Molecular doping of conjugated polymers with the strong lewis acid Tris(pentafluorophenyl)borane

In the past years, conjugated polymers have acquired a great deal of interest as semiconductors in electronic devices due to their attractive properties such as mechanical flexibility and easy processability. Efficient charge transport through the semiconductor layers is a crucial requirement in electronic devices. Molecular doping provides an easy means of tuning and improving the transport properties of these materials. In this work, doping of the conjugated polymer poly(3-hexylthiophene) (P3HT) with the strong Lewis acid tris(pentafluorohenyl)borane (BCF) is studied using UV-Vis spectroscopy and conductivity measurements. By assigning the sub-band gap signals observed in the optical spectra of the doped solutions and layers to the known spectral signatures of the charged species in P3HT, the nature of charged species formed, the predominant mechanism of charge transfer between the dopant and polymer, and the doping efficiency is analyzed. While charge transfer is rather inefficient in solution, ca. 50 % of the employed dopant molecules undergo integer charge transfer with the polymer in solid state. Comparison with the well-known acceptor F4TCNQ reveals important differences with regard to the degree of ionization and the quality of the doped layer, which we assign to the better solubility and bulky structure of BCF.