(52a) Contact Doping with Strong Polyelectrolytes for Organic Photovoltaics

Interfacial barriers at electrode-semiconductor contacts limit charge collection efficiency and hamper performance of organic electronic devices. 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 for polymer donors, such as poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT). The performance of contact-doped organic photovoltaics nearly matches the performance of devices comprised of traditional hole transport layers such as PEDOT:PSS. By varying the pendant acidic groups of the polyelectrolyte between aromatic sulfonic acid, trifluoromethane sulfonimide, and perfluorosulfonic acid, we find the effectivenessof 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 10²⁰ 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.