Synthesis and photophysical properties of N-alkyl dithieno[3,2-b:2 ',3 '-d]pyrrole based donor/acceptor-pi-conjugated copolymers for solar-cell application

Abstract

Two kinds of donor-acceptor pi-conjugated copolymer based on poly{[N-hexyl-dithieno(3,2-b:2 ',3 '-d)pyrrole-2,6-diyl]alt-[isoindigo]} (PDTP-IID) and poly{[N-hexyl-dithieno(3,2-b:2 ',3 '-d)pyrrole-2,6-diyl]alt-[thiazol-2,5-diyl]} (PDTP-Thz) were investigated. These copolymers were synthesized via a Stille coupling reaction. The results showed the structure-property relationships of different donor-acceptor (D-A) combinations. The polymer structures and photophysical properties were characterized by H-1 NMR, TGA, DSC, UV-vis absorption spectroscopy, AFM, CV, and XRD measurement. Through UV-vis absorption and cyclic voltammetry (CV) measurements, it showed that the copolymers exhibit not only a low bandgap of 1.29 eV and 1.51 eV but also a deep highest occupied molecular orbital (HOMO) of -5.49 and -5.11 eV. Moreover, photovoltaic properties in combination with the fullerene derivatives were investigated. The device based on the copolymers with PC71BM exhibited higher maximum power conversion efficiency and higher maximum short-circuit current density of 0.23% with 1.64 mA cm(-2) of PDTP-IID:PC71BM and 0.13% with 1.11 mA cm(-2) of PDTP-Thz:PC71BM than those of the copolymers with PC61BM. Measurements performed for N-hexyl-dithieno(3,2-b:2 ',3 '-d)pyrrole-based copolymers proved the potential of these polymers to be applied in optoelectronic applications.