Niobium-doped NiO as p-type nanostructured layer for transparent photovoltaics

Abstract

Niobium-doped nickel oxide (NiO:Nb) thin films were investigated as p-type layer for all oxide transparent solar cells. The NiO:Nb films were grown by sputtering on room-temperature substrates in plasma containing 50% Ar and 50% O2 gases. The undoped NiO film was oxygen-rich, single-phase cubic NiO, exhibiting transmittance of less than 20%. Upon doping with Nb, the film remained p-type, the hole concentration increased from 4.3 × 1018 cm−3 to 1.2 × 1019 cm−3 but the optical properties remained almost the same with the undoped film. The changes in the properties of the films such as structural disorder, energy band-gap, Urbach states and resistivity were correlated with the incorporation of Nb in their structure. Improvement of the optical properties was achieved by thermal treatment at 300 °C. The visible transmittance increased to around 50% and the films showed a direct band-gap of 3.65–3.71 eV depending on the amount of Nb in the Ni-O structure. The optimum p-NiO:Nb film was used to form a diode with a spin-coated, mesoporous on top of a compact, TiO2 film. The p-NiO:Nb/n-TiO2 heterojunction became transparent after thermal treatment, showing rectifying characteristics. The diode showed photovoltaic behavior upon illumination with ultraviolet light exhibiting short-circuit current density 1.5 μA/cm2 and open-circuit voltage 200 mV. Transparent NiO:Nb films can be realized for all-oxide ultraviolet photovoltaics, tandem solar cells, solar-blind photodetectors and other optoelectronic devices.