Characterization of Dopant Diffusion within Semiconducting Polymer and Small-Molecule Films Using Infrared-Active Vibrational Modes and Attenuated Total Reflectance Infrared Spectroscopy

Understanding dopant diffusion within organic and polymeric semiconductors is of great importance toward the development of organic photovoltaic and electronic devices, many of which require layered structures with controlled doping profiles (e.g., p-n and p-i-n structures). The current paper demonstrates a new method to determine the diffusion and permeability coefficients for dopant diffusion within polymeric and small-molecule organic semiconductors using attenuated total reflectance infrared (ATR-IR) spectroscopy and taking advantage of the intense IR-active vibrational bands created when dopants such as iodine accept charge from a semiconducting polymer to generate polaronic species. The diffusion and permeability coefficients for iodine within poly(3-hexylthiophene) (P3HT) are determined to be 2.5 x 10(-11) +/- 1.2 x 10(-11) cm(2)/s and 2.4 x 10(-8) +/- 1.2 x 10(-8) cm(2)/s.atm, respectively. The approach is applied to P3HT/PCBM (1:1 mass ratio) films, and the diffusion and permeability coefficients through these composite films are determined to be 7.8 x 10(-11) +/- 2.8 x 10(-11) cm(2)/s and 4.8 x 10(-8) +/- 1.3 x 10(-8) cm(2)/s.atm, respectively. Finally, the approach is extended to determining iodine diffusion within the polycrystalline semiconductor tetraphenylporphyrin (TPP) in a bilayer film with P3HT, and the diffusion coefficient of iodine through TPP is determined to be 7.1 x 10(-14) +/- 1.1 x 10(-14) cm(2)/s. Although the current paper determines diffusion and permeability for the dopant iodine, this approach should be applicable to a wide array of dopants and polymeric and small-molecule semiconductors of interest in photovoltaic and electronic applications.