Active-matrix Backplane Research Articles

Strain-Sensor-In-Pixel Technology for Resolution-Sustainable Stretchable Displays.

One of the limitations of stretchable displays is the severe degradation of resolution or the decrease in the number of pixels per unit area when stretched. Hence, we suggest a strain-sensor-in-pixel (S-SIP) system through the adoption of hidden pixels that are activated only during the stretch mode for maintaining the density of on-state pixels. For the S-SIP system, the gate and source electrodes of InGaZnO thin-film transistors (TFTs) in an existing pixel are connected to a resistive strain sensor through the facile and selective deposition of silver nanowires (AgNWs) via electrohydrodynamic-jet-printing. With this approach, the strain sensor integrated TFT functions as a strain-triggered switch, which responds only to stretching along the designated axes by finely tuning the orientation and cycles of AgNW printing. The strain sensor-integrated TFT remains in an off-state when unstretched and switches to an on-state when stretched, exhibiting a large negative gauge factor of -1.1 × 1010 and a superior mechanical stability enduring 6000 cycles, which enables the efficient structure to operate hidden pixels without requiring additional signal processing. Furthermore, the stable operation of the S-SIP in a 5 × 5-pixel array is demonstrated via circuit simulation, implying the outstanding applicability and process compatibility to the conventional active-matrix display backplanes.

Monolithic Integration of Perovskite Photoabsorbers with IGZO Thin‐Film Transistor Backplane for Phototransistor‐Based Image Sensor

AbstractMonolithic integration of perovskite materials and their optoelectronic devices with well‐developed thin‐film transistor (TFT) backplane is leading to new applications in displays and image sensors. Herein, a scalable polyimide assisted patterning approach for monolithic integration of perovskite based high‐sensitive phototransistor array on indium gallium zinc oxide (IGZO) active matrix backplane is introduced. Polyimide vias are first formed by conventional photolithography process, through which uniform perovskite films of arbitrary patterns with feature size less than 20 µm are fabricated by spin‐on‐patterning method. Using this technique, patterns of quasi 2D perovskite photoabsorbing layer are precisely deposited onto the channel area of the photosensing IGZO TFT, forming high‐performance phototransistors with responsivity and detectivity reaching 835.7 A W−1 and 5.4 × 1012 Jones, respectively. An image sensor with 8 × 8 pixels array containing both photosensing perovskite/IGZO transistors and switching IGZO transistors is demonstrated, in which the switching IGZO transistor elements on the backplane are protected by the non‐patterned region of the polyimide encapsulation layer. This whole fabrication process is compatible with TFT manufacturing process and will significantly reduce the cost needed for constructing next generation high‐resolution image sensors.

Effect of X-ray irradiation on a-IGZO and LTPS thin-film transistors for radiography applications

The effects of X-ray irradiation on amorphous indium-gallium-zinc oxide (a-IGZO) and low-temperature polysilicon (LTPS) thin-film transistors (TFTs) were investigated. A comparison of the electrical characteristics of the TFTs at different stages of exposure to X-rays was conducted. After X-ray irradiation, the a-IGZO TFTs maintained their performance, while the LTPS TFTs showed considerable performance degradation. A distinct negative threshold voltage shift occurred in the LTPS TFTs under X-ray irradiation. In addition, the electrical parameters including mobility, subthreshold swing and off-current were significantly deteriorated. In addition, the binding energy shift of Si 2p peaks in the phase transition region of the interlayer between Si and SiO2 was confirmed by XPS depth profile analysis. This result indicates that permanent damage to the LTPS crystalline structure was induced under X-ray exposure. Under the same doses of X-ray irradiation, the a-IGZO film did not exhibit any notable change, indicating the tolerance of a-IGZO to X-ray irradiation. Therefore, we expect a-IGZO to be applied in the active matrix backplane of medical X-ray detectors for use for prolonged periods of time. This work not only provides systemic studies on the effects of X-rays on semiconductor materials but also proposes an alternative material to highly endurable X-ray detectors.

Technology progress on quantum dot light-emitting diodes for next-generation displays.

Quantum dot light-emitting diodes (QD-LEDs) are widely recognised as great alternatives to organic light-emitting diodes (OLEDs) due to their enhanced performances. This focus article surveys the current progress on the state-of-the-art QD-LED technology including material synthesis, device optimization and innovative fabrication processes. A discussion on the material synthesis of core nanocrystals, shell layers and surface-binding ligands is presented for high photoluminescence quantum yield (PLQY) quantum dots (QDs) using heavy-metal free materials. The operational principles of several types of QD-LED device architectures are also covered, and the recent evolution of device engineering technologies is investigated. By exploring the fabrication process for pixel-patterning of QD-LEDs on an active-matrix backplane for full-colour display applications, we anticipate further improvement in device performance for the commercialisation of next-generation displays.

Amorphous oxide thin-film transistors and inverters enabled by solution-processed multi-layers as active channels

This paper reports an indium zinc oxide (IZO) thin film transistor (TFT) with a multi-stacked structure that showed improved performance compared to the TFT with a single active-layer structure. Devices with a multi-stacked active-layer structure were fabricated using an IZO solution process. The determination results showed that the electrical operation and environmental stability of multi-stacked IZO TFTs were improved significantly compared to those of single active-layer IZO TFTs. The roughness of the device surface was measured by atomic force microscopy. The root mean square calculation showed that the multi-stacked active-layer IZO TFT had a smoother surface. The multi-stack structured IZO TFT showed an exceptional electron mobility of 7.75 ± 0.2 cm2/V s, an on–off current ratio of 4.67 × 105 ± 0.55 × 105, and a subthreshold swing of 1.11 V/decade. In addition, a weak threshold voltage (0.35 ± 0.42 V) that is more conducive to device driving was obtained. These results highlight the great potential of multi-stack structured IZO TFTs for applications in active-matrix backplane displays.

A portable driving system for high‐resolution active matrix electrowetting display based on FPGA

AbstractHow to display pictures and even videos on electrowetting displays (EWDs) still needs improvement. Therefore, we seek to develop a robust, portable and scalable system for the realization of high‐resolution EWDs. In this paper, a driving system for an 8 inch active matrix electrowetting display (AM‐EWD) based on a Field‐Programmable‐Gate‐Array (FPGA) is proposed, where the key components are an active matrix backplane, an FPGA driving waveform and driver integrated circuits (ICs). We successfully demonstrate an AM‐EWD with 1024×768 resolution and 16‐level gray‐scale realized by unique dynamic and asymmetric sub‐frame of FPGA. The whole system is just powered by a 3.7V 1100mAH lithium battery. Such a system has not been reported before, also well‐suited for transferring to a higher performance portable EWD in the future.

Test System for Thin Film Transistor Parameter Extraction in Active Matrix Backplanes

Thin film transistor (TFT) active matrix backplanes are used in large area electronic systems, such as displays and image sensors. With backplanes being fabricated on wearable and flexible substrates, the possibilities of operational faults in backplanes have increased. These faults could either be hard faults, such as line opens or shorts or could be softer faults, such as time dependent variations in the TFT transfer characteristics. Real time diagnosis of these faults require built-in-self-test systems. While many such systems have been demonstrated to diagnose hard faults, an easily realizable system to identify soft faults, such as variations in transistor transconductance remain an open challenge. In this paper, we discuss a system that extracts the transconductance by charging and then discharging the pixel capacitor at various gate voltages for an active matrix liquid crystal display backplane. This permits a plot of the time averaged current versus the gate voltage from which the spatial variation of transconductance can be extracted. The details of the design are discussed and a proof of concept with a $3\times 4$ amorphous silicon backplane is demonstrated.

(Invited) New p-Type Oxide Semiconductor with High Hole Mobility over 7 cm2/Vs Formed By Solution Process

We report new p-type metal oxide semiconductor thin-film transistor with high field-effect hole mobility fabricated in solution process in this work. Metal oxide semiconductors have long attracted large attention as large-area electronic devices by virtue of high electrical and optical performances such as transparency, large electron mobility, good uniformity and low-temperature processability since the pioneering work of Prof. Hosono group in 2004, and their mass production has now been successfully realized in the active matrix backplane of flat-panel or flexible display. Few p-type metal oxide semiconductors with high performance, however, have been reported compared with n-type metal oxide semiconductors such as InGaZnO, InZnSnO, etc. This is mostly ascribed as the localization of valence band maximum which mainly consists of oxygen p orbitals. To enhance the dispersion of valence band, it has been theoretically proposed that the filled diffuse metal d or s orbital to hybridize with oxygen 2p orbital can be utilized and indeed it was experimentally demonstrated that Sn 5s orbital and Cu 3d orbital played such a role in SnO and CuO, respectively. But, SnO phase is very vulnerable to disproportion reaction to metallic Sn and SnO2 which behaves as an n-type semiconductor so that the process window to realize it in electron devices was very narrow and the electrical performance in the devices such as thin-film transistor was very limited, e.g low on/off ratio. Inspired by the theoretical work of high throughput materials screening performed by the first principles calculation, we fabricated the alkali metal doped SnO thin-film with solution process and low-temperature annealing less than 400 oC. We found that the oxidation state of Sn in thin-films changed from +4 state (SnO2 phase) to +2 state (SnO phase) depending on the content of alkali metal ions from x-ray photoelectron spectroscopy (XPS). Furthermore, Hall-effect measurement indicated the type conversion of the majority carrier from n-type to p-type as the content of alkali metal ion increases, in consistence with XPS results. The high hall mobility of 51 cm2/Vs was obtained at the optimal alkali-metal ion doped SnO thin film and we successfully fabricated thin-film transistors with alkali-metal ion doped SnO with a field-effect hole mobility over 7 cm2/Vs. We hope that this work will contribute to open a new era for transparent large area electronics based on oxide semiconductors with high functionalities such as complementary logic circuit.

Visible and Near‐Infrared Imaging with Nonfullerene‐Based Photodetectors

AbstractThe solution‐processed organic photodetectors underpin an emerging technology with inherent implications in the biological sensors and imaging displays. Conventional organic photodiodes, the core element of an organic photodetector, rely mainly on fullerene‐based acceptors, which in combination with high and middle bandgap donors, limit the spectral photosensitivity to the visible range. Even in the case of low bandgap polymers the oscillator strength and thus the extinction coefficient are usually limited in the NIR due to the nature of molecular orbital hybridization. Instead, it is showed that pairing prototypical poly(3‐hexylthiophene) (P3HT) with rhodanine‐benzothiadiazole‐coupled indacenodithiophene (IDTBR), a nonfullerene electron acceptor absorbing beyond 750 nm, as the photoactive material of a simple photodiode results in a highly efficient organic photodetector with a record responsivity of 0.42 A W−1 and external quantum efficiency (EQE) of 69% in the NIR (755 nm). Nonfullerene‐based photodiodes are processed on amorphous silicon active matrix backplanes to realize large area flat panel photodetector imagers able to detect objects under visible and NIR light conditions with an exceptional combination of responsivity, dynamic response and image crosstalk.

SuPR-NaP Technique for Printing Ultrafine Silver Electrodes and its Use for Low-Voltage Operation of Organic Thin-Film Transistors

ABSTRACTRecently, an epoch-making printing technology called “SuPR-NaP (Surface Photo-Reactive Nanometal Printing)” that allows easy, high-speed, and large-area manufacturing of ultrafine silver wiring patterns has been developed. Here we demonstrate low-voltage operation of organic thin-film transistors (OTFTs) composed of printed source/drain electrodes that are produced by the SuPR-NaP technique. We utilize an ultrathin layer of perfluoropolymer, Cytop, that functions not only as a base layer for producing patterned reactive surface in the SuPR-NaP technique but also as an ultrathin gate dielectric layer of OTFTs. By the use of 22 nm-thick Cytop gate dielectric layer, we successfully operate polycrystalline pentacene OTFTs below 2 V with negligible hysteresis. We also observe the improvement of carrier injection by the surface modification of printed silver electrodes. We discuss that the SuPR-NaP technique allows the production of high-capacitance gate dielectric layers as well as high-resolution printed silver electrodes, which provides promising bases for producing practical active-matrix OTFT backplanes.

Fast optical inspection of operations of large-area active-matrix backplane by gate modulation imaging

Abstract We report herein a fast optical inspection technique for the operation of large-area active-matrix (AM) backplanes by means of gate modulation (GM) imaging measurements. The technique is based on highly sensitive difference-image sensing between alternately biased gate-on and gate-off states for optical microscope images with many AM pixels. The use of a high-frame-rate complementary-metal-oxide-semiconductor image sensor and a high-speed image-subtraction module facilitates the bulk inspection of 30,000 pixels within 3 min, as is demonstrated using an all-printed and flexible AM backplane composed of one-transistor–one-capacitor cells arranged at a 150 pixel-per-inch resolution. We also demonstrate that the technique is very useful and effective not only in quickly identifying both defective transistors and capacitors by examining a zoomed-out image, but also in identifying the origin of defects within channel layers or capacitors by investigating an expanded image.

Integrated Active-Matrix Capacitive Sensor Using a-IGZO TFTs for AMOLED

We report a capacitive touch sensor using a-IGZO TFTs as active components to improve touch sensitivity and output robustness. The output potential difference between touched/untouched states is 1.7 V without readout buffer. Additionally, due to the fully compatible gate driving clocks with conventional active-matrix backplane, the integration of the capacitive touch sensor with AMOLED display without additional driver ICs can be realized. An $8 \times 8$ array with only one output pin using pixel-by-pixel readout has been developed, which benefits the full flexible applications with simple connection and readout method.

63‐3: Capacitive Touch Sensor using a‐IGZO TFTs for Flexible AMOLED

We propose a capacitive touch sensor using a‐IGZO TFTs on plastic. With only two transistors and one capacitor, the proposed circuit achieves high aperture ratio while maintaining robust output voltage variation over 1.6V. Furthermore, the required driving clocks are fully compatible with active matrix backplane, which enables an easy integration with flexible AMOLED display.

Continuous Color Reflective Display Fabricated in Integrated MEMS-and-TFT-on-Glass Process

The single mirror interferometric MEMS display is a full-color, daylight-visible, video-rate reflective display with high brightness that consumes power only during content updates. It consists of an array of identical pixels each of which can produce a continuous range of colors through precise analog positioning of one moving element, a mirror. Electrostatic actuation is driven with an integrated active-matrix backplane of indium-gallium-zinc oxide thin-film transistors. Displays of 384 × 384 pixels built on a surface micromachining process on glass substrates have been demonstrated on both 6-in wafers and Gen 4.5 panels (730 mm × 920 mm) showing good yield and uniformity.

Fully Screen-Printed, Large-Area, and Flexible Active-Matrix Electrochromic Displays Using Carbon Nanotube Thin-Film Transistors.

Semiconducting single-wall carbon nanotubes are ideal semiconductors for printed electronics due to their advantageous electrical and mechanical properties, intrinsic printability in solution, and desirable stability in air. However, fully printed, large-area, high-performance, and flexible carbon nanotube active-matrix backplanes are still difficult to realize for future displays and sensing applications. Here, we report fully screen-printed active-matrix electrochromic displays employing carbon nanotube thin-film transistors. Our fully printed backplane shows high electrical performance with mobility of 3.92 ± 1.08 cm2 V-1 s-1, on-off current ratio Ion/Ioff ∼ 104, and good uniformity. The printed backplane was then monolithically integrated with an array of printed electrochromic pixels, resulting in an entirely screen-printed active-matrix electrochromic display (AMECD) with good switching characteristics, facile manufacturing, and long-term stability. Overall, our fully screen-printed AMECD is promising for the mass production of large-area and low-cost flexible displays for applications such as disposable tags, medical electronics, and smart home appliances.

Active Matrix Magneto-Optical Spatial Light Modulator Driven by Spin-Transfer-Switching

We have developed a magneto-optical spatial light modulator (SLM) driven by spin-transfer-switching (STS), called spin-SLM, using a field-effect-transistor array active-matrix backplane, which plays an important role for developing large scale spin-SLM display devices. The fabricated spin-SLM device has 10 × 5 array pixel layout using magnetic tunneling junctions with sizes of 0.5 μm × 0.5 μm that are driven by a mosfet backplane. We have successfully demonstrated its electrical operation with current densities as low as 0.6 MA/cm2, one of the lowest levels ever achieved with STS devices, and confirmed changes in optical contrast by magnetization switching of the light modulation elements. We have demonstrated the potential for large-scale active-matrix spin-SLM technology as a potential enabler in the realization of electro-holographic displays.

Numerical Simulation and Analysis of the Switching Performance for Printable Low-Voltage Organic Thin-Film Transistors in Active-Matrix Backplanes

To achieve low-voltage organic thin-film transistors (OTFTs) with printing or coating processes over large area, a thick gate dielectric layer is preferred. Taking this into consideration, a common solution for low-voltage OTFTs is using large-permittivity (high- $k$ ) dielectric material to enlarge the gate dielectric capacitance. Our recent work shows that reducing the subgap density of states (DOS) at the channel can also help to achieve low-voltage OTFTs. This approach does not need large gate dielectric capacitance, and thus a low- $k$ and thick gate dielectric layer can be used. In this work, device/circuit mixed-mode simulations are carried out to compare the performance of the two type low-voltage OTFTs as switches in active-matrix backplanes. It is shown that the low-voltage OTFT with reduced subgap DOS can achieve faster charging and discharging. The less parasitic and storage capacitance with the low- $k$ dielectric layer would also reduce the required dynamic power.

52-2:Invited Paper: OLED Microdisplays - Enabling Advanced Near-to-Eye Displays, Sensors, and Beyond

OLED-on-Silicon technology has become vital to emissive microdisplays in near-to-eye displays, e.g., upcoming smart glasses. High-resolution OLED micro-patterning, embedded sensors and emission spectra outside the visible enable advanced features. Low-power active-matrix circuitry CMOS backplane architecture broadens the application range. Recent developments will be reported here.

Paper No P12: Transparent Conductive Electrode Based on Hydrogen Doped Zinc Oxide for OLED Application

AbstractA transparent and conductive electrode based on hydrogen (H) doped ZnO has been proposed for active‐matrix (AM) display applications. With optimal H plasma power and treat time, the resultant ZnO:H exhibits a low sheet resistance of 200 ohm/sq at a thickness of 150 nm. The demonstrated transparent and conductive ZnO:H can serve as electrode for LCD or OLED as well as S/D electrode for ZnO TFT. With proposed ZnO:H electrode, two photomask steps can be reduced in the fabrication of ZnO TFT‐based AM backplanes.

A Solution-Processed Organic Thin-Film Transistor Backplane for Flexible Multiphoton Emission Organic Light-Emitting Diode Displays

An active matrix backplane based on solution-processed organic thin-film transistors (OTFTs) has been developed for flexible displays having multiphoton organic light-emitting diodes (OLEDs). The OTFT device has a bottom-gate/bottom-contact type structure consisting of a polyethylene naphthalate-based flexible substrate covered with patterned gate electrodes and a gate dielectric of UV-cured cardo-polymer. Teflon AF was used for a hydrophobic bank structure that defines active regions of the OTFTs, and the organic semiconductor was coated by solution shearing. The OTFT backplane fabricated here had $384 \times 128$ pixels at a resolution of 300 dpi; images and video were successfully displayed on the resulting flexible multiphoton-emission OLED display.