Most electrical sensor and biosensors elements require reliable transducing elements to convert small potential changes into easy to read out current signals. Offering inherent signal magnification and being operable in many relevant environments field-effect transistors (FETs) are the element of choice in may cases. In particular using electrolyte gating numerus sensors and biosensors have been realized in aqueous environments. Over the past yeas electrolyte gated FETs have been fabricated using a variety of semiconducting materials including graphene, ZnO as well as conjugated molecules and polymers. In particular using conducting polymers top performing devices have been achieved. Here we present an approach to use a transition metal dichalcogenide (TMDCs) based monolayer device. Using MoS2 monolayers we show that such electrolyte gated devices may be regarded as very promising future transducing elements for sensor and biosensor applications.
Organic field-effect transistors (OFETs) are highly promising candidates for chemical and biological sensing applications. Many organic semiconductor compounds are solution-processable at low temperatures on a variety of substrates, which allows for cost-effective fabrication methods, leading to smart (disposable) sensor tags in the field of health-, food- and environmental monitoring. Concerning the detection of ions or biological molecules in aqueous solutions, a water-stable operation of OFET sensor elements is crucial. Thus low voltage operation is a prerequisite. In this context electrolyte-gated OFETs (EGOFETs) seem to be the transducing devices of choice. Yet, many EGOFETs suffer from bias stress induced degradation of the organic semiconductor. In this contribution we will therefore benchmark EGOFETs as the transducing devices against other state of the art devices such as classical CMOS FETs implemented in an electrolyte-gated sensor configuration.
Electrolyte-gated organic field-effect transistors (EGOFETs) are the transducer of choice for many ion- and biosensor applications. Due to the formation of an electric double layer at the electrolyte/organic semiconductor interface, they exhibit a very high capacitance allowing for low-voltage and therefore the necessary stable operation in aqueous environment. We show that also using poly(3-hexylthiophene) (P3HT) based EGOFET devices, one can overcome oxygen and water induced degradation processes, normally observed for this polymer, when operating the device in the right gate potential window, avoiding electrochemical processes at the polymer water interface. Moreover, the use of a polymeric blend of P3HT with poly(methyl methacrylate) (PMMA) as the active layers showed improve device stability, as it was tested. We also tested the response of the device as function of the distance between the active EGOFET and the gate wire, aiming to use the device in different sensor applications. Based on the transfer curves of the devices, it was found that the choice of a proper operational window is the most critical parameter and seems to limit both P3HT and P3HT:PMMA systems to the same gate potential which seem to be more important than choice of the semiconductor material as such. Moreover, we could show that the EGOFET device performance is almost independent of the distance between the gate wire and the device pixel within the correct gate potential window.
The increasing interest in non-volatile memory devices has extended the exploration towards new materials, such as organic-inorganic hybrids. Devices based on organic semiconductors and embedded metal nanoparticles (MNPs) were found to display resistive bistability, suitable for programmable electronic applications. Different models were developed to explain the resistive switching mechanism occurring in the devices. Charging/ de-charging of MNPs and concomitant resistivity changes was mainly proposed as mechanism, despite the lack of solid experimental evidence.[ ] In this contribution we report on the role of the space-charge field due to charged MNPs in two-terminal devices, via electrical characterization. Devices comprise 4,4-bis[N-(1-naphthyl)-N-phenyl-amino]diphenyl (α-NPD) with embedded gold nanoparticles (AuNPs). The electrical characterization (current vs bias) of the devices was conducted with and without illumination during operation. Due to the energy level alignment of the chosen materials, the AuNPs behave as deep charge carrier traps. The induced space-charge spontaneously sets the device to the low conductivity state. The de-charging of the AuNPs can then be dynamically induced through illumination, setting the device to a high conductivity state.
Despite the ability to optically control the charging state of the AuNPs, the devices do not display any bistability. This finding provides evidence that the commonly proposed MNPs charging/de-charging mechanism can be excluded as the cause for electrical bistability in two-terminal devices, and that other mechanisms, such as filament formation, should be evoked.
Herein we report on the fabrication and the properties of two highly efficient blue light emitting multilayer polymer light emitting diodes (PLEDs). The first device structure combines a thermally stabilized polymer with a material processed from an orthogonal solvent, allowing for the fabrication of a triple layer structure from solution. The well known poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB), which can be stabilized in a bake-out procedure, was used as a hole transporting layer. A novel pyrene – triphenylamine (PPyrTPA) copolymer was used as emissive layer. The stack was finalized by a poly(fluorene) - derivative with polar side-chains, therefore being soluble in a polar solvent which allows for the deposition onto PPyrTPA without redissolving. The resulting PLED showed bright-blue electroluminescence (CIE1931 coordinates x=0.163; y=0.216) with a high efficiency of 1.42 cd/A and a peak luminescence of 16500 cd/m². The second presented device configuration comprises a thermally stabilized indenofluorene – triphenylamine copolymer acting as hole transporter, and an emissive copolymer with building blocks specifically designed for blue light emission, effective charge carrier injection and transport as well as for exciton generation. This multilayer PLED led to deep-blue emission (CIE1931 x=0.144; y=0.129) with a remarkably high device efficiency of 9.7 cd/A. Additionally, atomic force microscopy was carried out to investigate the film morphology of the components of the stack and x-ray photoemission spectroscopy was performed to ensure a full coverage of the materials on top of each other. Ultraviolet photoemission spectroscopy confirmed the desired type-II band level offsets on the individual interfaces.
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