Regioregularity and Electron Deficiency Control of Unsymmetric Diketopyrrolopyrrole Copolymers for Organic Photovoltaics

Manipulating the electron deficiency and controlling the regioregularity of pi-conjugated polymers are important for the fine-tuning of their electronic and electrochemical properties to make them suitable for an organic solar cell. Here, we report such a molecular design of unsymmetric diketopyrrolopyrrole (DPP) based copolymers with different aromatic side units of either thiophene (Th), pyridine (Py), or fluorobenzene (FBz). The unsymmetric electron acceptors of Th-DPP-Py and Th-DPP-FBz were polymerized with the electron donor of two-dimensional benzobisthiophene (BDT-Th), affording two regiorandom DPP copolymers. They exhibited contrasting molecular orbital levels and bulk heterojunction morphology in methanofullerene-blended films, leading to power conversion efficiencies of 3.75 and 0.18%, respectively. We further synthesized a regioregular DPP copolymer via sandwiching the centrosymmetric BDT-Th unit by two Th-DPP-Py units in an axisymmetric manner. The extensive characterization through morphology observation, X-ray diffraction, and space-charge-limited current mobilities highlight the case-dependent positive/negative effects of regioregularity and electron deficiency control.