Effect of seven different terthiophene π-spacers on dye performance in dye-sensitized solar cells

To ensure high photocurrents from dye-sensitized solar cells (DSSC), it is important that the dye absorbs as much of the energy from the sunlight as possible. To achieve a wide absorption and cover larger parts of the solar spectrum, dyes are frequently fitted with oligothiophene π-spacers. We wish to examine the terthiophene motif as π-spacers in triarylamine dyes and evaluate the suitability of using these in modern copper based DSSC devices. The foundation for this analysis will be the series of seven novel dyes (DMA-6 – DMA-12) which all are fitted with the same donor and acceptor, but linked with seven different terthiophene motifs. The photovoltaic performance of the dyes show that the smallest π-spacer in the series, dithieno[3,2-b:2′,3′-d]thiophene, was the most efficient. The DSSC device sensitized by this dye achieved a power conversion efficiency of 4.4% (Jsc = 7.0 mA cm−2, Voc = 0.95 V, FF = 0.66). The photovoltaic performance of the dyes was tested in devices with two different TiO2 thicknesses, and we established that the thinner TiO2 yielded the best DSSC performance for all the dyes. Electrochemical impedance spectroscopy revealed that the thicker semiconductor layer resulted in a reduction of the effective electron diffusion length.