Comparison of the photoelectrochemical properties of RDS NiO thin films for p-type DSCs with different organic and organometallic dye-sensitizers and evidence of a direct correlation between cell efficiency and charge recombination

Undyed mesoporous NiO in the configuration of thin film (thickness 2-3 mu m) presents photoelectrochemical activity as cathode of a p-type dye-sensitized solar cell (p-DSC) towards the reduction of triiodide to iodide under irradiation with a solar simulator. The photoelectroactivity of the oxide prepared via microwave plasma sintering (or rapid discharge sintering, RDS) has been observed in the spectral range 300-500 nm with the incident photon-to-current conversion efficiency (IPCE) reaching a maximum of 8.7 % at 375 nm. Upon sensitization, the characteristic photoelectrochemical activity of NiO can be either enhanced or depressed depending on the nature of the dye-sensitizer. The comparative analysis of the JV and IPCE curves of the p-DSCs based on bare NiO and four differently sensitized NiO cathodes reveals that N719, black dye (BD), and commercial squaraine 2 (SQ2) decrease the efficiency of conversion of dyed NiO with respect to bare NiO in the range of photoelectroactivity of the latter (300-500 nm). The fourth dye P1 represents the sole exception since its employment brings about an enhancement of the quantum efficiency of P1-sensitized vs. unsensitized NiO up to a maximum of 21 % within the spectral interval of reference for NiO (300-500 nm). Outside the range of NiO photoelectrochemical activity, i.e., lambda > 500 nm, only N719 does not introduce a gain of quantum efficiency with respect to bare NiO despite the observation of spectral sensitization up to 580 nm for N719-sensitized NiO. The impedance spectra recorded under illumination shows a direct proportionality between the overall efficiency (eta) of the variously sensitized p-DSCs and the amplitude of the semicircle which is generally associated with the process of charge recombination at the electrode/electrolyte interface with eta decreasing with the increase of the recombination resistance.