Opto-electrical and density functional theory analysis of poly(2,7-carbazole-alt-thieno[3,4-c]pyrrole-4,6-dione) and photovoltaic behaviors of bulk heterojunction structure

To increase open-circuit voltage (Voc), we introduced thieno[3,4-c]pyrrole-4,6-dione (TPD) moieties which has unsubstituted and substituted thiophene spacers. Poly(2,7-carbazole-alt-TPD) derivatives, namely PCDTTPD and PCDHTTPD, were copolymerized with 2,7-carbazole through Suzuki coupling reaction. An increase in the molecular weight and a decrease in the highest occupied molecular orbital (HOMO) energy level are confirmed. Steric hindrance caused by rotational dynamics was measured by density functional theory calculation. When the resulting polymers, PCDHTTPD, were used to fabricate a bulk heterojunction device with PC71BM, Voc increased to 0.84 V, whereas the short-circuit current density (Jsc) decreased to 0.93 mA/cm2 because of poor charge dissociation.