We report improved device performance of poly(3-hexylthiophene) (P3HT) and [6,6]phenyl C₆₁ butyric acid methyl ester (PCBM)-based inverted bulk-heterojunction (BHJ) solar cells through the modified interface of the TiO₂/BHJ with a series of carboxylic acid functionalized self-assembled monolayers (SAMs). The SAMs reduce the series resistance and improve the shunt resistance of the cell leading to increased fill factor and photocurrent density. Different aspects of device improvement can be affected depending on the nature of the SAMs. Modification with a C₆₀-SAM shows the largest enhancement leading to a 35% improvement (η = 3.78%) over unmodified inverted devices (η = 2.80%). This SAM serves multiple functions to affect the photoinduced charge transfer at the interface to reduce the recombination of charges, passivation of inorganic surface trap states, improve the exciton dissociation efficiency at the polymer/TiO₂ interface as well as a template to influence the overlayer BHJ distribution of phases, morphology and crystallinity leading to better charge selectivity and improved solar cell performance.