Abstract
In the last decade, dye-sensitized solar cells (DSSCs), based on organic compounds containing metal atoms as a dye material, have become more popular due to their low-cost production and high efficiency. However, the cost and environmental pollution associated with the metal-containing DSSCs are found to be very high, which demand researchers to look for an alternative source of dye, such as pure organic DSSC dyes. Among various dye materials, polythiophene derivatives are of great interest due to their unique combination of original electronic properties, environment stability, and structure versatility. Recently, our lab have made a DSSC containing polythiophene with butylthio substuient as a dye material. The photoelectric conversion efficiency of the DSSC was found to be up to 1.41%, which is much higher than the photo-electric efficiency of the device made up of poly(3-hexylthiophene) as the dye. The very low efficiency of poly(3-hexylthiophene) containing device could be due to the poor adsorption nature of the dye on TiO2 material. In this work, the synthetic methods for two new polymers, i.e., poly[3-(phenylthio)thiophene] and poly[3-(2-pyridinylthio)thiophene] were developed following the previous work done in our laboratory by Mr. Ming Kee Ho. We found through theoretical calculations that polythiophenes with electron withdrawing substituents at 3-position may provide an efficient way to transfer electrons from the main chain to their side chains during their photo-oxidation. While the polythiophene main chain itself holds a very good hole transporting properties and electrical conductivity. Thus, the combination of the polythiophene main chain and the electron withdrawing side chain substituent should render the dye with high DSSC efficiency. The in-house prepared monomers of 2,5-dibromo and 2-bromo derivatives of 3-(phenylthio)thiophene and 3-(2-pyridinylthio)thiophene were polymerized by GRIM and McCullough method, respectively, to give rise to regioregular polythiophenes. Both 2,5-dibromo-3-(arylthio)thiophene and 2-bromo-5-iodo-3-(aryl- thio)thiophene monomers were synthesized and polymerized by GRIM method, and their polymerizations results, based on the reactivity difference, were compared. The optimal yield was found to be 60% through the adjustment of process, and the molecular weight of the polymers were found to be between 1000 to 2500 by gel permeation chromatography (GPC) using polystyrene standards in THF eluant. DSSC devices have been prepared by using these polythiophene polymers as dye and efficiency measurements were performed for their corresponding devices. The devices have shown a maximum efficiency of up to 1.19% and 0.25% respectively, for poly[3-(phenylthio)thiophene] and poly[3-(2-pyridinylthio)thiophene].