Effects of ω-guanidinoalkyl acids as coadsorbents in dye-sensitized solar cells

Dye-sensitized solar cells (DSCs) based on nanocrystalline TiO₂ have been fabricated with an amphiphilic ruthenium sensitizer NaRu(2,2′-bipyridine-4-carboxylic acid-4′-carboxylate)(4,4′- dinonyl-2,2′-bipyridine)-(NCS)₂, coded as Z-907, and a series of ω-guanidinoalkyl acids as coadsorbents. The addition of guanidinoalkyl acids as coadsorbents increased the open-circuit voltage of the DSCs and had no adverse effect on the photocurrent if an appropriate amount was used. Phototransient measurements showed that the addition of these guanidino coadsorbents slowed down the charge recombination and that the increase in the open-circuit voltage was due to suppression of the recombination or an upward shift of the TiO₂ band-edge to negative potentials. Thus, for the first time, a class of coadsorbents has been demonstrated to not only shield the surface electrons against recombination but also to shift the band edge to negative potentials, which is essential for future improvements in the performance of DSCs.