Sensitized Zinc–Cobalt–Oxide Spinel p-Type Photoelectrode

Abstract

Tandem dye-sensitized solar cells that use nickel oxide as a hole-transport layer have limited performance due to the poor transport properties of NiO; alternative p-type oxides with the correct band alignment, good hole mobility, and stability in corrosive electrolyte must be identified. In this paper, we present our study on the ternary oxide material (zinc–cobalt–oxide spinel) as a hole conductor in a sensitized architecture. Zinc substitution for cobalt in cobalt(II,III) oxide results in good p-type conductivity and a spinel crystal structure. The flatband potential of Zn–Co–O thin films was determined to be around 0.7 V vs NHE (pH 5) and varies slightly depending on the conducting substrate. The films were used as the hole-transport layer in a dye-sensitized solar cell, and the hole-injection current was generated upon sensitization by coumarin-343. The cells exhibited an IQE of >90% and an open-circuit voltage of around 260 mV. The increase in photon-to-electron conversion efficiency of Zn–Co–O over NiO was due to an increased diffusion length, lower recombination, and better transport within the semiconductor-electrolyte device. Although the films were assembled here as liquid electrolyte dye-sensitized photovoltaic devices, they are also applicable to dye-sensitized photoelectrochemical cells for solar fuels.