Preparation and photovoltaic properties of pyrene-thieno[3,4-c]pyrrole-4,6-dione-based donor-acceptor polymers

Four new donor-acceptor conjugated copolymers, containing pyrene moieties flanked by thienyl or bithienyl groups as a donor units and thieno[3,4-c]pyrrole-4,6-dione (TPD) as acceptor units, were successfully prepared via a direct arylation polymerisation method. While all polymers prepared had 2-ethylhexyloxy-substituents on the pyrene repeat units, two different alkylsusbtituents (octyl or 4-hexylphenyl groups) were attached to their TPD moieties. The influence of these different substituents as well as the number of thienyl units linking the pyrene and TPD units along polymer chains on the photophysical, electronic and photovoltaic properties of these materials was investigated. All polymers displayed good thermal stability up to 315°C. The optical band gap of the four polymers, PPEHDT-TPDO, PPEHDT-TPDHP, PPEHDT2-TPDO and PPEHDT2-TPDHP, were estimated to be 2.00, 2.06, 1.94 and 1.91 eV, respectively. Polymers that possessed a single thiophene unit attached to the pyrene unit, PPEHDT-TPDO and PPEHDTTPDHP, displayed deeper HOMO levels compared to those with bithiophene units, PPEHDT2- TPDO and PPEHDT2-TPDHP. Photovoltaic devices were fabricated from all polymers. PPEHDT2-TPDO boasted the highest efficiency with a PCE (2.06 %), a FF of 53.07 %, a Jsc of 4.66 mA/cm2 and a Voc of 0.83 V.