Spectroscopic studies of dopant-induced conformational changes in poly (3-hexylthiophene) thin films
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Research Letter
Spectroscopic studies of dopant-induced conformational changes in poly (3-hexylthiophene) thin films Annabel R. Chew and Alberto Salleo, Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA Address all correspondence to Alberto Salleo at [email protected] (Received 21 June 2017; accepted 22 August 2017)
Abstract
The effect of p-type doping at ultra-low concentrations (∼10−4–10−5 monomer mol fraction) of the polymer poly(3-hexylthiophene) (P3HT) is studied using charge modulation (CM) spectroscopy. Quantitative analysis of CM spectra of doped P3HT show that dopants induce measurable changes in the P3HT local chain conformation. We find that the dopants reside in both the aggregate and amorphous regions of the polymer, not just in the amorphous regions, as previously assumed. With increased doping, the P3HT intrachain disorder grows, causing the P3HT chains to become more oligomer-like, which we postulate leads to the drop in mobility commonly observed in literature.
Introduction The phenomenal success of Silicon technology hinged fundamentally on the ability to locally control its conductivity by doping. Today, semiconducting polymers have the potential to enable next-generation solution-processable and flexible optoelectronic devices.[1] Doping has been instrumental in enhancing the performance of organic light-emitting diodes, leading to their commercial success, but a well-developed doping technology that would enable controlled engineering of electronic properties is still lacking for instance, in polymeric thin film transistors. In organic semiconductors, the doping process is the result of a complex interplay of electronic and morphological effects caused by the interactions between the semiconductor matrix and the doping molecules. Therefore, fundamental studies of polymer–dopant interactions are necessary in order to develop a robust doping technology. To date, studies of the model system consisting of the semiconducting polymer poly(3-hexylthiophene) (P3HT), and p-type dopant tetrafluoro-tetracyanoquinodimethane (F4TCNQ), have provided much useful information about the doping mechanism. Unlike with silicon, doping in organic semiconductors has traditionally involved the addition of sizeable amounts of dopants (upwards of one-tenth P3HT monomer mole fraction). These films are fabricated by co-mixing the dopants and polymer in a suitable solvent, yielding highly conductive P3HT:F4TCNQ films of up to 1 S/cm.[2] The strong doping accessible through this method leads to significant changes in the macroscopic structural and optical properties of the doped P3HT,[3–5] with a new crystal structure emerging above a certain doping threshold.[2] Other doping methods have been proposed to overcome these limitations.[6–8] At the opposite end of the spectrum, ultra-low doping (∼10−4–10−6 dopant molar ratio) is of interest due to its
potential to fill electronic traps. The expectation is that carrier mobilities in the semiconductor would increase because of tra
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