Influence of fluorination on the microstructure and performance of diketopyrrolopyrrole‐based polymer solar cells

The solar cell performance and microstructure of DPP-based polymers with different degrees of fluorination are reported. DPP-based polymers with thiophene–phenyl–thiophene comonomer and thiophene flanking units are studied, with the degree of fluorination of the phenyl unit varied. With bifluorination of the phenyl ring, a higher open circuit voltage is achieved whilst maintaining or even improving the overall solar cell efficiency. While tetrafluorination leads to a further 0.1 V increase in VOC, reaching a high photo voltage of 0.81 V, overall solar cell performance significantly drops. Microstructural studies using AFM, TEM, Grazing incidence wide-angle X-ray scattering (GIWAXS), and Resonant soft X-ray scattering (R-SoXS) reveal that bifluorination largely preserves the microstructure of the nonfluorinated system, whereas tetrafluorination results in coarse phase separation between the polymer donor and the fullerene acceptor. Our results demonstrate that the use of an extended comonomer is a promising strategy for optimizing the beneficial effects of fluorination for DPP-based polymer solar cells, especially in improving the open circuit voltage. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 49–59