A-π-A structured non-fullerene acceptors for stable organic solar cells with efficiency over 17%

With the development of photovoltaic materials, especially the small molecule acceptors (SMAs), organic solar cells (OSCs) have made breakthroughs in power conversion efficiencies (PCEs). However, the stability of high-performance OSCs remains a critical challenge for future technological applications. To tackle the inherent instability of SMA materials under the ambient conditions, much effort has been made to improve OSCs stability, including device modification and new materials design. Here we proposed a new electron acceptor design strategy and developed a "quasi-macromolecule" (QM) with an A-pi-A structure, where the functionalized pi-bridge is used as a linker between two SMAs (A), to improve the long-term stability without deteriorating device efficiencies. Such type of QMs enables excellent synthetic flexibility to modulate their optical/electro-chemical properties, crystallization and aggregation behaviors by changing the A and pi units. Moreover, QMs possess a unique long conjugated backbone combining high molecular weight over 3.5 kDa with high purity. Compared with the corresponding SMA BTP-4F-OD (Y6-OD), the devices based on newly synthesized A-pi-A type acceptors QM1 and QM2 could exhibit better device stability and more promising PCEs of 17.05% and 16.36%, respectively. This kind of "molecular-framework" (A-pi-A) structure provides a new design strategy for developing high-efficiency and -stability photovoltaic materials.