Stepwise co-sensitization of two metal-based sensitizers: probing their competitive adsorption for improving the photovoltaic performance of dye-sensitized solar cells

How to graft co-sensitizers with different binding strengths onto TiO2 surfaces for enhancing the performance of dye-sensitized solar cells (DSSCs) has not been discussed very much. Herein a ruthenium-based sensitizer (N719) and a porphyrin molecule (LP-2) with complementary absorption spectra (300-750 nm) have been chosen to investigate how the dye loading procedure would influence the photovoltaic performance of co-sensitized solar cells. Interestingly, it is found that 54.7% of the loading amounts of pre-adsorbed LP-2 are replaced by the post-adsorption of N719. The replacement adsorption is not observed when the two molecules are loaded in reverse order, which is attributed to their different adsorption configurations and binding energies. The competitive adsorption between co-sensitizers is thus systematically investigated by UV-visible absorption spectroscopy, energy dispersive spectrometry (EDS) and electron probe microanalysis (EPMA). Upon optimization, the device sequentially sensitized with LP-2 and N719 exhibits efficiency (7.72%) enhancement of 38.6% and 18.0% compared with those fabricated with single LP-2 and N719, respectively. The results provide a new vision on the stepwise sensitization of TiO2 films using co-sensitizers with a difference in adsorption properties, suggesting that complementary spectral absorption of co-sensitizers can lead to excellent cell performance by choosing an appropriate dye loading procedure.