Novel a‐Si:H TFT pixel circuit for electrically stable top‐anode light‐emitting AMOLEDs

Abstract— A novel pixel circuit for electrically stable AMOLEDs with an a‐Si:H TFT backplane and top‐anode organic light‐emitting diode is reported. The proposed pixel circuit is composed of five a‐Si:H TFTs, and it does not require any complicated drive ICs. The OLED current compensation for drive TFT threshold voltage variation has been verified using SPICE simulations.     

Letter: A new compensation pixel circuit with metal oxide thin‐film transistors for active‐matrix organic light‐emitting diode displays

This paper presents a novel compensation pixel circuit for active‐matrix organic light‐emitting diode displays, in which the coupling effect mask technology is developed to compensate the threshold voltage of driving thin‐film transistor whether it is positive or negative. Twenty discrete compensation pixel circuits have been fabricated by In‐Zn‐O thin‐film transistors process. It is measured that the non‐uniformity of the proposed pixel circuit is significantly reduced with an average value of 8.6%. Furthermore, the organic light‐emitting diode emission current remains constant during 6 h continuous operation, which also confirms the validity of the proposed pixel circuit.     

A new a‐Si:H TFT pixel circuit employing data‐reflected negative‐bias annealing for a stable and uniform AMOLED

Abstract— A new a‐Si:H pixel circuit to reduce the VTH degradation of driving a‐Si:H thin‐film transistors (TFTs) by data‐reflected negative‐bias annealing (DRNBA) is presented. The new pixel circuit compensates VTH variation induced by non‐uniform degradation of each a‐Si:H pixel due to various electrical stress. The proposed pixel circuit was verified by SPICE simulations. Although the VTH of the driving a‐Si:H TFT varies from 2.5 to 3.0 and 3.5 V, the organic light‐emitting diode (OLED) current changes by only 1.5 and 2.8% in the emission period, respectively. During the negative‐bias annealing period, the negative VGS is applied to the driving TFT by using its own data signal. It is expected that the VTH shift of the driving TFT can be effectively reduced and the VTH shift can be compensated for in our new pixel circuit, which can contribute to a stable and uniform image from an a‐Si:H TFT active‐matrix OLED.     

Active‐matrix organic light‐emitting diode using inverse‐staggered poly‐Si TFTs with a center‐offset gated structure

Abstract— A low‐cost active‐matrix backplane using non‐laser polycrystalline silicon (poly‐Si) having inverse‐staggered TFTs with amorphous‐silicon (a‐Si) n⁺ contacts has been developed. The thin‐film transistors (TFTs) have a center‐offset gated structure to reduce the leakage current without scarifying the ON‐currents. The leakage current of the center‐offset TFTs at Vg = ?10 V is two orders of magnitude lower than those of the non‐offset TFTs. The center‐offset length of the TFTs was 3 μm for both the switching and driving TFTs. A 2.2‐in. QQVGA (1 60 × 1 20) active‐matrix organic light‐emitting‐diode (AMOLED) display was demonstrated using conventional 2T + 1C pixel circuits.     

Novel analog feedback current programming architecture compatible with 2‐transistor 1‐capacitor pixel for active matrix organic light‐emitting diode displays

A new feedback current programming architecture is described, which is compatible with active matrix organic light‐emitting diode (AMOLED) displays having the 2T1C pixel structure. The new pixel programming approach is compatible with all TFT technologies and can compensate for non‐uniformities in both threshold voltage and carrier mobility of the pixel OLED drive TFT. Based on circuit simulations, a pixel drive current of less than 10 nA can be programmed in less than 50 µ. This new approach can be implemented within an AMOLED external or integrated display data driver.     

An active‐matrix organic light‐emitting‐diode pixel circuit for the compensation of I × R voltage drop in power line

Abstract— A new voltage‐driving active‐matrix organic light‐emitting diode (AMOLED) pixel circuit is proposed to improve the display image‐quality of AMOLED displays. Because OLEDs are current‐driven devices, the I × R voltage drop in the power lines is evitable. Accordingly, the I × R voltage‐drop compensation scheme should be included in the pixel‐driving method when a voltage‐compensation method is used. The proposed pixel was designed for the compensation of an I × R voltage drop in the power lines as well as for the compensation of the threshold‐voltage non‐uniformity of low‐temperature polycrystalline‐silicon thin‐film transistors (LTPS TFTs). In order to verify the compensation ability of the proposed pixel, SPICE simulation was performed and compared with those of other conventional pixels. When the Vss voltage varies from 0 to 1 V, the drain current of the proposed pixel decreased by under 1% while that of conventional Vth compensation methods without Vss compensation decreased by over 60%. 2.2‐in. QCIF⁺ full‐color AMOLED displays, which employ the proposed pixel, have been also developed. It was verified by comparison of the display image quality with a conventional panel that our proposed panel successfully overcame the voltage‐drop problems in the power lines.     

Pixel structures using the negative feedback method for high‐resolution active matrix organic light‐emitting diode displays

Novel two pixel structures are proposed for high‐resolution active matrix organic light‐emitting diode displays. The proposed two pixels (pixel structures A and B) use the negative feedback method for high‐resolution displays that requires to have small‐sized storage capacitance. The proposed pixel structures A and B improve the luminance uniformity by reducing the voltage distortion in the storage capacitor. However, the proposed pixel structure A is vulnerable to the organic light‐emitting diode (OLED) degradation because the anode voltage of the OLED affects the emission current. In order to compensate the OLED degradation, the proposed pixel structure B stores the turn‐on voltage of OLEDs in the storage capacitor. The simulation results show that the emission current error of the proposed pixel structure B is improved by four times in comparison with the proposed pixel structure A when the OLED turn‐on voltage increases by 0.1 V. Also, the emission current error of the proposed pixel structure B when the threshold voltage of driving thin‐film transistors varies from ?2.2 to ?1.8 V is from ?0.69 least significant bit (LSB) to 0.13 LSB, which shows the excellent luminance uniformity. The proposed pixels are designed for 5.5‐in. full high‐definition displays.     

Clamped‐inverter circuit architecture for luminescent‐period‐control driving of active‐matrix OLED displays

Abstract— An innovative pixel‐driving technology for high‐performance active‐matrix OLED flat‐panel displays is described. Called “clamped‐inverter circuit architecture,” it uses luminescent‐period‐control driving to reduce the inter‐pixel non‐uniformity caused by the device‐to‐device variability of low‐temperature poly‐Si TFTs. A prototype full‐color display shows a luminous deviation of less than 1.6%, which corresponds to only the LSB‐error in 6‐bit gray‐scale.     

Letter: Dual‐mode oxide thin film transistor shift register for external compensation in active‐matrix organic light emitting diode displays

This letter describes a dual‐mode oxide thin film transistor shift register for active‐matrix organic light emitting diode displays, which provides two separate scanning and sensing pulses on one line. At a 240 Hz full‐high definition (HD) timing, estimated power consumptions are 2.44 and 2.81 mW for scanning and sensing shift registers of 16 stages at V_TH = ?1.56 V, respectively.     

Flexible high‐resolution full‐color top‐emitting active‐matrix organic light‐emitting diode display

We developed a high‐performance 3.4‐in. flexible active‐matrix organic light‐emitting diode (AMOLED) display with remarkably high resolution using an oxide semiconductor in a backplane, by applying our transfer technology that utilizes metal separation layers. Using this panel, we also fabricated a prototype of a side‐roll display for mobile uses. In these AMOLED displays, a white OLED combined with a color filter was used in order to achieve remarkably high resolution. For the white OLED, a tandem structure in which a phosphorescent emission unit and a fluorescent emission unit are serially connected with an intermediate layer sandwiched between the emission units was employed. Furthermore, revolutionary technologies that enable a reduction in power consumption in both the phosphorescent and fluorescent emission units were introduced to the white tandem OLED.     

A novel TFT‐OLED integration for OLED‐independent pixel programming in amorphous‐Si AMOLED pixels

Abstract— The direct voltage programming of active‐matrix organic light‐emitting‐diode (AMOLED) pixels with n‐channel amorphous‐Si (a‐Si) TFTs requires a contact between the driving TFT and the OLED cathode. Current processing constraints only permit connecting the driving TFT to the OLED anode. Here, a new “inverted” integration technique which makes the direct programming possible by connecting the driver n‐channel a‐Si TFT to the OLED cathode is demonstrated. As a result, the pixel drive current increases by an order of magnitude for the same data voltages and the pixel data voltage for turn‐on drops by several volts. In addition, the pixel drive current becomes independent of the OLED characteristics so that OLED aging does not affect the pixel current. Furthermore, the new integration technique is modified to allow substrate rotation during OLED evaporation to improve the pixel yield and uniformity. The new integration technique is important for realizing active‐matrix OLED displays with a‐Si technology and conventional bottom‐anode OLEDs.     

An LTPS active‐matrix process with PECVD doped N+ drain/source areas

Abstract— A low‐temperature polysilicon active‐matrix process without the need for ion implantation to dope drain and source areas of TFTs has been developed. A doped silicon layer is deposited by PECVD and structured prior to the deposition of the intrinsic silicon for the channel. The dopant is diffused and activated during the excimer‐laser crystallization step. N‐channel test TFTs with different geometries were realized. The TFT properties (mobility, on/off ratio, saturation, etc.) are suitable to realize AMLCDs and AMOLED displays and to integrate driver electronics on the displays. In addition to simple TFTs, a full‐color 4‐in. quarter‐VGA AMLCD was realized. The complete display (including photolithographic masks, active‐matrix backplane, and color‐filter/black‐matrix frontplane), and an addressing system were developed and manufactured at the Chair of Display Technology, University of Stuttgart, Germany. The substitution of ion doping by PECVD deposition overcomes a major limitation for panel sizes in poly‐Si technology and avoids large investment costs for ion‐implantation equipment.     

Displacement‐current analysis of organic light‐emitting diodes

Abstract— Organic light‐emitting diodes (OLEDs) having multiple organic layers were fabricated to analyze the physical phenomena occurring in an OLED according to the amplitude of the applied voltage. The staircase voltage with both an increasing period and a constant period was designed and applied to an OLED. The displacement current began to change at a voltage where the conduction current began to change, and partly originated from the formation of space charge due to the low mobility of the majority carrier. The displacement current was shown to be constant at low voltage and decreased after showing a maximum value as the applied voltage increased. The exact voltage for the injection of two types of carriers and light emission could be obtained from the variation in the displacement current.     

Ultra‐low‐power LTPS TFT‐LCD technology using a multi‐bit pixel memory circuit

Abstract— Two types of low‐temperature poly‐Si TFT LCDs, which integrate a multi‐bit memory circuit and a liquid‐crystal driver within a pixel, have been developed using two different TFT process technologies. Both a 1.3‐in. 116‐ppi LCD having a 2‐bit pixel memory and a 1.5‐in. 130‐ppi LCD having a 5‐bit pixel memory consume very little power, less than 100 μW, which indicates that this technology is promising for mobile displays.     

A new current‐mirror pixel circuit employing poly‐Si TFTs for active‐matrix organic light‐emitting‐diode displays

Abstract— A novel active‐matrix organic light‐emitting‐diode (AMOLED) display employing a new current‐mirror pixel circuit, which requires four‐poly‐Si TFTs and one‐capacitor and no additional signal lines, has been proposed and sucessfully fabricated. The experimental results show that a new current mirror can considerably compensate luminance non‐uniformity and scale down a data current more than a conventional current‐mirror circuit in order to reduce the pixel charging time and increase the minimum data current. Compared with a conventional two‐TFT pixel, the luminance non‐uniformity induced by the grain boundaries of poly‐Si TFTs can be decreased considerably from 41% to 9.1%.     

High resolution photolithography for direct view active matrix organic light‐emitting diode augmented reality displays

High‐resolution RGB organic light‐emitting diode frontplane is a key enabler for direct‐view transparent augmented reality displays. In this paper, we demonstrate 1250 ppi passive displays and semi‐transparent active displays. Organic light‐emitting diode photolithography can provide pixel density above 1000 ppi while keeping effective emission area high because of high aperture ratio. Patterns with 2 μm line pitch were successfully transferred to emission layers, indicating possible further pixel density scaling. Lifetime after patterning, key parameter enabling industrialization, is above 150 h (T90 at 1000 nit).     

A new threshold‐voltage compensation technique of poly‐Si TFTs for AMOLED display pixel circuits

Abstract— A new threshold‐voltage compensation technique for polycrystal line‐silicon thin‐film transistors (poly‐Si TFTs) used in active‐matrix organic light‐emitting‐diode (AMOLED) display pixel circuits is presented. The new technique was applied to a conventional 2‐transistor—1‐capacitor (2T1C) pixel circuit, and a new voltage‐programmed pixel circuit (VPPC) is proposed. Theoretically, the proposed pixel is the fastest pixel with threshold‐voltage compensation reported in the literature because of the new compression technique implemented with a static circuit block, which does not affect the response time of the conventional 2T1C pixel circuit. Furthermore, the new pixel exhibits all the other advantages of the 2T1C pixel, such as the simplicity of the peripheral drivers and improves other characteristics, such as its behavior in the temperature variations. The verification of the proposed pixel is made through simulations with HSpice. In order to obtain realistic simulations, device parameters were extracted from fabricated low‐temperature poly‐Si (LTPS) TFTs.     

Novel voltage programming n-channel TFT pixel circuit for low power and high performance AMOLED displays

This paper proposes a novel pixel circuit for high resolution, high frame rate, and low power AMOLED displays that is implemented with one driving n-channel TFT, six switching n-channel poly-Si TFTs, and a storage capacitor. The proposed pixel circuit adopts the voltage programming scheme for threshold voltage compensation. Because the whole line time is in use only for charging the data voltage, this pixel circuit is applicable to high resolution and frame rate displays. In addition, it compensates voltage variation of OLEDs and voltage drop of supply lines at lower power consumption. On the average, the non-uniformity of a proposed circuit is reduced to 2.5%, compared to 7.1% of the previous one at a 240 Hz full-HD display. On the other hand, the compensation voltage error, which is caused by feed-through and charge injection noises from falling control signals of switching TFTs, is much less in the proposed scheme than in the previous 5T2C structure. The average error of the proposed circuit is reduced to 0.18 V, compared to 0.75 V of the previous one. The initialization power consumption of the 7T1C circuit is reduced to 98 mW, compared to 530 mW of the 5T2C circuit and the average dynamic power saving ratio of data drivers is estimated in the 7T1C pixel as 98.7% over the 5T2C one for 24 test images.     

Active‐matrix organic light‐emitting displays employing two thin‐film‐transistor a‐Si:H pixels on flexible stainless‐steel foil

Abstract— An active‐matrix organic light‐emitting diode (AMOLED) display driven by hydrogenated amorphous‐silicon thin‐film transistors (a‐Si:H TFTs) on flexible, stainless‐steel foil was demonstrated. The 2‐TFT voltage‐programmed pixel circuits were fabricated using a standard a‐Si:H process at maximum temperature of 280°C in a bottom‐gate staggered source‐drain geometry. The 70‐ppi monochrome display consists of (48 × 4) × 48 subpixels of 92 ×369 μm each, with an aperture ratio of 48%. The a‐Si:H TFT pixel circuits drive top‐emitting green electrophosphorescent OLEDs to a peak luminance of 2000 cd/m².     

A LTPS-TFT pixel circuit for active matrix organic light emitting diode based on improved current mirror

In this paper, a voltage-driving and current compensation method for active matrix organic light emitting diode (AMOLED) displays is proposed. An improved current mirror is introduced into the pixel circuit to overcome the channel length modulation effect of TFTs. The SPICE simulation results show that the proposed pixel circuit not only effectively compensates for non-idealities related with deviations of μ and V_T in TFTs, the OLED degradation, but also offers a less setting time and guarantees a good liner relationship between V_DATA and I_OLED.