Schottky diode behaviour with excellent photoresponse in NiO/FTO heterostructure

Highlights: • Sol-gel processed p-NiO/n-FTO metal oxide photo diode. • Delocalized holes from Ni vacancies in NiO. • Excellent Schottky diode behavior of p-NiO/n-FTO heterostructure. • Remarkable photoresponse in reverse characteristics. • Supportive energy band alignment. - Abstract: Delocalization of charge carriers through formation of native defects in NiO, to achieve a good metal oxide hole transport layer was attemted in this work and thus a heterojunction of p-type NiO and n-type FTO have been prepared through sol-gel process on FTO coated glass substrate. The synthesis process was stimulated by imparting large number of OH⁻ sites during nucleation of Ni(OH)₂ on FTO, so that during oxidation through annealing Ni vacancies are introduced. The structural properties as observed from X-ray diffraction measurement indicate formation of well crystalline NiO nanoparticles. Uniform distribution of NiO nanoparticles has been observed in the images obtained from scanning electron microscope. The occurrence of p-type conductivity in the NiO film was stimulated through the formation of delocalized defect carriers originated from crystal defects like vacancies or interstitials in the lattice. Ni vacancy creates shallow levels with respect to the valance band maxima and they readily produce holes. Thus a native p-type conductivity of NiO originates from Ni vacancies. NiO was thus obtained as an auspicious hole transport medium, which creates an expedient heterojunction at the interface with FTO. Excellent rectifying behavior was observed in the electrical J–V plot obtained from the prepared heterojunction. The results are explained from the band energy diagram of the NiO/FTO heterojunction. Remarkable photoresponse has been observed in the reverse characteristics of the heterojunction caused by photon generated electron hole pairs.