Efficient dye-sensitized solar cell with a pure thin film of a hybrid polyoxometalate covalently attached organic dye as a working electrode in a cobalt redox mediator system

Polyoxometalates (POMs) are a class of inorganic metal–oxygen clusters comprising many elements, with unique structures and molecular weights, which possess similar energy band structures to metal oxides. In this research, for the first time, a Keggin-type hybrid polyoxometalate [SiW₁₁O₃₉(Si(CH₂)₃NH₂)₂O] (Hybrid-POM) is used as a pure semiconductor thin film in the working electrode of an efficient dye-sensitized solar cell (DSC) which was fabricated with a D35 organic dye as the sensitizer and a one-electron fast redox mediator cobalt complex. The D35 dye is attached to the surface of the POM by a strong amide bond (D35@Hybrid-POM). For comparison, two DSCs were also fabricated with mesoporous anatase TiO₂, with and without a titania blocking layer, under the same conditions of electrolyte, sensitizer and counter electrode. J–V measurements of the DSCs were obtained under AM1.5G irradiation and photoelectrochemical parameters like open-circuit voltage (Vₒc), short-circuit current density (Jₛc), fill factor (FF), voltage decay and the overall efficiency were measured and compared to the mesoporous titania cells. The J–V curves show a promising enhancement overall in the DSC with the D35@Hybrid-POM structure in comparison with other prevalent mesoporous titania cells. Incident photon to current conversion efficiency spectra of the D35@Hybrid-POM DSCs and mesoporous titania with TBL (titania blocking layer) working electrodes were also obtained. A maximum IPCE of about 91% was found for the DSC employing D35@Hybrid-POM which is about 30% higher than the DSC with the mesoporous titania layer. The voltage decay measurements, calculated electron lifetime and effective recombination order (β) showed that the electron lifetime becomes promisingly longer when we use the Hybrid-POM structure in the photoanode. Thus, these results show that the rates of recombination and electron transfer of the cobalt mediator system at the surface of Hybrid-POM are very slow.