fullerene-based OSCs. Despite the rapid progress in NF-OSCs from SA treatment, the working mechanism of SAs in the morphological optimization of NF-OSCs and their role in affecting the photovoltaic performance are still controversial, and systematic investigations are needed. In this work, two kinds of SA, non-aromatic solvent additives (NSA: DIO, DBrO) and aromatic solvent additives (ASA: DPE, CN), were applied to the benzo[1,2-b:4,5-b′]difuran-based polymer PBDF-BDD and non-fullerene acceptor IT-4F to systemically investigate their effects on photovoltaic performance and their roles in the open-circuit voltage (V_oc) and fill factor (FF). The SAs have a positive impact on the power conversion efficiencies of NF-OSCs, but completely opposite trends of V_oc and FF were observed for NF-OSCs processed with NSAs and ASAs. The devices with NSA obtained higher FF than devices with ASA. However, the NSA-added devices exhibited a V_oc drop of about 0.1 V with a large energy loss (E_loss); in contrast, devices with ASAs showed minor change on V_oc and thereby on E_loss. Further investigations indicated that NSA and ASA have different effects on inducing the crystallization of either PBDF-BDD or IT-4F, resulting in different donor/acceptor interactions and interfaces, thus producing the changes in E_g, E_CT and energetic disorder, which were further investigated through GIWAXS and energy loss mechanism.