The interface of planar TiO2/polymer photovoltaic cells was modified with two carboxylated polythiophenes having different densities of carboxylic acid groups. Both of the interface modifiers increase the photocurrent of the cells but lower the open-circuit voltage. The work function of the TiO2, measured using a Kelvin probe, increases with increasing density of carboxylic acid groups due to the formation of interfacial dipoles pointing toward the TiO2 surface. The formation of interfacial dipoles results in a shift in the band offset at the TiO2/polymer interface, which explains the decrease in the open-circuit voltage. This work demonstrates that care must be taken when using carboxylic acid side groups to attach polymers to titania surfaces in photovoltaic cells. If the density of attachment groups is just enough to attach the polymer, then the benefits of the interface modifier can be realized without substantially decreasing the open-circuit voltage.