P-type dye sensitized solar cells (p-DSCs) suffer low performances compared to the n-type counterpart^[1]. One of the main causes of this is the fast recombination reaction occurring between the photoinjected holes in the valence band of the p-type semiconductor (usually NiO) and the reduced form of the redox shuttle (typically I^-). As a matter of fact, recombination phenomena at the NiO/electrolyte interface heavily limit both photovoltage and photocurrent. Different approaches have been adopted to minimize such an unwanted process: these range  from the pretreatment of the electrode surface with NaOH^[2] to the employment of passivating organic molecules (e.g. CDCA) in the sensitizing solution and/or in the electrolyte solution.^[3]

The present contribution describes the implementation of metal oxide (MO_x in which M is Al, Zr, Y or Ce) nanoparticles (NPs) as anti-recombination agent in NiO-based photocathodes for p-DSCs. MO_x NPs (diameter, Ø = 20 nm) and NiO nanoparticles (Ø < 50 nm) were dispersed together in a methanol solution and spray-deposited onto FTO. Different MO_x/NiO molar ratio (ranging from 1 to 20%) were considered. The as deposited films were annealed at 450 °C in a conventional oven or by Microwave-assisted Rapid Discharge Sintering (RDS). The latter method was proved to be more effective by assuring the formation of a more porous, able to load more sensitizer (i.e. P1) molecules, and electroactive film. Scanning Electron Microscopy (SEM) was used to check the morphology of the electrodes and the effective dispersion of MO_x NPs. It was proved that the nature and the electrochemical and optical properties of MO_x deeply influence the photoelectrochemical behavior of photocathodes. Indeed, the optimal MO_x/NiO ratio is strongly dependent on the nature of the metal oxide but all tend to form aggregates and macrostructures if their relative concentration exceeds 10%.

Among the investigated MO_x NPs, Zirconium Oxide (ZrO₂) was found to be the most performing additive: an 80% enhancement of the photoconversion compared to unmodified photocathode was obtained following on from the minimization of recombination reactions at the electrode/electrolyte interface as proved by Electrochemical Impedance Spectroscopy. Very interestingly, the addition of Y₂O₃ (ratio 0.02%) has a positive effect on the open circuit voltage (i.e. > 0.145 mV) of the device whereas Al₂O₃ (ratio = 0.10%) allows to boost the photocurrent powered (i.e. > 3.5 mA*cm^-2) by the cell.

The most performing cell (i.e. ZrO₂/NiO-based electrode sintering by RDS and sensitized with P1) produced a J_SC up to 3.6 mA*cm^-2, a V_OC of 129 mV leading to an overall efficiency of 0.164%. Furthermore, this photocathode was coupled with a VG10-sensitized TiO₂-based photoanode to produce a tandem DSC with an overall efficiency close to 2% (J_SC = 4.2 mA*cm^-2, a V_OC = 678 mV and FF = 0.66%).