Efficient electrical doping of organic semiconductors is a necessary prerequisite for the fabrication of high performance organic electronic devices. In this work, we study p-type doping of poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) spin-cast from two different solvents. Using electron diffraction, we find strong dopant-induced π−π-stacking for films from the solvent chloroform, but not from chlorobenzene. This image is confirmed and expanded by the analysis of vibrational features of P3HT and polaron absorptions using optical spectroscopy. Here, a red-shifted polaron absorption is found in doped films from chloroform, caused by a higher conjugation length of the polymer backbone. These differences result in a higher conductivity of films from chloroform. We use optical spectroscopy on the corresponding blend solutions to shed light on the origin of this effect and propose a model to explain why solutions of doped P3HT reveal more aggregation of charged molecules in chlorobenzene, whereas more order is finally observed in dried films from chloroform. Our study emphasizes the importance of solvent parameters exceeding the bare solubility of pure dopant and host material for the preparation of highly conductive doped films.
The diffusivity of dopants in semiconducting polymers is of high interest as it enables methods of sequential doping but also affects device stability. In this study, we investigate the diffusion of a bulky sequentially deposited p-dopant in poly(3-hexylthiophene) (P3HT) thin films using nondestructive in situ infrared (IR) spectroscopy and photoelectron spectroscopy (PES). We probe dopant diffusion into the polymer film at varying coverage by differentially evaluating electron transfer in the bulk and at the surface. Thereby it is possible to determine dopant coverages at which both electron transfer and incorporation of dopants are saturated. By use of PES, neutral and charged dopants can be distinguished, revealing that charged dopants are less mobile in the diffusion process than neutral molecules. We further compare the diffusivity in semicrystalline and fully amorphous P3HT. We find that at high coverage semicrystalline P3HT seems to yield a higher capacity for dopants than fully amorphous P3HT. A temperature-dependent measurement of sequential doping shows directly that the incorporation of dopants is thermally activated and requires temperatures close to room temperature.
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