Improving the color switch contrast in PEDOT:PSS-based electrochromic displays

Poly(3,4-ethylenedioxythiophene) chemically doped with poly(styrene sulfonic acid) (PEDOT:PSS) is a material system commonly used as a conductive and transparent coating in several important electronic applications. The material is also electrochemically active and exhibits electrochromic (EC) properties making it suitable as the active element in EC display applications. In this work uniformly coated PEDOT:PSS layers were used both as the pixel electrode and as the counter electrode in EC display components. The pixel and counter electrodes were separated by a whitish opaque and water-based polyelectrolyte and the thicknesses of the two EC layers were varied independently in order to optimize the color contrast of the display element. A color contrast (ΔE^∗, CIE L^∗a^∗b^∗ color space) exceeding 40 was obtained with maintained relatively short switching time at an operational voltage less than 2 V.     

印制电子技术

概述了印制电子的关键技术、印制技术、安装技术和全世界的开发动向以及未来的印制电子。     

Stable fully-printed polymer resistive read-only memory and its operation in mobile readout system

The read-only memory (ROM) is a key component for a wide range of printed electronics applications. The resistive type ROM based on conductive polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT: PSS) would be a promising technology of choice, which can be “manufactured-on-demand” via digital printing for high throughput and material saving. However, the instability issues associated with the conventional PEDOT: PSS and its interface with contact electrodes would be a critical hurdle preventing the technology from practical applications. This work proves that, by removing the hydrophilic acidic groups in conventional PEDOT: PSS, these instability issues can be well addressed. The ROM tags fabricated based on the modified PEDOT: PSS of neutral pH and inkjet printed silver electrodes present extremely stable performance under both aging and electrical stress tests in air ambient. A self-designed memory readout circuit board, communicating with mobile phone through near field communication, is also implemented to demonstrate the feasibility of using the ROM tags in real mobile systems. It is shown that, without any encapsulation, the ROMs can have stable output under high humidity condition (>60% RH), after either being stored in the ambient condition for 30 days or being operated after 20000 reading cycles.     

Speed considerations for electrochromic displays

The elect rochromic effect based on the reversible electro-deposition of a viologen radical cation has been applied to information display systems. Problems of multiplexing can be overcome by the use of an auxiliary switch, such as a thin-film transistor. The question of speed is the subject of this paper, which discusses the factors influencing the contrast ratio of an electrochromic display. The contrast ratio is determined by electrochemical and optical considerations, the former governing the amount of material deposited and the latter the visual effect of the deposit. Electrochemically, the deposition may be influenced by a variety of mechanisms, including diffusion, migration, electrode kinetics and deposit resistance. Multiple pulse driving gives increased speed in diffusion-limited cases. Optically, the perceived contrast is maximised by increasing the absorbance of the deposited material and by optimising the diffuse reflectance of the display electrode.     

Solution modification of PEDOT:PSS inks for ultrasonic spray coating

PEDOT:PSS is a high-conductivity hole-transporting polymer that is widely used in polymer and perovskite photovoltaic devices, as well as in a host of other antistatic applications. Here we show that modification of PEDOT:PSS inks using ternary solvents and by the addition of small amounts of a high molecular weight polymer make it possible to deposit highly uniform thin films via ultrasonic spray coating. Such films can be deposited using a single pass in the wet phase without the use of surfactants; a process that greatly simplifies their deposition. Using this technique we create films having thickness and roughness comparable to that of spin coated films, whilst properties such as the conductivity and stability can be improved.     

Effect of electrode surface on the electrochromic properties of electropolymerized poly(3,4-ethylenedioxythiophene) thin films

Electrochromism or electrochromic contrast in electropolymerized thin films of dioxythiophenes based conjugated polymers is known to be sensitive to the structure of monomer and the polymerizing conditions. However in our studies we found that it is also sensitive to the electrode surface wherein a significantly high electrochromic contrast is observed in the electropolymerized thin films of poly(3,4-ethylenedioxythiophene), PEDOT deposited on platinum (71%) as compared to that on indium tin oxide, ITO, coated glass surface (54%). This is attributed to the formation of more conjugated polymer on the metallic surfaces as confirmed by narrow and red shifted absorption peak for PEDOT on platinum compared to broad and blue shifted peak on ITO in the UV–vis absorption spectra. This difference in the electrochromic properties of electropolymerized PEDOT thin films on the two surfaces is investigated by studying their electrochemical growth using UV–vis absorption, Raman spectroscopy and atomic force microscopy techniques. These results suggest the deposition of more conjugated polymer in the initial stages of growth (≤3 mC/cm²) on both the substrates whereas it continues the same way into the intermediate stages (up to ∼15 mC/cm²) only on platinum, thereby, resulting in higher electrochromic contrast on platinum. The coloration efficiency of PEDOT thin film was also found to be improved on platinum (465 cm²/C) compared to that on ITO (230 cm²/C). Moreover, we observed that the EC contrast of electropolymerized PEDOT thin films on platinum was found to be insensitive to polymerizing solvent that is generally not the case when polymerized on ITO. The cyclic stability of PEDOT-Pt films are better compared to that of PEDOT-ITO which is attributed to the improved reversibility of these films with respect to potential switching.     

Low-cost and flexible printed graphene–PEDOT:PSS gas sensor for ammonia detection

This work presents a simple, low-cost and practical inkjet-printing technique for fabricating an innovative flexible gas sensor made of graphene–poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite film with high uniformity over a large area. An electronic ink prepared by graphene dispersion in PEDOT:PSS conducting polymer solution is inkjet-printed on a transparency substrate with prefabricated electrodes and investigated for ammonia (NH₃) detection at room temperature. Transmission electron microscopy, Fourier transform infrared spectroscopy, UV–visible spectrometer and Raman characterizations confirm the presence of few-layer graphene in PEDOT:PSS polymer matrix and the present of π–π interactions between graphene and PEDOT:PSS. The ink-jet printed graphene–PEDOT:PSS gas sensor exhibits high response and high selectivity to NH₃ in a low concentration range of 25–1000 ppm at room temperature. The attained gas-sensing performance may be attributed to the increased specific surface area by graphene and enhanced interactions between the sensing film and NH₃ molecules via π electrons network. The NH₃-sensing mechanisms of the flexible printed gas sensor based on chemisorbed oxygen interactions, direct charge transfers and swelling process are highlighted.     

Synergistic effects of buffer layer processing additives for enhanced hole carrier selectivity in inverted Organic Photovoltaics

Solution based inverted Organic Photovoltaic (OPVs) usually use Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) derivatives combined with pristine processing additives as hole selective contact on top of the hydrophobic conjugated polymer:fullerene active layer. In this study, PEDOT:PSS based hole selective contact is treated with two different boiling point additives, 2,5,8,11-tetramethyl-6-dodecyn-5,8-diol ethoxylate (Dynol) and Zonyl FS-300 fluorosurfactant (Zonyl). Although corresponding inverted OPVs using the above PEDOT:PSS:Additives show similar power conversion efficiency (PCE) values, the mechanisms of their implementation on inverted OPV operation are not identical. By understanding the synergistic effects of PEDOT:PSS processing additives on the hole selectivity of inverted OPVs we demonstrate a novel combination of PEDOT:PSS additives mixture as an effective route to further increase the hole selectivity, reliability andpower conversion efficiency of inverted OPVs.     

Exploring the ultrasonic nozzle spray-coating technique for the fabrication of solution-processed organic electronics

The ultrasonic nozzle (US) spray method was investigated for its utility in fabricating organic electrodes composed of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), a standard conductive polymer material used to produce large-area low-cost OFETs. The US spray technique involves generating a solution spray by first passing the solution through a head and nozzle subjected to ultrasonic vibrations that induce atomization. This method is advantageous in that the resulting spray comprises extremely small solution droplets a few micrometers in diameter, unlike the spray produced using conventional air spray methods. The PEDOT:PSS US solution spraying process was optimized by controlling the flow rate of the N₂ carrier gas and the substrate temperature while monitoring the quality of the resulting PEDOT:PSS electrode films. The pentacene field-effect transistors prepared using the US spray method displayed a maximum field-effect mobility of 0.47 cm²V⁻¹s⁻¹ (with an average value of 0.31 cm²V⁻¹s⁻¹), 35% better than the mobilities achieved using the conventional air spray method. In addition, the device-to-device reproducibility was improved, as indicated by a decrease in the standard deviation of the mobility values from 30% for the air spray devices to 24% for the US spray devices. These results indicated that the US spray technique is efficient and superior to the conventional air spray method for the development of low-cost large-area organic electronics.