A printed OTFT-backplane for AMOLED display

An AMOLED panel driven by an OTFT-backplane is an attractive display because OTFTs and OLEDs use organic materials with unique characteristics such as low temperature and solution processing ability, and thus are able to implement the key features of future displays. In this study we applied some printing technologies to fabricate an OTFT-backplane for AMOLEDs. Screen printing combined with photolithography with Ag ink was used for the gate electrodes and scan bus lines and contact pads. Ag metal lines with a width of 20 μm and thickness of 60 nm and resistivity of 3.0 × 10^?5 Ω cm were achieved. Inkjet printing was applied to deposit TIPS-pentacene as an organic semiconductor. The OTFT-backplane using the Ag gate electrodes and TIPS-pentacene exhibited uniform performance over 17,500 pixels on a 7 in. panel. The mobility was 0.31 ± 0.05 cm²/V s with a deviation of 17%. The AMOLED panel successfully demonstrated its ability to display patterns.     

External Compensation of Nonuniform Electrical Characteristics of Thin-Film Transistors and Degradation of OLED Devices in AMOLED Displays

The variation of electrical characteristics of polycrystalline-silicon thin-film transistor (TFT) and degradation of organic light-emitting-diode (OLED) device cause nonuniform intensity of luminance and image sticking in active-matrix OLED (AMOLED) displays. An external compensation method that senses and compensates variations of threshold voltage and mobility of TFTs and degradation of OLED device is proposed. The effect of the external compensation method on AMOLED pixel is experimentally verified by measuring the luminance of OLEDs and the electrical characteristics of TFTs in AMOLED pixels.      

Electrical Compensation of OLED Luminance Degradation

This letter presents a stable compensation scheme for active-matrix organic light-emitting-diode (AMOLED) displays based on the observed strong interdependence between the luminance degradation of organic light-emitting diodes (OLEDs) and its current drop under bias stress. This feedback-based compensation provides 30% improvement in luminance stability under 1600 h of accelerative stress. To employ this scheme in AMOLED displays, a new pixel circuit is presented that provides on-pixel electrical access to the OLED current without compromising the aperture ratio.     

Power Consumption and Temperature Increase in Large Area Active-Matrix OLED Displays

We model and analyze the power consumption and resulting temperature rise in active-matrix organic-light-emitting device (AMOLED) displays as a function of the OLED efficiency, display resolution and display size. Power consumption is a critical issue for mobile display applications as it directly impacts battery requirements, and it is also very important for large area applications where it affects the display temperature rise, which directly impacts the panel lifetime. Phosphorescent OLEDs (PHOLEDs) are shown to offer significant advantage as compared to conventional fluorescent OLEDs due to high luminous efficiency resulting in lower pixel currents, reducing both the power consumed in the OLED devices and the series connected driving thin-film transistor (TFT). The power consumption and temperature rise of OLED displays are calculated as a function of the device efficiency, display size, display luminance and the type of backplane technology employed. The impact of using top-emission OLEDs is also discussed.     

Fabrication of organic thin film transistors on Polyethylene Terephthalate (PET) fabric substrates

In this paper organic thin film transistors (OTFTs) are directly fabricated on fabric substrates consisting of Polyethylene Terephthalate (PET) fibers. A key process is coating the polymer layers on the fabric in order to reduce the large surface roughness of the fabric substrate. Two polymers, i.e. polyurethane (PU) and photo-acryl (PA), are used to reduce the large surface roughness and simultaneously improve the process compatibility of the layers with the subsequent OTFTs processes while also retaining the original flexibility of the fabric. The surface roughness of the PU/PA-coated fabric is significantly reduced to 0.3 μm. Furthermore, the original flexibility of the PET fabric remained after coating of the PU/PA polymer layers. The mobility of the OTFTs fabricated on the PU-PA coated fabric substrate is 0.05 ± 0.02 cm²/V s when three PA layers and 90 nm thick pentacene layer were used. The performance does not vary even after 30,000 bending test.     

Fabricating large-area white OLED lighting panels via dip-coating

Large-area lighting panels based on white organic light-emitting diodes (OLEDs) with emission area of 72 × 72 mm² have been fabricated in air through dip-coating process. By studying the effects of the solution concentration, the panel withdrawal speed, as well as improving the panel structure, uniform hole transport layer and emitting layer are deposited with appropriate thickness. The 9-point luminance uniformity and the opto-electrical performance of the dip-coated OLED panel are as same as that of the spin-coated panel. However, the spin-coated panel exhibits dim emission at the corner region and the metal grid region due to panel rotation and material pile-up, which the dip-coated panel shows a uniform light emission across the whole panel.     

一种针对AMOLED器件劣化的电学补偿技术

AMOLED显示屏经过长时间的使用,不可避免地会有OLED器件劣化的问题。同一显示屏内不同位置的像素器件的劣化程度不同,造成显示屏整体亮度下降和显示残像。使用越久、亮度越高的像素,器件劣化就越严重。本文提出一种电学补偿技术,用以改善OLED器件劣化造成的亮度下降和显示残像。采用电学侦测的方法获得各个像素的OLED器件的VTH,将其存储于闪存芯片内,显示时通过查找表找到对应的OLED发光效率,计算得出各个像素的补偿后灰阶值,实现实时补偿的效果。本技术可以使劣化越严重的像素,获得越大的像素电流,从而弥补由于劣化造成的发光效率下降,最终得到理想的像素亮度。使用该方法可以补偿由于OLED劣化造成的亮度下降和显示残像,残像指标JND小于3.1。     

Luminance uniformity compensation for OLED panels based on FPGA

Aiming at the problem of luminance uniformity for organic lighting-emitting diode （OLED） panels, a new brightness calculating method based on bilinear interpolation is proposed. The irradiance time of each pixel reaching the same lumi- nance is figured out by Matlab. Adopting the 64×32-pixel, single color and passive matrix OLED panel as adjusting luminance uniformity panel, a new circuit compensating scheme based on FPGA is designed. VH L is used to make each pixel＇s irradiance time in one flame period written in program. The irradiance brightness is controlled by changing its irradiance time, and finally, luminance compensation of the panel is realized. The simulation result indicates that the design is reasonable.     

High‐Transconductance Organic Thin‐Film Electrochemical Transistors for Driving Low‐Voltage Red‐Green‐Blue Active Matrix Organic Light‐Emitting Devices

Switching and control of efficient red, green, and blue active matrix organic light‐emitting devices (AMOLEDs) by printed organic thin‐film electrochemical transistors (OETs) are demonstrated. These all‐organic pixels are characterized by high luminance at low operating voltages and by extremely small transistor dimensions with respect to the OLED active area. A maximum brightness of ≈900 cd m^?2 is achieved at diode supply voltages near 4 V and pixel selector (gate) voltages below 1 V. The ratio of OLED to OET area is greater than 100:1 and the pixels may be switched at rates up to 100 Hz. Essential to this demonstration are the use of a high capacitance electrolyte as the gate dielectric layer in the OETs, which affords extremely large transistor transconductances, and novel graded emissive layer (G‐EML) OLED architectures that exhibit low turn‐on voltages and high luminescence efficiency. Collectively, these results suggest that printed OETs, combined with efficient, low voltage OLEDs, could be employed in the fabrication of flexible full‐color AMOLED displays.     

Effect of various microlens parameters on enhancement of light outcoupling efficiency of organic light emitting diode

Since organic light emitting diode (OLED) is a multilayer device where each layer has different refractive index, total internal reflection (TIR) plays an important role in limiting the efficiency of an OLED. Due to the presence of TIR, a major portion of light is trapped within the device in various wave guiding modes. Of the total light trapped in an OLED, we address only the part that is lost due to wave guiding mode arising from refractive index mismatch at the glass-air interface. Microlens array, to improve luminance, is a method that can be externally applied to the OLEDs without altering its electrical characteristics and is easy to use. Microlens arrays ranging from 10 to 40 μm have been fabricated using an organic elastomeric material polydimethylsiloxane (PDMS) by mold transfer technique. Maximum improvement of 25% in outcoupling efficiency for blue OLED is reported upon using the microlens array with diameter 10 μm. For a given diameter of microlens, out-coupling efficiency of OLED increases as height to diameter (H/D) ratio of microlens array approaches 0.5 (perfect hemisphere). It is also observed that outcoupling efficiency increases with the diameter of microlens for a given H/D ratio. The best luminescence improvement was observed for blue OLED, which can be explained by the higher refractive index of PDMS at lower wavelengths.     

A New Poly-Si TFT Current-Mirror Pixel for Active Matrix Organic Light Emitting Diode

A new poly-Si thin-film-transistor (TFT) current-mirror-active-matrix-organic-light-emitting-diode (AMOLED) pixel, which successfully compensates for the variation of the threshold voltage as well as mobility in the excimer laser annealed poly-Si TFT pixel, is designed and fabricated. The OLED current$(I_ OLED)$of the proposed pixel does not depend on the operating temperature. When the temperature of pixel is increased from 27$^circhboxC$to 60$^circhboxC$, the$I_ OLED$of the new pixel circuit composed of four TFTs and one capacitor increases only about 1.5%, while that of a conventional pixel composed of two TFTs and one capacitor increases about 37%. At room temperature, nonuniformity of the$I_ OLED$in the proposed circuit was also considerably suppressed at around 9%. We have successfully fabricated a 1.2-in AMOLED panel$(hbox96 times hbox96 times hboxred green blue)$to evaluate the performance of the proposed pixel. A troublesome residual image caused by the hysteresis phenomenon of the poly-Si TFT was almost eliminated in the proposed AMOLED panel as a result of current programming.     

Soluble pentacene thin-film transistor using a high solvent and heat resistive polymeric dielectric with low-temperature processability and its long-term stability

Solution processable organic thin-film transistors (OTFTs) were fabricated using 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene) and low-temperature processable polyimide gate dielectric. The TIPS-pentacene OTFT with the dielectric was found to have a field-effect mobility of 0.15 cm²/Vs, which is comparable to that of OTFT with an inorganic dielectric. The OTFTs with the polyimide dielectric did not show any significant performance degradation as time passed. A field-effect mobility of the OTFTs in 60 days was found to be almost identical to that of pristine OTFT. The combination of TIPS-pentacene and our polyimide gate dielectric can be one of the potential candidates for the fabrication of stable OTFTs for large-area flexible electronics.     

Novel Deep-Blue Hybridized Local and Charge-Transfer Host Emitter for High-Quality Fluorescence/Phosphor Hybrid Quasi-White Organic Light-Emitting Diode

A new hybrid local and charge transfer (HLCT) molecule 2TPA-PPI is obtained for constructing the high-performance organic light-emitting diodes (OLEDs) in this work. 2TPA-PPI possesses the sufficient emission/charge-transporting properties, thus it is used as a neat emitter achieving an efficient deep-blue OLED with very high external quantum efficiency (EQE) up to 10.7%, as well as a multi-functional emitting host matrix constructing the high-performance phosphorescent OLEDs. More importantly, a high-efficiency candle light-style OLED adopting the HLCT/phosphor hybrid strategy is realized, where 2TPA-PPI acts as not only a blue emitter, but also a universal host sensitizing both yellow and red phosphors. This quasi-white OLED represents almost the highest EQE/PE level of 25.2%/49.7 lm W⁻¹ at the practical luminance level of 1000 cd m⁻² for the white OLEDs with the excellent color rendering index values of more than 80 reported.     

Stacked inverted top-emitting green electrophosphorescent organic light-emitting diodes on glass and flexible glass substrates

Stacked inverted top-emitting green electrophosphorescent organic light-emitting diodes (OLEDs) are demonstrated on glass and flexible glass substrates. A single-unit OLED is shown to have a current efficacy of 46.8 cd/A at a luminance of 1215 cd/m². When two of these OLEDs are stacked, the double-unit OLED exhibits a current efficacy more than twice that of the single-unit OLED, with a current efficacy of 97.8 cd/A at a luminance of 1119 cd/m². With the addition of an optical outcoupling layer of N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (α-NPD) on top of the semitransparent gold anode, the double-unit stacked OLED achieves a maximum current efficacy of 205 cd/A at a luminance of 103 cd/m², maintaining a high current efficacy of 200 cd/A at a luminance of 1011 cd/m². These stacked inverted OLED combine the advantages of inverted OLEDs with the benefits of having a stacked architecture.     

A novel 3-TFT voltage driving method of compensating VTH shift for a-Si:H TFT and OLED degradation for AMOLED

This work demonstrates the feasibility of a novel pixel circuit by using three a-Si:H TFTs. The proposed circuit can stabilize the OLED current and provide an additional driving current to ameliorate the brightness degradation of the AMOLED. Measurement results indicate that the current degradation of the proposed circuit, caused by V_TH variations, is less than 5% over more than 50,000 s at 60 °C, whereas that of a conventional 2T1C pixel circuit is larger than 34%. Furthermore, to ameliorate the decrease in luminance owing to the OLED degradation, the OLED current can be increased by 10% by analyzing the current degradation and modulating the detected voltage appropriately.     

Restraining for switching effects in an AC driving pixel circuit of the OLED-on-silicon

The AC driving scheme for OLEDs,which uses the pixel circuit with two transistors and one capacitor(2T1C),can extend the lifetime of the active matrix organic light-emitting diode(AMOLED) on silicon,but there are switching effects during the switch of AC signals,which result in the voltage variation on the storage capacitor and cause the current glitch in OLED.That would decrease the gray scale of the OLED.This paper proposes a novel pixel circuit consisting of three transistors and one capacitor to realize...     

Vertical natural convection models and their effect on failure analysis in electro-thermal simulation of large-surface OLEDs

Besides classical inorganic LEDs, intelligent light sources can be also based on organic LEDs. OLEDs function as surface light sources and manufacturing of large area light sources is feasible with OLED technologies. Despite their lower luminous efficacy their other properties make OLEDs still an attractive option especially in high end indoor applications. In natural convection environment the temperature difference in the same OLED panel can reach 20–30 °C which can result in up to 30–40% difference in current density and thus, in the luminance. This difference in temperature and current density leads to differential ageing of the organic materials. CFD simulation is the obvious way to investigate natural convection environments but integration of a CFD solver in an OLED simulator may be difficult and the solution times are high. As a possible workaround to this problem, in this paper the application of five natural convection models for vertical plates in an electro-thermal field solver based OLED simulator as thermal boundary condition are presented. Steady state and transient simulation results of a free-standing 50 × 50 mm² active surface OLED, surrounded by still air, are compared with measurement results. A typical failure type of OLEDs is thermal runaway caused by e.g. manufacturing problems, operational damages or overcurrent. The paper presents the effect of the natural convection model on the overcurrent caused thermal runaway simulation results.     

High-current stressing of organic light-emitting diodes with different electron-transport materials

We conducted accelerated reliability tests of electron-only devices (EODs) and organic light-emitting diodes (OLEDs) differing only in their electron-transport material (ETM). High current stressing of EODs at 50 mA/cm² showed that Bphen ~ Alq₃ > TPBi > TAZ in terms of intrinsic material stability. In addition, the lowest unoccupied molecular orbital (LUMO) level and electron mobility have been identified as two other key material factors affecting the degradation rate of OLEDs. TAZ has a low electron mobility, a LUMO level misaligned with the Fermi level of the cathode, and poor material stability, leading to extremely poor reliability of devices with a TAZ electron-transport layer (ETL). In contrast, the OLED with a Bphen ETL exhibited more stable operation and a 76 × longer luminance lifetime. Due to its relatively high electron mobility and good stability as well as perfect energy level alignment with the cathode, Bphen has proven to be the most desirable ETM from the standpoint of OLED reliability.     

Flexible AM OLED panel driven by bottom-contact OTFTs

Active matrix organic-light-emitting-diode (AM OLED) panels, driven by organic thin-film transistors (OTFT), have been successfully fabricated on a flexible plastic substrate. The pixel circuit consists of two bottom-contact pentacene OTFTs working as switching and driving transistors. The panel has 16 /spl times/ 16 pixels, each of which have an OLED using a phosphorescent material with an emission efficiency of 30 cd/A. A tantalum oxide (Ta/sub 2/O/sub 5/) film with a dielectric constant of 24, prepared by the anodization of Tantalum (Ta), was used as the gate insulator of the OTFTs. The passivation layer on the OTFTs was formed by a layer of silicon dioxide (SiO/sub 2/) and two layers of polyvinyl alcohol. Using OTFTs with a Ta/sub 2/O/sub 5/ gate insulator, the authors have realized a flexible active matrix OLED panel driven with a low voltage of -12 V.     

Copper(I) Complex as Sensitizer Enables High-Performance Organic Light-Emitting Diodes with Very Low Efficiency Roll-Off

Organic light-emitting diodes (OLEDs) utilizing purely organic thermally activated delayed fluorescence (TADF) sensitizers have recently achieved high efficiencies and narrow-band emissions. However, these devices still face intractable challenges of severe efficiency roll-off at practical luminance and finite operational lifetime. Herein, a carbene-Cu(I)-amide complex, (MAC*)Cu(Cz), is demonstrated as a TADF sensitizer for both fluorescent and TADF OLEDs. The (MAC*)Cu(Cz)-sensitized fluorescent OLED not only achieves a high external quantum efficiency (EQE) of 14.6% with an extremely low efficiency roll-off of 12% at the high luminance of 10 000 nits, but also delivers a 15 times longer operational lifetime than that of the non-sensitized reference device. More importantly, utilizing the (MAC*)Cu(Cz) sensitizer in the multi-resonance (MR) TADF OLED results in a record-high EQE of 26.5% together with a full-width at half maximum of 46 nm and an emission peak at 566 nm. This value is the state-of-the-art efficiency for yellow-emitting MR-TADF OLEDs. The photophysical analysis proved that the fast reverse intersystem crossing process of (MAC*)Cu(Cz) is the key factor to suppress triplet exciton involved quenching at high luminance. This finding firstly demonstrates the use of Cu(I) complex as an efficient TADF sensitizer and paves the way for practical applications of TADF sensitized OLEDs.