Amorphous silicon TFT-based active-matrix organic polymer LEDs

We report active-matrix organic polymer light-emitting displays (LEDs) based on a three hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) pixel electrode circuit that supplies a continuous output current to organic polymer light-emitting devices. The output current level drift induced by either process variations or device aging can be reduced in this design by adjusting the driver TFT operating point with the active resistor. Our first green light-emitting engineering prototype had a brightness of 120 cd/m/sup 2/ and fill factor of about 45%.     

A novel current-scaling a-Si:H TFTs pixel electrode circuit for AM-OLEDs

Hydrogenated amorphous silicon thin-film transistor (a-Si:H TFT) pixel electrode circuit with a function of current scaling is proposed for active-matrix organic light-emitting displays (AM-OLEDs). In contrast to the conventional current mirror pixel electrode circuit, in this circuit a high data-to-organic light-emitting device (OLED) current ratio can be achieved, without increasing the a-Si:H TFT size, by using a cascade structure of storage capacitors. Moreover, the proposed circuit can compensate for the variations of TFT threshold voltage. Simulation results, based on a-Si:H TFT and OLED experimental data, showed that a data-to-OLED current ratio larger than 10 and a fast pixel programming time can be accomplished with the proposed circuit.     

Stability analysis of current programmed a-Si:H AMOLED pixel circuits

In this paper, we present self-compensating current mirror-based pixel circuits, and analyze basic stability issues to provide a deeper understanding of circuit operation, and the impact of thin film transistor bias nonidealities, which can lead to the long-term (and gradual) instabilities in pixel drive current. The analysis also provides the circuit designer a means to tailor the pixel drive current stability to the long-term brightness degradation characteristics of the organic light-emitting diode.     

有源OLED像素电路的设计与仿真

设计了有源OLED显示用非晶硅薄膜晶体管恒流型4-TFT像素驱动电路，并给出了驱动方法。应用HSPICE仿真了恒流型像素驱动电路的工作过程，详细分析了源（Source）电压VDD、存储电容Cs，以及开关晶体管T1、驱动晶体管T3的宽长比等参数对电路的输出特性的影响。仿真结果表明，此电路可以在整个帧周期持续供给OLED器件电流，并且解决了由于各像素驱动管阈值电压的差异带来的OLED亮度的不均匀问题。     

Amorphous Silicon Display Backplanes on Plastic Substrates

Amorphous silicon (a-Si) thin-film transistor (TFT) backplanes are very promising for active-matrix organic light-emitting diode displays (AMOLEDs) on plastic. The technology benefits from a large manufacturing base, simple fabrication process, and low production cost. The concern lies in the instability of the TFTs threshold voltage (V_T) and its low device mobility. Although V_T-instability can be compensated by means of advanced multi-transistor pixel circuits, the lifetime of the display is still dependent on the TFT process quality and bias conditions. A-Si TFTs with field-effect mobility of 1.1 cm²/Vmiddots and pixel driver circuits have been fabricated on plastic substrates at 150 degC. The circuits are characterized in terms of current drive capability and long-term stability of operation. The results demonstrate sufficient and stable current delivery and the ability of the backplane on plastic to meet AMOLED requirements     

High performance active image sensor pixel design with circular structure oxide TFT

We report a high-performance active image sensor pixel design by utilizing amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) with a circular structure. The TFT, configured with the inner electrode as source and outer electrode as drain, typically exhibits good saturation electrical characteristics, where the device has a constant drive current despite variations in drain voltage. Due to the very high output resistance exhibited by this asymmetric TFT structure with a circular shape, the pixel circuit considered here in common-drain configuration provides a higher gain of operation than a pixel circuit implemented with rectangular a-IGZO TFTs. They can be used as driving TFTs in active image sensor circuits. They are, therefore, good candidates for digital X-ray detectors in applications such as medical diagnostic procedures.     

Current-source a-Si:H thin-film transistor circuit for active-matrix organic light-emitting displays

In this letter, we describe a four thin-film-transistor (TFT) circuit based on hydrogenated amorphous silicon (a-Si:H) technology. This circuit can provide a constant output current level and can be automatically adjusted for TFT threshold voltage variations. The experimental results indicated that, for TFT threshold voltage shift as large as /spl sim/3 V, the output current variations can be less than 1 and 5% for high (/spl ges/0.5 /spl mu/A) and low (/spl les/0.1 /spl mu/A) current levels, respectively. This circuit can potentially be used for the active-matrix organic light-emitting displays (AM-OLEDs).     

Driving schemes for a-Si and LTPS AMOLED displays

Design of stable active matrix organic light-emitting diode (AMOLED) displays comes with significant challenges that stem from the electrical property of the backplane materials, line parasitics in the matrix, and the opto-electronic property of the organic light-emitting diode (OLED). This paper reviews voltage and current programming schemes for AMOLEDs. Following a systematic review of pixel circuits, design considerations are examined for both current and voltage schemes with focus on stability and programming speed for both amorphous silicon (a-Si) and low temperature polysilicon (LTPS) pixel circuits. In particular, spatial parameter variations and stability, which hinder reliable operation of AMOLED display backplanes, are discussed. Analysis shows that while driving schemes reported hitherto maybe suitable for small and medium size displays, new schemes are critically needed for large-area high-resolution AMOLED displays.     

Design and fabrication of AlGaInP LED array with integrated GaAsdecode circuits

A two-dimensional (2-D) AlGaInP light-emitting diode (LED) array with monolithic integration of one-to-four GaAs MESFET decode circuits has been developed as an image source for portable virtual displays. The epitaxial layers of AlGaInP LEDs with light emission at a wavelength of 605 nm were grown on a semi-insulating GaAs substrate by organometallic vapor phase epitaxy. LED arrays consisting of 240 columns and 144 rows for a total of 34560 pixels were then fabricated on such epitaxial wafers. One-to-four GaAs MESFET decode circuits consisting of eight MESFET's for each decode circuit and a total of 768 MESFET's for a 34 K decode array were fabricated on the semi-insulating GaAs substrate with removal of LED epitaxial layers around the periphery of the LED array. LED arrays with the integrated decode circuits provide a great reduction in I/O terminals. The I/O count of the demonstrated 34 K decode LED array is 104, which is much less than 384 for a comparable array without the integrated decode circuits. The pixel pitch of the LED array is 20 μm and each LED pixel has 10×10 μm² emitting area. The output power of LED pixel is 50 nW at an operation current of 50 μA. The address voltages used to activate the column decode circuits are 3 V for high and -3 V for low, while the address voltages used to activate the row decode circuits are 0 V for high and -3 V for low. The operating voltage of the decode LED array ranges from 3 to 5 V, and the total power dissipation of the decode LED array is less than 16 mW     

Improvement of pixel electrode circuit for active-matrix OLED by application of reversed-biased voltage

In this paper, an improved ac pixel electrode circuit for active-matrix organic light-emitting display (AMOLED) has been proposed by adding a thin-film transistor. This circuit can provide an ac driving mode for AMOLED and makes the OLED in a reversed-biased voltage during the reverse cycle. And a circuit design for understanding ac driving mode was presented. The circuit simulation results indicate that this circuit is feasible. The circuit structure is practical for the AMOLED pixel driver; it can improve the performance of OLED.     

Active-Matrix Amorphous-Silicon TFTs Arrays at 180$^circhboxC$on Clear Plastic and Glass Substrates for Organic Light-Emitting Displays

An amorphous-silicon thin-film transistor (TFT) process with a 180$^circhboxC$maximum temperature using plasma-enhanced chemical vapor deposition has been developed on both novel clear polymer and glass substrates. The gate leakage current, threshold voltage, mobility, and on/off ratio of the TFTs are comparable with those of standard TFTs on glass with deposition temperature of 300$^circhboxC$–350$^circhboxC$. Active-matrix pixel circuits for organic light-emitting displays (LEDs) on both glass and clear plastic substrates were fabricated with these TFTs. Leakage current in the switching TFT is low enough to allow data storage for video graphics array timings. The pixels provide suitable drive current for bright displays at a modest drive voltage. Test active matrices with integrated polymer LEDs on glass showed good pixel uniformity, behaved electrically as expected for the TFT characteristics, and were as bright as 1500$hboxcd/hboxm^2$.     

Device and circuit level optimization for high performance a-Si:H TFT-based AMOLED displays

Active matrix organic light-emitting diode (AMOLED) displays with amorphous hydrogenated silicon (a-Si:H) thin-film transistor (TFT) backplanes are becoming the state of art in display technology. Though a-Si:H TFTs suffer from an intrinsic device instability, which inturn leads to an instability in pixel brightness, there have been many pixel driving methods that have been introduced to counter this. However, there are issues with these circuits which limit their applicability in terms of speed and resolution. This paper highlights these issues and provides detailed design considerations for the choice of pixel driver circuits in general. In particular, we discuss the circuit and device level optimization of the pixel driver circuit in a-Si:H TFT AMOLED, displays for high gray scale accuracy, subject to constraints of power consumption, and temporal and spatial resolution.     

A new a-Si:H TFT pixel circuit compensating the threshold voltage shift of a-Si:H TFT and OLED for active matrix OLED

We propose a new hydrogenated amorphous silicon thin-film transistor (a-Si:H TFT) pixel circuit for an active matrix organic light-emitting diode (AMOLED) employing a voltage programming. The proposed a-Si:H TFT pixel circuit, which consists of five switching TFTs, one driving TFT, and one capacitor, successfully minimizes a decrease of OLED current caused by threshold voltage degradation of a-Si:H TFT and OLED. Our experimental results, based on the bias-temperature stress, exhibit that the output current for OLED is decreased by 7% in the proposed pixel, while it is decreased by 28% in the conventional 2-TFT pixel.     

A New Pixel Circuit Compensating for Brightness Variation in Large Size and High Resolution AMOLED Displays

A new pixel design and driving method for active-matrix organic light-emitting diode (AMOLED) display using low-temperature polycrystalline silicon thin-film transistor (LTPS-TFT) is proposed. The new circuit consists of five TFTs and one capacitor to eliminate the variation in the threshold voltage of the TFTs, and the drop in the supply voltage in a single frame operation. The proposed pixel circuit has been verified to realize uniform output current by the simulation work using HSPICE software. The simulated error rate of the output current is also discussed in this paper. The novel pixel design has great potential for use in large size and high resolution AMOLED displays.     

Investigation on the current nonuniformity in current-mode TFT active-matrix display pixel circuitry

Detailed analysis of the mechanisms that cause deviations of the emission current from the input data current in polycrystalline silicon (poly-Si) thin-film transistor (TFT) current-copier active matrix carbon nanotube field emission display (FED) pixel circuits is presented. These effects make the modulated emission current sensitive to the process variations of the circuit elements. Monte Carlo circuit analysis with a Gaussian statistical distribution of all related process parameters in the pixel circuit shows the emission current nonuniformity, and therefore illustrates the importance of improving the poly-Si TFT process for the design of high-resolution and high-brightness current-mode active matrix addressed displays. The analysis is also suitable for design and optimization of other reported current-mode active matrix addressed FED or organic light-emitting displays.     

Three-terminal light-emitting device with adjustable emission color

A three-terminal organic light-emitting device with a periodic interrupted middle electrode is developed to allow for an adjustable emission color. The emission results from three independent light-emitting diodes with one diode utilizing exciplex emission. An equivalent electrical circuit is suggested taking the current–voltage characteristics and the direction of current flow through the organic structure into account. Two diodes are formed between the embedded middle electrode and the LiF/Al top and ITO bottom electrode, respectively, and the third diode utilizes that part of the device without the middle-electrode exhibiting exciplex emission. It will be shown that the spectrum of the emitted light can be tuned from blue to orange by controlling the applied potentials to the device terminals.     

En Route to Wide Area Emitting Organic Light-Emitting Transistors for Intrinsic Drive-Integrated Display Applications: A Comprehensive Review

Organic light-emitting transistors (OLET) evolved from the fusion of the switching functionality of field-effect transistors (FET) with the light-emitting characteristics of organic light-emitting diode (OLED) that can simplify the active-matrix pixel device architecture and hence offer a promising pathway for future flat panel and flexible display technology. This review systematically analyzes the key device/molecular engineering tactics that assist in improving the electrode edge narrow emission to wide-area emission for display applications via three different topics, that is, narrow to wide-area emission, vertical architecture, and impact of high-κ dielectric on the device performance. Source–drain electrode engineering such as symmetric/asymmetric, planar/non-planar arrangement, semitransparent nature, multilayer approach comprising charge transport, and work function modification layers enable widening the emission zone. Vertical OLET architecture offers short channel lengths with a high aperture ratio, pixel type area emission, and stable light-emitting area. Transistors utilizing high-κ dielectric materials have assisted in lowering the operating voltage, enhancing luminance and air stability. The promising development in achieving wide-area emission provides a solid basis for constructing OLET research toward display applications; however, it relies on developing highly luminescent and fast charge transporting materials, suitable semitransparent source/drain electrodes, high-κ -dielectrics, and device architectural engineering.     

Amorphous silicon thin film transistor circuit integration for organic LED displays on glass and plastic

This paper presents design considerations along with measurement results pertinent to hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) drive circuits for active matrix organic light emitting diode (AMOLED) displays. We describe both pixel architectures and TFT circuit topologies that are amenable for vertically integrated, high aperture ratio pixels. Here, the OLED layer is integrated directly above the TFT circuit layer, to provide an active pixel area that is at least 90% of the total pixel area with an aperture ratio that remains virtually independent of scaling. Both voltage-programmed and current-programmed drive circuits are considered. The latter provides compensation for shifts in device characteristics due to metastable shifts in the threshold voltage of the TFT. Various drive circuits on glass and plastic were fabricated and tested. Integration of on-panel gate drivers is also discussed where we present the architecture of an a-Si:H based gate de-multiplexer that is threshold voltage shift invariant. In addition, a programmable current mirror with good linearity and stability is presented. Programmable current sources are an essential requirement in the design of source driver output stages.     

Efficiency enhancement of organic light-emitting devices by using honeycomb metallic electrodes and two-dimensional photonic crystal arrays

In this paper, a facile fabrication technique called nanosphere lithography combining with two-step reactive ion etching method for patterning honeycomb metallic electrode with high transparency and excellent uniformity is demonstrated. The patterning silver electrode with 15-nm film thickness and 68.6% fill-factor was used as the organic light-emitting diode (OLED) anode, which showed an average transmittance of 77.4% and sheet resistance of 30.7 Ω/□. The current efficiency is 8.35 cd/A for the OLED with patterned silver anode under 100 cd/m² operation brightness, which was 47% higher than the device with indium tin oxide (ITO) anode. After applying the polystyrene nanosphere to form a photonic crystal array onto the device, the extracted light from organic mode can be further coupled out from device substrate mode. The overall luminous enhancement of the device with the combination of internal honeycomb metallic anode and external photonic crystal array is 115% higher than the traditional ITO-based OLED.     

Enhancement of Brightness Uniformity by a New Voltage-Modulated Pixel Design for AMOLED Displays

This letter presents a new pixel design and driving method for active-matrix organic light-emitting diode (AMOLED) displays using low-temperature polycrystalline silicon thin-film transistors (TFTs). The proposed pixel circuit consists of five TFTs and one capacitor to eliminate the variation in the threshold voltage of the TFTs, and the drop in the supply voltage in a single frame operation by the source-follower-type connection and the bootstrap. The proposed pixel circuit has been verified to realize uniform output current by the simulation work using the HSPICE software. The novel pixel design has great potential for use in large-size and high-resolution AMOLED displays.