Synthesis and characterization of conjugated copolymer containing 2,5-bis(3-decylthiophen-2-yl)thiazolo[5,4-d]thiazole and 2,6-[(1,5-didecyloxy) naphthalene for polymer solar cells

A new alternating donor-acceptor conjugated copolymer, poly{(2,5-bis(3- decylthiophen-2-yl)thiazolo[5,4-d]thiazole-5,5′-diyl)-alt-(2,6-[(1, 5-didecyloxy)naphthalene])} (PNTzTz) based on thiazolothiazole and didecyloxynaphthalene units was synthesized using a typical Suzuki coupling polymerization method and was explored as a donor polymer in bulk heterojunction (BHJ) solar cells. The copolymer was characterized using differential scanning calorimetry, thermogravimetric analysis, UV-Vis absorption and cyclic voltammetry. It possesses moderate molecular weights and excellent thermal properties with a 5% weight loss around 340 °C. It also exhibits good optical absorption with an absorption peak at 475 nm and an absorption onset at 560 nm in the film state. Photovoltaic properties of the copolymer were investigated blended with electron acceptor, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). The optimized photovoltaic device with a device structure of ITO/PEDOT:PSS/PNTzTz:PC₆₁BM (1:2, w/w)/LiF/Al shows a power conversion efficiency up to 0.99% with a short circuit current density of 3.3 mA cm^-2, an open circuit voltage of 0.96 V and a fill factor of 0.33 under the illumination of AM 1.5, 100 mW cm^-2.