Efficient triphenylamine dyes for solar cells: Effects of alkyl-substituents and π-conjugated thiophene unit

Several triphenylamine dyes, both with and without an n-hexyl-substituted oligothiophene as π-conjugated linker, were designed, synthesized and utilised in dye-sensitized, nanocrystalline, TiO2 solar cells. The effects of both the length of the π-conjugated unit and of the alkyl chain upon the photo-physical, electrochemical and photovoltaic properties of the dyes were studied. Between 1 and 3 thiophene units impart superior solar-to-electricity conversion efficiency and the hexyl chains also markedly improve both short-circuit photocurrent density and open-circuit photovoltage, which result in relatively high solar-to-electricity conversion efficiency. In EtOH solution, a dye-sensitized solar cell based on 2-cyano-3-(5-(4-(diphenylamino) phenyl)-4-hexylthiophenyl-2-yl) acrylic acid with one hexyl-substituted thiophene, π-conjugated unit, exhibited the largest observed maximum solar-to-electricity conversion efficiency of 7.26% under simulated AM 1.5 G solar irradiation (100 mW/cm2).