Morphology-driven oxygen evolution performance of NiO _x nanostructures and implications for hole transport in...

Morphology-controlled nanostructures provide an effective strategy to modulate both oxygen evolution reaction (OER) activity and photovoltaic performance in perovskite solar cells (PSCs). However, achieving low OER overpotentials and high power conversion efficiency (PCE) simultaneously through morphology engineering remains challenging. In this work, nickel oxide (NiO _x ) nanostructures with spindle-like (NiO _x -NS) and plate-like (NiO _x -NP) morphologies were synthesized and evaluated as bi-functional OER catalysts and hole transport layers (HTLs) in inverted PSCs. Structural and thermal analyses reveal that NiO _x -NS crystallizes into a cubic phase at a lower temperature (300 °C), whereas NiO _x -NP requires higher calcination temperatures, reflecting differences in precursor microstructure. Electrochemical measurements indicate that NiO _x -NS calcined at 300 °C delivers the lowest OER overpotential (395 mV at 10 mA cm^-2), outperforming NiO _x -NP calcined at 400 °C (565 mV) and 500 °C (474 mV). This enhanced activity is ascribed to favorable surface strain, increased defect density, and advantageous facet exposure. When used as HTLs, NiO _x -NS also delivers the highest PCE (13.25%) among all tested devices, exceeding those based on NiO _x -NP and commercial NiO _x , owing to improved hole extraction and interfacial contact. Overall, this study highlights the importance of morphology control and thermal processing in tailoring NiO _x for multifunctional nanomaterials in electrocatalytic and photovoltaic applications.