Enhanced photovoltaic performance of perovskite solar cells by Co-doped spinel nickel cobaltite hole transporting layer

A solution combustion synthesized hole transport layer (HTL) of spinel nickel cobaltite (NiCo2O4) incorporating 3% Cu–2% Li was fabricated using the doctor-blading technique for planar inverted perovskite solar cells (PVSCs). PVSCs incorporating 3% Cu–2% Li-doped NiCo2O4 showed an increase in Jsc and Voc device performance parameters compared to unmodified NiCo2O4, leading to power conversion efficiency (PCE) of 16.5%. X-ray photoelectron spectroscopy measurements revealed the tendency of Cu cations to replace preferably the surface Ni atoms by changing the surface stoichiometry of NiCo2O4, inducing a cathodic polarization. Ultraviolet photoelectron spectroscopy measurements unveiled the increase in the ionization potential by 0.1 eV for a co-doped NiCo2O4 film compared to unmodified NiCo2O4-based HTL. We attribute the enhanced PCE of the inverted PVSCs presented to the improved hole extraction properties of 3% Cu–2% Li NiCo2O4 HTL.