Synthesis and characterization of novel D–A porphyrin-containing copolymers for polymer solar cells

A novel asymmetrical D–A zinc porphyrin derivative with dimehtyl triphenylamine (donor unit) and methyl benzoate (acceptor unit) as para-arms was first synthesized. Then, two new copolymers (P1 and P2) containing D–A zinc porphyrin derivatives were synthesized by the Stille coupling method and applied in PSCs. Their structures, photophysical and electrochemical properties were characterized by 1H NMR, 13C NMR, gel permeation chromatography, thermogravimetric analysis, UV–vis absorption spectroscopy, photoluminescence spectroscopy, and cyclic voltammetry. The two copolymers exhibited good thermal stability and film-forming ability. The results showed that P1 containing D–A zinc porphyrin exhibits a strong absorption in the range of 400–500 nm. By the introduction of thiophene derivative with 4,7-di(4-hexylthiophen-2-yl)benzothiadiazole (T-DTBT) conjugated side-chain unit, P2 showed broader absorption in the region of 300–650 nm than P1. The photoluminescence spectra made clear that charge transfer between the whole main chain and side chain can be effective. Cyclic voltammograms revealed that the LUMO energy levels of P2 was reduced in comparison with P1 due to the introduction of electron-deficient T-DTBT conjugated side-chain unit, indicating that electron-injection and transporting properties have been improved. Polymer solar cells were fabricated based on the blend of the copolymers and methanofullerene[6,6]-phenyl C61-butyric acid methyl ester (PC61BM). The PSC based on P2:PC61BM (1:2, w/w) exhibited a power conversion efficiency of 1.26% under AM 1.5, 100 mW cm−2.