Layer-by-layer and non-halogenated solvent processing of benzodithiophene-free simple polymer donors for organic solar cells

• Two simple non-fused-ring polymer donors for LBL and halogen-free solvent processing. • PBTz-TC can well balance the independent film formation. • The efficiency of PBTz-TC device treated with non-halogenated solvent reaches 12.81%. High performance and easy-synthesis benzodithiophene (BDT)-free simple active layer materials, green non-halogen solvent processing and convenient device fabrication with batch-to-bath stability are all crucial to realize the industrial production of organic solar cells (OSCs) in the future. In this work, we develop two BDT-free polymer donors, PBTz-TC and PBTz-TTC, for green halogen-free solvent and layer-by-layer (LBL) processed high performance OSCs. The strong molecular aggregation of BTz units helps to improve the crystallinity and film-forming properties of the polymers, while the introduction of ester groups assists in enhancing the solubility of conjugated polymers in both halogen and non-halogen solvents and down-shifting the energy levels. The synergistic implementation of LBL method and orthogonal solvents can significantly reduce the scouring effect during the sequential deposition process, which is conducive to construct planar heterojunction. Due to the more planar conformation, PBTz-TTC possesses more robust film-forming property with superior corrosion resistance to the upper-layer than PBTz-TC. While, PBTz-TC better balances the independent film formation and moderate miscibility with IT-4F acceptor to favor stronger interaction at the donor/acceptor interfaces. Consequently, PBTz-TC shows much better performance than PBTz-TTC in both bulk heterojunction and LBL devices, ascribed to the better energy alignment, more favorable morphology and more efficient charge dynamic process. Notably, when treated with halogen-free solvent, an outstanding power conversion efficiency (PCE) of 12.81% with improved stability is achieved for PBTz-TC/IT-4F-based LBL device, which is the highest value of non-halogenated solvent processed devices based on benzodithiophene-free simple polymer donors. These findings provide a guideline for rational design of the environmental friendly simple polymer donor materials for further acceleration of the industrial production of OSCs.