Donor polymer based on alkylthiophene side chains for efficient non-fullerene organic solar cells: insights into fluorination and side chain effects on polymer aggregation and blend morphology

Reducing the voltage loss is considered a promising route to improve the performance of organic solar cells (OSCs). In this work, two donor polymers PT4FB and PTT4FB based on a tetra-fluorinated benzene building block are synthesized and applied to non-fullerene OSC devices, which achieve high open-circuit voltage (V_OC). When PT4FB is combined with IDIC, the blend exhibits large fiber-like domains (size ∼39 nm) due to the excessive aggregation of PT4FB, which results in a relatively low short-circuit current (J_SC) of 14.06 mA cm⁻² and a moderate power conversion efficiency (PCE) of 9.81%. To weaken the aggregation of PT4FB, we replace the alkyl chains on the polymer backbone with alkylthiophene side chains to afford a novel donor polymer PTT4FB with reduced packing and crystallinity. As a result, the PTT4FB:IDIC-based blend shows significantly smaller domain sizes (24 nm) than the PT4FB-based device, contributing to a remarkably increased J_SC and an overall higher PCE of 10.60%. The synergistic effects of abundant fluorination and alkylthiophene side chains can be utilized as a general method to tune the morphological and energetic properties when designing novel donor polymers to achieve both high V_OC and favorable morphology for efficient non-fullerene OSCs.