# Facile Synthesis of Highly Conductive Vanadium-Doped NiO Film for Transparent Conductive Oxide
## 1 Introduction  
Nickel oxide (NiO) is one of the rare metal oxide semiconductors, exhibiting a wide bandgap of \(\sim\)3.6 eV [1; 2; 3]. It has been used in numerous practical applications, such as organic light-emitting diodes [4], sensors [5], supercapacitors [6], and solar cells [7]. There are many literature reports available on the synthesis of NiO nanoparticles (NPs) [8; 9; 10; 11]. However, there have been difficulties in preparing a well-dispersed ultra-small NiO ink for transparent conductive oxide (TCO) applications [12; 13]. In reality, electrode fabrication using pre-annealed NiO nanoparticles is difficult due to their lower dispersibility and poor adhesion on TCO substrates. Capping agents like oleylamine and oleic acid added during the synthesis of the metal oxide are beneficial for the suppression of particle growth and can eventually result in monodispersed metal oxide nanoparticles with great dispersion in DI water.  
In recent reports, NiO has been used as a hole transport layer (HTL) in inverted perovskite solar cells [14; 15; 16] and organic light-emitting diodes (LEDs) [17]. In principle, to enhance the performance of solar cells and OLEDs, one needs to meet the following general requirements: better electrical conductivity, efficient charge transport, solution processing under low temperature, and bandgap tuning [18]. Improved electrical conductivity and hole extraction can be achieved through doping [19]. Different dopants have been used to improve the conductivity of NiO, such as Cs [19], Cu [20], Nb [21], Y [22], Ag [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24], Co [25], N [26], and K [27]. Among all these dopants, Cu is widely used for light doping in various TCO-based applications.  
In a study of Ag\({}_{\rm x}\)Ni\({}_{\rm x}\)O film, [PERSON] et al. reported that the Ni\({}^{+3}\)/Ni\({}^{+2}\) ratio was enhanced by varying concentrations of Ag content [23]. They also reported that an increased Ni\({}^{+3}\)/Ni\({}^{+2}\) ratio significantly increased the hole concentration. Another study by [PERSON] et al. reported that the hole concentration of NiO is enhanced by light Ag doping, and the product was applied in heterojunction diodes (HJDs) [24].  
A notable study by [PERSON] et al. [20] demonstrated that Cu doping on NiO drastically enhances the electrical conductivity of NiO. More recently, [PERSON] et al. reported that N-doped NiO showed improved conductivity as compared to undoped NiO due to the influence of Ni vacancies [26]. The work of [PERSON] et al. also demonstrated NiO film with superior conductivity due to a higher ratio of Ni\({}^{3+}\) to Ni\({}^{2+}\)[28].  
In this work, for the first time (to the best of our knowledge), we studied the optical, morphological, and electrical conductivity properties of V:NiO nanoparticles. Our main aim is to investigate how to modify NiO film conductivity on the TCO substrate. So, we substituted Ni with varying atomic percentages of vanadium in the NiO lattice. A comparative study of pristine NiO and 3 at.%V:NiO showed improvements in film morphology and conductivity for 3 at.%V:NiO as compared to pristine NiO film. Both the pristine and V-doped NiO were prepared by the solvothermal decomposition method. Ultra-small-sized, highly crystalline, pre-annealed nanoparticles were used for deposition on the TCO substrate. However, the effect of vanadium doping on NiO has not been widely studied.