C. David Heinrich, Paul M. Reichstein, Mukundan Thelakkat*
Applied Functional Polymers, Macromolecular Chemistry I, University of Bayreuth, Universitätsstr. 30, 95440 Bayreuth, Germany
*E-mail of corresponding author: mukundan.thelakkat@uni-bayreuth.de
Prepared for submission
Monolayer Brushes for Highly Efficient Polymeric SAMFETs
150
Abstract
Densely surface grafted poly(3-hexylthiophene) (P3HT) monolayer brushes were prepared by click-chemistry. For this, alkyne-functionalized P3HT was coupled to a surface immobilized self-assembled monolayer (SAM) with azide functionality. The grafted P3HT-Alkyne with a molecular weight of Mn,MALDI = 11400 g mol-1 (SEC:
17400 g mol-1) and a narrow distribution of Đ = 1.15, has the highest reported molecular weight for surface immobilized P3HT brushes. We show the successful grafting of P3HT on the substrate surface with AFM. From the film thickness, we could calculate a grafting density that is high enough to form a monolayer in the true brush regime. The aggregation behavior of the films is characterized by UV-Vis spectroscopy and compared to linear P3HT and the bottlebrush copolymer PS-g-P3HT. SAM based organic field-effect transistors (SAMFETs) with P3HT as active materials were optimized, and characterized. A high field effect mobility of 1.6.10-3 cm2 V-1 s-1 was achieved, which is nearly two orders of magnitude higher than reported values on polymer based SAMFETs.
Introduction
The spontaneous organization of molecules on a surface –self-assembly- is widely used for creating functional surfaces. For example, self-assembled monolayers (SAMs) are used in organic electronics to functionalize surfaces with materials of minimal thickness.1 In the field of organic photovoltaics, these self-assembled layer function as charge extraction layers for holes or electrons.2,3 In organic field effect transistors (OFETs) SAMS are routinely used to treat the oxidic dielectric in order to tune the wettability of the substrate and to reduce the amount of redox active reactive sites at the interface between the dielectric and the active semiconductor material.4 Materials for SAMs consist of an anchoring group which chemisorbs on the surface and a linker for example an aliphatic chain.5 Established anchoring groups for oxide substrates are
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chlorosilanes, methoxysilanes and ethoxysilanes for SiO2, phosphonic acids for Al2O3
and thiols for gold.6,7 Backbones with functional end groups can be used instead of a hydrophobic aliphatic chain in order to tune the wettability.7 Paoprasert et al.8 developed an azide functionalized silane, thereby introducing a reactive functional group, which can be used to fabricate active surfaces which can be further modified by copper-catalyzed alkyne-azide cycloaddition (CuAAC).9
While SAMs are predominantly used to tune the interface between the substrate and the semiconductor material,10 it is also possible for SAMs to act as the active material in the organic semiconductor devices themselves. Several SAM based organic field effect conductors (SAMFETs) have been reported.11,12 Different kinds of SAMs (p- or n-type) can be used according to the desired type of charge transport. Highly efficient p-type SAMFETs and integrated circuits based on small molecule oligothiophene derivatives with charge carrier mobilities up to 2.0.10-2 cm2 V-1 s-1 have been reported by Blom & de Leeuw et al.12 However, the highest charge carrier mobilities of a SAMFET based on poly(3-hexylthiophene) (P3HT)8 was reported to be 5.10-5 cm2 V-1 s-1 which is several orders of magnitude lower than the reported values of thicker spin-cast films.13 This is mainly due to low degree of grafting in self-assembled mono-layers and lack of enough pi-pi stacking in such ultra-thin layers. The best n-type SAMFETs on the other hand were reported by Ringk et al. with a mobility of 1.5.10-3 cm2 V-1 s-1.14 The inherent advantage of an end-on aligned polymeric semiconductor for charge transport is realizable in a SAM only with a high degree of grafting as in the brush regime.
Aside from applications as transistors, SAMs of organic semiconductors may also be used as charge extraction layers for organic photovoltaics (OPV).2,3or to modify the work function of metals in order to minimize the energy barriers for injection or ectraction of charges.15 Such interlayers are used to improve the wettability and the contact with the active material and introduce a selectivity of the electrode towards positive or negative charges.Chemical bound interlayers which are inherently very thin and stable towards a solution based processing of the active layer may be beneficial in terms of performance and processing of the device.3
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The click chemistry concept proposed by Paoprasert et al. can be used to introduce any kind of acceptor or donor material with an alkyne functionality.8 P3HT was chosen as we could report high charge carrier mobilities in molecular bottle-brush type polymers.16 For this, P3HT with a high molecular weight (11400 g mol-1) was chosen. In this molecular weight range P3HT shows the best electronical properties in linear polymers17,18 as well as brush systems.16 We characterized the absorption of the grafted films to gain information about the success of the grafting process. The occurrence of aggregation in these thin films can indicate a dense grafting and can be crucial for a lateral charge transport along the π-π stacked molecules. Record charge carrier mobility was obtained for SAMFETs using surface-grafted conjugated polymer, P3HT.
Monolayer Brushes for Highly Efficient Polymeric SAMFETs