Synthesis and photovoltaic property of copolymers with phenanthrothiadiazole moiety

A new accepter unit, phenanthro[9,10-c][1,2,5]thiadiazole, was synthesized and utilized for the syntheses of the conjugated polymers containing electron donor–acceptor pairs for OPV devices in order to describe the effect of changing the coupling position with cyclopentadithiophene unit. This novel phenanthro[9,10-c][1,2,5]thiadiazole unit with a phenanthene group possesses an electron deficient thiadiazole group and can promote planarity of the compound. The solid films of PCPDTPTm with meta linkage and PCPDTPTp with para linkage show absorption bands with maximum peaks at about 460 and 484 nm and the absorption onsets at 558 and 579 nm, corresponding to band gaps of 2.22 and 2.14 eV, respectively. The absorption spectrum of the PCPDTPTp was red shifted as compared to that of PCPDTPTm, which reveals that PCPDTPTp has longer conjugated length caused by the coupling position. The electron density of the PCPDTPTm HOMO is distributed not only on the donor moiety but also on the thiadiazole moiety to generate intermolecular charge transfer from cyclopentadithiophene unit to phenanthrothiadiazole unit. The device with PCPDTPTm:PC71BM with TiOx layer showed VOC value of 0.97 V, JSC value of 5.50 mA/cm2, and FF of 0.33, giving power conversion efficiency of 1.80%.