We adsorbed fluorinated-alkyl and hydrogenated-alkyl phosphonic acid derivatives onto indium tin oxide (ITO) to form self-assembled monolayers (SAMs). Polymer solar cells having these treated ITOs as anodes display open-circuit voltages (Vocs) that are higher than those with bare ITO as anodes. Although the work function of ITO can be significantly tuned by SAM adsorption, the position of the Fermi level of the anode with respect to the hole transport level in the polymer active layer is essentially the same in all of the devices, suggesting that changes in the work function of the anode are not responsible for the Voc variation. Rather, the barrier for minority carrier transport to ITO is altered through SAM adsorption. The adsorption of fluorinated-alkyl phosphonic acid on ITO, in particular, induces a barrier of 2.4 eV for minority carrier transport, which effectively increases carrier selectivity at the anode and increases the Voc in polymer solar cells comprising such treated ITO as anodes compared to those with untreated anodes. © 2013 American Chemical Society.