Flexible Organic Electronic Devices Research Articles

Slot-die coating of sol–gel-based organic–inorganic nanohybrid dielectric layers for flexible and large-area organic thin film transistors

The slot-die coating method has enormous potential owing to the industrial applicability of its high throughput capability to the large-area fabrication of solution-processed films. Herein, we demonstrated a slot-die-coated sol–gel-based organic–inorganic (O–I) nanohybrid dielectric film for organic thin film transistors (OTFTs) with highly uniform properties, such as thickness, morphology, and electrical insulating characteristics. Through the modification of the solution properties and coating speed conditions, the slot-die coating process was used to successfully fabricate defect-free dielectric films with uniform morphology, ranging from below 50 to 800 nm. Additionally, from the OTFT behavior, we identified the clockwise hysteresis by the insulator bulk for the deposition of thick O–I nanohybrid dielectrics. Based on these analyses, we can produce flexible large-area OTFT arrays with optimum slot-die-coated dielectric layers, showing an average field-effect mobility of 0.675 cm2 V−1 s−1 with a low standard deviation of 0.052017 cm2 V−1 s−1. These results suggest that this printing technology has considerable potential in the manufacturing of flexible organic electronic devices that require industrial reliability and large-area processing.

Dye-doped poly(3,4-Ethylenedioxythiophene)-Poly(Styrenesulfonate) electrodes for the application in organic light-emitting diodes

A facile way to enhance the electrical conductivity of conductive polymers via solution-processed low-concentration doping, is suggested. An organic cationic azo dye, methyl red was used as a p-type dopant for poly(3, 4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to realize highly conductive electrodes for organic light-emitting diode applications. By optimization of the dopant concentration, the doped PEDOT:PSS films exhibited electrical conductivities enhanced by three orders of magnitude compared to the pristine films. Ultraviolet-visible absorption spectroscopy studies indicate that charge transfer complexes between PEDOT:PSS and the dopant were formed. It was found that the dye exhibits high tendency to oxidize the polymer in a single-blend system which is necessary for the efficient doping. Our research proposes a simple and low-cost doping agent to improve the conductivity and sheet resistance of PEDOT:PSS in order to apply it as a promising electrode material for various kinds of highly flexible organic electronic devices.

Continuous and Segmented Semiconducting Fiber‐like Nanostructures with Spatially Selective Functionalization by Living Crystallization‐Driven Self‐Assembly

AbstractFiber‐like π‐conjugated nanostructures are important components of flexible organic electronic and optoelectronic devices. To broaden the range of potential applications, one needs to control not only the length of these nanostructures, but the introduction of diverse functionality with spatially selective control. Here we report the synthesis of a crystalline‐coil block copolymer of oligo(p‐phenylenevinylene)‐b‐poly(2‐vinylpyridine) (OPV5‐b‐P2VP44), in which the basicity and coordinating/chelating ability of the P2VP segment provide a landscape for the incorporation of a variety of functional inorganic NPs. Through a self‐seeding strategy, we were able to prepare monodisperse fiber‐like micelles of OPV5‐b‐P2VP44 with lengths ranging from 50 to 800 nm. Significantly, the exposed two ends of OPV core of these fiber‐like micelles remained active toward further epitaxial deposition of OPV5‐b‐PNIPAM49 and OPV5‐b‐P2VP44 to generate uniform A‐B‐A and B‐A‐B‐A‐B segmented block comicelles with tunable lengths for each block. The P2VP domains in these (co‐)micelles can be selectively decorated with inorganic and polymeric nanoparticles as well as metal oxide coatings, to afford hybrid fiber‐like nanostructures. This work provides a versatile strategy toward the fabrication of narrow length dispersity continuous and segmented π‐conjugated OPV‐containing fiber‐like micelles with the capacity to be decorated in a spatially selective way with varying functionalities.

Continuous and Segmented Semiconducting Fiber-like Nanostructures with Spatially Selective Functionalization by Living Crystallization-Driven Self-Assembly.

Fiber-like π-conjugated nanostructures are important components of flexible organic electronic and optoelectronic devices. To broaden the range of potential applications, one needs to control not only the length of these nanostructures, but the introduction of diverse functionality with spatially selective control. Here we report the synthesis of a crystalline-coil block copolymer of oligo(p-phenylenevinylene)-b-poly(2-vinylpyridine) (OPV5 -b-P2VP44 ), in which the basicity and coordinating/chelating ability of the P2VP segment provide a landscape for the incorporation of a variety of functional inorganic NPs. Through a self-seeding strategy, we were able to prepare monodisperse fiber-like micelles of OPV5 -b-P2VP44 with lengths ranging from 50 to 800 nm. Significantly, the exposed two ends of OPV core of these fiber-like micelles remained active toward further epitaxial deposition of OPV5 -b-PNIPAM49 and OPV5 -b-P2VP44 to generate uniform A-B-A and B-A-B-A-B segmented block comicelles with tunable lengths for each block. The P2VP domains in these (co-)micelles can be selectively decorated with inorganic and polymeric nanoparticles as well as metal oxide coatings, to afford hybrid fiber-like nanostructures. This work provides a versatile strategy toward the fabrication of narrow length dispersity continuous and segmented π-conjugated OPV-containing fiber-like micelles with the capacity to be decorated in a spatially selective way with varying functionalities.

In-Situ Imaging of Electrochemical Polymerization of Functionalized Thiophenes Using Transmission Electron Microscopy

There is considerable continuing interest in developing conjugated polymer materials like the functionalized thiophene poly(3,4-ethylenedioxythiophene) (PEDOT) to interface ionically conducting living tissue with electronically conducting metallic or semiconducting biomedical devices. These versatile conjugated polymers can also be used in organic solar cells, anti-static coatings and flexible organic electronic devices. Electrochemical polymerization can be used to synthesize thin-films of conjugated polymers. Typically, a 3-electrode system comprising of a working, counter and reference electrodes is used for this process. It is initiated by the electrochemical oxidization of the monomers, followed by the formation of higher molecular weight products that precipitate from solution, forming a thin-film of the conjugated polymer on the working electrode (anode). The reaction can be controlled either potentiostatically (constant voltage) or galvanostatically (constant current).This technique has several advantages over vapor-phase and chemical polymerization such as the facile control of polymer morphology (rough and bumpy under normal conditions) and the ability to coat patterned surfaces. However, the factors that determine the development of these structures during electrodeposition are not yet well established. This makes it difficult to design new systems and optimize device performance. Considering that the morphology of electrochemically-polymerized thin-films can be fine-tuned by controlling the early stage nucleation and growth of the oligomeric clusters, we have used in-situ, low dose Transmission Electron Microscopy to image and obtain a detailed understanding of the fundamental processes occurring at the electrode-solution interface, especially the evolution of the mobile oligomers that precede solid polymer film formation. The observation and quantification of these electrochemical reactions are made possible using a commercial holder designed by Protochips, Inc. The liquid (monomer solution) is sandwiched between a top chip and a spacer chip. The spacer chip is used to control the amount of liquid to be imaged (typically 500 nm – 1000 nm). This system can either be used in a static mode (no flow) or flowing mode depending on the requirements. The top chip is equipped with the electrical connections required to perform electrochemical reactions and watch the process in-situ. During the experiments, we observed individual nuclei forming, merging and growing in size and thickness with increasing charge density. These droplets correspond to more mobile, oligomer-rich clusters which further react with the monomers in the solution to form the solid polymer (PEDOT) deposit. This mechanism of precipitation and solidification provides a reasonable explanation for the usual bumpy morphology of the electrochemically polymerized films. We observed that the deposition occurs in 2 regimes. The first regime corresponds to an increase in nucleation density (the nucleation regime) and the second (the growth regime) where the clusters start to merge and increase in size causing a decrease in the nucleation density. During the deposition, certain droplets were found to gradually decrease in size and mass thickness, indicating some degree of reversibility of the process which was not previously appreciated. In addition to this, we have found substantial variations when the chemistry of the monomer is changed, including a dramatic increase in the nucleation density when a more hydrophilic, carboxylic acid substituted EDOT was used as a comonomer. The total electron doses we used for these experiments were around 10-20 mC/cm2. These were well below the critical dose for electron damage of the PEDOT, which is about 0.1 C/cm2. Further, using a macromolecular counter-ion, poly(acrylic acid) (PAA) during the electrodeposition facilitates the formation of highly anisotropic nano-fibrils of PEDOT thereby reducing the overall impedances due to increase in the effective surface area. The basic understanding is that PAA prevents the lateral growth of PEDOT which results in the nano-fibrillar morphology. Using operando electron microscopy, we have observed the nucleation and growth of these fibrils in an oriented, dendritic manner at the solution-electrode interface, allowing us to better understand the nano-fibril formation mechanisms. These experiments have presented us with unprecedented insights about the processes occurring at the nanoscale during the early stage nucleation and growth of electrodeposition of PEDOT and PEDOT copolymers. Figure 1

Gravure Printed Ultrathin Dielectric for Low Voltage Flexible Organic Field‐Effect Transistors

AbstractOrganic electronics requires dielectrics which can be processed over large areas at low temperature in ambient using simple solution based techniques. Here a crosslinkable acrylate‐based organic dielectric is reported, which is gravure printable on plastic substrates, patternable by conventional photolithography, and can form electrically functional films at thicknesses below 100 nm. Capacitors are successfully fabricated with film thicknesses of 50 and 70 nm, and organic field effect transistors with thicknesses of 70 nm, transistors with gold contacts patterned by lift‐off on top of the dielectric followed by spin‐cast 6,13‐bis(triisopropylsilylethynyl)‐pentacene showing near ideal behavior at operating voltages as low as −1 V. Importantly saturation, a requirement for digital logic, in transistors is achieved with gate lengths of 2.7 µm, at the limit of conventional photolithography. This demonstrates that such materials have potential in flexible organic electronic devices requiring digital signal processing and low voltages.

Time-dependent water vapor permeation through multilayer barrier films: Empirical versus theoretical results

Multilayer films that comprise alternating inorganic barrier and polymer layers deposited on a flexible substrate are often used to protect organic electronic devices from degradation caused by oxygen and water vapor. We tested films consisting of up to 11 inorganic silicon oxide barrier and polymer layers to characterize the water vapor permeability, solubility and diffusivity of each layer based on measurements of water sorption, transient water vapor permeation and lag times. The time-dependency of transient water vapor transmission rates (WVTR) of the structures containing a barrier-polymer-barrier sequence was also estimated using a so-called quasi-steady-state (QSS) approximation. The permeability values and lag times in multilayer structures predicted by QSS approximation agreed with the values determined empirically based on time-dependent WVTR measurements. The steady-state WVTR of the multilayer barrier films was dominated by the permeability of the inorganic barrier layers, whereas the solubility coefficient and thickness of the interleaved polymer layers determined the lag times. The barrier performance and lag time of multilayer structures can be predicted using the parameters obtained from sorption measurements, thus significantly reducing the time required for experiments. The results of this study will allow us to design multilayer barrier films according to the lifetime requirements of flexible organic electronic devices.

Excellent carrier transport materials produced by controlled molecular stacking and their application in flexible organic electronic devices

Research studies based on flexible organic electronic devices regarding the relationship between carrier transport and molecular stacking are actively reported.

Origin of the synthetic circuits and comparison effects of different dose malachite green oxalate doped hydrogel

Abstract Different concentration malachite green oxalate (MGO) dye doped stretchable hydrogel have been prepared for synthetic circuits in flexible organic electronic devices by using the polymerization method. Frequency evolution of the tangent factor, complex impedance, phase angles, electric modulus, capacitance, and ionic conductivity for MGO dye doped hydrogel were investigated by using the impedance spectroscopy (IS) in the frequency range of 100 Hz to 40 MHz at room temperature (RT). The total polarization effects, the electric modulus-based Cole-Cole diagrams and their adopted to Smith-Chart were analyzed by the ion-migration in the hydrogel channels. The critical frequency of the imaginary part of the electric modulus increases with increasing MGO concentrations from 50 mg/L to 100 mg/L but excluding 25 mg/L. It was observed the alternative behaviour for 75 mg/L doped hydrogel originated from the fluctuation behaviour of interview interaction between MGO cationic ions and hydrogel bond. The highest and lowest values of capacitance for 25 mg/L and 75 mg/L concentrations were recorded at low frequency (LF). It was found that the conductivity properties are related to the change in the grain size of hydrogel structure because of increasing MGO concentrations.

Encapsulation requirements to enable stable organic ultra-thin and stretchable devices

Abstract

An Asymmetric Furan/Thieno[3,2-b]Thiophene Diketopyrrolopyrrole Building Block for Annealing-Free Green-Solvent Processable Organic Thin-Film Transistors.

A new asymmetric furan and thieno[3,2-b]thiophene flanked diketopyrrolopyrrole (TTFDPP) building block for conjugated polymers is designed and used to generate a donor-acceptor semiconducting polymer, poly[3-(furan-2-yl)-2,5-bis(2-octyldodecyl)-6-(thieno[3,2-b]thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-thieno[3,2-b]thiophene] (abbreviated to PTTFDPP-TT), consisting of TTFDPP unit copolymerized with thieno[3,2-b]thiophene comonomer (TT), which is further synthesized. Results demonstrate that PTTFDPP-TT-based thin-film transistors in a bottom-gate bottom-contact device configuration exhibit typical hole-transporting property, with weak temperature dependence for charge carrier mobility from room temperature to 200 °C. In addition, the good solubility of PTTFDPP-TT due to the incorporation of a polar furan unit and an asymmetric conjugated structure makes it able to be solution processed with a less toxic nonchlorinated solvent such as toluene, demonstrating comparable performance with that prepared from chlorinated solution. These results suggest PTTFDPP-TT as a promising organic semiconductor candidate for annealing-free, environmentally benign, and less energy-consuming applications in large-area flexible organic electronic devices.

Anisotropic charge transport in highly oriented films of semiconducting polymer prepared by ribbon-shaped floating film

Fabrication of large area and uniform thin films with preferred edge-on orientation is highly demanded for flexible organic electronic devices. In this work, utilization of a newly designed slider in the floating film transfer method to assist unidirectional film spreading led to the fabrication of uniform and macroscopically edge-on oriented ribbon-shaped floating films (15 cm × 2 cm) having very large optical anisotropy (>20). The results pertaining to the in-plane grazing angle incident X-ray diffraction (GIXD) and φ-scan measurements revealed the well-stacked polymer backbone conformation with edge-on orientation without any inter-mixing of face-on oriented counterparts. Optical anisotropy dependent charge transport in organic field effect transistors using anisotropic poly(3,3‴-didodecyl-quaterthiophene) films has also been shown, and the results are supported by polarized electronic absorption and GIXD measurements.

Development of Annealing-Free, Solution-Processable Inverted Organic Solar Cells with N-Doped Graphene Electrodes using Zinc Oxide Nanoparticles.

An annealing-free process is considered as a technological advancement for the development of flexible (or wearable) organic electronic devices, which can prevent the distortion of substrates and damage to the active components of the device and simplify the overall fabrication process to increase the industrial applications. Owing to its outstanding electrical, optical, and mechanical properties, graphene is seen as a promising material that could act as a transparent conductive electrode for flexible optoelectronic devices. Owing to their high transparency and electron mobility, zinc oxide nanoparticles (ZnO-NP) are attractive and promising for their application as charge transporting materials for low-temperature processes in organic solar cells (OSCs), particularly because most charge transporting materials require annealing treatments at elevated temperatures. In this study, graphene/annealing-free ZnO-NP hybrid materials were developed for inverted OSC by successfully integrating ZnO-NP on the hydrophobic surface of graphene, thus aiming to enhance the applicability of graphene as a transparent electrode in flexible OSC systems. Chemical, optical, electrical, and morphological analyses of ZnO-NPs showed that the annealing-free process generates similar results to those provided by the conventional annealing process. The approach was effectively applied to graphene-based inverted OSCs with notable power conversion efficiencies of 8.16% and 7.41% on the solid and flexible substrates, respectively, which promises the great feasibility of graphene for emerging optoelectronic device applications.

Laminated Free Standing PEDOT:PSS Electrode for Solution Processed Integrated Photocapacitors via Hydrogen‐Bond Interaction

AbstractIn this work, a novel lamination method employing hydrogen‐bond interaction to assemble a highly conductive free standing poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film as a common electrode is demonstrated in a solution processed metal‐free foldable integrated photocapacitor (IPC) composed of a monolithic organic solar cell (OSC) and a capacitor. The highlights of the work are: (1) micrometer free standing PEDOT:PSS electrode is successfully laminated onto a relatively large area (1 cm2) OSCs; (2) a free standing capacitor based on the PEDOT:PSS electrode is achieved; (3) the IPC demonstrates an overall efficiency of 2% and an energy storage efficiency of 58%, which is comparable with those of IPCs based on metallic common electrodes; (4) the novel lamination method for PEDOT:PSS electrode enables free standing PEDOT:PSS broad applications in solution processed flexible organic electronics, especially tandem or/and integrated organic electronic devices. Furthermore, the IPC is foldable with excellent cycling stability (no decay after 100 recycles at 1 mA cm−2). These results indicate that free standing PEDOT:PSS film is a promising candidate as common electrodes for IPCs to break the restrictions of metal electrodes. The demonstrated lamination method will greatly extend the applications of PEDOT:PSS electrodes to large area flexible organic electronic devices.

Silicon Nitride Deposition for Flexible Organic Electronic Devices by VHF (162\u2009MHz)-PECVD Using a Multi-Tile Push-Pull Plasma Source

Depositing a barrier film for moisture protection without damage at a low temperature is one of the most important steps for organic-based electronic devices. In this study, the authors investigated depositing thin, high-quality SiNx film on organic-based electronic devices, specifically, very high-frequency (162 MHz) plasma-enhanced chemical vapor deposition (VHF-PECVD) using a multi-tile push-pull plasma source with a gas mixture of NH3/SiH4 at a low temperature of 80 °C. The thin deposited SiNx film exhibited excellent properties in the stoichiometry, chemical bonding, stress, and step coverage. Thin film quality and plasma damage were investigated by the water vapor transmission rate (WVTR) and by electrical characteristics of organic light-emitting diode (OLED) devices deposited with SiNx, respectively. The thin deposited SiNx film exhibited a low WVTR of 4.39 × 10−4 g (m2 · day)−1 for a single thin (430 nm thick) film SiNx and the electrical characteristics of OLED devices before and after the thin SiNx film deposition on the devices did not change, which indicated no electrical damage during the deposition of SiNx on the OLED device.

Insights on the Optical Properties of Poly(3,4-Ethylenedioxythiophene):Poly(styrenesulfonate) Formulations by Optical Metrology.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is among the most widely used polymers that are used as printed transparent electrodes for flexible Organic Electronic (OE) devices, such as Organic Photovoltaics (OPVs). The understanding of their optical properties and the correlation of the optical properties with their electronic properties and metallic-like behavior can lead to the optimization of their functionality as transparent electrodes in multilayer OE device architectures. In this work, we study the optical properties of different PEDOT:PSS formulations by non-destructive Spectroscopic Ellipsometry (SE), from the infrared to the far ultraviolet spectral regions. The optical response of PEDOT:PSS includes an intense optical absorption originated from the conductive part (PEDOT) at lower photon energies, whereas the electronic transition energies of the non-conductive PSS part have been measured at higher photon energies. Based on the different PEDOT:PSS formulations, the optical investigation revealed significant information on the relative contribution of conductive PEDOT and insulating PSS parts of the PEDOT:PSS formulation in the overall optical response, which can strongly impact the final device functionality and its optical transparency.

Synthesis and characterization of novel Cu2O/PVDF nanocomposites for flexible ferroelectric organic electronic memory devices

Copper oxide nanoparticles loaded Poly(vinylidene fluoride) (PVDF) nanocomposites, at concentrations from 1 to 9 wt%, were developed by casting technique. The effect of the Cu2O nanoparticles on the structure, thermal, mechanical and dielectric properties of the PVDF were inspected. The scanning electron microscopy (FE-SEM) revealed the well dispersion of the Cu2O nanoparticles into the PVDF matrix up to 5 wt%. The X-ray diffraction (XRD) patterns and the infrared spectroscopy (FTIR) measurements implied that the inclusion of the Cu2O nanoparticles into the PVDF matrix leads to a transform of the non-polar semi-crystalline α-phase of the neat PVDF to highly crystallized polar β-phase. Moreover, an increase of the thermal stability and crystallinity of the PVDF after the incorporation of the Cu2O nanoparticles was achieved. The Cu2O/PVDF nanocomposites exhibited excellent cyclic mechanical property compared to the neat PVDF. The introduction of 5 wt% Cu2O nanoparticles into the PVDF composites resulted in remarkable increase of the dielectric constant by 13 times while the dielectric loss was very low. A device based on Al/5 wt% Cu2O doped PVDF/Pt structure is developed. This device shows ferroelectric hysteresis with butterfly like shape and exhibited remanent polarization of 11.2 μC/cm2 with coercive field of 49 MV/m. These superior properties enable the developed Cu2O/PVDF nanocomposite films to be a potential candidate in the field of ferroelectric organic electronic memory devices.

Breakdown of Polarons in Conducting Polymers at Device Field Strengths

Conducting polymers have become standard engineering materials used in many electronic devices. Despite this, there is a lack of understanding of the microscopic origin of the conducting properties, especially at realistic device field strengths. We present simulations of doped poly(p-phenylene) (PPP) using a Su–Schrieffer–Heeger (SSH) tight-binding model, with the electric field included in the Hamiltonian through a time-dependent vector potential via Peierls substitution of the phase factor. We find that polarons typically break down within less than a picosecond after the field has been switched on, already for electric fields as low as around 1.6 mV/Å. This is a field strength common in many flexible organic electronic devices. Our results challenge the relevance of the polaron as charge carrier in conducting polymers for a wide range of applications.

Laminated aluminum thin-films as low-cost opaque moisture ultra-barriers for flexible organic electronic devices

Flexible aluminum moisture barrier films are shown to meet ultra-barrier requirements with water vapor transmission rates as low as 1*10−4 g/m2/day at 38 °C and 90% R.H. These low transmission rates are achieved by lamination of two films that are independently processed on individual substrates. The integration in organic electronic devices like organic solar cells or OLEDs is particularly easy in a single lamination step, since an existing aluminum electrode can be utilized as one half of the moisture barrier. The resulting laminate maintains flexibility of the device and improves the barrier quality by an order of magnitude compared to conventional stacking of two barriers. Organic solar cells with this type of encapsulation are manufactured and aged in controlled climate. They do not exhibit significant extrinsic degradation.

A systematic approach for the accurate and rapid measurement of water vapor transmission through ultra-high barrier films.

Flexible organic electronic devices are often protected from degradation by encapsulation in multilayered films with very high barrier properties against moisture and oxygen. However, metrology must be improved to detect such low quantities of permeants. We therefore developed a modified ultra-low permeation measurement device based on a constant-flow carrier-gas system to measure both the transient and stationary water vapor permeation through high-performance barrier films. The accumulation of permeated water vapor before its transport to the detector allows the measurement of very low water vapor transmission rates (WVTRs) down to 2 × 10-5 g m-2 d-1. The measurement cells are stored in a temperature-controlled chamber, allowing WVTR measurements within the temperature range 23-80 °C. Differences in relative humidity can be controlled within the range 15%-90%. The WVTR values determined using the novel measurement device agree with those measured using a commercially available carrier-gas device from MOCON®. Depending on the structure and quality of the barrier film, it may take a long time for the WVTR to reach a steady-state value. However, by using a combination of the time-dependent measurement and the finite element method, we were able to estimate the steady-state WVTR accurately with significantly shorter measurement times.