Interfacial barriers at electrode-semiconductor contacts can greatly limit charge collection efficiency and hamper device performance. Doping of the semiconductor near the interface can mitigate charge extraction or injection problems by allowing charge tunneling through barriers with reduced width. Here we demonstrate that polymer acids can act as p-type dopants near electrode interfaces for active layers containing poly(3-hexylthiophene-2,5-diyl) (P3HT). By varying the pendant acidic groups between aromatic sulfonic acid, trifluoromethane sulfonimide, and perfluorosulfonic acid, we find the effectiveness of doping the conjugated polymer at the interface depends on the strength of the pendant acid group with stronger acid moieties being capable of creating more carriers in the doped system. Deposition of acidic polymeric dopants at the anode allows high carrier densities, of order 10(20) cm(-3), to be obtained in polymer semiconductors near the electrode interface. The charge carrier density also depends on the miscibility between polymeric dopants and conjugated polymers. The overall doping efficacy near electrodes therefore depends on the interplay between the strength of pendant acid groups and miscibility between polymeric dopants and conjugated polymers.