Improved photon-to-current conversion efficiency with a nanoporous p-type NiO electrode by the use of a sensitizer-acceptor dyad

A peryleneimide sensitizer and a covalently linked peryleneimide- naphthalenediimide dyad were prepared and characterized by absorption and emission spectroscopies, electrochemistry, and spectroelectrochemistry. These compounds were chemisorbed on nanoporous nickel oxide electrodes and then studied by femtosecond transient absorption spectroscopy in the presence of a redox active electrolyte (I₃^-/I^-). In both compounds, upon excitation of the peryleneimide unit, an electron is efficiently ejected from the valence band of NiO to the dye with an average time constant of approximately 0.5 ps. In the case of the dyad, the excess electron is shifted further onto the naphtalenediimide unit, creating a new charge separated state. The latter exhibits a substantial retardation of the charge recombination between the hole and the reduced molecule compared with the peryleneimide sensitizer. The photoaction spectra of a sandwich dye-sensitized solar cell (DSSC) composed of NiO films and these new dyes were recorded, and the absorbed-photon to current conversion efficiency (APCE) was three times higher with the dyad than with the peryleneimide dye: 45%. The maximum APCE of approximately 45% is the highest value reported for a DSSC based on a nanostructured metal oxide p-type semiconductor.