Defect Engineering in π-Conjugated Polymers

The optoelectronic properties of thin films of poly(3-hexylthiophene) (P3HT) are markedly improved by treating the polymer in solution with either electrophiles or nucleophiles. The work function is strongly decreased by treatment with lithium aluminum hydride, consistent with hydride ion addition and the observed ∼400-fold decrease in p -type defect density. The exciton diffusion length ( L ex ) doubled to 14 nm, while the bulk carrier mobility (μ) improved by more than an order of magnitude to 2.4 × 10 −3 cm 2 /(V s). The work function is increased after treatment with dimethylsulfate, which is consistent with methyl cation addition and, again, both L ex and μ are increased substantially. Both treatments greatly improve the polymer’s stability against photo-oxidation. We discuss these results in the framework of defect engineering in organic semiconductors, in analogy to defect engineering studies of inorganic semiconductors.