Stacked OLED microdisplay with low-voltage silicon backplane

Abstract

A microdisplay comprising a light-emitting OLED stack on top of a silicon-based backplane having individually addressable pixels and control circuitry, wherein the light-emitting OLED stack has three or more OLED cells between a top electrode and a bottom electrode; and the control circuitry of the silicon-based backplane comprises at least two transistors, wherein the channels of the at least two transistors are connected in series to an external power supply V DD Between the bottom electrode of the OLED stack. The light-emitting OLED stack preferably has a Vth of at least 7.5 V or higher. The control circuit may include a protection circuit consisting of a p-n junction diode, preferably a bipolar junction transistor.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Application No. 62 / 966,757, filed on January 28, 2020, entitled “Stacked OLED Microdisplay with Low Voltage Silicon Backplane,” under attorney docket no. OLWK-0021-USP, and U.S. Provisional Application No. 63 / 054,387, filed on July 21, 2020, entitled “Stacked OLED Microdisplay with Low Voltage Silicon Backplane,” under attorney docket no. OLWK-0021-USP2. Background Art

[0003] Typically, microdisplays are ultra-small display sizes with a diagonal size of less than two inches (approximately 5 cm) or even less than 0.25. In most cases, microdisplays have a high resolution, with a pixel pitch typically ranging from 5 to 15 microns. They first came into commercial use in the late 1990s, typically for rear-projection televisions, head-mounted displays, and digital camera viewfinders. In recent years, devices such as smartwatches have taken advantage of the high resolution and low power consumption of these displays. The global market for microdisplays is expected to grow at a compound annual growth rate of 20% over the next few years. One of the trends driving this growth will be the increasing adoption of near-eye displays, augmented reality devices, and virtual reality devices, such as head-mounted displays (HMDs), head-up displays (HUDs), and electronic viewfinders (EVFs).

[0004] There are two main categories of microdisplays. The first is projection microdisplays, which involve highly magnified images projected onto a surface. Types of projection microdisplays include rear-projection televisions and compact data projectors. The second category is near-eye displays (NEDs), which consist of highly magnified virtual images viewed through an eyepiece (such as a virtual reality headset or a camcorder viewfinder). These displays are increasingly being used in HMDs and HUDs, particularly in the military and medical industries.

[0005] Both types of microdisplays offer significant advantages over conventional direct-view displays, such as flat-panel LCDs. The advantages of microdisplays include the ability to produce large images from very small, lightweight source display units, making them easy to integrate into space-constrained technologies, such as wearables; large pixel capacity, resulting in high resolution and clarity; and greater power efficiency compared to other display types. The higher the resolution and brightness, and the lower the power consumption, the better the quality of the microdisplay. However, challenges for microdisplay manufacturers are relatively high production costs, as well as the need for high brightness and contrast, and a long operating life.

[0006] Microdisplays can be made using a variety of display technologies, including liquid crystal on silicon (LCoS), liquid crystal display (LCD), digital micromirror device (DMD), digital light processing (DLP), and more recently, microLED (light emitting diode) and organic light emitting diode (OLED).

[0007] In recent years, LCDs have dominated the microdisplay market. LCD technology offers high brightness, relatively low cost, and a relatively simple manufacturing process. By using LCDs, device manufacturers have been able to reduce the size of microdisplay components over time. LCD displays are currently used in some HMDs, HUDs, EVFs, and thermal imaging glasses and wearable devices. However, LCD microdisplays require a light source or backlight to form an image in conjunction with a liquid crystal array to modulate the light. The technology has limitations such as polarization, color space, maximum brightness limits, LC temperature sensitivity, viewing angle, LCD transmittance and extinction ratio, and system size constraints, which may not provide all the required performance characteristics.

[0008] Microdisplays based on microLED technology (microLED) can offer advantages over LCD microdisplays, such as self-luminescence, a larger color gamut, a wide viewing angle, better contrast, a faster refresh rate, lower power consumption (depending on the image), and a wide operating temperature range. Currently, microLED microdisplays are based on standard gallium nitride (GaN) wafers adopted from standard LEDs. This approach has the potential to provide high-brightness display devices without lifetime issues at a relatively low price. Typically, a standard GaN wafer is patterned into an array of microLEDs. The microLED display is then produced by integrating the microLED array with a transistor. However, this approach has several manufacturing issues, including: monolithic formation of the microLEDs on the transistors, pixel pitch, color generation, and spatial uniformity due to color and brightness variations between individual microLEDs.

[0009] OLED technology has many of the attractive features of micro-LED technology for microdisplays. It is self-emissive, has excellent image quality, is very efficient compared to LCDs or LCoS, and has ultra-high color reproduction and a wide color space. Self-emissive OLED devices have a significant advantage over backlit devices (such as LCDs) because each pixel produces only the intensity required for the image, while backlit pixels produce maximum intensity and then absorb unwanted light. In addition, because the OLED layers can be vacuum deposited or coated directly onto the transistor backplane, forming OLEDs on transistors is much easier and less expensive than forming micro-LEDs. On the other hand, OLEDs can have limited brightness and lifespan.

[0010] Resolving motion blur is also important for the control circuitry in OLED microdisplays (sample-and-hold displays) (see “Why Do Some OLEDs Have Motion Blur?” at https: / / www.blurbusters.com / faq / oled-motion-blur / , dated December 28, 2018, and “Is Motion Resolution an Issue with OLED TVs?” at https: / / www.soundandvision.com / content / motion-resolution-issue-oled-tvs, dated January 15, 2015).

[0011] The only way to reduce motion blur caused by sample-and-hold is to shorten the amount of time a frame is displayed. This can be achieved by using additional refreshes (higher Hz) or by introducing black periods (flickering) between refreshes. For OLED microdisplays, the best solution is to "turn off" the displayed image, either by simultaneously turning off the entire active area or by using a "rolling" technique (turning off only portions of the displayed image at a time in a sequential manner). The "rolling" technique is preferred. The pixel is off for very short periods of time, well below the threshold of detection by the human eye, to avoid perceptible flicker. This is achieved in the control circuit by including a shutter transistor, which, when activated by a select line, prevents current from flowing through the OLED and "turns off" the emission of the OLED pixel for the desired period of time. In other words, the shutter transistor is a switching transistor, as it only turns the pixel "on" or "off" without regulating voltage or current. However, since the average brightness of the OLED over the frame is perceptible to the eye when it is "on," this solution (turning off the pixel for a portion of the time the image is displayed, often referred to as the frame time)) only increases the need for increased brightness from the OLED when it is "on." The motion blur-reducing shutter can be applied to any method of powering the OLED stack; such as current control or PWM.

[0012] OLED microdisplays using silicon backplanes are very attractive from the perspective of cost and manufacturability. See, for example, Ali et al., “Recent advances in smallmolecule OLED-on-Silicon microdisplays,” Proc. of SPIE, vol. 7415 74150Q-1, 2006; Jang et al., J. Information Display, 20(1), 1-8 (2019); Fujii et al., “4032ppi High-Resolution OLED Microdisplay,” SID 2018 Abstracts, p. 613; US 2019 / 0259337; Prache, Display, 22(2), 49 (2001); Vogel et al., 48th European Solid-State Devices Research Conference, 2018 48 th European Solid-State Device Research Conference, 90 pages, September 2018; and Wartenberg et al., “High Frame-Rate1" WUXGA OLED Microdisplay and Advanced Free-Form Optics for Ultra-Compact VR Headsets,” SID Proceedings, 49 (1), Paper 40-5, 514(2018).

[0013] Microdisplays require very high brightness to be useful in all environmental conditions, such as outdoors in bright sunlight. Even in controlled environmental conditions, such as in VR headsets, very high brightness is required to create an immersive visual experience. The extremely high brightness from the display allows the use of less efficient optics that are smaller, lighter, and less expensive, resulting in more competitive headsets. Currently, state-of-the-art OLED microdisplays do not provide as much brightness as is required.

[0014] For example, a press release from one manufacturer of tandem OLED microdisplays describes a full-color product that could deliver up to 2.5k nits, but acknowledges that 5k nits would be a more ideal target (see https: / / www.kopin.com / kopin-to-showcase-latest-advances-in-its-lightning-oled-microdisplay-line-up-at-ces-2020 / dated January 7, 2020). Some manufacturers suggest the target should be 10,000 nits or higher (see https: / / hdguru.com / calibration-expert-is-10,000-nits-of-brightness-enough / dated July 26, 2018). A recent press release from June 20, 2020 (https: / / www.businesswire.com / news / home / 20200630005205 / en / Kopin-Announces-Breakthrough-ColorMax%E2%84%A2-Technology-Unparalleled-Color) described a tandem (two-stacked) OLED display with an emission of >1000 nits. It also announced that "By optimizing OLED deposition conditions, further improvements in brightness (>2000 nits) and color fidelity are expected. By incorporating structures that improve output coupling efficiency, the brightness of OLED microdisplays could be increased to >5000 nits within a few years."

[0015] One solution for increasing the total amount of light emitted from an OLED device is to stack multiple OLED cells on top of each other, so that the total light emitted from the stack is the sum of the light emitted by each individual cell. However, while the total light emitted from such an OLED stack is additive based on the total number of individual OLED light-emitting cells, the voltage required to drive the OLED stack is also additive based on the voltage required to drive each individual OLED cell. For example, if a light-emitting OLED cell requires 3V to produce 250 nits at a given current, then a stack of two such cells would require 6V to provide 500 nits at the same current, a stack of three cells would require 9V to provide 750 nits, and so on.

[0016] OLED stacks are well known; for example, US7273663, US9379346, US9741957; US9281487 and US2020 / 0013978 all describe stacked OLED stacks with multiple light-emitting OLED units, each separated by an intermediate connecting layer or charge generation layer. Springer et al., Optics Express, 24 (24), 28131 (2016) reported OLED stacks with two and three light-emitting units, each with a different color. OLED stacks with up to six light-emitting units have been reported (Spindler et al., "High Brightness OLED Lighting," SID Display Week 2016, San Francisco, CA, May 23-27, 2016).

[0017] However, this multi-stacked OLED approach, which requires higher drive voltages, is difficult to apply in microdisplay applications. The problem is that microdisplays also need to have high resolution, which requires that the size of each pixel must be as small as possible, and the active (light-emitting) area of ​​the microdisplay must contain as many pixels as possible. This requires the transistors in the backplane's control circuitry to be small, but large enough to handle the required voltages and currents without causing permanent damage or current leakage.

[0018] Typically, as transistors become smaller, they cannot handle higher powers due to leakage current and other failure mechanisms, so they have lower voltage ratings. Smaller low-voltage transistors have thinner insulating layers at the gate, so they also have more static current leakage. For this reason, larger transistors that can handle higher voltages are typically used in OLED devices that require higher voltages. WO2008 / 057372 discusses problems and prior art related to reducing the size of pixel circuits in microdisplays. See also, for example, O. Prache, Journal of the Society for Information Display, 10(2), 133 (2002); O. Prache, "Active Matrix Molecular OLED Microdisplays," Displays, 1996. 22, 49-56 (2001); and Howard et al., "Microdisplays based upon organic light emitting diodes," IBM J. of Res. & Dev., 45 (1), 15 (2001) discuss the need to provide high brightness microdisplays on silicon backplanes with large contrast ratios at low voltages.

[0019] In addition, when using a MOSFET p-channel transistor from V DD The power supply has a cathode voltage of V CATHODE When supplying a constant current to an OLED, the total voltage must be large in order to power the transistor and turn the OLED "on" to high brightness. However, if a low voltage p-channel transistor is used to control a 12V OLED, the current leakage through the transistor will be high when trying to turn the OLED off to form a black pixel, and the (V 阳极 -V 阴极 ) will remain above the OLED threshold, so the voltage OLED will continue to emit light. In an OLED microdisplay, because the OLED pixel will continue to emit light when it should remain dark, current leakage through the drive transistor will reduce contrast. This effect will cause pure black (no emission) to appear gray and reduce the size of the tonal range between pure black and pure white. This is undesirable.

[0020] There is a need to improve the performance of OLED microdisplays on silicon backplanes by utilizing OLED stacks with high brightness and high voltage requirements for light emission. However, the control circuitry on the silicon backplane must be able to handle the increased voltage and current requirements without a significant increase in size in order to maintain the resolution and pixel pitch within the active area of ​​the OLED. Specifically, the control circuitry should maintain contrast by preventing or minimizing current leakage through the TFTs. Contrast is the difference in light emission when a pixel should be "off," "black," or not emitting light (typically, image signal code value (CV) = 0) and when the pixel should be fully "on," "white," or at maximum light emission (typically, image signal CV = 255).

[0021] In the semiconductor foundry industry, which manufactures backplanes, analog transistors with an operating range of 5V or less are generally considered standard "low-voltage" (LV) transistors. A 10% safety limit is typically placed on the voltage rating to allow reliable operation up to 5.5V without degrading the life of this "5V transistor"; 5.5V is high enough to allow for some overvoltage within the OLED's dynamic voltage range and the driver circuit's overhead voltage. While the voltage limit generally applies between any pair of contacts of the transistor (gate, source, drain, body (also called bulk or well)), the voltage limit specifically applies to the maximum gate-drain voltage, ensuring that the transistor's performance remains within the specified range under these conditions for a typical 43,000 hours of operation. Sometimes, depending on the transistor's design, the voltage limit for other pairs of contacts may be higher (e.g., 7V), but the transistor is still referred to as an LV or 5V transistor. 5V analog transistors are widely available throughout the industry due to their compatibility with the traditional TTL logic voltage levels used for communication between integrated circuit (IC) chips. With the trend toward lower voltages for input and output communications (e.g., the 3.3V and 1.8V standards), these 5V transistors are sometimes referred to as medium voltage (MV) transistors, changing the LV label to the new "lower voltage" analog transistors. Although relative labels such as LV and MV may change over time, in this patent application, the term LV or "low voltage" refers to transistors rated at 5V or less, and the term MV or "medium voltage" refers to transistors with a rated voltage exceeding 5V. Higher voltage analog transistors are also common, but the exact voltage has not been standardized throughout the integrated circuit (IC) manufacturing industry as it has been for 5V transistors. For example, industries such as automotive often require higher voltage transistors.

[0022] Currently, silicon backplanes with low voltage 5V driver transistors are available that emit light using OLED stacks in series (two light-emitting OLED units separated by a CGL). See, for example, Cho et al., Journal of Information Display, 20 (4), 249-255, 2019; https: / / www.ravepubs.com / oled-silicon-come-new-joint-venture / , published in 2018; Xiao, “Recent Developments in Tandem White Organic Light-Emitting Diodes,” Molecules, 24 , 151(2019). This example is not bright enough to meet the technical requirements.

[0023] Han et al., “Advanced Technologies for Large-Sized OLED Displays,” Chapter 3, 10.5772 / intechopen.74869 (2018). This review article describes three stacked white OLED concepts and advances in backplane technology, including a backplane with two series-connected transistors, although these technologies are separate rather than combined. The reference also notes that such two-transistor backplanes “are difficult to implement in large, high-resolution panels due to large line loads and short charging times,” and therefore employs a different kind of backplane circuitry for their devices.

[0024] Liu et al., Journal of Central South University of Technology, 19 , 1276-1282 (2012) discloses a 3T1C circuit with two p-channel transistors connected in series in an OLED on a Si chip microdisplay,

[0025] Zeng et al., “A Novel Pixel Circuit with Threshold Voltage Variation Compensation in Three-Dimensional AMOLED on Silicon Microdisplays,” P-27, SID 2019 Digest, page 1313, describes a 4T1C pixel circuit for driving an AMOLED display on a silicon backplane. It discloses the use of two p-channel low-voltage transistors connected in series between a power supply and the OLED. The first transistor is used to drive the OLED and control the current supplied to the OLED. The second transistor is a switching transistor for turning off the OLED during programming operations. The above circuit also includes a circuit for determining and compensating V th Variation of the additional transistor.

[0026] US9,066,379 describes a control circuit suitable for an OLED microdisplay. It discloses the use of two p-channel low-voltage transistors connected in series between a power supply and the OLED. The first transistor is used to drive the OLED and control the current supplied to the OLED. The second transistor is a switching transistor used to disconnect (turn off) the OLED. The circuit also requires a third transistor between the switching transistor and the OLED, which is a medium-voltage or high-voltage p-channel transistor. The purpose of this third transistor seems to be to prevent the OLED from being turned off during the "off" period (V ss -V 阴极 ) is greater than the OLED threshold voltage, current flows to the OLED.

[0027] Pashmineh et al., "High-voltage circuits for power management on 65nm CMOS" (Adv. Radio Sci, 13 , 109-120, 2015) describe a high voltage circuit based on stacked low voltage CMOS transistors connected in series (also commonly referred to as "stacked" transistors).

[0028] Dawson et al., "The Impact of the Transient Response of Organic Light Emitting Diodes on the Design of Active Matrix OLED Displays" (International Electronic Devices Mtg., 1998, 875-878) describe a pixel circuit having two p-channel transistors connected in series.

[0029] Kwak et al., “Organic Light-Emitting Diode-on-Silicon Pixel Circuit Using the Source Follower Structure with Active Load for Microdisplays” (Japan Journal of Applied Physics, 50, 03CC05 (2011)) describes a pixel circuit having three p-channel transistors connected in series and an overvoltage protection circuit. The reference states that because the operating voltage of the OLED is higher than that of the MOSFET, an “overvoltage protection circuit” is required to prevent breakdown of the metal oxide semiconductor field effect transistor (MOSFET). U5760477 and US9066379 also describe the use of a protection circuit.

[0030] Vogel et al., SID 2017 Digest (Article 77-1, pp. 1125-1128) disclose the use of protection circuits in low-voltage OLED silicon microdisplays to extend the OLED voltage operating range.

[0031] Other references including overvoltage protection for OLEDs are disclosed in US6580657, WO2009072205 and CN200488960. US9059123, US9299817, US9489886, US20080316659 and US20200202793 disclose the use of np junction diodes, such as bipolar junction transistors, in pixel control circuits of OLED displays.

[0032] Other patent references that disclose two transistors connected in series include: CN109166525, US20140125717, US7196682, US6229506, US9262960, US7443367, US6930680, US10614758, WO2019019590, US6946801, US7755585, US8547372, US8786591, US8797314, US9818344, US2018 / 0211592, and US10269296. The following references disclose pixel circuits having three transistors connected in series: US9324266, US9384692, US10600366, US7180486, US9858863, and US20190279567.

[0033] US9059123 and US9489886 disclose the use of np junction diodes, such as bipolar junction transistors, in pixel control circuits of OLED displays. Summary of the Invention

[0034] Described is a microdisplay comprising a light-emitting OLED stack on top of a silicon-based backplane having individually addressable pixels and control circuitry, wherein the light-emitting OLED stack has three or more OLED cells between a top electrode and a bottom electrode; and for each individually addressable pixel, the control circuitry of the silicon-based backplane comprises at least two transistors, wherein the channels of the transistors are connected in series to an external power supply V DD Between the bottom electrode of the OLED stack.

[0035] The microdisplay described above, wherein the threshold voltage V th It is at least 7.5 V or greater, or preferably at least 10 V or greater.

[0036] Any microdisplay as above, wherein the OLED stack comprises four or more OLED light emitting units.

[0037] Any microdisplay as described above, wherein the OLED light-emitting units are each separated from each other by a charge generation layer (CGL), or wherein the bottom electrode is segmented and each segment is in electrical contact with the control circuitry in the backplane, or wherein the OLED stack is top-emitting.

[0038] Any microdisplay as described above, wherein the OLED stack forms a microcavity, wherein the physical distance between the segmented bottom electrode and the top electrode is constant across all pixels.

[0039] Any microdisplay as described above, wherein the transistors whose channels are connected in series are rated at 5V or less, or wherein the transistor closest to the power supply is a drive transistor and is rated at 5V or less, and the transistor closest to the bottom electrode of the OLED is a switching transistor and is rated at greater than 5V.

[0040] Any microdisplay as described above, wherein the two transistors whose channels are connected in series are p-channel transistors, or wherein the two transistors whose channels are connected in series are each located in a separate well.

[0041] Any microdisplay as described above, wherein the control circuit further comprises a protection circuit comprising a p-channel transistor or diode. The protection circuit may comprise a pn junction diode, and wherein the cathode of the pn junction diode is connected to the node of the bottom electrode of the OLED stack and the anode is connected to the voltage reference V REF Or current reference I REF .

[0042] The protection circuit as described above, wherein the diode is a bipolar junction transistor. The bipolar junction transistor may be an NPN transistor, wherein the base is connected to a voltage source V PROTECT or current source I PROTECT , the emitter is connected to the node, the node is connected to the bottom electrode of the OLED stack, and the collector is connected to the voltage source V DD The base of the BJT can be isolated.

[0043] A protection circuit as described above, wherein the two transistors whose channels are connected in series with the bipolar junction transistor are located in separate wells, or wherein the two transistors whose channels are connected in series are both p-channel transistors and are each located in a separate n-well, and the bipolar junction transistor is an NPN transistor located in a separate p-well.

[0044] Any microdisplay as described above, further comprising an RGB color filter array (CFA) located on top of the OLED stack, wherein individual R, G, B color filters are aligned with the segmented bottom electrodes to form R, G, B pixels; or alternatively comprising an RGBW color filter array (CFA) located on top of the OLED stack, wherein individual R, G, B and clear or absent color filters are aligned with the segmented bottom electrodes to form R, G, B and W pixels.

[0045] Microdisplays offer very high brightness and contrast at small pixel pitch sizes. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] FIG. 1 shows a simple prior art control circuit for an OLED.

[0047] Figure 2 A basic control circuit with two transistors having their channels connected in series is shown, which circuit is suitable for an OLED stack with at least three OLED cells.

[0048] Figure 3 Shown Figure 2 An embodiment of an intrinsic body diode connection.

[0049] Figure 4 Shown Figure 2 Another embodiment of the intrinsic body diode connection.

[0050] Figure 5A A basic control circuit with three transistors having their channels connected in series is shown, which circuit is suitable for an OLED stack with at least three OLED cells. Figure 5B Similar to Figure 5A , except that the drive transistor is controlled via a level shift circuit.

[0051] Figure 6 A basic control circuit suitable for an OLED stack having at least three OLED cells is shown, with an additional protection circuit comprising a diode-connected p-channel MOSFET transistor.

[0052] Figure 7 A similar protection circuit including diodes is shown.

[0053] Figure 8 A basic control circuit is shown which is suitable for an OLED stack with at least three OLED cells and has a protection circuit with a bipolar junction transistor (BJT).

[0054] Figure 9 A schematic cross section through a silicon backplane is shown, demonstrating Figure 8Connection locations, connections, and transistor wells for the circuit shown.

[0055] Figure 10 Shown is an RGB microdisplay with three adjacent single-color RGB OLED stacks 100 Cross-sectional view, each individual stack has three OLED light-emitting units.

[0056] Figure 11 Shown is a microdisplay with a multimode microcavity OLED stack with three OLED light-emitting units and an RGB color filter array. 200 Cross-sectional view of .

[0057] Figure 12 A microdisplay with a multimode microcavity OLED stack with three OLED light-emitting units (with an additional blue light-emitting layer) and an RGB color filter array is shown. 300 Cross-sectional view of .

[0058] Figure 13 Shown is a microdisplay with a multimode microcavity OLED stack with five OLED light-emitting units and an RGB color filter array. 400 Cross-sectional view of .

[0059] Figure 14 Graphs of current density versus voltage are shown for some comparative OLED devices and an OLED device of the present invention.

[0060] Figure 15 Shown are the spectra (plotted as intensity versus wavelength) for some comparative OLED devices and an OLED device of the present invention.

[0061] Figure 16 A graph showing the peak current density required for each color to produce a D65 white point for a comparative OLED device with color filters and an OLED device of the invention.

[0062] Figure 17 Graphs of brightness versus voltage are shown for a comparative OLED device and an OLED device of the present invention when the device is driven to produce black and again at peak white brightness.

[0063] Figure 18 Graphs of current density versus voltage are shown for some of the inventive OLED devices.

[0064] Figure 19 Shown are the spectra (plotted as intensity versus wavelength) for some of the inventive OLED devices.

[0065] Figure 20A graph showing the peak current density required for each color to produce a D65 white point for some of the inventive OLED devices with color filters.

[0066] Figure 21 Graphs of brightness versus voltage are shown for some of the inventive OLED devices as the device is driven to produce black and again at peak white brightness. DETAILED DESCRIPTION

[0067] For the purposes of this disclosure, the terms "on" or "over" mean that the structure in question is located above another structure, that is, on the side opposite to the substrate. "Top", "uppermost" or "upper" refer to the side or surface farther from the substrate, while "bottom", "lowermost" or "bottom" refer to the side or surface closest to the substrate. Unless otherwise specified, "on" should be interpreted as meaning that two structures may be in direct contact or that there may be an intermediate layer between them. With respect to "layer", it should be understood that a single layer has two sides or surfaces (an uppermost layer and a lowermost layer); in some cases, "layer" may refer to multiple layers considered as a whole and is not limited to a single layer.

[0068] For light-emitting units or light-emitting layers, R represents a layer that emits primarily red light (>600nm, desirably in the range of 620 to 660nm), G represents a layer that emits primarily green light (500-600nm, desirably in the range of 540 to 565nm), and B represents a layer that emits primarily blue light (<500nm, desirably in the range of 440 to 485nm). It is important to note that the R, G, and B layers can produce some degree of light outside the specified range, but the amount produced is always less than the primary colors. Y (yellow) represents a layer that emits a large amount of R and G light and a much smaller amount of B light. "LEL" refers to light-emitting layer. Unless otherwise specified, wavelengths are expressed in vacuum values ​​rather than in-situ values.

[0069] A single OLED light emitting unit can produce a single "color" of light (i.e., R, G, B, or a combination of 2 or more primary colors such as Y, C (cyan), or W (white)). Monochromatic light can be produced within an OLED unit by a single layer having one or more emitters of the same color or multiple layers where each layer has the same or different emitters whose primary emissions fall within the same color. A single OLED unit can also provide a combination of two colors (i.e., R+G, R+B, G+B) in a single OLED unit by having: one layer with a single emitter that emits two colors of light, one layer with two different emitters, or a combination of multiple individual layers where each layer emits a single but different color. A single OLED unit can also provide white light (a combination of R, G, and B) by having: one layer that emits all three colors of light or a combination of multiple individual layers where each layer emits a single (but different) color whose sum is white. Individual OLED light emitting units can have a single light emitting layer or can have more than one light emitting layer (either directly adjacent to each other or separated from each other by intermediate layers). Each light-emitting unit may also contain various non-emitting layers, such as hole transport layers, electron transport layers, blocking layers, and other non-emitting layers known in the art to provide desired effects, such as promoting emission and managing charge transfer throughout the light-emitting unit.

[0070] Because OLED light-emitting cells can contain multiple layers, individual cells are sometimes referred to as "stacks," which can be confusing with OLED devices with multiple cells. In this application, a "stacked" OLED has at least two OLED light-emitting cells stacked on top of each other on a substrate, thus creating multiple light sources within the device. In the stacked OLEDs of the present invention, the individual OLED light-emitting cells are separated from each other by a charge generation layer (CGL) rather than by separate, independently controlled intermediate electrodes. To be considered an OLED light-emitting cell, it must be separated from another light-emitting cell by a CGL. Therefore, a light-emitting layer adjacent to one of the OLED light-emitting cells but not separated from it by a CGL is not considered a separate cell. Within a stack, all or some of the individual OLED light-emitting cells can be identical, or they can all be different. Within an OLED stack, the individual OLED light-emitting cells can be placed in any order between the top and bottom cathodes. Stacked OLEDs can be monochromatic (each pixel of the OLED stack emits primarily the same color; for example, green light) or can have multimodal emission (where each pixel emits two or more colors of light (for example, yellow or white), or where different pixels emit different colors of light so that the overall emission includes two or more colors of light).

[0071] In some cases, the threshold voltage (V) of the OLED stack can be estimated by linear extrapolation of the IV curve after significant light emission begins to return to the voltage axis. th ). Since the IV response curve of an OLED may not be completely linear across its response range, the value calculated in this way is not exact. A typical range for this specification is + / - 10%. More precisely, the threshold voltage can be defined as the voltage where the current density is no greater than 0.2 mA / cm3 of the exposed anode layer. 2 And there is at least some voltage with reliably detectable brightness (ie at least 5 cd / A). This is the approach used in this application.

[0072] In the following, transistors may be referred to as being "on" or "off". In an "off" transistor, the signal sent to the gate is intended so that no current will flow through the terminal; in other words, the signal (typically CV = 0) is an indication that the signal requires any current not to flow through the transistor, so that the transistor is adjusted to "off" rather than being switched "off". In this case, even though the transistor may be "off", there may still be some current leakage. Similarly, in an "on" transistor, the signal sent to the gate is intended so that at least some current will flow through the terminal; in other words, the signal (typically CV = greater than 0 but less than 255) is an indication that the image requires a certain degree of emission from the pixel, so that the transistor is adjusted to "on" rather than being switched "on". In a similar way, a pixel or OLED may be referred to as being "on" or "off", depending on the requirements of the image, and the appropriate signal is sent to the pixel or OLED accordingly.

[0073] Silicon backplanes are derived from silicon wafers (also called slices or substrates). They are thin slices of semiconductors (such as crystalline silicon (c-Si)) used to make integrated circuits. The wafers serve as the substrate for the microelectronic devices built into and on the wafers. The wafers undergo numerous microfabrication processes, such as doping with various materials, ion implantation, etching, thin film deposition, and photolithographic patterning. Finally, the individual microcircuits are separated and packaged into integrated circuits by wafer dicing. Wafers are grown from crystals with a regular crystal structure, where silicon has a rhombic cubic structure with a lattice spacing. When cut into wafers, the surfaces are aligned in one of several relative directions called crystal orientations. Silicon wafers are typically not 100% pure silicon, but are formed with initial impurity doping concentrations of boron, phosphorus, arsenic, or antimony, which are added to the melt and define the wafer as either bulk n-type or p-type. For background, see "Flat Panel Display Manufacturing" (Souk, L., Ed., 2018, Chapter 7). The silicon backplane is preferably a single crystal silicon wafer.

[0074] To provide control circuitry for the operation of stacked OLEDs, thin-film transistors (TFTs) and other components, such as capacitors, resistors, interconnects, or bus bars, are disposed on the surface of a silicon wafer. See, for example, T. Arai, "High Performance TFT Technologies for the AM-OLED Display Manufacturing," thesis, Nara Institute of Science and Technology, 2016; M.K. Han, Proc. of ASID'06, October 8-12, New Delhi; US Pat. No. 9,066,379; and US Pat. No. 1,016,3998. It will be understood that the TFTs may or may not be incorporated into the silicon wafer as part of the TFT structure, or may be made of a separate material deposited on the surface.

[0075] TFTs can be made from a variety of semiconductor materials. The characteristics of silicon-based TFTs depend on the crystalline state of the silicon; that is, the semiconductor layer can be amorphous silicon, microcrystalline silicon, or it can be annealed to polycrystalline silicon (including low-temperature polycrystalline silicon (LTPS) and laser annealing).

[0076] The manufacture of silicon backplanes with suitable control circuitry is a very well-known, understood, and predictable technology. However, due to the cost and complexity of the manufacturing processes and equipment, it is often impractical to build facilities specifically for manufacturing specific backplanes. Instead, foundry models have been widely adopted in industries where the functional characteristics of microelectronic devices have become more standardized. This standardization allows for the separation of design from manufacturing. Designs that follow appropriate design rules can be manufactured more easily and cheaply by different companies with compatible manufacturing methods. For this reason, the control circuitry on silicon backplanes is typically limited to the use of standard components selected from a range of options provided by the backplane manufacturer. For example, a manufacturer of a silicon backplane may offer the option of incorporating various standard designs of transistors rated for 1.8V, 2.5V, 3.3V, 5V, 8V, and 12V into a customer's design, but cannot (without significant expense) provide transistors that are not included in the provided design.

[0077] For the purposes of this application, "low voltage" (LV) is defined as those analog microelectronic components that are sized, designed, and rated to operate safely and reliably at voltages of 5V or less. "High voltage" (HV) microelectronic devices are generally considered to be in the range of 18 to 25V. "Medium voltage" (MV) microelectronic devices are generally considered to be those microelectronic devices between LV and HV. It should be noted that these voltage ratings are set by the manufacturer, and the manufacturer does not recommend exceeding the maximum set voltage of each transistor.

[0078] Complementary Metal Oxide Semiconductor (CMOS) technology uses p-type and n-type Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) to implement complex integrated circuits. Depending on the manufacturing process, there are different voltage domains that can be used (i.e., 1.8V, 2.5V, 3.3V, 5V, 12V, etc.). In all voltage domains, a MOSFET transistor has a drain, source, gate, and body / well. The base of the MOSFET is the substrate; for an n-channel FET, the substrate is a p-type doped substrate or a well with a low doping rate; for a p-channel FET, the substrate is an n-type doped well with a low doping rate. The source and drain regions are formed by highly doped regions with either n-type or p-type for n-channel FETs or p-channel FETs. A controlled channel is formed between the source and drain, isolated by a thin oxide and typically covered by a polysilicon layer that acts as a gate. All four terminals of the FET (source, drain, gate, substrate / well) are connected to a metal interconnect layer through metal contacts, which is ultimately connected to the OLED.

[0079] The emissive area of ​​a microdisplay is small, and achieving the necessary pixel pitch limits the space available for the control circuitry for each pixel. For a full-color microdisplay, the space occupied by the control circuitry for each individual pixel should be no larger than 100 square microns, and preferably no larger than 50 square microns. For a monochrome microdisplay, in which all pixels emit the same color, the space for the control circuitry can be three to four times larger because fewer pixels are needed.

[0080] In a suitable low-voltage 5V transistor, the maximum voltage between any pair of terminals can not exceed 5V without damaging the device. A typical safety margin of 10% overvoltage is acceptable for short periods of time. Medium-voltage transistors rated for voltages greater than 5V (for example, a transistor rated for 7.5V) typically have the same configuration as 5V transistors, but with thicker gate oxides and larger geometries (channel width and length) to withstand the higher voltage. Therefore, MV transistors will generally be larger and take up more space than their LV counterparts.

[0081] Although transistors can be manufactured in any size range without regard to their voltage rating, the total area of ​​a low-voltage MOS transistor rated for 5V suitable for microdisplay applications does not exceed 20 square microns, and preferably does not exceed 10 square microns. A suitable channel area (channel length x channel width) for a 5V transistor for microdisplay applications should not exceed 1 square micron, and preferably does not exceed 0.30 square microns. The two transistor contacts should each not exceed 1 square micron, and preferably not exceed 0.30 square microns.

[0082] For the purposes of this application, the BJTs suitable for the protection circuit are generally arranged vertically for both NPN and PNP types. For the NPN BJT, the collector is formed as a low-doped deep n-well with low p-doping within a common silicon substrate (body). The base is formed as a p-well inside the deep n-well and is connected through a highly doped p-region. The emitter of the BJT is formed by a highly doped n-region inside the p-well. The typical dimensions of the emitter region are about 500nm x 500nm, and preferably do not exceed 0.30 square microns to allow for smaller pixel sizes. The maximum voltage between any pair of all terminals of the BJT (body, base, emitter, collector) may not exceed 5V without damaging the device. A typical safety margin of 10% overvoltage is acceptable for short periods of time.

[0083] An OLED microdisplay (commonly referred to as an "AMOLED") consists of an active matrix of OLED pixels that produce light (luminescence) when electrically activated, and that have been deposited or integrated onto an array of transistors located on a silicon chip, where the array acts as a series of switches to control the current flowing to each individual pixel. Typically, this continuous current is controlled by at least two transistors at each pixel (to trigger the luminescence), one of which starts and stops the charging of a storage capacitor along with controlling another transistor that provides a voltage source at the level required to produce a constant current for the pixel. In some embodiments, the transistor (commonly referred to as the scan or select transistor) that controls the current provided by another transistor (commonly referred to as the drive transistor) is controlled by a select line that is common to all pixels spanning along the row. The signal passed by the scan transistor is provided by a data line that is common to all pixels spanning along the column.

[0084] This is illustrated in Figure 1, which represents the simplest form of prior art AMOLED pixel design. The simplest AMOLED pixel with pixel memory uses two transistors and a capacitor. The current drive transistor MP2 is typically fed from a supply voltage V DD Connected to the anode of the OLED. As shown in the figure, one transistor (MP2) drives the current of the OLED, and the other transistor MP1 (also called the scan transistor) acts as a switch to sample and hold the voltage on the storage capacitor C1. There is a control circuit that controls the current V flowing through the drive transistor MP2. DD The data line (provides V 数据 ). There is a select line that controls MP1 and therefore controls the charging of capacitor C1. Typically, transistors have intrinsic capacitance, so depending on the intrinsic capacitance of the transistor and the leakage current through the transistor, additional capacitance may not be needed.

[0085] For clarity, any capacitors present have been omitted from the diagrams in the figures following Figure 1. The presence of capacitors is optional, and the circuit may have no capacitors or any number of capacitors as desired. The reference voltage for the capacitors may be V DD or some other voltage.

[0086] It should be noted that the threshold voltage (V th ), carrier mobility or series resistance will directly affect the current uniformity of the OLED driver TFT and thus affect the brightness of the display. A major factor affecting the non-uniform current is the threshold voltage (V th ) variations. For pixels, the control circuitry may need to perform other types of compensation; for example, aging, degradation or ageing of the OLED material over time, non-uniformity or unevenness across the active area, or voltage drops in metal interconnects. Additionally, the control circuitry provided by the TFTs may need to control the timing of the delivery of current to the pixel, for example, via PWM. The design of control circuits for OLEDs, including various types of compensation and drive schemes, has been of great interest, and many approaches have been proposed. Such compensation circuitry may also be present in control circuits for stacked OLEDs having 3 or more stacks.

[0087] Surprisingly, it was discovered that the high voltages and currents required for stacking three or more stacked OLEDs can be handled by a control circuit comprising at least two transistors whose channels are connected in series. This transistor is sometimes referred to as a "stacked transistor." Ideally, one of the series-connected transistors is a low-voltage driver transistor, and the other is a switching transistor. This arrangement allows the OLED pixels to be driven without significant current leakage through the TFTs, resulting in high brightness without a loss of contrast.

[0088] Specifically, the driving TFT and the switching transistor are connected in series so that the first terminal of the driving transistor is electrically connected to (and closer to) an external power source, the second terminal of the driving transistor is electrically connected to the first terminal of the switching transistor, and the second terminal of the switching transistor is electrically connected to (and closer to) the bottom electrode of the OLED stack. This arrangement allows the OLED stack to operate at a higher voltage than a single transistor is designed for without causing significant current leakage or physical damage to the low-voltage transistor.

[0089] The basic control circuit is arranged as Figure 2 As shown, the p-channel driver transistor T1 is connected to V at its first terminal (source for a p-channel transistor). DD(external power supply) and connected to switching transistor T2 at its second terminal (the drain in a p-channel transistor). The gate of drive transistor T1 is controlled via the data line and series select transistor T3, whose gate is controlled by select line Select 1. Switching transistor T2 receives current at its first terminal (source) from T1's second terminal (when T1 is "on") and is connected to the bottom electrode of the OLED at its second terminal (drain). The gate of switching transistor T2 is directly controlled by select line Select 2. Drive transistor T1 and switching transistor T2 are connected in series. When both T1 and T2 are selected to be "on," current flows to the bottom electrode of the OLED stack, and it emits light. If T2 is selected to be "off," no current will flow to the OLED stack, regardless of whether T1 is "on" or "off." In some embodiments, switching transistor T2 can provide a shutter function to prevent motion blur. If T1 is "off," it does not matter whether T2 is "on" or "off"; the OLED will not emit light.

[0090] exist Figure 2 In V DD The control circuit of the backplane having at least two transistors connected in series with the bottom electrode of the OLED electrode is a drive transistor and a switch transistor. In the present application, the drive transistor has the function of regulating the voltage and current flowing to the OLED pixel to appropriate levels according to the image displayed, and the purpose of the switch transistor is to switch the current flowing to the OLED pixel on and off in order to provide a shutter function. For example, during operation of the microdisplay, the switch transistor T2 interrupts the power flowing from the drive transistor T1 to the bottom electrode of the OLED stack, so that no light is emitted (black) for a part of the frame time. This is different from the scan transistor T3, which is turned on only for a very small part of the frame time (for example, 1 / 1200 for a display with 1200 rows) in order to power the storage capacitor ( Figure 2 Typically, for the same maximum operating voltage, the switching transistor can be smaller than the driving transistor. However, if the switching transistor is required to handle higher voltages, as in a control circuit with three or more stacked OLEDs, it can be larger and have a higher voltage rating than the driving transistor.

[0091] Desirably, the driving transistor (T1) is closer to the power supply V compared to the switching transistor (T2) which is located closer to the bottom electrode of the OLED. DDPositioning. Preferably, at least the drive transistor is a p-channel MOSFET transistor, and more preferably, both the drive transistor and the switch transistor are p-channel MOSFET transistors. The two transistors connected in series can be two LV, two MV, or one LV and one MV transistor. Preferably, two LV transistors are used to reduce the excessive size of the pixel circuit. However, in some embodiments, the drive transistor is LV and the switch transistor is MV.

[0092] Despite Figure 2 Only p-channel transistors are shown, but n-channel transistors or a mixture of n-channel and p-channel transistors may be used. In this case, it will be necessary to rearrange the circuit appropriately to take into account the difference in polarity between n-channel and p-channel transistors. In addition, other microelectronic components such as other transistors, capacitors, and resistors may be included in the control circuit as needed.

[0093] As is known in the art, MOSFET transistors require intrinsic body diode connections to operate as required. Due to the structure of the MOSFET transistor, the parasitic diode is inherently present and can affect the operation of the transistor. Typically, the intrinsic body diode is connected to a power supply internally or externally to apply bias. These body connections are also called "body connections" or "transistor wells" and other terms. This is in Figure 3 As shown in FIG, IBD1, IBD2 and IBD3 are connected to a separate voltage source V DD2 Intrinsic body diode connection (for T1, T2 and T3). For intrinsic body diode connection, these transistors can share the same well. However, the same power supply V used to power T1 and T2 DD It can also be used for IBD; that is, V DD and V DD2 It is a public power supply.

[0094] However, it may be desirable to float one or more transistors in their own separate wells on the silicon backplane to avoid exceeding the operating voltage range of any component. Specifically, when the first drive transistor and the switch transistor are both p-channel transistors, each transistor can be located in its own separate n-well. This allows a larger dynamic voltage range to control the OLED than can be achieved using two transistors in the same n-well. The use of isolated, floating or different wells for transistors connected in series is known in the art, for example, see US9066379, US5764077, US7768299, US9728528 and JP2016200828.

[0095] This is Figure 4As shown in the figure, T1 occupies its own well as shown by the dotted line and is connected to V DD2 and T2 occupies a different well as shown by the dotted line, which is biased by the connection with the transistor source (V DD ) are biased by different individual connections IBD2. Figure 4 In the embodiment of FIG. 5 , the bias voltage applied to each IBD is different, and therefore, each n-well is independent of the others.

[0096] Figures 2 to 4 The embodiment shown has several advantages in design and operation for driving stacked OLEDs with three or more. In design, the circuit can be very compact because all transistors are relatively small LV transistors, commonly found in most foundries. All transistors can be p-channel transistors with all n-wells biased to V DD , which eliminates the need for an isolated well or floating well. These features can allow for very compact pixel circuit designs for microdisplay designs with very high resolution in small sizes.

[0097] Figure 5A The control circuit of the three transistors in series is shown. DD In addition to the third transistor T4 connected in series with the source of the driving transistor T1, Figure 5A The circuit shown is Figure 2 As shown, the gate of T4 is controlled via the data line and the series select transistor T5, the gate of which is controlled by the select line select3. Figure 5A Lines Select 2 and Select 3 in can provide signals at the same or different voltages, can switch at the same time or at different times, or can remain unswitched depending on operating conditions.

[0098] Desirably, the added transistor T4 may be a second driving transistor so that the power supply V DD Between the bottom electrode of the OLED are two drive transistors (T4 and T1) and a switch transistor (T2), all connected in series. In this case, the signal provided to the gate of T4 can also be the same as the signal provided to T1 and can be provided via T3 / Select 1. Although Figure 5A A design with one data line and two select lines for data control is shown, but a similar design with two data lines and one select line is also possible.

[0099] In embodiments where there are three or more transistors in series, these transistors can be a combination of both LV and MV transistors. Preferably, these transistors are all LV transistors to reduce the size of the pixel circuit. These multiple transistors can be p-channel, n-channel, or a mixture of transistors. Preferably, all are p-channel. If there is a mixture of n-channel and p-channel transistors, then preferably there is at least one driver transistor that is a p-channel transistor, and more preferably, it is an LV transistor. The switching transistor is the preferred LV transistor; however, when it is necessary to extend the operating range of the backplane circuit while observing the voltage limitations of a single transistor, it may be desirable to use one or more MV switching transistors.

[0100] In the case where any of the multiple transistors in series are of a single type, multiple common transistors of the same type can share the same well to reduce the size of the design. However, to extend the operating range of the backplane circuit while observing the voltage limitations of the individual transistors, it may be necessary to place transistors of the same type in separate well areas.

[0101] Figure 5B shows a similar Figure 5A Schematic diagram of the control circuit of two driver transistors shown, where the two driver transistors are controlled by a level shift circuit (LSC). A level shift circuit is a circuit used to convert a signal from one logic level or voltage domain to another logic level or voltage domain, and is often used to resolve voltage incompatibilities between parts of a system. For clarity, internal components or all connections to the LSC (such as V DD , ground, and other possible input connections). In this embodiment, the LSC sets the gate voltages of driver transistors T4 and T1 based on signal Select 1 and the signal from the data line so that the total voltage across T4 and T1 is always divided approximately equally between the two driver transistors. As is well known, small logic transistors can be used for this function.

[0102] OLED-based microdisplays typically include protection circuitry in the MOSFET-based control circuitry of the backplane to limit the amount of power flowing through the transistors to prevent damage. For this purpose, it is desirable to include protection circuitry in the control circuitry of the backplane of a microdisplay having OLEDs with three or more stacked cells, since the power required to cause such a device to emit light is relatively high. The protection circuitry should maintain, or "limit," the voltage at the bottom electrode of the OLED so that it does not fall below a desired voltage level when the OLED is not emitting light. This protection circuitry may also be referred to as a "voltage maintenance" circuit.

[0103] To protect the low voltage transistors present in the control circuitry and remain within the specified operating range of the transistors as set by the foundry, the protection circuitry would be expected to maintain a black level current (CV = 0 or pixel "off") at the bottom electrode of the stacked OLED for pixels below 4 μA / cm2 (or more desirably 2 μA / cm2 or less for a 3-cell stacked OLED with a Vth of approximately 7.5 V). For a 4-cell stacked OLED device, a similar black level current is required, and a typical Vth is approximately 10 V.

[0104] Figure 6 Shows something like Figure 2 The schematic control circuit is similar to the control circuit shown in FIG, with the addition of a protection circuit. In this particular embodiment, the protection circuit comprises a p-channel transistor connected at one end to a node between the drain of T2 and the bottom electrode of the OLED, and at the other end to a power supply (in this example, a reference voltage V REF (common across all pixels). The gate of T6 is also connected to this node. Therefore, the diode-connected transistor T6 limits the minimum drain voltage of T2 to prevent it from breaking down. There may be other electronic components not shown as part of this circuit. In addition, one end of the p-channel transistor is grounded instead of connected to V REF Otherwise, this arrangement is similar to that described by Kwak et al.

[0105] However, the issues caused by the higher voltage requirements of using OLEDs with 3 or more cells still apply to the use of MOSFET transistors such as Figure 6 In this embodiment, the use of a LV or MV MOSFET transistor (i.e., T6) to implement protection for the LV drive and switch transistors does not result in a significantly smaller pixel design compared to using MV drive and switch transistors and eliminating the protection circuitry.

[0106] Except that the p-channel transistor T6 is replaced by the diode D4, Figure 7 Shown with Figure 6The protection circuit is similar to the protection circuit shown in FIG. Desirably, the diode D4 in the protection circuit is a pn junction diode. A pn ​​junction diode is a circuit element that allows current to flow in one direction but not in the other (opposite) direction. A pn ​​junction diode can have a forward bias in the direction where current flows easily, or a reverse bias where little or no current flows. Such a pn junction diode can be formed inside a CMOS project in different ways; for example, as a vertical diode with a highly doped p region in an n-well. One end of D4 is connected to a node located between the drain of T2 and the bottom electrode of the OLED, and the other end is connected to a power supply 45, which is a reference voltage V REF Or reference current I REF .

[0107] Figure 8 Shows something like Figure 7 The protection circuit shown in Figure 8 In the protection circuit, the diode D4 is specifically a BJT1 (BJT (bipolar junction transistor)), whose collector (C) is connected to V DD , whose emitter (E) is connected to the node between T2 and the bottom electrode of the OLED stack, and whose base (B) is connected to the power supply 50, which is a voltage source V 保护 or current source I 保护 One benefit of using a BJT in a protection circuit is that the current is amplified from the base current to the collector current. If desired, the collector of the BJT can be connected to a different source than V DD A separate power supply. Power supply V 保护 or I 保护 It can be shared by all pixels. It should be noted that V 保护 or I 保护 May or may not be constant, but may vary depending on whether the OLED is to emit light or not. This may be advantageous when used with a switching transistor to reduce the persistence of the display through the shutter. Figure 8 In the figure, BJT1 is an "NPN" type BJT transistor (preferably), but it can also be a "PNP" transistor after appropriate changes in design.

[0108] For NPN type BJT, whenever the emitter voltage V E Greater than the base voltage V B And V B Less than the collector voltage V C (V E >V B <V C ), the BJT will be in off mode and no current will flow. However, whenever the voltage VE Less than voltage V B And V B Less than V C (V E <V B <V C ), the BJT will be in forward active mode. In this mode, the base-emitter junction is forward biased and the base-collector junction is reverse biased, and therefore the collector-emitter current will be approximately proportional to the base current.

[0109] Therefore, in Figure 8 If BJT1's V C It is V DD And V 保护 (It is V B ) is set to be smaller than the V th , whenever the voltage V at the bottom electrode E Higher than V th +V 阴极 , BJT1 is “cut off”, but whenever V E falls below V th +V 阴极 (i.e. whenever T1 or T2 is “off”), the voltage at the bottom electrode is maintained at V th Therefore, the protection circuit is designed to provide enough current when needed to protect the driving transistor and the switching transistor so that the voltage across their terminals does not exceed their rated value whenever the cathode voltage drops (more negative voltage) below a certain value. In addition, by increasing the base voltage B (V 保护 ) is set to be lower than the OLED’s on-state voltage (V on the cathode). th ), whenever the voltage delivered to the bottom electrode of the OLED is greater than or equal to V th , power loss is minimized.

[0110] In use Figure 8 In some embodiments of the protection circuit shown, the base voltage of the BJT (V B ) is isolated from any external power supply. That is, the power supply V 保护 or I 保护 There is no electrical connection to the base of the BJT. The BJT physically exists with the existing collector and emitter connections, such as Figure 8 As shown, the base connection, while still present, is not connected to any external source. In this case, V B It is not intentionally maintained at any particular value, nor is any voltage or current intentionally applied to it. B is allowed to be independent of the voltage V C and V EThe OLED is "floating" and is kept as part of the active control circuitry that operates the OLED. However, there may be parasitic current paths within the backplane that bias the base internally with high impedance. It should be noted that in this type of embodiment, V B may vary during operation of the OLED.

[0111] In other embodiments, the BJT's V B Can be self-biased (sometimes referred to as having "base bias"). When a BJT is self-biased, no externally controlled input signal is applied to the base of the BJT, but instead the base is controlled by a constant supply voltage (i.e., V DD ) and the value of any bias resistors connected to the transistor to set the signal applied to the base of the BJT. One method for achieving self-biasing of the base of a BJT is to form a "fixed base bias circuit." In this arrangement, the base of the BJT is connected to a constant power supply (i.e., V) through a single current limiting resistor. DD ). For a given V DD value, which will allow the BJT base current (I B ) remains constant, and therefore the operating point of the BJT also remains fixed. Alternatively, a simple voltage divider network can provide the required bias voltage. It should be noted that in this type of embodiment, the bias voltage and the resulting parasitic current within each pixel will respond to the operation of the OLED. This response can provide effective protection for the transistor.

[0112] Figure 9 is used for Figure 8 Schematic diagram of a cross section of the transistor wells of the circuit shown. It should be noted that T1, T2, and T3 can each be located in a separate, but not isolated, or floating n-well connected to V DD And all separated from the p-well of BJT1 (NPNBJT). For convenience, the source (s), gate (g), and drain (d) regions are labeled for T1, T2, and T3. Similarly, the emitter (e), base (b), and collector (c) regions are labeled for BJT1. The collector (c) region of BJT1 is shown connected to V through a deep n-well. DD , but in some embodiments it may alternatively be connected directly to V DD

[0113] Figure 9 Also indicated are IBD5, which is the V to the deep n-well of the silicon substrate where all transistors are located, and IBD6. DDThe intrinsic body diode connection of IBD6 is the intrinsic body diode connection of the entire silicon substrate to ground. It should be noted that the BJT in the protection circuit can be an NPN or PNP transistor. If the BJT is an NPN transistor, it is in the p-well, while if the BJT is a PNP transistor, it is in the n-well.

[0114] A protection circuit comprising a BJT is preferably used. When a BJT transistor is used as part of a control circuit comprising at least two transistors whose channels are connected in series, the pixel size of the BJT transistor design is smaller. In addition, the inherent amplification factor of the forward-active-mode BJT means that a relatively small parasitic current can generate a much larger protection current. As a result, the current drawn from the reference / protection voltage source is much smaller, and the voltage drop across the reference voltage interconnect is significantly reduced.

[0115] In addition to maintaining the minimum voltage at the bottom electrode below the threshold voltage of the OLED by including other appropriate circuit components, the protection circuit can also be designed to prevent other undesirable influences, such as short circuit protection, transient spikes, etc. However, in some cases, it may still be useful to include this known type of protection circuit in the pixel circuit when using the stacked OLED of the present invention.

[0116] like Figures 2 to 5B As shown, for many OLED stacks, especially those including three light-emitting units, it is desirable that in the control circuit, there be no intervening transistor in the electrical connection between the second terminal of the switching transistor and the bottom electrode of the OLED. By intervening, it is meant that in the electrical connection between the switching transistor and the OLED, current does not flow directly through the other transistor (i.e., one terminal of the transistor is connected to the switching transistor and the other terminal is connected to the OLED, such that current would flow through the intervening transistor at some point during operation of the OLED). Other (non-transistor) microelectronic components may be connected in series between the switching transistor and the OLED.

[0117] There can be a branched (meaning not directly in series) connection between the switching transistor OLED connection and the first terminal of the microelectronic component (i.e., the non-intervening transistor or diode), wherein the OLED operating current does not flow directly through the non-intervening transistor or diode. Figures 4 to 6 , where one side of the non-intervening component is connected to the T2-OLED connection and the other side is connected to the power supply.

[0118] Desirably, the drive transistor is a low voltage (LV) transistor. That is, it is designed and sized to operate safely and efficiently at voltages of 5V or less, without regard to the actual load in the circuit. This is necessary to help minimize pixel size in order to maximize microdisplay resolution. It should be noted that an OLED stack with three or more OLED cells driven by a drive transistor will typically require voltages in excess of 7.5V. If desired, two or more low voltage drive transistors in series in separate wells (such as Figure 4 as shown) to further mitigate the effects of higher voltages.

[0119] The drive transistor is preferably a p-channel thin film transistor (also known as a p-channel MOSFET (metal oxide semiconductor field effect transistor)). The structure, characteristics, and preparation of p-channel transistors are well known. When the drive transistor is a p-channel transistor, its source is electrically connected to an external power supply, and its drain is electrically connected to the first terminal of the switching transistor. The gate of the drive electrode is controlled by a data line that is separate from the power supply.

[0120] When the OLED stack includes three OLED light-emitting units, it is desirable that the switching transistor be a low-voltage transistor. That is, the switching transistor should be designed and sized to operate safely and efficiently at voltages of 5V or less, regardless of the actual load in the circuit. The current required for the operation of the OLED flows through the transistor and drives it.

[0121] However, if the OLED stack contains four or more OLED light-emitting units, the operating voltage may be much higher than 7.5V as needed, and the switching transistors can be medium voltage (e.g., designed for 7.5V to 12V) or high voltage (designed for 18 to 25V). Alternatively, two or more switching transistors connected in series, selected together or independently, can be used to mitigate the effects of higher voltages. The multiple switching transistors connected in series can all be LV or can be a mix of LV and MV transistors.

[0122] The switching transistor is preferably a p-channel transistor. When the switching transistor is a p-channel transistor, its source is connected to the drain of the drive transistor, and its drain is electrically connected to the bottom electrode of the OLED. The gate of the switching electrode is controlled by a select line that is separate from the data line and select line that control the power supply and drive transistor.

[0123] In some embodiments, it is desirable that both the drive transistor and the switch transistor are p-channel transistors, where both are low voltage transistors, or the drive transistor is low voltage and the switch transistor is medium voltage or high voltage. In some embodiments, it is also desirable that all transistors in the control circuit are p-channel transistors.

[0124] Although the drive transistor and the switch transistor are electrically connected in series, there may be other intervening microelectronic components between the two transistors. The intervening components may be connected in series, where the current between the drive transistor and the switch transistor flows directly through the series components. There may also be other microelectronic components electrically connected to the connection between the drive transistor and the switch transistor, such that the current flowing from the drive transistor also flows to the additional components and the switch transistor.

[0125] In its most basic form of operation, a voltage is supplied to the drive transistor from an external power source. A select line is set to allow the correct data voltage from the data line to be applied to the gate of the drive transistor, thereby allowing current from the power source to flow through the drive transistor to the switching transistor. The select line for the switching transistor is set to allow the correct charge to be applied to the switching transistor, thereby allowing current to flow through the switching transistor to the bottom anode of the OLED stack in each individual pixel. This causes the pixel to "turn on" and emit light based on the current it receives. When the image being displayed requires that an individual pixel emit no light or reduced light, the data voltage of the drive transistor can be changed to prevent or limit current flow through the driven pixel.

[0126] Other components in the control circuit can help control the current provided by the drive transistor, either to control the current intended to power the OLED when "on", or to control current leakage through the drive transistor when the OLED is intended to be "off".

[0127] To prevent or minimize motion blur by providing a shutter function, a switching transistor can prevent current from flowing to the OLED regardless of the operation of the drive transistor (which controls the power supplied to its pixels); specifically, when the drive transistor is "on" (passing current). At the appropriate time for the pixel, the switching transistor can be selected so that the OLED pixel remains "off" even if the image to be displayed requires that pixel to be "on." This allows all or a rotating portion of the pixels in the display to be "off" (not emitting light), which minimizes the perception of motion blur in the microdisplay.

[0128] In a microdisplay, power from an external power supply can be delivered as a variable current or voltage to the drive transistor in order to drive the OLED stack to deliver the desired level of brightness. This is typically stored by charging a storage capacitor during a write operation. The power level can be controlled at the external power supply, or if the power delivered is constant, it can be set to the appropriate level by other microelectronic circuits within the backplane. This is called "current control" and is commonly used to power most OLED devices. Alternatively, the power supplied to the OLED stack can be constant, and the total amount of light emitted within a set time period (frame) is controlled by the time the OLED pixel is fully "on" compared to the time it is "off". This is called pulse width modulation or PWM control.

[0129] Since the described control circuit is able to handle higher voltage and current demands than at least the drive transistor is designed or rated for without causing significant leakage or damage, the use of OLED stacks with increased light emission (and higher voltages) is enabled. th The OLED device with two light-emitting units in series does not emit the same amount of light as the one with a relatively high V th The described circuit can be used with an OLED having a threshold voltage (V th ) is used together with a stacked light-emitting OLED; more desirably, the light-emitting OLED stacked V th is at least 10 V or greater. Alternatively, the circuit can be used with a stacked OLED that provides a full color microdisplay having an emission of at least 2500 nits, or preferably at least 5000 nits.

[0130] There are two basic approaches to making pixelated OLED microdisplays where the brightness of each individual pixel must be controlled by powering one of the pixel electrodes via a control circuit on the backplane. The first approach involves having each pixel produce red, green, or blue light (R, G, B, respectively) or the same color if it is a monochrome display. In this case, the light-emitting OLED stack can be arranged so that all stacked light-emitting units above a single bottom electrode segment emit the same color of light (selected from R, G, or B light) to form R, G, and B pixels. In some embodiments with this feature, each color pixel forms a microcavity in which the distance between the segmented bottom electrode and the top electrode depends on the color of the light emitted. In this case, the length of the microcavity will depend on the color of the emitted light and will be different for red, green, and blue pixels.

[0131] The second approach is to have a common multi-mode (white) emitting OLED layer across all pixels with a color filter array (CFA) to create separate RGB pixels. This approach has an advantage over the first approach because it does not have to form individual OLED pixels of different design, and therefore will reduce manufacturing costs.

[0132] The number of individual OLED light-emitting cells within a stacked OLED is limited only by the overall thickness of the OLED and the ability of the control circuitry to handle the power required to operate the OLED. As the number of OLED cells increases, the total amount of light emitted increases, but the thickness of the package, the complexity of the manufacturing process, and the threshold voltage also increase. An OLED with at least three stacked light-emitting cells will provide increased brightness over a tandem OLED (two OLED cells). However, OLEDs with at least four stacked OLED light-emitting cells are preferred, and more preferred are OLEDs with at least five stacked OLED light-emitting cells. OLEDs with as many as six to ten or more stacked OLED light-emitting cells are contemplated.

[0133] To minimize the increase in voltage required to drive the OLED stack, a charge generation layer (CGL; sometimes also called a connector or intermediate layer) is located between individual OLED light-emitting units. This is because the structure of the CGL is such that electrons and holes are generated when voltage is applied and injected into the adjacent organic emission layer. Therefore, the use of a CGL has the potential to convert one injected electron into multiple photons, allowing for higher brightness. Specifically, it is desirable that a CGL is located between each light-emitting unit within the stack. However, it is not necessary for a light-generating unit to have adjacent CGLs on both sides. The OLED light-emitting units on the top and bottom of the stack will typically have only one adjacent CGL. Although a CGL can be used when desired, it is generally not necessary to use a CGL between a light-emitting unit and one of the top or bottom electrodes.

[0134] Many different types of CGLs have been proposed and can be used in OLED stacks. For example, see US7728517 and US2007 / 0046189. To form a CGL, an np semiconductor heterojunction located at the interface of the n-type and p-type layers is generally required to generate charges. Therefore, the CGL will have two or more layers. For example, n-doped organic layer / transparent conductive layer, n-doped organic layer / insulating material, n-doped organic material layer / metal oxide layer, and n-doped organic material layer / p-doped organic material layer have all been reported. The desired metal oxide for CGL is MoO3. In some cases, the n-layer and p-layer can be separated by a thin intermediate layer. Typically, the CGL is arranged so that the n-layer is closer to the anode and the p-layer is closer to the cathode.

[0135] An ideal concept for a CGL has three layers; an electron transport material doped with an n-type dopant (e.g., Li), a thin intermediate layer of the same (but undoped) electron transport material, and a hole transport material doped with a p-type dopant. Suitable electron transport and hole transport materials, along with n-type and p-type dopants suitable for CGLs, are well known and commonly used. These materials can be organic or inorganic. The choice of suitable material is not critical and any material can be chosen based on its properties. The thickness of the CGL should ideally be between 200 and 1000 nm. In many instances, a CGL will have an ETL on the anode side and a HTL on its cathode side to help improve charge transport and to help separate the charge-generating dopants (if present) from the LEL in the light-emitting cell.

[0136] While using a CGL helps minimize the voltage rise when OLED light-emitting cells are stacked on top of each other, the total voltage required for the stack still increases by approximately the voltage required for each individual cell. An OLED with at least three stacked OLED light-emitting cells would be expected to require voltages exceeding the recommended operating range of the 5V drive transistor.

[0137] In one embodiment, all OLED light emitting cells within an OLED stack above a single bottom electrode segment emit the same color; for example, red, green, or blue. This results in a pixelated RGB microdisplay. Figure 10 shows a microdisplay 100 , the microdisplay 100 Three different OLED sub-pixel stacks are used to form R, G, and B pixels. Each OLED sub-pixel stack contains three OLED light-emitting units that emit the same color, where each unit is vertically separated from the other by a CGL.

[0138] In microdisplays 100 In the present invention, there is a silicon backplane 3, which includes a silicon backplane 3 such as Figures 2 to 8, and other necessary components that will power the sub-pixels based on the input signals. On layer 3 with the transistors and control circuitry, there may be an optional planarization layer 5. Above layer 5 (if present) are individual first electrode segments 9 connected by electrical contacts 7, which extend through the optional planarization layer to form electrical contacts between the individual bottom electrode segments 9 and the control circuitry in layer 3. The individual bottom electrode segments 9 are electrically isolated from each other in the laterally direction by the pixel definition layer 1. Above the segmented bottom electrode segments 9 are non-luminous OLED layers 11, such as electron or hole injection (EIL or HIL) layers or electron or hole transport (ETL or HTL) layers. A first light-emitting OLED unit 13 is above the OLED layer 11. Layer 15 is a charge generation layer that is located between the first light-emitting layer 13 and the second light-emitting OLED unit 17 and separates them. Above the second light-emitting OLED unit 17, there is a second charge generation layer 19 that is located between the second light-emitting OLED unit 17 and the third light-emitting OLED unit 21 and separates them. Above the third light-emitting layer 21 are non-emissive OLED layers 23 such as electron or hole transport layers or electron or hole injection layers and a transparent top electrode 25 through which light can be transmitted. The OLED microcavity is protected from the environment by an encapsulation layer 27. In the embodiment shown, all organic layers in a single OLED stack are horizontally separated from adjacent stacks by a pixel definition layer 1, but the top electrode 25 and encapsulation 27 are common and extend across the entire active area. However, the top electrode 25 does not need to be continuous and can be segmented as required. In microdisplays 100 In FIG, all OLED light emitting cells 13, 17, 21 above the same bottom electrode segment 9 emit the same color of light (B, G or R). This particular microdisplay is not a microcavity device; however, a similar pixelated RGB design utilizing the microcavity effect can be used.

[0139] A well-known method for improving the brightness and color purity of OLED light emission is by utilizing the optical microcavity effect. This effect is based on the formation of an optical resonator between a reflective surface and a semi-reflective surface, which allows some light to pass through. Depending on the optical distance between the two surfaces, multiple reflections between the two surfaces form a standing wave, which will enhance certain wavelengths of light and reduce other wavelengths of light due to constructive and destructive interference effects that will occur depending on whether the emission occurs at the antinode or node of the standing wave, respectively. The antinode will appear at different positions depending on the total space between the reflectors and the wavelength to be optimized. However, the light emitted from the microcavity can exhibit severe angular dependence, where color shift and brightness loss can occur as the viewing angle deviates from perpendicular to the viewing surface. This is generally not a problem for NED applications due to the limited angle of incidence of the projection optics.

[0140] It is hoped that the brightness of OLED stacks can be further increased by using the microcavity effect. Figure 10 The microdisplay shown 100 It can be redesigned to produce a microcavity effect by using a reflective bottom electrode or a reflective layer under the bottom electrode, thereby making the top electrode semi-transparent so that it has a certain degree of reflectivity and adjusting the distance between the uppermost surface of the reflective element (bottom electrode 9 or the underlying reflective layer) and the bottommost surface of the top electrode to form a microcavity suitable for light of that specific color.

[0141] Figure 11 shows a microdisplay 200 , which uses a multi-mode (white) OLED microcavity shared across all pixels together with a color filter array (CFA) to form R, G, and B pixels. Multi-mode OLEDs produce more than one color of light. Ideally, a multi-mode OLED produces white light with approximately equal amounts of R, G, and B light. Typically, this would correspond to a CIE 0.33, 0.33 x 、CIE y However, some variation in these values ​​may still be acceptable or even desirable, depending on the characteristics of the color filters used to form the RGB pixels. 200 A microcavity effect is also incorporated. In this embodiment, a multimode OLED stack contains three OLED light emitting units emitting different colors, where each unit is vertically separated from the other by a CGL, where the distance between the reflective surface and the top electrode is constant over the active area.

[0142] In microdisplays 200 In the present invention, there is a silicon back plate 3, which includes a silicon back plate such as Figures 2 to 8, and the necessary components to provide power to the sub-pixels based on the input signals. Above layer 3 with transistors and control circuitry, there may be an optional planarization layer 5. Above layer 5 (if present) are individual first electrode segments 9 connected by electrical contacts 7, which extend through the optional planarization layer to form electrical contacts between the individual bottom electrode segments 9 and the control circuitry in layer 3. In this embodiment, the bottom electrode segments 9 have two layers, a reflective layer 9B closer to the substrate 1 and an electrode layer 9A closer to the OLED layer. The individual bottom electrode segments 9 are electrically isolated from each other in the lateral direction. Above the segmented bottom electrode segments 9 are non-emissive OLED layers 11, such as electron or hole injection layers or electron or hole transport layers. The red OLED light generating unit 13A is above the OLED layer 11. Layer 15 is a first charge generating layer that is located between the red OLED light generating unit 13A and the green OLED light generating unit 17A and separates them. Above the green light-emitting layer 17A, there is a second charge generation layer 19, which is located between and separates the green OLED light generating unit 17A and the blue OLED light generating unit 21A. Above the blue OLED light generating unit 21A are non-emissive OLED layers 23, such as electron or hole transport layers or electron or hole injection layers, and a semi-transparent top electrode 25. This forms an OLED microcavity 30 that extends from the uppermost surface of the reflective surface 9B to the lowermost surface of the semi-transparent top electrode 25, which is also a semi-reflective electrode. The OLED microcavity is protected from environmental influences by an encapsulation layer 27. In this embodiment, there is a color filter array with color filters 29B, 29G and 29R that filters the multi-mode emission generated by the OLED microcavity 30 so that B, G and R light are emitted depending on the power provided to the bottom electrode segment 9.

[0143] Figure 12 shows a microdisplay 300 , for microdisplays 200 A variation of the blue OLED light emitting layer 22 having an additional blue OLED light emitting layer 22 between the blue OLED light emitting unit 21A and the OLED layer 23. In this embodiment, there is no CGL between the blue OLED unit 21A and the additional blue light emitting layer 22. 200 Same, microdisplay 300There are three (not four) OLED light-emitting cells because blue cell 21A and blue layer 22 are not separated by a CGL (and therefore, layer 22 is not considered a separate light-emitting OLED cell and should be considered part of blue light-emitting cell 21A). Even if blue cell 21A and blue LEL 22 were separated by a non-charge generating intermediate layer, there would still be only three OLED cells in the stack. However, if there were an intermediate CGL located between blue light-emitting cell 21A and the additional blue light-emitting layer 22, the OLED stack would contain four OLED cells.

[0144] Figure 13 It shows a micro-display with five OLED light-emitting units. 200 Microcavity effect for multi-mode OLED microdisplays 400 In this case, some OLED cells emit light of the same color, while other cells emit light of different colors. Specifically, a multi-mode microdisplay 400 There are two blue OLED light-emitting units, one green OLED light-emitting unit, one yellow OLED light-emitting unit, and one red OLED light-emitting unit, all of which are separated by CGLs.

[0145] In microdisplays 400 In the example, OLED cell 13A emits red light. OLED cell 13A is separated from yellow-emitting OLED cell 16 by CGL 15. Yellow-emitting OLED cell 16 is separated from green-emitting OLED cell 17A by CGL 14. Green-emitting OLED cell 17A is separated from first blue-emitting OLED cell 21A by CGL 19. Second blue-emitting cell 32 is separated from first blue-emitting OLED cell 21A by CGL 24. All other layers are connected to the same Figure 12 The same as in .

[0146] As previously mentioned, OLED microdisplays are built on a silicon backplane that serves as a substrate. Generally, the backplane will be flat and of uniform thickness. Because silicon backplanes are typically opaque, the OLED stack is preferably top-emitting. However, transparent backplanes are known; in this case, the OLED stack can be top-emitting or bottom-emitting. The top surface of the substrate is the surface facing the OLEDs. The silicon backplane can have various types of underlying layers (i.e., planarization layers, light management layers, light blocking layers, etc.), which can be patterned or unpatterned and can be on the top or bottom surface.

[0147] The bottom electrode segment (9 or 9a) can be an anode or a cathode and can be transparent, reflective, opaque or translucent. If the OLED is top-emitting, the bottom electrode can be made of a transparent metal oxide or a reflective metal such as Al, Au, Ag or Mg or alloys thereof and have a thickness of at least 30 nm, desirably at least 60 nm.

[0148] In microcavity applications where the first electrode is above a reflective layer, the first electrode should be transparent. However, in other applications, the first electrode layers 9A and 9B can be folded into a single reflective electrode so that its uppermost reflective surface forms one side of the optical microcavity (i.e., Figure 10 30 in the ).

[0149] When the OLED stack is a top-emitting microcavity and the bottom electrode is transparent, there should be a reflective layer below the bottom electrode that defines the first side of the microcavity 30. When the transparent anode is located above the reflective surface, the transparent anode is part of the optical cavity. The reflective layer 9B can be a reflective metal such as Al, Au, Ag, Mg, Cu or Rh or their alloys, a dielectric mirror or a highly reflective coating. The dielectric mirror is composed of multiple thin layers of materials such as magnesium fluoride, calcium fluoride and various metal oxides deposited on the substrate. The highly reflective coating is composed of multiple layers of two materials, one with a high refractive index (such as zinc sulfide (n=2.32) or titanium dioxide (n=2.4)) and one with a low refractive index (such as magnesium fluoride (n=1.38) or silicon dioxide (n=1.49)). The thickness of the layer is typically one-quarter wave relative to the wavelength of the light being reflected. It is desirable that the reflective layer reflects at least 80% of the incident light, and most preferably at least 90% of the incident light. The preferred reflective layer is Al or Ag with a thickness of 300 to Most preferably, 800 to

[0150] Ideally, when the OLED stack is bottom-emitting, the bottom electrode is a transparent anode and should transmit as much visible light as possible, preferably with a transmittance of at least 70% or more desirably at least 80%. Although the bottom transparent electrode can be made of any conductive material, a metal oxide such as ITO or AZO or a thin metal layer such as Ag is preferred. Materials with poor conductivity (such as TiN) can be used, provided they are made thin.

[0151] Suitable for non-emissive layers such as hole injection layers, hole transport layers, or electron injection layers or electron transport layers (ie, Figure 10The electron transport and hole transport materials of 11 and 23 in the embodiment of the present invention are well known and commonly used. These layers can be mixtures of such materials and can contain dopants to change their properties. Since these layers are non-luminescent, they do not contain luminescent materials and are transparent. The selection of suitable materials is not critical and any material can be selected based on its performance.

[0152] In embodiments utilizing the microcavity effect, because the spacing between the various OLED units within the microcavity and the size of the microcavity are important for maximizing efficiency, the thickness of the various non-emissive layers must generally be selected to provide the desired spacing. Preferably, the spacing between the OLED units and the size of the microcavity are adjusted by using appropriate thicknesses of the organic non-emissive layers (such as the hole transport layer).

[0153] The light-emitting layer typically has a host material (or a mixture of host materials) and a light-emitting compound, which is the main component of the layer. Ideally, the light-emitting compound is phosphorescent because they have higher efficiency. However, in some cases, some LELs may use fluorescent or TADF (thermally activated delayed fluorescence) compounds as materials for light emission, while others use phosphorescent materials. Specifically, blue light OLED layers may use fluorescent or TADF compounds or a combination thereof, while non-blue light-emitting layers may use green, yellow, orange or red phosphorescent compounds or a combination thereof. The light-emitting layer may use a combination of light-emitting materials. The selection of suitable materials for LELs is well known, not critical, and any material may be selected based on its performance and luminescent characteristics. When using phosphorescent emitters, it is sometimes necessary to confine the excitons generated by the phosphorescent emitter to the layer. Therefore, if necessary, an exciton blocking layer on either or both sides of the phosphorescent LEL may be used. Such materials and their applications are well known. In addition, it may be desirable to add HBL (hole blocking layer) and EBL layers (electron blocking layer) around the light-emitting layer, especially the blue light-emitting layer, to improve lifetime and brightness efficiency.

[0154] If the OLED stack is top-emitting, the top electrode (i.e. Figure 1025) should be transparent, if the OLED stack is bottom emitting then the top electrode should be reflective, and in the case of the OLED stack being a microcavity then the top electrode should be translucent as well as translucent: that is, it reflects part of the light and transmits the rest. In the case of a microcavity, the bottommost inner surface of the top electrode defines the second side of the microcavity 30. Desirably, the translucent top electrode reflects at least 5% of the light emitted by the LEL, and more desirably at least 10%, in order to establish the microcavity effect. The thickness of the translucent second electrode is important because it controls the amount of light reflected and the amount transmitted. However, the second electrode cannot be too thin because then the second electrode may not be able to transfer charge into the OLED effectively or may suffer from pinholes or other defects. The thickness of the top electrode layer is ideally 100 to 1000 nm. and more ideally 125 to

[0155] It is desirable that the top electrode be a thin layer of metal or metal alloy. Suitable metals include Ag, Mg, Al and Ca or their alloys. Of these, Ag is preferred because it has a relatively low blue absorption capacity. To aid electron transport and stabilization, an adjacent layer of a transparent metal oxide such as ITO, InZnO or MoO3 may be present on the electrode surface. Alternatively, a metal halide such as LiCl, an organometallic oxide such as lithium quinolate or other organic materials may be used.

[0156] There may be a protective layer or a spacer layer above the upper electrode ( Figures 10 to 13 not shown) to prevent damage during the packaging process.

[0157] The encapsulation 27 is deposited or placed above the top electrode 25 and any optional protective layer (if present). The encapsulation should completely cover the light emitting area on the top and sides at a minimum and be in direct contact with the substrate. The encapsulation should be impervious to air and water penetration. The encapsulation can be transparent or opaque. The encapsulation should not be conductive. The encapsulation can be formed in situ or can be added as a separate preformed plate with the side edges sealed together. An example of in situ formation would be thin film encapsulation. Thin film encapsulation involves depositing multiple layers of alternative layers with inorganic materials and polymer layers until the desired degree of protection is achieved. The concepts and methods for forming thin film encapsulation are well known and any concept and method can be used as needed. Alternatively, the encapsulation can be provided using a preformed plate or cover sheet attached over at least the sealing area and the enclosed area. The preformed plate can be rigid or flexible. It can be made of glass (including flexible glass), metal or organic / inorganic barrier layer. It should have a thermal expansion coefficient close to that of the substrate to achieve a more robust connection. The preformed package plate may need to be attached over the sealing area using a breathable and waterproof adhesive (such as a silicone adhesive or epoxy adhesive) or by thermal means (such as ultrasonic welding or glass frit welding) which may require an additional sealant such as solder or glass frit. The side edges and bottom edges of the cover sheet can be specifically designed to better fit into the sealing area or promote a better seal. The cover sheet and the sealing area can be designed together so that they partially fit or lock in place before the seal is formed. In addition, the cover sheet can be pre-treated to promote better adhesion to the sealing area.

[0158] Although this application describes the use of OLEDs as light-emitting elements in microdisplays, the same control circuitry can be used in any self-emissive display technology that will require a relatively high voltage to emit light. The present invention is not limited to OLEDs, but is applicable to any other display technology that will require greater than 5V, preferably greater than 7.5V, or even greater than 10V to provide at least 1000 nits, or preferably at least 5000 nits of light.

[0159] Crosstalk in a display is a situation where the brightness of the light emitted by one pixel is unexpectedly affected by another pixel. This is undesirable because the affected pixel no longer provides the correct brightness according to the image signal, and as a result, image quality may be degraded. Depending on the amount and nature of the crosstalk, important factors such as color reproduction, contrast (the difference between maximum and minimum brightness), grayscale, resolution, and "ghosting" in the display can all be negatively affected. Typically, crosstalk in microdisplays is particularly problematic where the pixel pitch is small. It is desirable that the amount of crosstalk between pixels is 15% or less, preferably 5% or less, and most preferably 3% or less.

[0160] The stacked microcavity OLED devices described herein must be thick enough to have a defined spacing between the internal light emitting units. This is necessary to provide a microcavity effect to enhance the R, G, and B emissions within a single microcavity. Increased crosstalk can result from both optical and chemical / electrical mechanisms. Some optical processes that can increase the amount of crosstalk include light scattering and waveguiding within the OLED device. Some chemical / electrical processes that can increase crosstalk include lateral carrier migration from an active pixel area to an adjacent inactive pixel area within the same layer.

[0161] We believe there are multiple modes contributing to crosstalk. Short-range (0.2 to 0.7 μm) interactions appear to be a combination of lateral carrier and optical mechanisms. Medium-range (3 to 7 μm) interactions appear to be primarily due to lateral carrier migration, but may also be partially due to optical mechanisms. Long-range (50 to 200 μm) interactions appear to be primarily due to light scattering from the active pixel area to the inactive area. We also believe there are even longer-range optical contributions to crosstalk based on the waveguides according to the pixel spacing.

[0162] Some useful methods for minimizing crosstalk problems caused by carrier migration in OLED devices include:

[0163] - Lateral carrier migration is reduced by varying the layer thickness and composition (to increase the "resistance of the plate") in layers with high carrier mobility (e.g., HIL, HTL, CGL, ETL, and EIL). Specifically, carriers (holes or electrons) are generated in the active region and can move laterally across the gap between the illuminated and non-illuminated areas. The problem appears to occur primarily in layers next to or near one of the electrodes. It is believed that the common HIL and HTL layers above the anode may be the biggest contributor to the problem. It appears that once holes are generated in the illuminated area of ​​the HIL on one anode pad, they can migrate to the non-illuminated area of ​​the adjacent anode pad, and the voltage generated due to the holes can exceed the V of the OLED. th , and therefore the (nominally unilluminated) pixel will emit light anyway. Furthermore, these holes can enter the conductive anode pad as electrons and flow laterally through the anode with very little lateral resistance. At the far side of the unilluminated anode pad, the current can flow back into the HIL (as holes) in order to jump to the next unilluminated anode pad. Therefore, the problem of carrier migration may not be limited to the short distance between adjacent anode pads, but may also have a longer distance component. Therefore, special attention should be paid to the thickness and composition of the two electrodes, especially the anode. Thinner organic layers with smaller carrier mobility can help minimize these undesirable carrier migration processes.

[0164] - Material selection for organic layers with high carrier mobility. Specifically, materials can be selected to minimize their contribution to crosstalk. In this regard, the type and level of p-type dopants added to the HIL (e.g., F4-TCNQ, F6-TCNNQ, or HAT-CN) and the choice of HTM in the HIL or HTL (e.g., aromatic amine compounds such as NPB or spiro-TTB) may be important. A p-doped or undoped HIL alone may also be effective. In some cases, a non-doped HIL and a p-doped HTL can be used. Inorganic HIL materials (such as MoO3) (which can be mixed with organic materials) may also have advantages.

[0165] Some useful methods for minimizing crosstalk problems caused by optical processes within OLED devices include:

[0166] -Optimized color filters to reduce light waveguides between the air / glass interface and the reflective anode, including the use of specially designed optical filter layers to absorb light traveling at high angles from the substrate normal.

[0167] - Reduce light scattering by reducing the number of scattering sites. Specifically, the number of small particle debris on or near the anode should be minimized. Scattering can also be caused by the roughness of the cathode, which can depend on the composition and process used for deposition. (See, for example, Shen et al., "Efficient Upper-Excited State Fluorescence in an Organic Hyperbolic Metamaterial," Nano Lett, 2018, 18, 3, pp. 1693-1698)

[0168] -The entire anode surface should be as flat and smooth as possible both over the active pixel area and between pixels. Specifically, it is well known that protrusions, bumps or other structures that form a PDL (pixel definition layer) between pixels and extend above the surface of the anode within the pixel area can be used to scatter light back into the pixel area and prevent it from entering the adjacent (unilluminated) pixels. However, this approach does not work well when there is a thicker OLED layer covering the structure. Light trapped within the thicker layer is more likely to be reflected internally within the layer so that it can travel over the structure to the other side. If the anode and covering OLED layers are uniformly flat, then light waveguided within the layers of the display is more likely to continue uninterrupted until it is absorbed or reaches the edge of the display.

[0169] -Use absorbers for waveguiding light.

[0170] - Light absorption by the dielectric of the backplane.

[0171] - The design of the HIL and the anode forms a barrier for the charges entering the anode from the HIL.

[0172] To control crosstalk, it may be useful to add additional, separate pixel control circuitry to the node between the second switching transistor and the anode of the OLED that controls the voltage at the anode whenever the OLED is driven so that it is nominally "off" (not powered and therefore not emitting light). In particular, the pixel control circuitry should reduce the voltage to V of the OLED only when the drive circuitry requires that the pixel emit no or very little light. th In this way, any voltages developed by lateral carrier migration can be minimized.

[0173] Experimental results

[0174] In the following examples, unless otherwise stated, the number preceding each material (e.g., 200 HIL) is the physical layer thickness in angstroms. All % are by weight. All devices were packaged after cathode deposition using the same procedure. All materials were purchased commercially.

[0175] The backplane used for the following experimental equipment is based on single crystal silicon and is received with Figure 8 and Figure 9 The backplane contains a driver transistor (T1) and a switch transistor (T2), which are low-voltage (rated at 5V) p-channel transistors connected in series, as well as a protection circuit with an NPN bipolar junction transistor (BJT1) for each pixel. In all experimental examples, the base of BJT1 is isolated; that is, the base is not connected to an external power supply, so that the base voltage of the BJT is not intentionally controlled or set to any specific voltage. The backplane includes a reflective metal layer as the bottom electrode / anode.

[0176] Comparative 2-unit stacked OLED device A-1 was prepared as follows:

[0177] Layer 1 (reflective bottom electrode / anode): As supplied

[0178] Layer 2 (hole injection layer HIL): 200% hole transport material HTM1 + 6% p-type dopant PD1

[0179] Layer 3 (hole transport layer HTL1): 410 hole transport material HTM1

[0180] Layer 4 (HTL2): 50% hole transport material HTM2

[0181] Layer 5 (green LEL): 100 green matrix GH1 and 3% green phosphorescent dopant GPD1 Layer 6 (yellow LEL): 100 GH1 + 10% GPD1 + 3% red phosphorescent dopant RPD1 Layer 7 (electron transport layer ETL1): 200 electron transport material ETM1

[0182] Layers 8 to 10 (charge generation layer CGL1): 270 three layers

[0183] Layer 11A (HTL3): 300HTM1

[0184] Layer 11B (HTL4): 280HTM1

[0185] Layer 12 (blue LEL2): 200 blue host BH1 + 4% fluorescent blue dopant BFD1 Layer 13 (ETL2): 150 ETM2

[0186] Layer 14 (electron injection layer EIL): 150ETM1+2%Li

[0187] Layer 15 (semi-transparent cathode): 105 75% Ag / 25% Mg

[0188] A second comparative OLED device, A-2, was prepared in a similar manner, except that the thickness of layers 11A and 11B were increased to 1090 and 4090 Å, respectively. Both A-1 and A-2 are 2-cell stacked OLEDs, where layer 5 (G LEL) and layer 6 (red LEL) together represent one cell (GY stack), and layer 11 (blue LEL) represents a second cell (B stack) separated from the first cell by a CGL. Because layers 5 and 6 are adjacent LELs that are not separated from each other by a CGL (layers 8 and 9), they are a single cell. Both A-1 and A-2 are microcavity devices, where the microcavity thickness of A-1 is 2320 Å, and the thickness of the microcavity in A-2 is 6920 Å. These two-cell stacked OLED devices represent the current state of the art.

[0189] A 3-unit stacked OLED B-1 of the present invention was prepared in the same manner as A-1, except that the following additional layer was inserted between layers 11A (whose thickness was adjusted to 470 Å) and 11B (at 4090 Å):

[0190] Layer 16 (BLEL1): 200BH1 + 4% BFD1

[0191] Layer 17 (ETL3): 150ETM2

[0192] Layers 18 to 20 (CGL2): 270 three layers (same concept as CGL1)

[0193] OLED device B-1 is a 3-cell stacked OLED, where layers 5 and 6 are the first GY unit (same as in A-1), layer 16 is an additional second B unit, and layer 12 (same as in A-1) is a third B unit. The second unit (layer 16) is separated from the first unit (layers 5 and 6) and the third unit (layer 12) by a CGL. That is, compared to A-1, B-1 contains an additional B unit and, therefore, will have higher brightness, but with an increased voltage requirement. OLED B-1 is a microcavity device with the same microcavity thickness (6920 Å) as device A-2. Figure 14 The characteristic IV curves (in mA / cm2) of OLEDs A-1, A-2 (comparative 2-cell device) and B-1 (3-cell device of the present invention) are shown. 2 The current density is expressed in units of voltage), and is also presented in Table 1.

[0194] Table 1 – List IV data for OLEDs

[0195]

[0196]

[0197] *I = current density on the anode pad, in mA / cm 2 Unit

[0198] V = voltage

[0199] L = luminous efficiency, in cd / A

[0200] exist Figure 14 As shown in Table 1, OLEDs A-1 and A-2 have approximately the same V th However, OLEDB-1 with the additional B unit has a significantly higher V th , and requires an even higher voltage (about 2.5V higher) to produce the same current density as A-2.

[0201] exist Figure 15 The advantages of a three-cell stacked OLED with a microcavity over a two-cell stacked OLED can be seen in Figure 1, which compares the spectral output (intensity vs. wavelength in nm) without color filters A-1, A-2, and B-1. OLED A-1, with its finer cavity, has relatively less B and R emission compared to G emission. OLED A-2, with a microcavity for all three colors, has more B and R emission compared to G, but still has a limit on B emission. OLED B-2, with its additional B cell, has significantly increased blue emission compared to A-2.

[0202] The above results relate to white light OLED devices before the addition of color filters that will create the R, G, and B pixels for the microdisplay. As is well known in the art, regardless of any inherent imbalance in the relative amounts of R, G, and B light produced by the WOLED, the desired R, G, and B balance in the display (typically expressed in terms of white brightness, such as a D65 white point) can be achieved by driving each color pixel to the appropriate brightness for that color. For example, if the W OLED produces insufficient blue light and too much red light compared to green light, the B pixel can be driven at a higher voltage to produce more blue light, while the R pixel can be driven at a lower voltage to produce less red light.

[0203] Figure 16 A graph showing the peak current density required for each color of OLED devices A-1, A-2, and B-1 using color filters to produce a D65 white point (assuming the areas of the R, G, and B sub-pixels are equal). The current density in B required by the 2-unit devices (A-1 and A-2) is much greater than that of the 3-unit device B-1. The current density in R and G is approximately the same in all 3 devices. The 3-unit device B-1 requires less current density, and therefore less power, to achieve the same target brightness at the same white point. The lifetime of the display is determined by the efficiency drop rate of the OLED with continuous operation, which is primarily a function of current density and temperature. Therefore, the lifetime of display B-1 will be more than twice that of A-1 or A-2.

[0204] In microdisplays, given the need to produce relatively high brightness due to their small size, it is desirable to display white at maximum display brightness while keeping the total current (and current density) for each color roughly the same (or as close as possible). This provides many advantages in terms of: neutral fading (each color of the subpixel loses brightness efficiency at approximately the same rate over the life of the device), high brightness (each pixel will be driven at its maximum brightness resulting in increased efficiency), high contrast (the full dynamic range of each pixel is available), and increased life (the current load is evenly shared by each color of the pixel).

[0205] However, if the required voltage increases, the advantage of using an OLED with 3 or more cells to provide increased brightness can be offset. In a display, the power supply for each pixel is provided and controlled by a control circuit in the backplane. However, in a microdisplay that needs to have a large number of pixels in a reduced area, there is only limited physical space for transistors and other circuit components. Generally speaking, higher voltage and current requirements must be handled by larger, thicker or more robust electronic circuits. Currently, the size of OLED microdisplays requires that the electronic circuits be rated to handle voltages of 5V or less in order to achieve the required pixel size and density (pixel pitch).

[0206] Electronic circuits, such as transistors, can fail catastrophically if excessive power is applied to their gates for too long. However, not all levels or durations of overvoltage will cause permanent transistor failure; in many cases, transistors fail due to leakage. Increased leakage is a common failure mode caused by non-catastrophic overstress in semiconductor devices, when the junction or gate oxide suffers permanent damage that is insufficient to cause catastrophic failure. Overstressing the gate can result in stress-induced leakage current.

[0207] Addressing motion blur in microdisplays used in virtual reality and similar products requires that the OLED provide very high brightness for very short periods of time and then be off for another period (the total on / off period being less than human perception to avoid flicker). This means that the control circuitry in the backplane must provide high voltage / current for one period to produce the high brightness, and then shut down the OLED for another period. If too much power is applied to the transistor in the control circuitry during the period when the OLED is on, current can leak through the transistor when it should be "off" and the OLED cannot fully shut off. The difference between the brightness of an OLED when it is on and the brightness when it is supposed to be off is often referred to as "contrast ratio." High contrast ratio is desirable in microdisplays because it makes images appear clear and sharp; low contrast ratio makes images appear dim and dull.

[0208] Figure 17 A set of (luminance versus cathode voltage) curves is shown for a 2-cell comparison device and a 3-cell device of the present invention. The top set of curves is when the device is driven at full brightness (white = CV255) with the drive transistor "on". The bottom set of curves is when the device is driven at no brightness (black = CV0) with the drive transistor nominally "off". However, leakage of current through the drive transistor causes some small amount of brightness to be generated, and the voltage at the cathode is non-zero. The difference between the two sets of curves represents the "contrast ratio" of the device, and it is desirable to increase the contrast ratio as much as possible by minimizing the brightness caused by leakage of current through the drive transistor. From Figure 17 It can be seen that the contrast ratios of all three devices are roughly the same over a range of cathode voltages (slightly less than 10 5 ), and is relatively constant.

[0209] It should be noted that despite requiring a higher operating voltage beyond the design limits of the drive transistors, the 3-cell device of the present invention maintains approximately the same overall contrast ratio as the comparative 2-cell device. Figure 16 As shown, although less current density is required to achieve full white for B-1 than for A-1 or A-2, B-1 still requires more voltage (about 2.5V, see Figure 14There is no evidence that the higher voltage OLEDs used in the present invention's backplanes increase leakage. Furthermore, there is no evidence of any catastrophic transistor failures in B-1 due to the higher voltage requirements (see Table 1), and there appears to be no impact on the device life of the control circuitry. These results are unexpected because the transistors in the backplanes used in A-1, A-2, and B-1 were designed with transistors suitable for driving 1 or 2 unit OLEDs (where low voltage transistors are acceptable and commonly used).

[0210] Using the same low-pressure backplane as A-1, A-2, and B-1, another 3-unit OLED device B-2 of the present invention was prepared as follows:

[0211] Layer 1 (bottom electrode / anode):

[0212] Layer 2 (HIL): 200HTM1+6% PD1

[0213] Layer 3 (HTL1): 410HTM1

[0214] Layer 4 (HTL2): 50HTM2

[0215] Layer 5 (Green LEL): 100GH1 and 3% GPD1

[0216] Layer 6 (yellow LEL): 100GH1+10% GPD1+3% RPD1

[0217] Layer 7 (ETL1): 200ETM1

[0218] Layers 8 to 10 (CGL1): 270 three layers

[0219] Layer 11 (HTL3): 570HTM1

[0220] Layer 12 (blue LEL2): 250BH1 + 4% BFD1

[0221] Layer 13 (ETL2): 150ETM2

[0222] Layers 14 to 16 (CGL2): 270 three layers (same concept as CGL1)

[0223] Layer 17 (HTL4): 3840HTM1

[0224] Layer 18 (BLEL1): 250BH1 + 4% BFD1

[0225] Layer 19 (ETL3): 150ETM2

[0226] Layer 20 (EIL): 100ETM2+2%Li

[0227] Layer 21 (semi-transparent cathode): 105 75% Ag / 25% Mg

[0228] OLED B-2 is a 3-cell device like B-1, in that it contains a GY cell (layers 5 and 6) separated from a second blue cell (layer 12) by a CGL (layers 8 to 10), which in turn is separated from a first blue cell (layer 18) by a second CGL (layers 14 to 16) in the order YG / B / B (from the backplane). Like B-1, B-2 is a 3-cell device with Microcavity for white light-generating OLEDs.

[0229] In a similar manner, a 4-cell OLED device C-1 of the present invention was prepared as follows:

[0230] Layers 1 to 11: Same as layers 1 to 11 in B-2

[0231] Layer 12 (blue LEL2): 200BH1 + 4% BFD1 (reduced )

[0232] Layer 13 (ETL2): 100ETM2 (lower )

[0233] Layers 14 to 16: Same as layers 14 to 16 in B-2

[0234] Layer 17 (HTL4): 420HTM1 (reduced )

[0235] Layer 18 (BLEL1): 200BH1 + 4% BFD1 (reduced )

[0236] Layer 19 (ETL3): 100ETM2 (lower )

[0237] Layers 20 to 22 (CGL3): 270 three layers (same concept as CGL1 and CGL2)

[0238] Layer 23 (HTL5): 2620HTM1

[0239] Layer 24 (HTL6): 50HTM2

[0240] Layer 25 (yellow LEL): 200GH1+10% GPD1+3% RPD1

[0241] Layer 26 (ETL4): 480ETM1

[0242] Layers 27 to 28: Same as layers 20 to 21 in B-2

[0243] OLED C-1 is a 4-cell device because OLED C-1 contains an additional Y unit (layer 25) in B-2 along with the same GY and two B units in the order YG / B / B / Y (from the backplane). In C-1, the additional Y unit is separated from the first B unit (layer 18) by CGL3 (layers 20 to 22). C-1 is a 4-cell device with Microcavity for white light-generating OLEDs.

[0244] In a similar manner, a 5-cell OLED device D-1 of the present invention was prepared as follows:

[0245] Layers 1 to 10: Same as layers 1 to 11 in C-1

[0246] Layer 11 (HTL3): 370HTM1 (reduced )

[0247] Layers 12 to 16: Same as layers 12 to 17 in C-1

[0248] Layer 18 (ETL2): 100ETM2 (lower )

[0249] Layers 14 to 16: Same as layers 14 to 16 in C-1

[0250] Layer 17 (HTL4): 620HTM1 (increase )

[0251] Layers 18 to 22: Same as layers 18 to 22 in C-1

[0252] Layer 23 (HTL5): 610HTM1 (reduced )

[0253] Layer 24 (BLEL3): 200BH1 + 4% BFD1

[0254] Layer 25 (ETL3): 100ETM2

[0255] Layers 26 to 28 (CGL4): 270 three layers (same concept as CGL1, CGL2, and CGL3)

[0256] Layer 29 (HTL6): 1440HTM1

[0257] Layers 30 to 34: Same as layers 24 to 28 in C-1

[0258] OLED D-1 is a 5-cell device because OLED D-1 contains an additional B cell (layer 24) in C-1 along with the same Y, GY, and two B cells. It has a total of 3 B cells, one Y cell, and one GY cell, all separated by the CGL in the order YG / B / B / B / Y (from the backplane). C-1 is a 5-cell device with Microcavity for white light-generating OLEDs.

[0259] Figure 18 Characteristic IV curves for OLEDs B-2, C-1, and D-1 are shown and also prepared as Table 2.

[0260] Table 2 – List IV data for OLEDs

[0261]

[0262]

[0263] *I = current, in mA / cm 2 Unit

[0264] V = voltage

[0265] L = luminance, in cd / A

[0266] like Figure 18 As shown in Table 2, adding each unit in the OLED provides greater brightness, but also increases V th And the operating voltage is increased by about 2.5V.

[0267] exist Figure 19 The advantages of adding additional cells to a microcavity 3-cell OLED can be seen in Figure 2, which compares the spectral output (without color filters) of B-2 (3-cell), C-1 (4-cell), and D-1 (5-cell). OLED B-2 has a G / Y cell plus two B cells. Adding another Y cell to C-1 increases the amount of G and R emission relative to B-2, but does not increase B emission much. Adding a third B cell to D-1 further increases blue emission while maintaining high G and R emission.

[0268] and Figure 16 Similar results are shown in Figure 20 The results show that OLEDs B-2, C-1 and D-1 can be used at 1500 cd / m 2The relative amount of peak current density for each color pixel required to produce balanced white brightness (using color filters) at 100 nm is given by the formula (A). The addition of an additional Y unit in C-1 reduces the current required to produce the necessary amount of R and G brightness compared to B-2. The addition of an additional B unit in D-1 further reduces the voltage required to produce the necessary amount of B light compared to C-1. Because these OLEDs with 3 or more units require less current density to achieve the same brightness, their operating lifetime will be significantly greater than OLEDs with fewer units. The LT70 (time for emission output to be reduced by 70%) lifetime for these devices is expected to be at least 18,000 hours for average video content.

[0269] Figure 21 (Similar to Figure 17 ) shows a set of (luminance vs. cathode voltage) curves for OLEDs B-2, C-1, and D-1. In all three examples, the contrast ratio is roughly the same (slightly less than 10) over a range of cathode voltages. 5 ), and is relatively constant. Remarkably, the contrast is maintained even when these devices are operated at voltages 2 or 3 times the design limit of the drive transistors.

[0270] Without being bound by any particular theory or speculation, there are questions about why a relatively high V th The stacked OLED maintains high contrast but does not cause burnout or destruction when using a backplane with low voltage transistors, which may be related to the use of transistors connected in series. Current leakage in transistors is a well-known problem, and generally speaking, the higher the voltage and current involved, the greater the leakage. As previously discussed, if this leakage is greater than Vth, the leakage may cause light to be emitted from the OLED. Since the total leakage through the transistors connected in series will be (leakage of the first transistor) x (leakage of the second transistor), this multiplier effect may be enough to significantly reduce the leakage at the bottom electrode of the OLED so that the OLED does not emit light due to current leakage and thus maintains contrast.

[0271] In operation, a control circuit comprising at least two transistors, with their channels connected in series in the backplane of an experimental microdisplay, exemplifies a compact circuit that protects the drive transistor T1 from operating under conditions outside its specified range, even when driving three or more stacked OLEDs with a switching voltage range greater than the specified operating range of the LV transistor. If the OLED's on-off voltage range exceeds the LV operating range, only the switching transistor is exposed to these conditions. This is less detrimental to image quality than other pixel circuit designs, in which the drive transistor must operate outside its specified operating range.

[0272] As MOSFET devices age over time, this can cause a slow change in some of their characteristics, such as threshold voltage, subthreshold slope, and transconductance at saturation. These are all symptoms of negative bias temperature instability (NBTI) and hot carrier injection (HCI) found in p-channel transistors. By limiting the operation of a transistor to the specified voltage range for that transistor, it is ensured that the transistor characteristics will remain within the narrow specified range for a long period of time (such as 5 years). Operating the transistor outside the specified voltage range increases the rate of change of the transistor characteristics, thereby reducing the period of time that the characteristics are within the specified range.

[0273] It is important that the characteristics of the drive transistor remain within specified limits, otherwise the result can be image degradation (commonly known as mura). The operation of the switching transistor is more robust to changes in its performance characteristics and can be driven with a wide enough gate voltage to ensure satisfactory operation even when the transistor characteristics have moved outside the specified range. Figures 2 to 6 The circuit shown in the backplane included in the experimental stacked OLED microdisplay for the present invention is an example of a compact circuit designed to handle the higher switching voltages associated with 3 or more OLED cells without the expected degradation in display quality that would be expected due to exceeding the specified operating range of the transistors.

[0274] It is believed Figures 2 to 6 The circuit in Figure 1 does this by selecting the Select 2 voltage for emitter "on" and emitter "off." For emitter "off," a reasonable choice for the Select 2 voltage is V DD , which places T2 in the subthreshold operating region, effectively stopping the current. For emission to be “on”, the 2 voltage can be chosen to be as low as V DD Below 5V.

[0275] In the case where the OLED's switching voltage swing is less than 4V, then when the pixel displays "black," the black-level current will be set by the driver transistor in the conventional manner, and the drain of T1 will be above the Select 2 "on" voltage, and the switching transistor T2 will remain conductive. However, when the OLED's switching voltage swing is greater than approximately 4V (as in the case of a multi-cell stacked OLED with more than 4 cells), when the black-level current is formed, the drain voltage of T1 (which is also the source voltage of T2) is close to the Select 2 "on" voltage, and because the low drain voltage of T1 reduces the overvoltage of T2 (Vgs-Vth) to zero or slightly negative, the switching transistor T2 is turned off. This drives T2 into a subthreshold state, thereby forming a black level. Under these conditions generated by the multi-cell stacked OLED, the driver transistor T1 controls T2 to produce a black-level current.

[0276] For these reasons, it is believed Figures 2 to 6 The simple series-connected transistor design shown is capable of driving multi-cell stacked OLEDs having a switching voltage range that exceeds the voltage range of low-voltage transistors, which have less lifetime-related nonuniformity compared to single-transistor drive circuits, such as shown in FIG1 .

[0277] The protection circuit is designed to maintain the voltage at the anode of the OLED below a certain level whenever the OLED is intended to be "off," or not emitting light, and can help maintain a high contrast ratio. As the cathode voltage drops (more negative voltage), the protection circuit is designed to provide additional current in order to protect the drive transistor and the switching transistor from voltage levels that violate the maximum ratings of the device.

[0278] However, a protective effect was still observed in experimental examples where the base of the BJT was isolated and not connected to an external power supply, so that the base voltage was not intentionally controlled. It has been observed that in devices lacking a stacked OLED, the exponential slope of the black current versus voltage of the OLED display (0.75 decade / volt) is similar to that of the example where the OLED is present. This means that even when the base of the BJT is isolated, the protection circuit still provides some current and voltage control at the anode of the OLED. Without being bound by any particular theory or speculation, the adjacent n-well (e.g., the n-well of the switching transistor T2) may be a source of holes that migrate to the p-well (base) of the BJT BJT1. Once these holes are in the base, they will diffuse across the depletion region to the n-type emitter contact (OLED anode pad) that travels in the forward direction through the base-emitter diode. This will be supplemented by the diffusion of thermally excited electrons from the emitter into the base and their transport through the base and depletion region into the collector, which is facilitated by the large electric field potential between the base and collector. In this scenario, holes from the adjacent n-well (of the driver transistor or switch) donate charge (holes) to the base, which would typically come from an external base connection. When the OLED anode voltage drops to a very low level (e.g., to display black with 3 or more OLED cells), one of the driver circuit transistors (at V DD The potential difference between the adjacent n-well (under the OLED anode voltage) and the BJT base is very large, thereby increasing the flow of holes from the driver transistor well into the BJT base. This increase in base current increases the emitter current due to the amplification of the BJT.

[0279] However, the protection effect provided by the protection circuit is different for each frame and must be reset appropriately for each new frame of the image. This is not a problem when the base of the BJT is connected to an external power supply and actively controlled for each frame. For embodiments where the base of the BJT is isolated and not intentionally connected, a reset for each frame can be provided due to the shutter effect provided by the switching transistor when the pixel is "turned off" by the switching transistor. Therefore, since emission must be turned off during data loading (or during certain parts of the frame time) to reset, this aspect of the protection effect control movement depends on having a drive transistor and a switching transistor connected in series.

[0280] OLEDs with three or more cells can be designed so that the voltage range from black level (below Vth; for example 2uA / cm2) to white level (20mA / cm2) is relatively constant and less than about 6V. Figure 17 and Figure 21 As shown in Figure 1, this can result in a contrast ratio of approximately 10,000:1, with the contrast possibly being slightly less due to the drop in current efficiency at the high end of the current density. This voltage range is approximately within the allowable operating range of the LV transistors, while the protection circuit only becomes active at the low end of the current range when the drive transistor and / or the switch transistor stops the current. Therefore, at the low current end of the range, the protection circuit also prevents the current density through the OLED from dropping below approximately 2 uA / cm 2 While this effect is in addition to the protection provided by having at least two transistors connected in series as described above, it also limits the ability to achieve higher contrast ratios. In exchange for this slightly raised black level and reduced contrast, the protection circuitry allows the pixel driver circuitry to be pushed by reducing the cathode voltage to achieve higher peak brightness or to compensate for efficiency losses due to OLED aging, and to ensure that the LV transistors operate within their specified voltage ranges.

[0281] The above description describes many different embodiments that may involve different combinations of different individual features. As desired, unless incompatible, individual features from any embodiment may be combined in any order or degree without restriction.

[0282] In the above description, reference is made to the accompanying drawings that form a part of this description, and wherein by way of illustration, a specific embodiment that can be put into practice is shown. These embodiments are described in detail so that those skilled in the art can practice the present invention, and it should be understood that other embodiments can be utilized, and without departing from the scope of the present invention, structural, logical and electrical changes can be made. Therefore, the description of any example embodiment should not be considered as limiting. Although the present invention has been described for illustrative purposes, it should be understood that such details are only used for this purpose and, without departing from the spirit and scope of the present invention, those skilled in the art can change.

[0283] Partial List

[0284] MP1 switching transistor

[0285] MP2 driver transistor

[0286] C1 capacitor

[0287] V DD External power supply

[0288] Select 1–Select 3 selection line

[0289] T1 first p-channel driver transistor

[0290] T2 switching transistor

[0291] T3 series selection transistor

[0292] V 阴极 Cathode voltage

[0293] IBD1 to IBD6 Intrinsic body diodes

[0294] V DD2 External power supply

[0295] T4 Second p-channel switching transistor

[0296] T6 protection circuit P-channel transistor

[0297] D4 diode

[0298] V REF Reference voltage

[0299] BJT1 Bipolar Junction Transistor

[0300] LSC level shift circuit

[0301] 1 pixel definition layer

[0302] 3 Silicon backplane

[0303] 5 Optional planarization layer

[0304] 7 electrical contacts

[0305] 9 First electrode segment

[0306] 9A First electrode layer

[0307] 9B Reflective layer

[0308] 11.23 Non-emissive OLED layer

[0309] 13. First light-emitting OLED unit

[0310] 13A red light emitting OLED unit

[0311] 15, 19, 24 Charge generation layer

[0312] 16 yellow light units

[0313] 17 Second light-emitting OLED unit

[0314] 17A green light emitting OLED unit

[0315] 21 Third light-emitting OLED unit

[0316] 21A blue emitting OLED unit

[0317] 22 Blue light-emitting layer

[0318] 25 Top electrode

[0319] 27 Packaging

[0320] 29 Color filter array

[0321] 29B Blue Filter

[0322] 29G green filter

[0323] 29R red filter

[0324] 30 Microcavity

[0325] 32 Second blue light emitting unit

[0326] 45, 50 Power Supply

[0327] GND Ground

[0328] 100 RGB pixelated OLED

[0329] 200 Multi-mode OLED microcavity devices

[0330] 300Multi-mode OLED microcavity devices

[0331] 400 Multi-mode OLED microcavity devices

Claims

1. A microdisplay comprising a light-emitting OLED stack on top of a silicon-based backplane having individually addressable pixels and control circuitry, wherein: The light emitting OLED stack has three or more OLED units between a top electrode and a bottom electrode; as well as For each individually addressable pixel, the control circuit of the silicon-based backplane includes at least two transistors, wherein the channels of the at least two transistors are connected in series to an external power supply V DD and the bottom electrode of the light-emitting OLED stack, wherein the at least two transistors include a driving transistor closest to the power supply and rated at 5 V or less and a switching transistor closest to the bottom electrode of the light-emitting OLED stack.

2. The microdisplay according to claim 1, wherein: The light-emitting OLED stack V th is at least 7.5 V or greater.

3. The microdisplay according to claim 1, wherein: The light-emitting OLED stack V th is at least 10V or greater.

4. The microdisplay according to any one of claims 1 to 3, wherein: The light-emitting OLED stack includes four or more OLED light-emitting units.

5. The microdisplay according to claim 4, wherein: The OLED light-emitting units are each separated from each other by a charge generation layer.

6. The microdisplay according to claim 5, wherein: The bottom electrode is segmented and each segment is in electrical contact with the control circuitry in the backplate.

7. The microdisplay according to claim 6, wherein: The light-emitting OLED stack is top-emitting.

8. The microdisplay according to claim 7, wherein: The light emitting OLED stack forms a microcavity where the physical distance between the segmented bottom electrode and the top electrode is constant across all pixels.

9. The microdisplay according to any one of claims 1 to 3, wherein: The transistors having channels connected in series have a rated voltage of 5 V or less.

10. The microdisplay according to any one of claims 1 to 3, wherein: The rated voltage of the switching transistor is greater than 5V.

11. The microdisplay according to any one of claims 1 to 3, wherein: The two transistors having channels connected in series are both p-channel transistors.

12. The microdisplay according to claim 11, wherein: The two transistors having channels connected in series are each located in a separate well.

13. The microdisplay according to any one of claims 1 to 3, wherein: The control circuit also includes a protection circuit including a p-channel transistor.

14. The microdisplay according to any one of claims 1 to 3, wherein: The control circuit further includes a protection circuit including a pn junction diode.

15. The microdisplay according to claim 14, wherein: The cathode of the pn junction diode is connected to the node of the bottom electrode of the light emitting OLED stack, and the anode is connected to the voltage reference V REF Or current reference I REF .

16. The microdisplay according to any one of claims 1 to 3, wherein: The control circuit also includes a protection circuit including a bipolar junction transistor.

17. The microdisplay according to claim 16, wherein: The bipolar junction transistor is an NPN transistor where the base is connected to a voltage source V 保护 or current source I 保护 , the emitter is connected to the node connected to the bottom electrode of the light-emitting OLED stack, and the collector is connected to the voltage source V DD .

18. The microdisplay according to claim 16, wherein: The base of the bipolar junction transistor is isolated, the emitter is connected to the node connected to the bottom electrode of the light emitting OLED stack, and the collector is connected to the voltage source V DD .

19. The microdisplay according to claim 16, wherein: The two transistors of the bipolar junction transistor connected in series with their channels are located in separate wells.

20. The microdisplay according to claim 19, wherein: The two transistors having channels connected in series are both p-channel transistors and are each located in a separate n-well, and the bipolar junction transistor is an NPN transistor located in a separate p-well.

Images