Combinative solution processing and Li doping approach to develop p-type NiO thin films with enchanced electrical properties

The deposition of nickel oxide (NiOx) thin film from an acetylacetonate source using many solution-based techniques has been avoided owing to its poor solubility in alcohol solvents. From this perspective, this work provides a systematic investigation of the development of NiOx thin film, using a combinative approach of ultrasonic spray pyrolysis (USP) and Li dopant for the synthesis and optimization of structural and optoelectronic properties of the films. An in-depth comparative analysis of nickel acetylacetonate-based precursor, employing acetonitrile and methanol as solvents, is provided. It is demonstrated that USP from acetylacetonate precursor yielded uniform, well-compact, and transparent films, with polycrystalline cubic NiOx crystal structures. By screening the deposition temperature in the range of 300–450°C, a temperature of 400°C was identified as an optimal processing temperature leading to uniform, compact, highly transparent, and p-type conductive films. At optimized deposition conditions (400°C), lithium-doped NiOx (Li:NiOx) thin film was deposited. The shift of the main (200) XRD peak position from 43.48° (0-Li:NiOx) to 43.56° (60-Li:NiOx) indicated Li incorporation into the NiOx lattice. An X-ray photoelectron spectroscopy (XPS) study was employed to unravel the incorporation of Li into the deposited Li:NiOx thin films. With the deconvolution of the Ni 2p core level for the as-deposited (0, 60)-Li:NiOx films, the intensity of Ni3+ related peak was found to increase slightly with Li doping. Furthermore, all the deposited Li:NiOx thin films showed p-type conductivity behavior, and the resistivity was reduced from 104 Ωcm (0-Li:NiOx) to 102 Ωcm (60-Li:NiOx). Based on these results, the deposited NiOx and Li:NiOx thin films suggested that USP-deposited Li:NiOx is highly suitable for application in inverted structure solar cells as the hole transport layer.