In this study, we utilized the carboxylic acid functionalized molecules, namely, benzoic acid (BZA) and C-61-dicarboxylic acid (CDCA) as interfacial modifiers to form self-assembled monolayers (SAM) in polymer hybrid solar cell (HSC) consisting of poly (3-hexylthiophene) (P3HT) and zinc oxide (ZnO). We found that the short circuit current density (J(sc)) and the open circuit voltage (V-oc) of the device could be drastically increased by using CDCA-SAM and BZA-SAM, respectively. On the other hand, the optimized mixture of CDCA-SAM and BZA-SAM showed better improvement in both the J(sc) and V-oc. The devices modified with mixed SAMs in an optimized ratio showed significant increment in the overall conversion efficiencies compared to unmodified devices consisting of both bi-layer and nanorod (NR) structured HSC. In particular, almost 1.2-fold improvement could be achieved on the NR device with the efficiencies of 033%. We attribute the improvement of J(sc), and V-oc to higher carrier mobility of CDCA-SAM and work function modulation of ZnO by BZA-SAM, respectively. (C) 2015 Elsevier Ltd. All rights reserved.