Diffusion of Brønsted Acidic Dopants in Conjugated Polymers

Depth-dependent doping levels in electrically doped organic semiconductor films are controlled by the relative rates of reaction and diffusion of the dopant. Many semiconductor devices (e.g., light emitters and photovoltaics) utilize heterojunctions that depend on dopant gradients to control charge transport. To understand the formation and stability of gradients requires understanding of dopant diffusion and the complex changes in polymer properties that arise during doping. Here, the doping of thin films of poly(3-hexylthiophene) (P3HT) using bistriflimide acid (HTFSI) from solution in methanol is evaluated. The contributions of the reaction rate and diffusion to doping were deconvoluted by examining the evolution in films of P3HT with varying thickness using a combination of spectroscopic techniques. The rate dependency of doping by hydrogenated and deuterated acids shows a significant kinetic isotope effect, indicating that doping is limited by proton transfer to the polymer. Dynamic secondary ion mass spectrometry (DSIMS) of doped films show that H/D is retained in doped films after the doping process in contrast to propositions in literature of hydrogen evolution. To assess diffusion limitations to the doping process, dopant concentration profiles were measured and quantified using complementary XPS and DSIMS depth profiling. The dopant concentration profiles show evidence of enrichment at the P3HT top surface. These limited concentration profiles suggest that dopant diffusivity varies inversely with dopant concentration due to doping-induced changes to the semiconducting polymer.