The Effect of Nitrogen as a Co-Dopant in p-Type NiO:Nb Films on the Photovoltaic Performance of NiO/TiO2 Transparent Solar Cells

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.