Efficient organic solar cells processed from a halogen-free solvent based on benzo[1,2-b:4,5-b′]difuran terpolymers

The low-cost and environment-friendly solvent processing in organic solar cells (OSCs) is critical for commercial applications. Herein, we selected the benzo[1,2-b:4,5-b′]difuran (BDF) terpolymers (PBDF4F-Th2 and PBDF4F-Th5) as the low-cost polymer donors and the small molecular semiconductor IT-4F as the acceptor to fabricate the OSCs using non-halogenated o-xylene (o-XY) as the processing solvent of the active layer. While introducing the 3-(ethyl carboxylate)thiophene (Th) unit as the third comonomer, terpolymers PBDF4F-Th2 and PBDF4F-Th5 exhibited good solubilities in the non-halogenated solvents such as o-XY and tetrahydrofuran (THF). The power conversion efficiencies (PCEs) of these devices based on PBDF4F-Th2:IT-4F(o-XY) and PBDF4F-Th5:IT-4F(o-XY) reached up to 11.70%, which are comparable with the devices processed from the chlorinated and toxic chlorobenzene (CB) solvent. However, the alternating polymer PBDF4F-based device only had a PCE of 3.32% while processing using the o-XY solvent due to its limited solubility. Furthermore, layer-by-layer devices were also fabricated via the sequential processing steps by dissolving PBDF4F-Th2 (or PBDF4F-Th5) into o-XY and IT-4F into THF. The PCE of 11.19% was obtained from the PBDF4F-Th5/IT-4F device. The high photovoltaic performance in these BDF terpolymer-based OSCs using non-toxic and environment-friendly non-halogenated solvents indicates that the ternary polymerization strategy is an effective route to develop low-cost and efficient BDF polymer donors that can be processed from halogen-free solvents via a simple processing technology.