Advancing MoOx and NiOx as hole transport layers for perovskite solar cells: Experimental and theoretical insights

We have developed crystalline thin metal oxide films (MoOx, NiOx) as hole transport layers with varying stoichiometries for perovskite solar cells applications. Reactive e-beam evaporation was employed to grow the oxides by vaporizing pure metals at different oxygen pressures, followed by thermal annealing at 200 °C. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, contact angle measurements, X-ray diffraction (XRD), and variable angle spectroscopic ellipsometry were used to analyze the grown films. The XRD findings confirm the presence of crystalline phases in the NiO_x thin films when processed at 200 °C, particularly in the most oxygen-rich films (deposited at 2e-4 Torr). In contrast, the MoOx layers exhibit an amorphous phase. Field emission SEM results confirm the production of dense and homogeneous films across the substrate's surface, free from cracks and pinholes. A numerical model utilizing the measured refractive indices suggests that optimizing the device design with these thin films can achieve power conversion efficiencies of over 25%.