Low bandgap diketopyrrolopyrrole-based polymers with an asymmetric unit of fluoridated phenylene-thiophene for efficient polymer solar cells

Two low bandgap copolymers (PDPP-PT and PDPP-PTF2) based on asymmetrical phenylene-thiophene as donor unit and diketopyrrolopyrrole as acceptor unit were developed for polymer solar cells. Their thermal, optical, electronic properties and photovoltaic performances were systematically investigated. Both polymers exhibited narrow bandgaps (below 1.5 eV) with a wide absorption range (350-900 nm). The introduction of the fluorine endows the polymer PDPP-PTF2 with better planarity, higher hole mobility and preferable morphology compared with its non-fluorinated analogue PDPP-PT. The polymer PDPP-PTF2 with asymmetric donor unit of fluoridated phenylene-thiophene displayed the power conversion efficiency of 3.0% with an open-circuit voltage of 0.68 V and a short circuit current of 7.24 mA cm-2. This study demonstrates that asymmetric unit of fluoridated phenylene-thiophene is promising building blocks for the development of conjugated polymers for high-performance optoelectronic devices.