  Solution combustion synthesized hole transport layer of co-doped spinel
nickel cobaltite were fabricated using doctor-blading technique for planar
inverted perovskite solar cells. Perovskite solar cells incorporating Co-doped
spinel nickel cobaltite shown an increase in device performance parameters
compared to un-doped spinel nickel cobaltite, leading to PCE of 16.5%. X-ray
photoelectron spectroscopy measurements revealed the tendency of Cu cations to
replace preferably the surface Ni atoms changing the surface stoichiometry of
spinel nickel cobaltite inducing a cathodic polarization. Ultraviolet
photoelectron spectroscopy measurements unveiled the increase of the ionization
potential by 0.1 eV for co-doped spinel nickel cobaltite film compare to
undoped spinel nickel cobaltite-based hole transporting layer. We attribute the
enhanced PCE of inverted perovskite solar cells presented due to improved hole
extraction properties of co-doped spinel nickel cobaltite hole transporting
layer.
