Photoexcitation and electron injection processes in azo dyes adsorbed on nanocrystalline TiO2 films

Dye-sensitized solar cells were fabricated using eight azo dyes which have different positions and/or numbers of carboxyl and hydroxyl groups. The short-circuit current density, photoabsorbance, absorptance and quantum yield for dyes-to-TiO2 electron injection were measured by photons ranging from 380 to 800 nm. X-ray and ultraviolet photoelectron spectroscopy of the photovoltaic electrodes were also conducted. The photon-to-current conversion efficiency of the cells was found to depend mostly on the relative position of the lowest unoccupied molecular orbital of the adsorbed dyes and partly by their concentration on the TiO2 nanoparticles.