Electrofluidic Display Research Articles

Design and synthesis of a terrylene diimide-based stable cyan dye for printable electrofluidic display

We synthesized a terrylene diimide (TDI)-based cyan dye TDIC, which exhibited excellent device performance with an extraordinarily high molar absorption coefficient up to 65 000 L moL−1 cm−1, high optical stability, and fast response in the EFD.

Toward Suppressing Charge Trapping Based on a Combined Driving Waveform with an AC Reset Signal for Electro-Fluidic Displays.

Digital microfluidic technology based on the principle of electrowetting is developing rapidly. As an extension of this technology, electro-fluidic displays (EFDs) have gradually become a novel type of display devices, whose grayscales can be displayed by controlling oil film in pixels with a microelectromechanical system (MEMS). Nevertheless, charge trapping can occur during EFDs' driving process, which will produce the leakage current and seriously affect the performance of EFDs. Thus, an efficient driving waveform was proposed to resolve these defects in EFDs. It consisted of a driving stage and a stabilizing stage. Firstly, the response time of oil film was shortened by applying an overdriving voltage in the driving stage according to the principle of the electrowetting. Then, a direct current (DC) voltage was designed to display a target luminance by analyzing leakage current-voltage curves and a dielectric loss factor. Finally, an alternating current (AC) reset signal was applied in the stabilizing stage to suppress the charge trapping effect. The experiment results indicated that compared with a driving waveform with a reset signal and a combined driving waveform, the average luminance was improved by 3.4% and 9.7%, and the response time was reduced by 29.63% and 51.54%, respectively.

Low voltage electrowetting of non-aqueous fluorescent quantum dot nanofluids

We present electrowetting studies of diverse non-aqueous CdSe quantum dot (QD) nanofluids with desirable optical properties for electrowetting and electrofluidic display applications. The QD-nanofluids investigated herein include QD-2.9 (CdSe/ZnS: average size 2.9 nm), QD-18 (CdSe/CdS: 2.9 nm core with thick gradient shell for average size 18 nm), and QD-T (CdSe/ZnS tetrapods: 4.7 nm and 3.5 nm average arm length and width respectively). Low voltage and reversible droplet wettability was achieved while maintaining the fluorescent properties of QD nanofluids in both dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Maximum contact angle (CA) change with minimal voltage application (Δθ10V = 78 ± 3) was achieved for QD-T and QD-2.9 DMF-based nanofluids which is desired for practical applications. Conversely, there was comparable wettability of QD-18 nanofluids in DMF and DMSO, stemming from QD size effects. The robust electrowetting of non-aqueous QD nanofluids has important implications for droplet-driven display and digital microfluidic applications.

Multi-Chromophore Dyes for Improving Light Stability of Electro-Fluidic Displays

Electro-fluidic display (EFD) is a new reflective display based on electrowetting phenomenon which is applied to the outdoor billboard. Organic dyes are the most important materials for the color gamut and reliability of the EFD devices, which are always synthesized based on mono-chromophore dye in previous work. In this paper, we report our research on azopyrazolone as the chromophore to construct a polychromic macromolecular dye system. The light stability and photo-electric properties of these new dyes are researched detailed. It is found that the light stability of multi-chromophore dyes and backflow properties are much better than corresponding mono-chromophore dye.

The Display Week 2021 Symposium Preview

The Display Week 2021 Symposium Preview

Electrofluidic displays based on inkjet printing and phase change filling

The patterned fluoropolymer film hydrophobicity and colored oil layer uniformity were critical for electrofluidic displays. Typically, reactive ion etching (RIE) is commonly used to pattern fluoropolymer films due to the chemically inert nature of fluorinated materials. However, the RIE process permanently changed the surface properties of the fluoropolymer film. These affect the function of the electrofluidic displays. The development of inkjet printing technology provides an innovative manufacturing pathway for functional materials, such as fluoropolymers. In this work, for the first time, an amorphous fluoropolymer (AF 1600) was patterned by a piezoelectric inkjet printer as a hydrophobic layer for an electrofluidic display. The pattered fluoropolymer films can be inkjet printed by varying the concentration of the fluoropolymer solution and adjusting printing parameters. Phase change filing was explored in combination with inkjet printing of colored oil, enabling the manufacture of electrofluidic cells in the air for the first time. The fluoropolymer and colored oil properties were systematically investigated to fulfill the inkjet printing requirements. The cost of manufacturing can be significantly reduced for large scale production by employing inkjet printing in combination with phase change filling. Furthermore, electrical and optical characterization of the printed electrofluidic cell shows better visual performance, dielectric stability, and fast electrical response and than the traditionally produced devices.

Reliability Studies on Multilayer Thin‐Film Materials and Seal Architecture for Flexible Electrofluidic Displays under Bending Stress

Electrofluidic Displays (EFD), as a promising reflective display technology, has unique advantage in flexibility because the mediums (oil&water) driven inside the pixel are completely soft maters. In this study, the reliability of the materials applied in the flexible EFD (FEFD) under bending were systematically evaluated. The mechanical neutral layer was designed to reduce the effect of stress. The UV acrylate, was chosen instead of traditional epoxy resin for sealing FEFD devices. Moreover, an open channel seal architecture was proposed to reduce the pressure building inside the cell during device bending.

Synthesis and a Photo-Stability Study of Organic Dyes for Electro-Fluidic Display.

Electro-fluidic display (EFD) is one of the most promising reflective displays because of its full color and video speed. Colored EFD oil, which normally consists of soluble organic dyes and non-polar solvent, plays a critical role in color, electro-optical behavior, and the reliability of the EFD devices. In this paper, we report our research on two kinds of electro-fluidic dyes based on anthraquinone and azo pyrazolone, including their synthesis, structure characterization, and application properties. Changes of absorbance curves, color coordinates of oils, and photoelectric responses of devices were studied in detail under accelerated irradiation to investigate the photo-stability and reliability properties of synthesized oil materials and devices. Photoelectric responses and photo stability of dyes are highly varied depending on their structures. We found that 1,4-dlialkylamino anthraqinone and mono azo pyrazolone dyes are much more stable than 1,8-dlialkylamino anthraqinone and corresponding bisazo pyrazolone dyes.

The Colored Oil Property Effect on Switching Behavior of Electro-Fluidic Display

The switchable oil layer driven by electrowetting gives visible color and light valve control, which is the basis of Electro-Fluidic Displays. The colored oil’s property is a key factor that influences the Electro-Fluidic Displays switching behavior. A purple oil was formulated by the oil-soluble purple dye in decane in this study. The dye molecule itself is nonpolar and it doesn’t dissolve in water. The concentration of colored oil influenced the oil/water interfacial tension and oil viscosity. The relationship of EFD switching behavior with oil/water interfacial tension, oil viscosity, and oil conductivity has been systematically investigated. The oil/water interfacial tension decreased with increasing oil concentration, in the meanwhile, the conductivity increased. Oil conductivity was one of the key factors that influenced the Electro-Fluidic Displays optical property. We found for the first time that at the lower oil concentration (2% - 10%), the interfacial tension plays a main role effect on the rupture voltage and response time, but as the conductivity of higher concentration of colored oil increased (at 20%), the rupture voltage-controlled both by conductivity and interfacial tension.

Novel perylene-based organic dyes for electro-fluidic displays

Electro-fluidic displays (EFDs) are one of the most promising reflective displays due to their full color and video speed.

Failure modes analysis of electrofluidic display under thermal ageing.

Dielectric failure as well as optical switching failure in electrofluidic display (EFD) are still a bottleneck for sufficient device lifetime. In this study, a dielectric redundancy-designed multilayer insulator of ParyleneC/AF1600X was applied in an EFD device. The reliability performance was systematically studied by tracking the applied voltage-dependent leakage current and capacitance changes (I–V and C–V curves) with thermal ageing time. The multilayer insulator shows a more stable performance in leakage current compared to a single-layer insulator. The failure modes during operation underlying the single-layer and the multilayer dielectric appear to be different as exemplified by microscopic images. The single-layer AFX shows significant detachment. In addition, by quantitatively analysing the C–V curves with ageing time, we find that for the single AFX device, the dominant failure mode is ‘no-opening’ of the pixels. For the multilayer device, the dominant failure mode is ‘no-closing’ of the pixels. This study provides tools for distinguishing the basic failure modes of an EFD device and demonstrates a quantitative method for evaluating the reliability performance of the device under thermal ageing.

Synthesis and application of an alkylated pyrazole‐based azo dye for electrofluidic display

AbstractElectrofluidic display (EFD) is one of the most promising reflective displays for its full color and video speed. Colored EFD oil, normally formulated by soluble organic dyes in non‐polar solvent, dominates the color, electro‐optical behavior, and reliability performances for EFD devices. In this paper, a novel yellow electrofluidic dye with excellent solubility in non‐polar EFD solvent was achieved based on the introduction of long alkyl chain into pyrazole azo dye. The resulting EFD device fabricated by the dye showed excellent application properties, such as fast switching speed (17.8 ms), high aperture ratio (68.5%), low threshold voltage (24v), good light stability (240 h under accelerated conditions), and low backflow phenomenon.

Oil Motion Control by an Extra Pinning Structure in Electro-Fluidic Display.

Oil motion control is the key for the optical performance of electro-fluidic displays (EFD). In this paper, we introduced an extra pinning structure (EPS) into the EFD pixel to control the oil motion inside for the first time. The pinning structure canbe fabricated together with the pixel wall by a one-step lithography process. The effect of the relative location of the EPS in pixels on the oil motion was studied by a series of optoelectronic measurements. EPS showed good control of oil rupture position. The properly located EPS effectively guided the oil contraction direction, significantly accelerated switching on process, and suppressed oil overflow, without declining in aperture ratio. An asymmetrically designed EPS off the diagonal is recommended. This study provides a novel and facile way for oil motion control within an EFD pixel in both direction and timescale.

11.4: The Fabrication of Flexible Electrofluidic Display Arrays Based on PEN Substrate

Flexible electrofluidic display (FLEFD), as one of the most promising reflective display technology, has drawn an increasing attention in recent years. In this study, we report the fabrication process for the FLEFD based on the flexible PEN film with ITO layer on the surface. The hyflon material was selected as the hydrophobic insulator layer to control the fabrication temperature below 170°C, and the photoresist was used to manufacture the pixel grids. After oil filing and packaging process, the electrical properties and optical properties of FLEFD were both taken into consideration. The results showed that FLEFD presented good display characteristics. The driving voltage of flexible device was 14V when the bending radius of curvature was 10cm. Furthermore, response time for the device was 18ms, and the aperture opening ratio was 42.5%.

Dynamic simulation of bistable electrofluidic device based on a combined design of electrode and wettability patterning

AbstractElectrowetting has shown its power in manipulating microfluidics. Of particular interest is a bistable electrowetting system, which enables versatile low power consumption applications. A combined design of electrode and wettability patterning, which includes a unique three‐electrode structure and wettability barrier, was proposed and introduced into as a new electrofluidic display (EFD) architecture. This architecture should be manufacturable in a fairly straightforward way within LCD cell‐making technology. For this architecture, an electrohydrodynamic two phase flow simulation was carried out using the COMSOL Multiphysics simulation software. The results show that this new EFD architecture could achieve a bistable display property. Compared with conventional EFD, the electro‐optical performance (the aspect ratio and response time) of this design has also been greatly improved.

Simplified dynamical model for optical response of electrofluidic displays

In this work we analyze the switching behavior of an electro-fluidic pixel in separate stages. For ‘on’ switching we consider the motion leading to oil film rupture (initiation stage), fast oil-dewetting and a slower droplet rearrangement stage. For ‘off’ switching we consider fast oil wetting and surface reforming to the flat (dark) state. A dynamic model derived from an overall energy balance analysis has been employed to describe the optical response inside an electrofluidic display (EFD) pixel for the oil dewetting and wetting stages. By comparison with the experimental electro–optic response data, the accuracy and shortcomings of this model can be illuminated. The optical response asymmetry between on and off-switching and optical response delay during the on-switching process are well described and explained. In addition, the liquid film reforming dynamics and electrohydrodynamic instability analysis are used to estimate the oil film rupture and film reforming times very well. This study provides a straightforward approach to describe the complicated electrofluidic switching dynamics inside an EFD pixel, which may guide the further optimization of EFD device design and driving schemes.

Forming Spacers in Situ by Photolithography to Mechanically Stabilize Electrofluidic-Based Switchable Optical Elements.

Electro-Fluidic Displays (EFD) have been demonstrated to be an attractive technology for incorporation into portable display devices. EFDs have excellent optical efficiency and fast switching enabling video content. Ensuring mechanical stability of EFD display cells is a key challenge and essential for developing large area as well as flexible displays. Although the electro-optic performance of an EFD, unlike a liquid crystal display (LCD), is insensitive to cell-gap, extreme changes in cell-gap can result in irreversible collapse of the cell. Here we use photolithography to develop spacers to prevent cell-gap collapse and provide the required mechanical stability for EFD devices. The spacer is formed directly on the cover plates (ITO/glass) after cell assembly with UV light induced phase separation polymerization in the illuminated area. Phase separation behavior between polar aqueous solution and polymer is closely related to the solubility of acrylate monomers. In this work, polyethylene glycol diacrylate (PEGDA) as cross-linker, 2-hydroxyethyl acrylate (HEA) and acrylic acid or acrylamide as co-monomers are investigated for fabricating the spacers. PEGDA was added to the mixtures in order to increase the mechanical strength of the spacer. The spacers showed excellent performance for cell-gap control in EFD devices.

Screen printing insulator coatings for electrofluidic display devices

The screen printing of amorphous fluoropolymer films as insulators for electrofluidic display device use is demonstrated. Screen printing enables the hydrophobic dielectric to be directly patterned at device level with the required film thickness as well as excellent material utilisation. The films give very good electrical performance, with high uniformity and dielectric strength, and the resulting devices exhibit comparable electro-optic behaviour to those fabricated with spin-coated films.

Novel Driving Methods for Manipulating Oil Motion in Electrofluidic Display Pixels

By applying driving voltage schemes with different rising gradient and the same final voltage in an electrofluidic display device (EFD), several kinds of oil patterns have been generated. The results show that, in the initial stage of oil film splitting and contraction, driving voltage has a crucial influence on the microfluidic behavior and oil distribution in each pixel. Oil patterns significantly affect the pixel aperture and, ultimately, the reflectivity of the display panel. The oil pattern with fewer oil droplets has a higher aperture ratio and higher reflectivity. The observation that oil patterns can be controlled by varying driving schemes offers a new way of manipulating oil motion in EFD pixels.

A Reflective Display Technology Based on Electrofluidics

In this paper we describe the design, driving and electro-optical behavior of an electrofluidic, or electrowetting, display panel. Electrofluidic displays involve the movement of a colored oil water interface as a result of polarizing the interface between water and a hydrophobic surface under the influence of external electric field. The proposed direct-segment drive approach successfully drives the display panel based on electrofluidic. The electro-optical behavior of electrofluidic display samples are measured and analyzed. The tonand toffof the pixel is 16 ms and 8 ms, respectively, which demonstrates that electrofluidic display panels have the capability to show video content.