The adsorption of self-assembled monolayers (SAMs) on metal oxide surfaces is a promising route to control electronic characteristics and surface wettability. Here, arylphosphonic acid derivatives are used to modulate the surface properties of vertically oriented ZnO nanowire arrays. Arylphosphonate-functionalized ZnO nanowires are incorporated into hybrid organic-inorganic solar cells in which infiltrated poly(3-hexylthiophene) (P3HT) serves as the polymer donor. Strong correlations between device short-circuit current density (J sc) and power conversion efficiencies (PCEs) with ZnO surface functionalization species are observed and a weak correlation in the open-circuit voltage (V oc) is observed. Inverted solar cells fabricated with these treated interfaces exhibit PCEs as high as 2.1%, primarily due to improvements in J sc. Analogous devices using untreated ZnO arrays having efficiencies of 1.6%. The enhancement in J sc is attributed to surface passivation of ZnO by SAMs and enhanced wettability from P3HT, which improve charge transfer and reduce carrier recombination at the organic-inorganic interface in the solar cells.