Modulation of the properties of pyrrolo[3,4-c]pyrrole-1,4-dione based polymers containing 2,5-di(2-thienyl)pyrrole derivatives with different substitutions on the pyrrole unit

Abstract

In this study, four new pyrrolo[3,4-c]pyrrole-1,4-dione (DKPP)-based polymers, P(DKPP-TPTH), P(DKPP-TPTE), P(DKPP-TPTA), and P(DKPP-TPTI), containing N-alkyl-2,5-di(2-thienyl)pyrrole (TPT) derivatives with four different substituents such as hydrogen, ester, amide, and imide groups on the 3,4-position of the pyrrole unit were prepared to tune the properties of the polymers. Opto-electrical studies showed that the incorporation of electron withdrawing substituents such as ester, amide and imide groups instead of hydrogen into the pyrrole backbone of the polymers increased the band gaps significantly from 1.31 eV to 1.42 eV, 1.37 eV and 1.37 eV, respectively, and reduced the highest occupied/lowest unoccupied molecular orbital (HOMO/LUMO) energy levels from −4.96 eV/−3.65 eV to −5.24 eV/−3.82 eV, −5.17 eV/−3.80 eV and −5.35 eV/−3.98 eV, respectively. Organic field effect transistors (OFETs) made from these polymers indicated that the incorporation of electron withdrawing functional groups into the polymer backbone reduced hole mobility. Polymer solar cells (PSCs) prepared using polymers as electron donors offered higher power conversion efficiency (PCE) for the polymer containing hydrogen on the TPT backbone, but the polymers incorporating electron withdrawing substituents into the TPT backbone showed a significantly higher open-circuit voltage (V_oc) though the PCE was relatively low.