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Conjugated Polymer- Based OFET Devices
Published in John R. Reynolds, Barry C. Thompson, Terje A. Skotheim, Conjugated Polymers, 2019
Mark Nikolka, Henning Sirringhaus
To understand the origin of high-performance in conjugated polymer OFETs, we have to take a look at the mechanism for charge transport in these materials. We should, however, firstly address the elephant in the room and point out that studies of charge transport are naturally entangled with the choice of device architecture and factors such as the device geometry, contact resistance and charge transport interfaces. Some of these factors have successfully been addressed through extensive research effort over the past decades and optimized device architectures have been conceived (Figure 1.4). For instance, the use of fluorinated polymer dielectrics with low dielectric constants such as CYTOP29 or TEFLON30 or conventional inorganic dielectrics such as aluminium oxide (Al2O3), hafnium oxide (HfO),31 benzocyclobutene (BCB) or silicon dioxide (SiO2)1 treated with dipole-shielding self-assembled monolayers (SAMs) based on octadecyltrichlorosilane (OTS),32 hexamethyldisilazane (HMDS) or phosphonic acid (PA)33 have given charge transport interfaces with low interfacial trap densities. Figure 1.5a illustrates the importance of shielding the charge carriers in the accumulation layer of the OFET from the randomly oriented dipole moments in the gate dielectric to avoid a widening of the density of states (DOS) at the crucial charge transport interface; this in turn has been shown to result in a detrimental localization of charge carriers and as a result, a reduction in charge carrier mobility. Evidence that careful optimization of both charge transport interface and the control of processing conditions have now become possible is served by the routine realization of notoriously sensitive n-type charge transport and light-emitting polymer OFETs that are very sensitive to electron trap states at the interface.34–37 A bottleneck in any OFET architecture still remains the injection/extraction of charge carriers into the semiconducting polymer. In contrast to silicon–MOSFETs where carriers are injected from highly doped regions of either n++ or p++ doped silicon, OFETs require a matching of the electrode’s work function to the polymer’s HOMO or LUMO level (Figure 1.5b). For many materials, this remains a challenge as the choice of (stable) electrode metals is limited; the resulting mismatch of energy levels can hence lead to considerable contact resistances. For many current generation donor–acceptor polymers thus far, the HOMO level is positioned between 5.0 and 5.3 eV which allows for relatively efficient injection from gold electrodes facilitating p-type OFETs with sufficiently low contact resistances.
Review of pH sensing materials from macro- to nano-scale: Recent developments and examples of seawater applications
Published in Critical Reviews in Environmental Science and Technology, 2022
Roberto Avolio, Anita Grozdanov, Maurizio Avella, John Barton, Mariacristina Cocca, Francesca De Falco, Aleksandar T. Dimitrov, Maria Emanuela Errico, Pablo Fanjul-Bolado, Gennaro Gentile, Perica Paunovic, Alberto Ribotti, Paolo Magni
One of the interesting advantages of carbon nanomaterials is the possibility to use conventional fabrication techniques to realize electronic devices and sensors on flexible substrates (Jung et al., 2014; Sharma & Ahn, 2013). Single wall nanotubes were employed for the fabrication of flexible FETs supported on polyethylene terephthalate (PET) films, using a layer-by-layer (LbL) approach. The film was obtained by LbL deposition of carboxylated SWCNT with two polyelectrolites, to work as the gate electrode. The response of the FET was found to be dependent on pH, although in a non-linear way (Lee & Cui, 2010). Mailly-Giacchetti et al. (2013) transferred graphene layers, grown by CVD, onto poly(ethylene 2,6-naphthalenedicarboxylate) (PEN), silicon modified with octadecyltrichlorosilane (OTS) and SiO2, to evaluate the influence of the substrate on sensing. Although the different devices showed different conductivities, the sensitivity to pH was around 22 mV/pH for all of them.