Manipulating the intermolecular stacking of polymeric donors for efficient organic solar cells

Abstract

Most polymer donors developed so far for high-performance non-fullerene OSCs are designed in planar molecular geometries containing BDT units. In this work, two donor–acceptor (D–A) conjugated polymers with wide band-gap, namely, PY3 and PY4, were designed and synthesized by introducing halogen atoms (F, Cl) into the donor units of polymers. We dismissed the π-bridge and fused it to the A unit (BTz), presenting a different electron-deficient unit labeled as fused-DTBTz, forming a D–A structure with halogenated BDT units. PY3 with fluorine has a deeper HOMO level and a slight bathochromic-shift compared with PY4 with a chlorine atom, and shows good thermal stability among the two polymers. The reasonable halogenated D-unit in D–A polymers is beneficial for effectively promoting the intermolecular packing and crystalline properties, and thus enhancing the charge separation and transfer as reported previously. The best-performing PY3:IT-4Cl-based device afforded a high PCE of 11.20% with a V_OC of 0.798 V, a J_SC of 19.09 mA cm⁻², and an FF of 73.5%, and the OSCs based on PY4:IT-4F delivered a PCE of 12.51%, with a noticeably elevated V_OC of 0.874 V, a J_SC of 20.26 mA cm⁻², and an FF of 70.7%. The results showed that chlorinated polymers have a more suitable energy level to match the acceptors than their fluorinated counterparts, thus effectively improving the FF and J_SC of photovoltaic devices. Therefore, there is much room for enhancement in photovoltaic properties with rational halogenation of the D moiety of D–A polymers.