Structural and optoelectronic properties of NiOx thin films synthesized via co-precipitation for hole transport layer applications

Abstract

Perovskite solar cells (PSCs) have emerged as promising next-generation photovoltaic devices due to their high power conversion efficiencies and low fabrication costs. However, the performance and stability of PSCs are strongly influenced by the quality of charge transport layers, particularly the hole transport layer (HTL). This study investigates the structural, morphological, and optoelectronic properties of nickel oxide (NiOx) thin films prepared via a chemical co-precipitation method and applied as hole transport layers (HTLs) in perovskite solar cells. NiOx films were spin-coated and thermally treated at different calcination temperatures to evaluate their effect on phase formation, surface morphology, and interfacial compatibility. X-ray diffraction (XRD) confirmed the formation of cubic NiO with increased crystallinity at higher calcination temperatures, while FTIR spectroscopy revealed the transformation of Ni(OH)₂ to NiOx through the disappearance of hydroxyl bands and the appearance of metal-oxygen stretching vibrations. Surface morphology assessed by FESEM and morphology analysis by ImageJ showed that films calcined at 300 °C presented uniform and fine-grain structure, while the 400 °C samples exhibited coarsening and increased roughness. UV-Vis spectroscopy demonstrated variations in optical absorption and band gap narrowing with increasing crystallinity. These optoelectronic improvements are critical for efficient hole extraction and transport. The optimized film at 300 °C provided a balance between crystallinity, morphology, and surface quality, making it a promising candidate for enhancing the stability and efficiency of perovskite solar cells.