The detailed characterization of solution-derived nickel (II) oxide (NiO) hole-transporting layer (HTL) films and their application in high efficiency organic photovoltaic (OPV) cells is reported. The NiO precursor solution is examined in situ to determine the chemical species present. Coordination complexes of monoethanolamine (MEA) with Ni in ethanol thermally decompose to form non-stoichiometric NiO. Specifically, the [Ni(MEA)2(OAc)]+ ion is found to be the most prevalent species in the precursor solution. The defect-induced Ni3+ ion, which is present in non-stoichiometric NiO and signifies the p-type conduction of NiO, as well as the dipolar nickel oxyhydroxide (NiOOH) species are confirmed using X-ray photoelectron spectroscopy. Bulk heterojunction (BHJ) solar cells with a polymer/fullerene photoactive layer blend composed of poly-dithienogermole-thienopyrrolodione (pDTG-TPD) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) are fabricated using these solution-processed NiO films. The resulting devices show an average power conversion efficiency (PCE) of 7.8%, which is a 15% improvement over devices utilizing a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. The enhancement is due to the optical resonance in the solar cell and the hydrophobicity of NiO, which promotes a more homogeneous donor/acceptor morphology in the active layer at the NiO/BHJ interface. Finally, devices incorporating NiO as a HTL are more stable in air than devices using PEDOT:PSS.