CoO_x is a promising hole-extracting layer (HEL) for inverted planar perovskite solar cells with device configuration ITO/CoO_x/CH₃NH₃PbI₃/PCBM/Ag. The devices fabricated according to a simple solution procedure showed the best photovoltaic performance attaining power conversion efficiency (PCE) 14.5 % under AM 1.5 G one-sun irradiation, which is significantly superior to those fabricated with a traditional HEL such as PEDOT:PSS (12.2 %), NiOx (10.2 %) and CuOx (9.4 %), under the same experimental conditions. We characterized the chemical compositions with XPS, crystal structures with XRD and film morphology with SEM/AFM techniques. Photoluminescence (PL) spectra and the corresponding PL decays for perovskite deposited on varied HEL films were recorded to obtain the hole-extracting characteristics, for which the hole-extracting times show the order CoO_x (2.8 ns) < PEDOT:PSS (17.5 ns) < NiO_x (22.8 ns) < CuO_x (208.5 ns) consistent with the trend of their photovoltaic performances. The reproducibility and enduring stability of those devices were examined to show the outstanding long-term stability of the devices made of metal oxide HEL, for which the CoO_x device retained PCE ~ 12 % for over 1000 h.