Nanocomposites of nickel oxide and zirconia for the preparation of photocathodes with improved performance in P-type dye-sensitized solar cells

In p-type dye sensitized solar cells (p-DSCs) with nickel oxide (NiO) based photocathodes one of the main causes of their relatively poor photoconversion performances is the fast recombination between the photoinjected holes in the valence band of the p-type semiconductor 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 to the employment of passivating organic molecules (e.g. CDCA) in the sensitizing solution and/or in the electrolyte solution. The present contribution describes the implementation of the addition of zirconia (ZrO2) nanoparticles in nanostructured NiO films as anti-recombination agent in p-DSCs due to the electro-inactivity of ZrO2. ZrO2 nanoparticles with diameter, Ø, of 20 nm, and NiO nanoparticles with Ø < 50 nm were dispersed together in the paste precursor for screen-printing. Different compositions of the mixture of NiO and ZrO2 nanoparticles were considered. From the combined analysis of the electrochemical and photoelectrochemical properties of different nanocomposites it was concluded that the molar ratio ZrO2/NiO had the optimal range of 2%-5% for realizing photocathodes more efficacious than sole nanostructured NiO. Among the nanocomposite photoelectrodes the one obtained from the inclusion of 2% of ZrO2 nanoparticles produced the better photoelectrochemical performance being the short-circuit current density JSC = 2.037 mA/cm2 and the overall efficiency η = 0.088% when P1 is the sensitizer. These results show an increase up to 40% compared to the un-modified NiO electrode. The unexpectedly low efficiency of electrode with molar ratio of zirconia in nickel oxide of 5% was associated to an insufficient dye-loading on NiO, in combination to the increase of the percentage of the photoelectrochemically inert ZrO2 additive. The electrochemical impedance spectroscopy (EIS) data of the complete device under illumination confirmed that the improvement is mainly due to an increase of the recombination resistance, Rrec, ongoing from sole nanostructured NiO (Rrec = 56.3 ) to the electrode obtained from the nanocomposite with molar ratio ZrO2/NiO = 0.02 (Rrec = 70.3 ). © 2019 The Electrochemical Society. [DOI: 10.1149/2.0691908jes]