Materials engineering based on metal oxides for manipulating the solar spectrum and producing solar energy have been under intense investigation over the last years. In this work, we present NiO thin films double doped with niobium (Nb) and nitrogen (N) as cation and anion dopants (NiO:(Nb,N)) to be used as p-type layers in all oxide transparent solar cells. The films were grown by sputtering a composite Ni-Nb target on room-temperature substrates in plasma containing 50% Ar, 25% O₂, and 25% N₂gases. The existence of Nb and N dopants in the NiO structure was confirmed by the Energy Dispersive X-Ray and X-Ray Photoelectron Spectroscopy techniques. The nominally undoped NiO film, which was deposited by sputtering a Ni target and used as the reference film, was oxygen-rich, single-phase cubic NiO, having a visible transmittance of less than 20%. Upon double doping with Nb and N the visible transmittance of NiO:(Nb,N) film increased to 60%, which was further improved after thermal treatment to around 85%. The respective values of the direct band gap in the undoped and double-doped films were 3.28 eV and 3.73 eV just after deposition, and 3.67 eV and 3.76 eV after thermal treatment. The changes in the properties of the films such as structural disorder, direct and indirect energy band gaps, Urbach tail states, and resistivity were correlated with the incorporation of Nb and N in their structure. The thermally treated NiO:(Nb,N) film was used to form a diode with a spin-coated two-layer, mesoporous on top of a compact, TiO₂ film. The NiO:(Nb,N)/TiO₂heterojunction exhibited visible transparency of around 80%, showed rectifying characteristics and the diode’s parameters were deduced using the I-V method. The diode revealed photovoltaic behavior upon illumination with UV light exhibiting a short circuit current density of 0.2 mA/cm² and open-circuit voltage of 500 mV. Improvements of the output characteristics of the NiO:(Nb,N)/TiO₂ UV-photovoltaic by proper engineering of the individual layers and device processing procedures are addressed. Transparent NiO:(Nb,N) films can be potential candidates in all-oxide ultraviolet photovoltaics for tandem solar cells, smart windows, and other optoelectronic devices.

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.

Doping engineering has been applied in niobium-doped NiO (NiO:Nb) by adding nitrogen (N) in its structure. The rf-sputtered films were made from a Ni-Nb composite target on unheated substrates at 300 W rf power and 5 mTorr total pressure. The plasma contained 50% Ar and 50% O₂ for the fabrication of the single-doped NiO:Nb film (AΝ film), and N₂ gas for the incorporation of N in the Ni-O-Nb structure. The N₂ in plasma was introduced by keeping constant the flow rates of O₂ and N₂ gasses (O₂/N₂ = 1) and reducing the amount of Ar gas, namely 94% Ar, 3% O₂, and 3% N₂ (film AN1); 50% Ar, 25% O₂, and 25% N₂ (film AN2); and 6% Ar, 47% O₂, and 47% N₂ (film AN3). All films had the single phase of cubic NiO and both Nb and N in the Ni-O structure were revealed by XPS experiments. The roughness of the films was increased with the increase in N in plasma. Post-deposition thermal treatment improved the crystallinity and reduced the structural disorder of the films. The AN2 film was found to be the most transparent of all films, exhibiting the widest band gap, 3.72 eV, and the narrowest Urbach tail states’ width, 313 meV. The AN and the AN2 films were employed to form NiO/TiO₂ heterostructures. The NiO:Nb/TiO₂ and NiO:(Nb,N)/TiO₂ heterostructures exhibited a visible transmittance of around 42% and 75%, respectively, and both showed rectification properties. Upon illumination with UV light, the NiO:(Nb,N)/TiO₂ diode exhibited enhanced photovoltaic performance when compared to the NiO:Nb/TiO₂ solar cell: the short-circuit current densities were 0.2 mA/cm² versus 1.4 μA/cm² and the open-circuit voltages were 0.5 V versus 0.2 V. The output characteristics of the p-NiO:(Nb,N)/n-TiO₂ UV photovoltaics can be further improved by proper engineering of the individual layers and device processing procedures.

Materials engineering based on metal oxides for manipulating the solar spectrum and producing solar energy have been under intense investigation over the last years. In this work, we present NiO thin films double-doped with niobium (Nb) and nitrogen (N) as cation and anion dopants (NiO:(Nb,N)) to be used as p-type layers in all oxide transparent solar cells. The films were grown by sputtering a composite Ni-Nb target on room-temperature substrates in plasma containing 50% Ar, 25% O2 and 25% N2 gases. The existence of Nb and N dopants in the NiO structure was confirmed by the Energy Dispersive X-Ray and X-Ray Photoelectron Spectroscopy techniques. The nominally undoped NiO film, which was deposited by sputtering a Ni target and used as the reference film, was oxygen-rich, single-phase cubic NiO, having a visible transmittance of less than 20%. Upon double doping with Nb and N the visible transmittance of NiO:(Nb,N) film increased to 60%, which was further improved after thermal treatment to around 85%. The respective values of the direct band gap in the undoped and double-doped films were 3.28 eV and 3.73 eV just after deposition and 3.67 eV and 3.76 eV after thermal treatment. The changes in the properties of the films such as structural disorder, direct and indirect energy band gaps, Urbach tail states and resistivity were correlated with the incorporation of Nb and N in their structure. The thermally treated NiO:(Nb,N) film was used to form a diode with a spin-coated two-layer, mesoporous on top of a compact, TiO2 film. The NiO:(Nb,N)/TiO2 heterojunction exhibited visible transparency of around 80%, showed rectifying characteristics and the diode’s parameters were deduced using the I-V method. The diode revealed photovoltaic behavior upon illumination with UV light exhibiting a short circuit current density of 0.2 mA/cm2 and open-circuit voltage of 500 mV. Improvements of the output characteristics of the NiO:(Nb,N)/TiO2 UV-photovoltaic by proper engineering of the individual layers and device processing procedures are addressed. Transparent NiO:(Nb,N) films can be potential candidates in all-oxide ultraviolet photovoltaics for tandem solar cells, smart windows and other optoelectronic devices.

In an attempt to identify solutions to advance net-zero energy actions and accelerate the deployment of cutting-edge low-carbon technologies, hybrid approaches of synthesizing and engineering materials towards solar energy harvesting and management have been developed. In this work, two different forms of TiO2 were synthesized and applied as electron transfer layer (ETL) in perovskite solar cells (PSCs). In addition, double-doped NiO was combined with these two forms of TiO2 and the fabricated NiO/TiO2 heterostructures were examined for their photocatalytic activities against decolorization of methylene blue (MB) probe-molecule pollutant. The two forms of TiO2 were the 1-D TiO2 nanorods (TiO2-NRs), synthesized by an aqueous hydrothermal technique, and the 3-D mesoporous TiO2 (m-TiO2), synthesized by spin-coating. The NiO was fabricated by sputtering and after doping-engineering the optimum double-doped NiO:(Nb,N) was used to form the NiO/TiO2 heterostructures. The PSC formed by the 1-D TiO2-NRs as ETL exhibited under AM1.5 illumination the same open-circuit voltage but twice the short-circuit current when compared to the PSC having the conventional m-TiO2 as ETL. The photocatalytic activity of the 1-D NiO/TiO2 heterostructure with the NRs was 23% faster than the respective 3-D NiO/TiO2 heterostructure, inducing at the same time around 83% more (MB) degradation. The effect of the increased effective surface area of the 1-D heterostructures employing the NRs, along with the enhanced NiO-TiO2 diode properties favouring and enhancing the absorption and the rates of photocatalytic reactions are discussed. These results provide a direct comparison between heterostructures synthesized via hybrid routes for optoelectronic applications in the fields of energy harvesting and photocatalysis.