Transparent Organic Field-effect Transistors Research Articles

Khaya gum – a natural and eco-friendly biopolymer dielectric for low-cost organic field-effect transistors (OFETs)

Nature provides a wide range of dielectric biopolymers that can be used in electronic devices. In this work, organic field-effect transistors (OFETs) using khaya gum (KG), a natural, biodegradable biopolymer that can be directly collected from khaya senegalensis trees, as the gate dielectric are demonstrated. The fabricated bottom gate/top contact poly (3,6-di (2-thien-5-yl)-2,5-di (2-octyldodecyl)-pyrrolo [3,4-c] pyrrole-1,4-dione) thieno [3,2-b] thiophene) (DPPTTT) –(polymethylmethacrylate) (PMMA) OFETs operate at 3 V with a saturation field-effect mobility (μsat) 0.3 cm2V−1 s−1, threshold voltage (Vth) -1.3 V, subthreshold swing (SS) 450 mV/dec, and current on/off ratios (ION/OFF) larger than 3 × 103. Significantly, the gate leakage current (IG) does not exceed 10–8 A for the gate-source voltage (VGS) $$\le$$ -3 V. UV–Vis spectra analysis shows that the prepared khaya gum films exhibit low absorbance and high transparency (up to 90%) with a calculated optical band gap of about 4.3 eV. Thermal characterization shows two stages of decomposition and a glass transition at around 60 °C. Characterization of metal–insulator-metal (MIM) capacitors using khaya gum reveals that the KG-based MIM capacitors possess a relatively high capacitance per unit area (Ci) of 130 ± 3 nF/cm2 at 1 kHz. As a result, khaya gum emerges as the dielectric of choice for low voltage, transparent OFETs where environmentally friendly device manufacturing is required.

Direct Observation of Confinement Effects of Semiconducting Polymers in Polymer Blend Electronic Systems.

The advent of special types of polymeric semiconductors, known as “polymer blends,” presents new opportunities for the development of next‐generation electronics based on these semiconductors' versatile functionalities in device applications. Although these polymer blends contain semiconducting polymers (SPs) mixed with a considerably high content of insulating polymers, few of these blends unexpectedly yield much higher charge carrier mobilities than those of pure SPs. However, the origin of such an enhancement has remained unclear owing to a lack of cases exhibiting definite improvements in charge carrier mobility, and the limited knowledge concerning the underlying mechanism thereof. In this study, the morphological changes and internal nanostructures of polymer blends based on various SP types with different intermolecular interactions in an insulating polystyrene matrix are investigated. Through this investigation, the physical confinement of donor–acceptor type SP chains in a continuous nanoscale network structure surrounded by polystyrenes is shown to induce structural ordering with more straight edge‐on stacked SP chains. Hereby, high‐performance and transparent organic field‐effect transistors with a hole mobility of ≈5.4 cm2 V–1 s–1 and an average transmittance exceeding 72% in the visible range are achieved.

The Critical Role of Materials' Interaction in Realizing Organic Field-Effect Transistors Via High-Dilution Blending with Insulating Polymers.

High-performance low-band-gap polymer semiconductors are visibly colored, making them unsuitable for transparent and imperceptible electronics without reducing film thickness to the nanoscale range. Herein, we demonstrate polymer/insulator blends exhibiting favorable miscibility that improves the transparency and carrier transport in an organic field-effect transistor (OFET) device. The mesoscale structures leading to more efficient charge transport in ultrathin films relevant to the realization of transparent and flexible electronic applications are explored based on thermodynamic material interaction principles in conjunction with optical and morphological studies. By blending the commodity polymer polystyrene (PS) with two high-performing polymers, PDPP3T and P (NDI2OD-T2) (known as N2200), a drastic difference in morphology and fiber network are observed due to considerable differences in the degree of thermodynamic interaction between the conjugated polymers and PS. Intrinsic material interaction behavior establishes a long-range intermolecular interaction in the PDPP3T polymer fibrillar network dispersed in the majority (80%) PS matrix resulting in a ca. 3-fold increased transistor hole mobility of 1.15 cm2 V-1 s-1 (highest = 1.5 cm2 V-1 s-1) as compared to the pristine material, while PS barely affects the electron mobility in N2200. These basic findings provide important guidelines to achieve high mobility in transparent OFETs.

Highly stable and flexible transparent conductive polymer electrode patterns for large-scale organic transistors

The application of conductive polymer polypyrrole (PPY) towards transparent and flexible electronics has been demonstrated by a photolithography-compatible technique. The oxygen plasma pretreatment was found to be important for successful fabrication of PPY electrode patterns on flexible poly(ethylene terephthalate) (PET). By the patterning process of PPY, the transparency of PPY electrode can be improved up to >80% over the visible spectrum, which combined with the excellent chemical and physical stability of PPY shows the huge potential of PPY electrode as flexible transparent conductive electrode. In addition, PPY provides better interface connection for uniform deposition of organic semiconductor thin film. These outstanding advantages in PPY, coupled with selection of a novel anti-solvent and water-tolerant elastic dielectric, enable the photolithographic PPY patterns to be used for fabrication of large-scale flexible transparent organic field-effect transistor arrays. These results open up the capability of PPY as flexible transparent electrode for flexible organic devices, and exhibit a strong potential of PPY electrode patterns for future large-scale high-precision flexible electronics.

The Preparation of Transparent Organic Field Effect Transistor Using a Novel EDOT Functional Styrene Copolymer Insulator With a PEDOT:PSS Gate Electrode

In this study, we have purposed to combine both polystyrene (PS) and 3,4-ethylenedioxythiophene (EDOT) groups as a gate dielectric and to fabricate a transparent organic field effect transistor (OFET) device with polymeric gate electrode using this novel organic dielectric material. The focus point of this work is to obtain a transparent OFET and to minimize the interface states between gate insulator and gate electrode. Firstly, we have synthesized EDOT functional polystyrene (PS-EDOT) copolymer as gate insulator via “click” chemistry between azide-functional styrene copolymer and propargyl-functionalized EDOT. We used the poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) conductive polymer mixture as a suitable alternative gate electrode instead of inorganic contacts, which is a new topic in the organic electronics. The contact resistance value was measured as 1/600 (S/cm)-1. At the end of the process transparent OFETs with different channel length were fabricated using spin coating method by which poly(3-hexylthiophene) (P3HT), novel PS-EDOT copolymer insulator and PEDOT:PSS were coated on prepatterned OFET substrate. Electrical characterizations of OFET devices were held in total darkness and in air ambient in order to achieve output and transfer current-voltage (I-V) characteristics. The main parameters such as the threshold voltage (V Th ), field effect mobility (μFET) and current on/off ratio (Ion/off) of the devices were extracted from capacitance-frequency (C-f) plot. It was found that fabricated PS-EDOT OFETs exhibit good device performances such as low V Th , remarkable mobility, and Ion/off values.

Strategy for preparation of transparent organic thin film transistors with PEDOT:PSS electrodes and a polymeric gate dielectric

Fully transparent organic field effect transistors (OFET) in top- and bottom-gate layouts were prepared. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was structured photolithographically as either gate- or source- and drain-contact on top of poly(4-vinylphenol) (PVP). The dielectric material was fitted to the sequence of materials by combining water soluble polyvinyl alcohol (PVA) and PVP. N-type conducting hexadecafluorophthalocyaninato-copper (F16PcCu) was used as organic semiconductor sensitive to interface traps. Critical boundary surfaces for film growth were examined by microscopic methods, which revealed smooth polymer surfaces as a result of our preparation and lithography steps characterizing the presented concept as a viable alternative to existing approaches.

Fully transparent organic transistors with junction-free metallic network electrodes

We utilize highly transparent, junction-free metal network electrodes to fabricate fully transparent organic field effect transistors (OFETs). The patterned transparent Ag networks are developed by polymer crack template with adjustable line width and density. Sheet resistance of the network is 6.8 Ω/sq and optical transparency in the whole visible range is higher than 80%. The bottom contact OFETs with DNTT active layer and parylene-C dielectric insulator show a maximum field-effect mobility of 0.13 cm2/V s (average mobility is 0.12 cm2/V s) and on/off ratio is higher than 107. The current OFETs show great potential for applications in the next generation of transparent and flexible electronics.

Polyol synthesis of silver nanostructures: Inducing the growth of nanowires by a heat-up process

The polyol synthesis of silver (Ag) nanostructures typically involves the rapid injection of Ag precursor to a preheated, ethylene glycol solution containing polymeric stabilizers and other additives. Here we report that Ag nanowires can be synthesized in high yields by applying a heat-up process in the polyol synthesis. Electron microscopy studies revealed that multiple-twinned Ag seeds were generated preferentially during the heat-up procedure, and then grew into nanowires. We also demonstrate that these Ag nanowires can be applied as electrode materials for the fabrication of flexible and transparent organic field-effect transistors with a reasonably high hole-mobility.

Correlation among Morphology, Crystallinity, and Charge Mobility in OFETs Made of Quaterthiophene Alkyl Derivatives on a Transparent Substrate Platform

In this Article, we present a comprehensive study of organic field-effect transistors (OFETs) made of thin films of methyl, n-butyl, and n-hexyl end-substituted quaterthiophenes on a transparent substrate platform. This particular platform has been already used for organic light-emitting diodes (OLEDs) but rarely employed for OFETs. In perspective, this is a very promising route for the development of field-effect photonic applications such as organic light-emitting transistors (OLETs). A systematic characterization of the organic films has been made by means of atomic force microscopy (AFM) and X-ray diffraction (XRD) to correlate morphology, crystallinity and charge mobility to the alkyl chain length. In particular, a charge mobility value of 0.09 cm2/(V s) has been obtained in transparent OFETs with a large area channel for DH4T grown at room temperature. This mobility exceeds the one obtained on silicon-oxide substrates and is likely due to a more favorable interaction of the DH4T molecules with the PMMA layer employed as gate dielectric.

ZnO as dielectric for optically transparent non-volatile memory

This paper discusses the application of a DC sputtered ZnO thin film as a dielectric in an optically transparent non-volatile memory. The main motivation for using ZnO as a dielectric is due to its optical transparency and mechanical flexibility. We have established the relationship between the electrical resistivity ( ρ) and the activation energy ( E a ) of the electron transport in the conduction band of the ZnO film. The ρ of 2 × 10 4–5 × 10 7 Ω-cm corresponds to E a of 0.36–0.76 eV, respectively. The k-value and optical band-gap for films sputtered with Ar:O 2 ratio of 4:1 are 53 ± 3.6 and 3.23 eV, respectively. In this paper, the basic charge storage element for a non-volatile memory is a triple layer dielectric structure in which a 50 nm thick ZnO film is sandwiched between two layers of methyl silsesquioxane sol–gel dielectric of varying thickness. A pronounced clockwise capacitance–voltage ( C–V) hysteresis was observed with a memory window of 6 V. The integration with a solution-processable pentacene, 13,6-N-Sulfinylacetamodipentacene resulted in an optically transparent organic field effect transistor non-volatile memory (OFET-NVM). We have demonstrated that this OFET-NVM can be electrically programmed and erased at low voltage (± 10 V) with a threshold voltage shift of 4.0 V.

Transparent organic field-effect transistors with polymeric source and drain electrodes fabricated by inkjet printing

Transparent organic field-effect transistors based on pentacene were fabricated on indium tin oxide (ITO)-coated glass using ITO as the gate electrode, Al2O3 grown by atomic layer deposition as the gate insulator, and an inkjet-printed conducting polymer poly(3,4-ethylenedioxythiophene):poly(4-styrenesulphonate) as the source and drain electrodes. The transistors combine an overall high transmittance (84% in the channel and 78% through source/drain electrodes) in the visible region, a field-effect mobility value of 0.3cm2∕Vs, a threshold voltage of −0.2V, a subthreshold slope of 0.9V/decade, and an on/off current ratio of 105.

Towards See‐Through Displays: Fully Transparent Thin‐Film Transistors Driving Transparent Organic Light‐Emitting Diodes

Fully transparent computer displays have, until now, been the vision of science-fiction movies. Nevertheless, there are numerous applications for these devices ranging, for example, from head-mounted displays to their integration in automotive windshields. In this paper we demonstrate that this vision is on the verge of becoming reality. The realization of entirely transparent displays requires both transparent light-emitting devices, in our case organic light-emitting diodes (OLEDs) with transparent contacts, and a driving scheme based on transparent thin-film transistors (TTFTs). Since the early reports on electroluminescence from multilayer, thin-film devices composed of vacuum-sublimed small organic molecules [1] and spin-coated conjugated polymers, [2] substantial research has been devoted to the improvement of device efficiencies, color purity, and lifetime. Incitement of this development is the potential use of OLEDs in future commercial flat-panel displays. OLEDs usually consist of a layer sequence of organic functional materials (charge transporters/blockers/emitters) with an overall thickness of the order of 100 nm. Most of these materials absorb light in the deep-blue or ultraviolet spectral region and are nearly transparent in the visible part of the spectrum. Organic layers applied to emit visible light are often based on so-called guest–host systems, in which a wide-bandgap host material (absorbing in the UVonly) is doped with a few weight percent of a light-emitting dye. [3] As a result, the emitting layer appears transparent. Transparent conductive oxides, most prominently indium tin oxide (ITO) and aluminum-doped ZnO (AZO), may be used as electrical contacts to OLEDs. Therefore, OLEDs seem to be promising devices for the realization of entirely transparent visible-light emitters. [4–6] OLED displays driven in passive-matrix mode are based on conventional bottom-emitting OLEDs and are considered as an approach to fabricate small-sized, low-information-content displays with moderate pixel counts. To accomplish largerarea, high-resolution OLED displays an active-matrix-addressing scheme has been suggested. [7] Conventional a-Si:H or poly-Si TFT backplanes are not suitable as drivers for transparent displays because they are opaque in the visible part of the spectrum. Putting pixels and transistors next to each other would compromise the displays’ fill factor. Organic field-effect transistors (OFETs) as pixel drivers for OLEDs have been discussed by Sirringhaus et al. [8] Transparent OFETs have also been reported. [9] However, their performance is still poor; transparent active pixels using OFETs have not yet been demonstrated. Therefore, in this paper we focus on the production of TTFTs based on the wide-bandgap oxide semiconductor zinc tin oxide (ZnO)x(SnO2)1–x ,a s a viable alternative to previously fabricated devices. Recently, research on TTFTs with channels made from ox