Probing the evolution of conductivity and structural changes in vapor-F4TCNQ doped P3HT

Abstract

Sequential vapor doping is a vital process in controlling the electronic transport properties of semiconducting polymers relevant to opto-electronic and thermoelectric applications. Here, we employed an in situ conductivity method to determine the temporal electronic conductivity (σ) profile when vapor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) doping poly(3-hexylthiophene) (P3HT) thin films held at a different temperatures. The temporal profile of σ first showed a fast exponential increase, followed by a brief linear increase until reaching a σ_max, and followed by a slow decay in σ. The σ profile were correlated to structural changes through a combination UV-vis-NIR spectroscopy, X-ray scattering, and Raman spectroscopy. We find that the timing for σ_max, and subsequent drop in σ of P3HT:F4TCNQ thin films corresponds to the evolution of doping in the crystalline (ordered) and amorphous (disordered) domains. Specifically, Raman spectroscopy resonant at 785 nm highlighted that the crystalline domains reached their saturated doping level near σ_max and subsequent smaller level of doping occurred in regions in the disordered domains. Overall, this study emphasizes the importance of granular understanding of σ and the corresponding structural changes in the crystalline and amorphous domains.