Improving the Photovoltaic Performance of Polymer Solar Cells Based on Furan-Flanked Diketopyrrolopyrrole Copolymers via Tuning the Alkyl Side Chain

Two furan-flanked diketopyrrolopyrrole copolymers, poly(3,6-difuran-2-yl-2,5-di(alkyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-altthienylenevinylene) with different alkyl side chains (PDVFs), have been synthesized and applied as a donor in polymer solar cells (PSCs). The PSC based on a blend of PDVF-8 with 2-octyldodecyl and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an active layer has shown a better device performance than the PSC based on a blend of PDVF-10 with 2-decyltetradecyl and PC71BM. Tuning the alkyl side chains attached on the PDVFs leads to an increase of power conversion efficiency from 3.57% (PDVF-10) to 4.56% (PDVF-8) due to enhancements of short circuit current and fill factor. The effect of different alkyl side chains on the phase separation of the PDVF/PC71BM thin film has been investigated by using atomic force microscopy, transmission electron microscopy, and X-ray photoemission spectroscopy depth profiling in details. Furthermore, impedance spectroscopy was used to analyze the relationship between the phase separation of the PDVF/PC71BM blend films and the PSCs performance.