Abstract
Nickel oxide (NiO) is a p-type oxide and nitrogen is one of the dopants used for modifying its properties. Until now, nitrogen-doped NiO has shown inferior optical and electrical properties than those of pure NiO. In this work, we present nitrogen-doped NiO (NiO:N) thin films with enhanced properties compared to those of the undoped NiO thin film. The NiO:N films were grown at room temperature by sputtering using a plasma containing 50% Ar and 50% (O2 + N2) gases. The undoped NiO film was oxygen-rich, single-phase cubic NiO, having a transmittance of less than 20%. Upon doping with nitrogen, the films became more transparent (around 65%), had a wide direct band gap (up to 3.67 eV) and showed clear evidence of indirect band gap, 2.50–2.72 eV, depending on %(O2-N2) in plasma. 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 nitrogen in their structure. The optimum NiO:N film was used to form a diode with spin-coated, mesoporous on top of a compact, TiO2 film. The hybrid NiO:N/TiO2 heterojunction was transparent showing good output characteristics, as deduced using both I-V and Cheung’s methods, which were further improved upon thermal treatment. Transparent NiO:N films can be realized for all-oxide flexible optoelectronic devices.
Highlights
Undoped Nickel oxide (NiO) was deposited as reference film at 300 W RF-power in 5 mTorr total pressure consisting of 50% Ar and 50% O2 gases (50% Ar + 50% O2 )
Under these oxygenrich deposition conditions, NiO was oxygen-rich, atomic percentage O/Ni~1.72 as revealed by energy-dispersive X-ray (EDX) measurements, and previous experiments showed that it exhibits p-type behavior [7]
The X-ray diffraction (XRD) pattern yielded one main diffraction peak at around 42.6◦ and a second one with much smaller intensity at around 62◦, which were identified as diffraction peaks arising from (200) and (220) crystallographic planes of the cubic NiO phase, respectively
Summary
Due to its chemical stability and non-toxicity, NiO has found numerous applications in photo-bio-catalysis [3,4,5], sensing [6,7], microbatteries [8] and transparent optoelectronics such as smart windows [9], ultra-violet (UV) photodetectors [10] and photovoltaics (PVs). In the case of PVs, NiO has been used in conventional PVs and ultra-violet UV-PVs, in addition to perovskite PVs, in which it is used as a hole transport layer [11,12,13,14]. If NiO is to be used for applications in which it can be applied as a coating or single layer, relatively low-cost chemical methods are generally employed for its fabrication, like sol-gel and spray pyrolysis. The formation of the oxide on unintentionally heated substrates is pursued if applications in the emerging and challenging field of flexible, foldable and stretchable optoelectronics (PVs, wearables, etc.) are sought
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