Effects of flanked units on optoelectronic properties of diketopyrrolopyrrole based π-conjugated polymers

• Two novel diketopyrrolopyrrole based conjugated polymers were synthesized.
• Optoelectronic properties can be affected by replacing phenyl with 2-pyridyl unit.
• Copolymer containing 2-pyridyl π-bridge has a power conversion efficiency of 3.26%.

Two novel donor–acceptor π-conjugated copolymers comprising (E)-1,2-di(thiophen-2-yl)ethylene as the electron-donating unit, and diketopyrrolopyrrole flanked with phenyl or 2-pyridyl moiety as the electron-accepting unit were synthesized through a palladium-catalyzed Stille polymerization. With respect to the copolymer based on diketopyrrolopyrrole flanked with phenyl unit (PTDPP-Ph), the copolymer counterpart containing the 2-pyridyl moiety (PTDPP-Py) as the flanked unit exhibited increased coplanarity, bathochromic shift in absorption, decreased lowest unoccupied molecular orbital energy levels, as well as the about one order of magnitude increase in hole mobility. Of particular interests is that the bulk heterojunction solar cell based on PTDPP-Py exhibited an improved power conversion efficiency of 3.26% relative to the power conversion efficiency of 2.51% as attained by PTDPP-Ph. These results indicated that the replacement of flanked phenyl with 2-pyridyl moiety of the diketopyrrolopyrrole core unit can be an effective strategy for the construction of conjugated polymers with improved optoelectronic properties.

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