Low band gap copolymers consisting of porphyrins, thiophenes, and 2,1,3‐benzothiadiazole moieties for bulk heterojunction solar cells

Two novel low band gap conjugated copolymers containing porphyrins, thiophenes, and 2,1,3-benzothiadiazole (BTZ) moieties were synthesized and applied in bulk heterojunction solar cells. The thermal, optical, electrochemical, and photovoltaic properties of the two copolymers were examined to investigate the effect of the introduction of BTZ moiety in the backbone of the porphyrin polymers. The copolymers exhibited good thermal stability and film-forming ability. The absorption spectra indicated that the BTZ moiety has significant influence on the UV―visible region spectra of the copolymers: with increasing the molar amount of BTZ moieties in conjugated main chain, the absorption in the range of 450― 700 nm is largely broadened and red-shifted compared to the similar polymers without BTZ moiety, and the optical band gaps of copolymers were narrowed to ∼1.50 eV. The photoluminescence spectra showed that there is effective charge transfer in the whole conjugated main chain. Cyclic voltammetry displayed that the band gaps were reduced effectively by the introduction of the BTZ moieties. The bulk heterojunction solar cells were fabricated based on the blend of the copolymers and [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) in a 1:2 weight ratio. The maximum power conversion efficiency of 0.91% was obtained by using P2 as the electron donor under the illumination of AM 1.5, 100 mW/cm 2 .