Synthesis and characterization of functionalized polythiophene for polymer-sensitized solar cell

The synthesis of Poly[1,5-naphthyridine-(3-hexylthiophene)] (1) semi-conducting polymer has been accomplished by adopting both conventional and microwave-assisted Suzuki-Miyaura cross-coupling reaction between 3-hexylthiophene-2,5-diboronic ester and 2,6-dibromo-1,5-naphthyridine. The synthesized copolymer was characterized by modern spectroscopic techniques including percent reflectance (%R), gel permeation chromatography (GPC), cyclic voltammetry (CV), Raman spectroscopy, thermogravimetric analysis (TGA), FTIR and NMR. The morphology of copolymer was examined via FESEM. Optical bandgap was calculated from the absorption edge of %R and found 2.26 eV. The electrochemical and transport properties of 1 were investigated both in the bulk as well as in thin film form. Cyclic voltammetry (CV) results of 1 as a bulk form in H2SO4 solution indicated that the concentration of the polymer in the solution is not well defined because of polymer poor solubility in aqueous solutions. On the other hand, the sulfur group which works as electron donating makes the system more electron-rich. This can be explained by the absence of the reduction peak. For the thin film, two single oxidation peaks were obtained at around 0 V and 0.3 V for both cases. Different solvents can tune the transport properties of the polymer as can be seen from the two CVs where BF3 exhibited enhanced transport properties over ACN. This polymer was also employed in dye-sensitized solar cells as a photosensitizer. The solar cell under AM 1.5 G illumination at 100 mW/cm2 shows power conversion efficiency of 0.67% with an open-circuit voltage of 621 mV, a short circuit current of 2.0 mA/cm2, and a fill factor of 55%.