Phenoxazine Dyes for Dye-Sensitized Solar Cells and Relationship Between Molecular Structure and Electron Lifetime

Metal-free org. dyes with a core phenoxazine chromophore have been synthesized and tested as sensitizers in dye-sensitized solar cells. Overall conversion efficiencies of 6.03-7.40% were reached under std. AM 1.5 G illumination at a light intensity of 100 mW cm-2. A clear trend in electron lifetime could be seen; a dye with a furan-conjugated linker showed a shorter lifetime relative to dyes with the acceptor group directly attached to the phenoxazine. The addn. of an extra donor unit, which bore insulating alkoxyl chains, in the 7-position of the phenoxazine could increase the lifetime even further and, together with additives in the electrolyte to raise the conduction band, an open circuit voltage of 800 mV could be achieved. From photoelectron spectroscopy and x-ray absorption spectroscopy of the dyes adsorbed on TiO2 particles, the excitation is mainly of cyano character (i.e., on av., the dye mols. are standing on, and pointing out, from the surface of TiO2 particles).