Nanocomposites of Nickel Oxide and Zirconia for the Preparation of Photocathodes with Improved Performance in p-Type Dye-Sensitized Solar Cells

Abstract

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 antirecombination 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/cm 2 and the overall efficiency  = 0.088% when P1 is the sensitizer. These results show an increase up to 40% compared to the unmodified 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 ).

Solar energy conversion using first row d-block metal coordination compound sensitizers and redox mediators

“…The most important use of nickel in DSCs is in the form of NiO as a p-type semiconductor in p-type or tandem DSCs. [286][287][288][289][290][291][292][293] In 2012, we overviewed Ni(II)-containing sensitizers for n-type DSCs that had been reported up to that date, 175 and here we focus on Ni(II) dyes reported since 2012. A number of dithiolate and dithiocarbamate complexes of nickel and bearing peripheral ferrocenyl units were described earlier in this review.…”

NiO/ZrO2 nanocomposites as photocathodes of tandem DSCs with higher photoconversion efficiency with respect to parent single-photoelectrode p-DSCs

“…58 Such sites are no longer available to the colorant for the further sensitization of the electrode. On the other hand, if blocking layers 59,60 or electronically insulating materials in the nanoparticulate form 61 are in direct contact with the dye-sensitized pSC there is still the inhibition of undesired phenomena of recombination at the dye-pSC/electrolyte interface. A consequence of the presence of extraneous insulating species within the SC is the prevention of intermolecular dye aggregation onto the sensitized surface of pSC (similarly to what CDCA effectuates).…”

“…For the controlled passivation of sensitized NiO cathodes against photohole recombination in operating p-DSCs, the inclusion of ZrO2 NPs in thin films of nanostructured NiO was considered. 61 The choice of screen-printed NiO/ZrO2 nanocomposites as cathodic substrates of p-DSCs led to the realization of p-type devices with overall photoconversion efficiencies that were almost 50 % higher than the typical efficiency of a p-DSC with sole nanostructured NiO as cathodic material. 61 This finding was accompanied by the increase of electron transfer resistance through the photocathode/electrolyte interface in passing from sole NiO to the nanocomposite NiO/ZrO2 cathode.…”

Recent advances in eco-friendly and cost-effective materials towards sustainable dye-sensitized solar cells

“…1b 46 ). Unfortunately, the photoconversion efficiency of the "inverted" geometry has never approached that of the conventional geometry (lower by one order of magnitude 47 ), despite the great efforts in tailoring photocathode materials [48][49][50][51] and the development of dedicated sensitizers [52][53][54][55] and redox couples. 56,57 This is mainly ascribed to the lower photocurrent powered by p-DSSCs, as expected by their hole-driven charge diffusion processes.…”