Structural, Optical and Electrical Properties of Co-Doped NiO Thin Films Grown by Molecular Beam Epitaxy for Optoelectronic Applications

Nickel oxide (NiO) is an attractive material for hole transporting layer (HTL) applications in optoelectronics due to its wide bandgap and robust chemical stability. This thesis presents a systematic investigation into the growth and characterisation of single-crystal NiO and Co doped NiO thin films, fabricated via MBE on both single-crystal MgO and flexible ITO-coated PET substrates. This work represents the first comprehensive study of Co doped NiO thin films grown by MBE, providing new insights into the impact of Co incorporation on their structural, optical, and electronic properties, with particular emphasis on their suitability as HTL materials. The research is divided into three core parts. The first examines Ni1-xCoxO thin films with Co concentrations ranging from 0 to 0.6 at.%. XRD confirms high crystalline quality with a systematic peak shift towards lower angles as Co content increases, indicating lattice expansion. XPS and TEM analyses demonstrate that Co2+ ions successfully substitute Ni2+ within the NiO lattice without inducing secondary phases, preserving structural integrity. UV-Vis absorption measurements show a progressive redshift of the absorption edge with higher Co levels, correlating with a reduced bandgap. PL studies reveal suppressed recombination rates with increasing Co content, suggesting that Co doping favourably modifies the optical and electronic behaviour, hence enhancing HTL performance. The second part extends this study to higher Co doping levels (x=12-32 at.%) to explore the structural and optoelectronic evolution at elevated dopant concentrations. XRD and TEM confirm that the single-crystal structure remains intact, with no evidence of secondary phase formation, as further supported by XPS results. A consistent bandgap narrowing is observed as Co content increases, while PL intensity diminishes significantly at high doping levels, indicating enhanced non-radiative recombination that can affect carrier dynamics. These findings deepen the understanding of the structure-property relationship in highly Co doped NiO films and their practical implications for device integration. The final part investigates the influence of film thickness on the properties of single-crystal NiO deposited on flexible ITO-coated PET substrates for advanced optoelectronic applications. SEM and AFM reveal improved surface uniformity and continuity with increasing film thickness, which is crucial for efficient absorber layer coverage and charge carrier transport. A slight reduction in bandgap from 3.96 eV (10 nm) to 3.91 eV (40 nm) is observed, alongside enhanced PL peak intensity at an optimal thickness of 30 nm, indicating reduced non-radiative recombination. This optimised configuration demonstrates significant potential for high-performance, flexible HTLs in next- generation solar cells.