Chemical and Physical Reduction of High Valence Ni States in Mesoporous NiO Film for Solar Cell Application
2017
D’Amario, Luca | Jiang, Roger | Cappel, Ute B. | Gibson, Elizabeth A. | Boschloo, Gerrit | Rensmo, Håkan | Sun, Licheng | Hammarström, Leif | Tian, Haining
The most common material for dye-sensitized photocathodes is mesoporous NiO. We transformed the usual brownish NiO to be more transparent by reducing high valence Ni impurities. Two pretreatment methods have been used: chemical reduction by NaBH₄ and thermal reduction by heating. The power conversion efficiency of the cell was increased by 33% through chemical treatment, and an increase in open-circuit voltage from 105 to 225 mV was obtained upon heat treatment. By optical spectroelectrochemistry, we could identify two species with characteristically different spectra assigned to Ni³⁺ and Ni⁴⁺. We suggest that the reduction of surface Ni³⁺ and Ni⁴⁺ to Ni²⁺ decreases the recombination reaction between holes on the NiO surface with the electrolyte. It also keeps the dye firmly on the surface, building a barrier for electrolyte recombination. This causes an increase in open-circuit photovoltage for the treated film.
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