Thickness dependent properties of NiO thin films on ITO/PET flexible substrates for solar cell applications

NiO thin films (10–40 nm) were grown on polyethylene terephthalate (PET) substrates coated with indium tin oxide (ITO) using molecular beam epitaxy (MBE) to evaluate their potential for flexible optoelectronic applications. The X-ray diffraction (XRD) analysis reveals a single NiO (002) textured growth with strain-related peak shifts arising from the flexible substrate. The X-ray photoelectron spectroscopy (XPS) analysis confirms that Ni²⁺ is the dominant oxidation state. Surface morphology and roughness were investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM), showing a clear evolution from discontinuous dome-like islands at ~ 10 nm thickness to a compact and smooth film at 40 nm, with the surface roughness decreasing from 2.96 to 0.26 nm, respectively. Optical characterisation using photoluminescence (PL) and UV-visible absorption spectra shows that film thickness strongly influences emission and absorption: PL intensity reaches a maximum for the 30 nm NiO film due to reduced non-radiative recombination losses. Thicker films exhibit a redshift and slight bandgap narrowing (3.98 → 3.92 eV), attributed to defect-induced band tails. Electrical measurements show a decrease in resistivity from 0.3806 to 0.1477 Ω·cm as thickness increases, consistent with enhanced hole concentration from Ni vacancies and oxygen interstitials. Overall, increasing the NiO thickness improves film uniformity, optical performance, and p-type conductivity, highlighting 30–40 nm NiO layers as promising hole-transport materials for flexible solar and optoelectronic applications.