Stacked OLED Microdisplay with Low-Voltage Silicon Backplane

Abstract

To provide a stacked OLED microdisplay having high luminance and contrast with a small pixel pitch dimension.SOLUTION: A microdisplay comprises a light emitting OLED stack on top of a silicon-based backplane with individually addressable pixels and control circuitry, where the light emitting OLED stack includes three or more OLED units between a top electrode and a bottom electrode, and the control circuitry of the silicon-based backplane includes at least two transistors whose channels are connected in series between an external power supply VDD and 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 circuitry may include a protection circuit consisting of a p-n diode, preferably a bipolar junction transistor.SELECTED DRAWING: Figure 11

Description

[Technical Field]

[0001] The present invention relates to a stacked OLED microdisplay with a low voltage silicon backplane. [Background technology]

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 62 / 966,757, entitled "STACKED OLED MICRODISPLAY WITH LOW-VOLTAGE SILICON BACKPLANE," filed January 28, 2020, and attorney docket number OLWK-0021-USP, and to U.S. Provisional Application No. 63 / 054,387, entitled "STACKED OLED MICRODISPLAY WITH LOW-VOLTAGE SILICON BACKPLANE," filed July 21, 2020, and attorney docket number OLWK-0021-USP2.

[0003] Typically, microdisplays are less than 2 inches (approximately 5 cm) diagonal, ranging from ultra-small display sizes of less than 0.25 inches diagonal. Microdisplays typically have high resolution, with pixel pitches typically ranging from 5 to 15 microns. First commercially available in the late 1990s, they are commonly used in rear-projection TVs, head-mounted displays, and digital camera viewfinders. In recent years, devices such as smartwatches have leveraged the high resolution and low power consumption of these displays. The global market for microdisplays is expected to grow rapidly, with a compound annual growth rate of 20% over the next few years. One of the trends driving this growth is 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 a highly magnified image projected onto a surface. Types of projection microdisplays include rear-projection TVs and small data projectors. The second is near-eye displays (NEDs), which consist of a highly magnified virtual image seen through an eyepiece (such as a virtual reality headset or camcorder viewfinder). These displays are increasingly being used in HMDs and HUDs, especially in the military and medical industries.

[0005] Both types of microdisplays offer significant advantages over traditional direct-view displays such as flat-panel LCDs. Microdisplay advantages include the ability to generate large images from extremely small, lightweight source display units, facilitating their integration 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 microdisplay quality. However, challenges for microdisplay manufacturers have been the relatively high production costs, along with the need for high brightness and contrast and long operating life.

[0006] Microdisplays can be made from 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] LCDs have dominated the microdisplay market in recent years. LCD technology offers high brightness, relatively low cost, and a relatively simple manufacturing process. The use of LCDs has allowed device manufacturers to gradually reduce the size of microdisplay components. LCD displays are currently used in some HMDs, HUDs, EVFs, and thermal imaging glasses and wearables. However, LCD microdisplays require a light source or backlight to create an image, along with a liquid crystal array to modulate the light. This technology has limitations, such as polarization, color space, maximum brightness limits, LC temperature sensitivity, viewing angle, LCD transmission and extinction ratios, system-limited dimensions, and others, which prevent it from providing all of the desired performance characteristics.

[0008] Microdisplays based on microLED technology may offer advantages over LCD microdisplays, such as self-luminance, a larger color gamut, a wide viewing angle, better contrast, faster refresh rates, lower power consumption (image-dependent), and a wide operating temperature range. Currently, microLED microdisplays are based on standard gallium nitride (GaN) wafers adapted from standard LEDs. This approach has the potential to provide high-brightness display devices at a relatively low cost and without lifetime issues. Typically, standard GaN wafers are patterned into arrays of microLEDs. MicroLED displays are then produced by integrating the microLED array and transistors. However, this approach has several manufacturing concerns, including monolithic formation of microLEDs on transistors, pixel spacing, color generation, and spatial uniformity due to color and brightness variations among individual microLEDs.

[0009] OLED technology shares many of the attractive features of microLED technology for microdisplays. It is self-emissive, has excellent image quality, is highly efficient compared to LCD or LCoS, and has extremely high color reproduction and a wide color space. Self-emissive OLED devices have a key advantage over backlit devices (such as LCDs) in that each pixel provides only the intensity required by the image, while backlit pixels provide maximum intensity, followed by absorption of unwanted light. Furthermore, forming OLEDs on top of transistors is much easier and less expensive than forming microLEDs, since the OLED layers can be vacuum-deposited or coated directly onto the transistor backplane. On the other hand, OLEDs have limited brightness and lifetime.

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

[0011] The only way to reduce the 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 (flicker) between refreshes. For OLED microdisplays, the best solution is to "shutter" the displayed image, either by simultaneously turning off the entire active area or by using a "rolling" technique, in which only portions of the displayed image are sequentially turned off at a time. The "rolling" technique is preferred. The time the pixel is turned off is very short, well below the threshold of human eye detectability to avoid perceptible flicker. This is achieved within the control circuitry by the inclusion of a shutter transistor, which, when activated through a select line, prevents current from flowing through the OLED, turning off light emission by the OLED pixel for the desired period of time. In other words, the shutter transistor is a switch transistor in that it only turns the pixel "on" or "off" and does not regulate voltage or current. However, this solution, in which the pixel is turned off for a portion of the time the image is displayed (commonly referred to as the frame time), only increases the need for increased brightness from the OLED whenever the pixel is "on," since it is the average brightness over the frame that is perceived by the eye. A shutter to reduce motion blur can be applied to any method of powering the OLED stack, for example, current control or PWM.

[0012] OLED microdisplays utilizing silicon backplanes are very attractive from a cost and manufacturability standpoint. For example, Ali et al., "Recent advances in small molecule 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 DIGEST, p. 613; U.S. Patent No. 2019 / 0259337, Prache, Displays, 22(2), 49(2001); Vogel et al., 2018 48th European Solid-State Device Research Conference, p. 90, September 2018; and Wartenberg et al., "High Frame-Rate 1" 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 ultra-high brightness to be useful under all environmental conditions, such as outdoors in bright sunlight. Even under controlled environmental conditions, such as in VR goggles, ultra-high brightness is needed to create an immersive visual experience. Ultra-high brightness from the display allows for the use of smaller, lighter, less expensive, and less efficient optical elements, resulting in more competitive headsets. Currently, prior art OLED microdisplays do not provide the desired brightness.

[0014] For example, a press release from one manufacturer of series OLED microdisplays describes a full-color product that may be capable of delivering as much as 2.5k nits, but acknowledges that 5k nits is a more desirable goal (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 that the goal should be 10k nits or higher (see https: / / hdguru.com / calibration-expert-is-10000-nits-of-brightness-enough / , dated July 26, 2018). A recent June 20, 2020 press release (https: / / www.businesswire.com / news / home / 20200630005205 / en / Kopin-Announces-Breakthrough-ColorMax%E2%84%A2-Technology-Unparalleled-Color) describes a series (two-stack) OLED display emitting >1000 nits. It also states that "Further improvements in brightness (>2000 nits) and color fidelity are expected through optimization of OLED deposition conditions. By incorporating structures to enhance output coupling efficiency, the brightness of OLED microdisplays may 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 units on top of each other, so that the total light emitted from the stack is the sum of the light emitted by each individual unit. However, while the total light emitted from such an OLED stack is additive based on the total number of individual OLED light-emitting units, the voltage required to drive the OLED stack is also additive based on the voltage to drive each individual OLED unit. For example, if a light-emitting OLED unit requires 3V to produce 250 nits at a given current, a stack of two such units will require 6V to deliver 500 nits at the same current, a stack of three units will require 9V to deliver 750 nits, and so on.

[0016] OLED stacks are well known, and for example, U.S. Patent No. 7,273,663, U.S. Patent No. 9,379,346, U.S. Patent No. 9,741,957, U.S. Patent No. 9,281,487, and U.S. Patent Application Publication No. 2020 / 0013978 all describe OLED stacks having multiple stacks of light-emitting OLED units, each separated by an intermediate connector or charge-generating layer. Springer et al., Optics Express, 24(24), 28131 (2016) reports OLED stacks having 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 approach of multi-stack OLEDs, which require higher driving 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 that the active (light-emitting) area of ​​the microdisplay contain as many pixels as possible. This requires that the transistors in the backplane's control circuitry be small, yet still large enough to handle the required voltages and currents without permanent damage or current leakage.

[0018] Typically, smaller transistors have lower voltage ratings due to their inability to handle high power due to leakage current and other failure mechanisms. Smaller, lower-voltage transistors have thinner insulating layers at the gate, so they also have higher static leakage currents. For this reason, larger transistors capable of handling higher voltages are generally used for OLED devices that require higher voltages. International Patent Publication No. WO2008 / 057372 discusses the problems and prior art associated with reducing pixel circuit size in microdisplays. See also, for example, O. Prache, Journal of the Society for Information Display, 10(2), 133(2002), O. Prache; "OLED Microdisplays, Displays, 22, 49-56(2001)"; and Howard et al., "Microdisplays based upon organic light emitting diodes", IBM J. of Res. & Dev., 45(1), 15(2001), which discuss the need for microdisplays on silicon backplanes that provide high brightness with a large contrast ratio at low voltages.

[0019] Furthermore, V DD From the power supply at V CATHODEWhen using a MOSFET p-channel transistor to provide a constant current to an OLED with a cathode voltage at 12V, the total voltage must be large 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 leakage current through the transistor will be high and the OLED will turn off to a high brightness (V anode -V cathode ) remains above the OLED threshold voltage, so it continues to emit light. In OLED microdisplays, leakage current through the drive transistor reduces contrast because the OLED pixel continues to emit light when the OLED microdisplay should remain dark. This effect makes pure black (no light emission) gray and reduces the number of gradations between pure black and pure white, which is undesirable.

[0020] There is a need to increase 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 demands without significantly increasing in size in order to maintain the resolution and pixel pitch within the active area of ​​the OLED. In particular, the control circuitry must maintain contrast by preventing or minimizing leakage current through the TFTs. Contrast is the difference in emission between when a pixel should be "off," "black," or non-emitting (typically, an image signal code value (CV) = 0) and when it should be fully "on," "white," or fully emitting (typically, an image signal CV = 255).

[0021] It is common in the semiconductor manufacturing industry that produces backplanes for analog transistors with an operating range of 5V or less to be considered standard "low-voltage" (LV) transistors. A 10% safety margin on the rated voltage is also common, allowing reliable operation up to 5.5V without degradation of the "5V transistor's" lifetime, which is high enough to allow for some overvoltage and drive circuit overhead voltage within the OLED dynamic voltage range. The voltage limit typically applies between any pair of contacts to the transistor (gate, source, drain, body (also called bulk or well)), but it specifically applies to the maximum gate-drain voltage such that the transistor's performance remains within the specified range under these conditions, typically for 43,000 operating hours. Sometimes, depending on the transistor design, the voltage limit for another contact pair may be higher (e.g., 7V), yet 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 legacy TTL logic voltage levels for communication between integrated circuit (IC) chips. With the downward trend in voltages for input-output communications (e.g., 3.3V and 1.8V standards), these 5V transistors are sometimes referred to as medium-voltage (MV) transistors, transitioning the LV label to newer "lower voltage" analog transistors. While 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 voltage ratings above 5V. Higher-voltage analog transistors are also commonly available, but the exact voltages are not standardized across the IC fabrication industry as 5V transistors. For example, higher-voltage transistors are often required for industries such as automotive.

[0022] Currently, silicon backplanes with low-voltage 5V drive transistors are available that use a series (two light-emitting OLED units separated by a CGL) OLED stack for light emission. 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). Such examples have insufficient brightness for the technology requirements.

[0023] Han et al., "Advanced Technologies for Large-Sized OLED Displays," Chapter 3, 10.5772 / intechopen.74869 (2018). This review article describes advances in three-stack white OLED formulations and backplane technologies, including those with two series-connected transistors, but separately and not in combination. The reference also notes that such two-transistor backplanes "are difficult to employ in large, high-resolution panels due to the large line load and short charging time," and therefore employs a different type of backplane circuit for those devices.

[0024] Liu et al., J. Cent. South Univ., 19, 1276-1282 (2012) discloses a 3T1C circuit with two series-connected p-channel transistors in an OLED on Si chip microdisplay. Zeng et al., "A Novel Pixel Circuit with Threshold Voltage Variation Compensation in Three-Dimensional AMOLED on Silicon Microdisplays," P-27, SID 2019 Digest, p. 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 for driving the OLED and controls the current supplied to the OLED. The second transistor is a switch transistor for turning off the OLED during a program operation. The circuit uses V th It also includes additional transistors to determine and compensate for variations.

[0025] U.S. Patent No. 9,066,379 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 source and an OLED. The first transistor is for driving the OLED and controls the current supplied to the OLED. The second transistor is a switch transistor for shuttering (turning off) the OLED. The circuit also requires a third transistor, a medium or high-voltage p-channel transistor, between the switch transistor and the OLED. The purpose of this third transistor is to provide a voltage (V ss -V cathode ) is greater than the OLED threshold voltage during the OLED's "off" period.

[0026] Pashmineh et al., "High-voltage circuits for power management on 65nm CMOS", Adv. Radio Sci., 13, 109-120, 2015, describe high-voltage circuits based on stacked low-voltage CMOS transistors connected in series (also commonly called "stacked" transistors).

[0027] 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, describes a pixel circuit having two series-connected p-channel transistors.

[0028] Kwak et al., "Organic Light Emitting Diode-on-Silicon Pixel Circuit Using the Source Follower Structure with Active Load for Microdisplays," Japanese Journal of Applied Physics, 50, 03CC05 (2011), describes a pixel circuit having three series-connected p-channel transistors and an overvoltage protection circuit. This reference states that an "overvoltage protection circuit" is needed to prevent breakdown of the metal-oxide-semiconductor field-effect transistor (MOSFET), because the operating voltage of OLEDs is higher than that of MOSFETs. U.S. Pat. No. 5,760,477 and U.S. Pat. No. 9,066,379 also describe the use of a protection circuit.

[0029] Vogel et al., SID 2017 DIGEST, Article 77-1, pp 1125-1128, disclose the use of protection circuitry in low-voltage OLED Si microdisplays to extend the OLED voltage operating range.

[0030] Other references involving overvoltage protection for OLEDs are disclosed in U.S. Patent No. 6,580,657, International Patent Publication No. WO2009072205, and Chinese Patent No. 200488960. U.S. Patent Nos. 9,059,123, 9,299,817, 9,489,886, 20080316659, and 20200202793 disclose the use of np junction diodes as bipolar junction transistors in the pixel control circuits of OLED displays.

[0031] Additional patent references disclosing two series-connected transistors include Chinese Patent No. 109166525, U.S. Patent No. 20140125717, U.S. Patent No. 7196682, U.S. Patent No. 6229506, U.S. Patent No. 9262960, U.S. Patent No. 7443367, U.S. Patent No. 6930680, U.S. Patent No. 10614758, International Patent Publication No. WO2019019590, U.S. Patent No. 6946801, U.S. Patent No. 7755585, U.S. Patent No. 8547372, U.S. Patent No. 8786591, U.S. Patent No. 8797314, U.S. Patent No. 9818344, U.S. Patent No. 2018 / 0211592, and U.S. Patent No. 10269296. The following references disclose pixel circuits having three series-connected transistors: U.S. Patent No. 9,324,266, U.S. Patent No. 9,384,692, U.S. Patent No. 10,600,366, U.S. Patent No. 7,180,486, U.S. Patent No. 9,858,863, and U.S. Patent No. 20190279567.

[0032] US Pat. No. 9,059,123 and US Pat. No. 9,489,886 disclose the use of np junction diodes as bipolar junction transistors in pixel control circuits of OLED displays. Summary of the Invention [Means for solving the problem]

[0033] A microdisplay comprising a light-emitting OLED stack on 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, and the control circuitry of the silicon-based backplane controls each individually addressable pixel to be powered by an external power source V DD A microdisplay is described that includes at least two transistors with channels connected in series between a first electrode and a second electrode of an OLED stack.

[0034] A microdisplay as described above, wherein the threshold voltage V th is at least 7.5V or more, or preferably at least 10V or more.

[0035] Any of the above microdisplays, wherein the OLED stack comprises four or more OLED light-emitting units.

[0036] Any of the above microdisplays, wherein the OLED light-emitting units are each separated from one another by a charge generation layer (CGL), or the bottom electrode is segmented and each segment is in electrical contact with control circuitry in the backplane, or the OLED stack is top-emitting.

[0037] Any of the above microdisplays, wherein the OLED stack forms a microcavity in which the physical distance between the segmented bottom and top electrodes is constant across all pixels.

[0038] In any of the above microdisplays, either the transistors whose channels are connected in series are both rated at 5 V or less, or the transistor closest to the power supply is the drive transistor and is rated at 5 V or less, and the transistor closest to the bottom electrode of the OLED is the switch transistor and is rated at more than 5 V.

[0039] In any of the above microdisplays, the two transistors whose channels are connected in series are both p-channel transistors, or the two transistors whose channels are connected in series are each located in a separate well.

[0040] In any of the above microdisplays, the control circuit additionally comprises a protection circuit having a p-channel transistor or diode. The protection circuit may comprise a p-n diode, the cathode of the p-n junction diode being connected to a node of the bottom electrode of the OLED stack and the anode being connected to a voltage reference V REF or current reference I REF is connected to.

[0041] In the protection circuit described above, the diode is a bipolar junction transistor. The bipolar junction transistor has a base connected to a voltage source V PROTECT or current source I PROTECT , with the emitter connected to the node that connects to the bottom electrode of the OLED stack, and the collector connected to a voltage source V DD The base of the BJT may be isolated.

[0042] In the above protection circuit, the bipolar junction transistor is located in a well separate from the two transistors whose channels are connected in series, or the two transistors whose channels are connected in series are both p-channel transistors each located in a separate n-well, and the bipolar junction transistor is an NPN transistor located in a separate p-well.

[0043] Any of the above microdisplays may additionally include an RGB color filter array (CFA) positioned above the OLED stack, where individual R, G, and B color filters match with the segmented bottom electrodes to create R, G, and B pixels, or an RGBW color filter array (CFA) positioned above the OLED stack, where individual R, G, B, and clear or missing color filters match with the segmented bottom electrodes to create R, G, B, and W pixels. [Effects of the Invention]

[0044] The microdisplay provides very high brightness and contrast at small pixel pitch dimensions. [Brief explanation of the drawings]

[0045] [Figure 1] FIG. 1 illustrates a simple prior art control circuit for an OLED. [Figure 2] FIG. 1 shows a basic control circuit with two transistors with channels connected in series, suitable for an OLED stack with at least three OLED units. [Figure 3] FIG. 3 illustrates one embodiment for the intrinsic body diode connection of FIG. 2. [Figure 4] 3 illustrates another embodiment for the intrinsic body diode connection of FIG. 2. [Figure 5A] FIG. 1 shows a basic control circuit with three transistors with channels connected in series, suitable for an OLED stack with at least three OLED units. [Figure 5B] FIG. 5B is similar to FIG. 5A, except that the drive transistor is controlled by a level shift circuit. [Figure 6] FIG. 1 shows a basic control circuit suitable for an OLED stack having at least three OLED units, with an additional protection circuit comprising a diode-connected p-channel MOSFET transistor. [Figure 7]FIG. 1 shows a similar protection circuit with a diode. [Figure 8] FIG. 1 shows a basic control circuit suitable for an OLED stack having at least three OLED units, with a protection circuit having bipolar junction transistors (BJTs). [Figure 9] FIG. 9 is a schematic cross section through a Si backplane illustrating the connection locations, connections, and transistor wells for the circuit shown in FIG. 8. [Figure 10] FIG. 1 is a cross-sectional view of an RGB microdisplay 100 with three adjacent monochromatic RGB OLED stacks, each individual stack having three OLED light-emitting units. [Figure 11] FIG. 2 is a cross-sectional view of a microdisplay 200 having a multimodal microcavity OLED stack with three OLED light-emitting units and an RGB color filter array. [Figure 12] FIG. 3 is a cross-sectional view of a microdisplay 300 having a multimodal microcavity OLED stack with three OLED light-emitting units with an additional blue light-emitting layer and an RGB color filter array. [Figure 13] FIG. 4 is a cross-sectional view of a microdisplay 400 having a multimodal microcavity OLED stack with five OLED light-emitting units and an RGB color filter array. [Figure 14] 1 is a plot of current density versus voltage for several comparative and inventive OLED devices. [Figure 15] FIG. 1 shows spectra (plotted as intensity versus wavelength) for several comparative and inventive OLED devices. [Figure 16] 1 is a graph of the peak current density required for each color for comparative and inventive OLED devices with color filters to produce a D65 white point. [Figure 17]1 is a plot of luminance versus voltage for several comparative and inventive OLED devices when the devices are driven to produce black and again at peak white luminance. [Figure 18] 1 is a plot of current density versus voltage for several inventive OLED devices. [Figure 19] FIG. 1 shows spectra (plotted as intensity versus wavelength) for several inventive OLED devices. [Figure 20] 1 is a graph of the peak current density required for each color for several invention OLED devices with color filters to produce a D65 white point. [Figure 21] 1 is a plot of luminance versus voltage for several invention OLED devices when the devices are driven to produce black and then again at peak white luminance. DETAILED DESCRIPTION OF THE INVENTION

[0046] For purposes of this disclosure, the terms "over" or "above" mean that the associated structure is located above another structure, i.e., on the opposite side from the substrate. "Top," "topmost," or "above" refers to the side or surface away from the substrate, while "bottom" or "bottom" refers to the side or surface closest to the substrate. As otherwise noted, "over" should be interpreted as meaning that two structures may be in direct contact or that there may be an intermediate layer between them. By "layer," it should be understood that a single layer has two sides or surfaces (top and bottom), and in some cases, "layer" may refer to multiple layers considered together and is not limited to a single layer.

[0047] For light-emitting units or layers, R indicates a layer that emits primarily red light (>600 nm, preferably in the range of 620-660 nm), G indicates a layer that emits primarily green light (500-600 nm, preferably in the range of 540-565 nm), and B indicates a layer that emits primarily blue light (<500 nm, preferably in the range of 440-485 nm). It is important to note that R, G, and B layers may produce some light outside the indicated ranges, but the amount is always less than the primary colors. Y (yellow) indicates a layer that emits large amounts of both R and G light and much less B light. "LEL" means light-emitting layer. Unless otherwise noted, wavelengths are expressed in vacuum values, not in situ values.

[0048] An individual OLED light-emitting unit can produce a single "color" of light (i.e., a combination of two or more primary colors, such as R, G, or B, or Y, C (cyan), or W (white)). Monochromatic light can be produced within an OLED unit by a single layer with one or more emitters of the same color, or by multiple layers each with the same or different emitters whose primary emission falls into the same color. A single OLED unit can also provide two-color combinations (i.e., R+G, R+B, G+B) within 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 separate layers that each emit 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 separate layers that each emit a single (but different) color that sums to white. An individual OLED light-emitting unit can have a single light-emitting layer or two or more light-emitting layers (either directly adjacent to each other or separated from each other by interlayers). Individual light-emitting units can also include various types of non-emissive layers, such as hole-transporting layers, electron-transporting layers, blocking layers, and others known in the prior art, to provide desired effects such as facilitating light emission and managing charge transport across the light-emitting unit.

[0049] Because an OLED light-emitting unit can have multiple layers, individual units are sometimes referred to as a "stack," which can be confused with an OLED device having multiple units. In this application, a "stacked" OLED has at least two OLED light-emitting units stacked on top of each other on a substrate, so that there are multiple light sources within the device. In the stacked OLED of the present invention, the individual OLED light-emitting units are separated from each other by a charge generation layer (CGL), not by separately and independently controlled intermediate electrodes. To be considered an OLED light-emitting unit, it must be separated from another light-generating unit by a CGL. Thus, an emitting layer adjacent to one of the OLED light-emitting units but not separated from it by a CGL is not considered a separate unit. Within a stack, all or some of the individual OLED light-emitting units can be the same, or they can all be different from each other. Within an OLED stack, the individual OLED light-emitting units can be arranged in any order between the top and bottom cathodes. Stacked OLEDs can be monochromatic (all pixels of the OLED stack emit predominantly the same color light, e.g., green light) or can have multimodal emission (either all pixels emit light of two or more colors (e.g., yellow or white), or different pixels emit light of different colors such that the overall emission includes light of two or more colors).

[0050] In some cases, the threshold voltage (V th ) can be estimated by linear extrapolation of the I-V curve after significant light emission begins to return to the voltage axis. This method is not accurate since the I-V response curves for OLEDs may not be perfectly linear over their response range, and therefore values ​​calculated in this manner are not precise. A typical range for this metric is + / - 10%. More precisely, the threshold voltage is the voltage at which a current density in the anode layer reaches 0.2 mA / cm 2where there is at least some reliably detectable luminance, i.e., at least 5 cd / A. This is the method used in this application.

[0051] Hereinafter, a transistor may be referred to as being "on" or "off." In an "off" transistor, the signal sent to the gate is intended to prevent current from passing through the terminal; in other words, this signal (typically CV=0) indicates that the signal requires that no current pass through the transistor, so that the transistor is regulated to "off," rather than switched "off." In such a case, even though the transistor may be "off," some leakage current may still exist. Similarly, in an "on" transistor, the signal sent to the gate is intended to allow at least some current to pass through the terminal; in other words, this signal (typically greater than CV=0 but less than 255) indicates that the image requires some light emission from the pixel, so that the transistor is regulated to "on," rather than switched "on." In a similar manner, a pixel or OLED may be referred to as being "on" or "on" according to the requirements of the image, so that an appropriate signal is sent to the pixel or OLED.

[0052] 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 in the fabrication of integrated circuits. The wafer serves as a substrate for microelectronic devices built within or on the wafer. It undergoes many microfabrication processes, such as doping, ion implantation, etching, thin-film deposition of various materials, and photolithographic patterning. Finally, the individual microcircuits are separated by wafer dicing and packaged as integrated circuits. Silicon has a diamond-cubic structure with lattice spacing, and wafers grow from crystals with a regular crystalline structure. When cut into wafers, the surfaces are aligned in one of several relative directions, known as crystal orientations. Silicon wafers are typically not 100% pure silicon; rather, they are formed with initial impurity doping concentrations of boron, phosphorus, arsenic, or antimony, which are added to the melt to define the wafer as bulk n-type or p-type. For background, see Chapter 7 of "Flat Panel Display Manufacturing", Souk, L., Ed., 2018. The silicon backplane is preferably a single crystal Si wafer.

[0053] To provide control circuitry for the operation of the stacked OLED, thin film transistors (TFTs) are provided on the surface of a silicon wafer along with other components such as capacitors, resistors, connecting wires or bus bars, and the like. 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, 8-12 Oct, New Delhi; U.S. Patent No. 9,066,379; and U.S. Patent No. 10,163,998. It should be understood that the TFTs may or may not incorporate the silicon wafer as part of the TFT structure, or may be prepared from separate materials deposited on the surface.

[0054] TFTs can be made using a wide variety of semiconductor materials. The properties of silicon-based TFTs depend on the crystalline state of the silicon; the semiconductor layer can be either amorphous silicon, microcrystalline silicon, or it can be annealed into polysilicon (including low-temperature polysilicon (LTPS) and laser annealing).

[0055] The fabrication of silicon backplanes with suitable control circuits is a very well-known, understood, and predictable technology. However, due to the cost and complexity of the manufacturing process and equipment, it is often impractical to build a facility specifically to manufacture a specific backplane. Instead, the industry has widely adopted a foundry model in which the functional characteristics of microelectronic devices have become more standardized. This standardization has allowed design to be separated from manufacturing. Designs that follow appropriate design rules can be more easily and inexpensively manufactured by different companies with compatible manufacturing methods. For this reason, control circuits on silicon backplanes are usually limited to the use of standard components selected from a variety of options offered by the backplane manufacturer. For example, a silicon backplane manufacturer may offer customers the option of incorporating various standard designs of transistors rated at 1.8V, 2.5V, 3.3V, 5V, 8V, and 12V into their designs, but they cannot offer (without significant expense) transistors not included in the provided design.

[0056] For purposes of this application, "low voltage" (LV) is defined as analog microelectronic components that are sized, designed, and rated to operate safely and reliably at or below 5 V. "High voltage" (HV) microelectronic devices are typically considered to be in the 18-25 V range. "Medium voltage" (MV) microelectronic devices are typically considered to be between LV and HV. Note that these voltage ratings are set by the manufacturer, and manufacturers do not recommend exceeding the set maximum voltage for each transistor.

[0057] Complementary metal-oxide semiconductor (CMOS) technology uses p-type and n-type metal-oxide field-effect transistors (MOSFETs) to realize complex integrated circuits. Depending on the manufacturing process, different voltage regimes (i.e., 1.8V, 2.5V, 3.3V, 5V, 12V, etc.) are available. In all voltage regimes, MOSFET transistors have a drain, source, gate, and bulk / well. The base of the MOSFET is the substrate. For n-channel FETs, the substrate is a p-type doped substrate or well with a low doping rate. For p-channel FETs, the substrate is an n-type doped well with a low doping rate. The source and drain regions are formed from highly doped regions with n- or p-type doping for n- or p-channel FETs. A controlled channel is formed between the source and drain, insulated by a thin oxide and typically covered with a layer of polysilicon that acts as the gate. All four terminals of the FET (source, drain, gate, and substrate / well) are connected by metal contacts to a metal interconnect layer that ultimately connects to the OLED.

[0058] The emissive area of ​​a microdisplay is small, and to achieve the required pixel pitch, the space available for per-pixel control circuitry is limited. For full-color microdisplays, the space occupied by the control circuitry for each individual pixel should not exceed 100 square microns, and preferably not exceed 50 square microns. For monochrome microdisplays, where all pixels emit the same color, the space for the control circuitry can be three to four times larger because fewer pixels are required.

[0059] In a preferred low-voltage 5V transistor, the maximum voltage between any pair of all terminals cannot exceed 5V without damaging the device. A typical safety margin of 10% overvoltage is acceptable for short periods of time. Medium-voltage transistors rated above 5V (e.g., those rated at 7.5V) generally have the same setup as 5V transistors, but with thicker gate oxides and larger geometries (channel width and length) to withstand the higher voltages. Thus, MV transistors are typically larger and occupy more space than corresponding LV transistors.

[0060] Although transistors can be fabricated in any size range regardless of their voltage rating, low-voltage MOS transistors rated at 5V suitable for microdisplay applications have a total area not exceeding 20 square microns, and preferably not exceeding 10 square microns. The preferred channel area (channel length x channel width) of a 5V transistor for microdisplay applications shall not exceed 1 square micron, and preferably not exceed 0.30 square microns. Each of the two transistor contacts shall not exceed 1 square micron, and preferably not exceed 0.30 square microns.

[0061] For the purposes of this application, the preferred BJT for protection circuits is vertical, with a typical setup for both NPN and PNP types. For an NPN BJT, the collector is formed as a lightly doped deep n-well inside a common silicon substrate (bulk) with low p-doping. The base is formed as a p-well inside the deep n-well and connected by a highly doped p-region. The emitter of the BJT is formed in a highly doped n-region inside the p-well. Typical sizes of the emitter regions are approximately 500 nm x 500 nm and preferably do not exceed 0.30 square microns to allow for small pixel sizes. The maximum voltage between any pair of all terminals (bulk, base, emitter, collector) of the BJT cannot exceed 5 V without damaging the device. A typical safety margin of 10% overvoltage is allowed for short periods of time.

[0062] OLED microdisplays (often called "AMOLED") consist of an active matrix of OLED pixels that generate light (emission) upon electrical activation. These pixels are deposited or integrated on a transistor array located on a silicon chip, which 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 in each pixel, one of which starts and stops charging a reservoir capacitor (to trigger light emission) in addition to controlling the other transistor, which provides a voltage source at the level required to create a constant current to the pixel. In some embodiments, the transistor (commonly referred to as the scan or select transistor) that controls the current provided by the other transistor (commonly referred to as the drive transistor) is controlled by a select line that is common to all pixels along a row. The signal passed by the scan transistor is supplied by a data line that is common to all pixels in a column.

[0063] This is illustrated in Figure 1, which shows 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 conventionally driven by a supply voltage V DD to the anode of the OLED. One transistor (MP2) drives the current for the OLED, and another transistor MP1 (also known as the scan transistor) acts as a switch to sample and hold the voltage onto the reservoir capacitor C1 as shown. A data line (V DATA There is a select line that controls MP1, and therefore the charging of capacitor C1. Generally, transistors have an intrinsic capacitance, so depending on the intrinsic capacitance of the transistor and the leakage current through the transistor, no additional capacitance may be needed.

[0064] In subsequent figures of Figure 1, any capacitors present have been omitted from the drawing for clarity. The presence of capacitors is optional, and the circuit may have zero or any number of capacitors, as desired. The reference voltage for the capacitors is V DD Or it may be some other voltage.

[0065] Threshold voltage (V th It should be noted that variations in the OLED drive transistor threshold voltage (V), carrier mobility, or series resistance will directly affect the current uniformity of the OLED drive TFT and, consequently, the brightness of the display. One major factor affecting the non-uniform current is the threshold voltage (V) of the OLED drive transistor. th) variations. Other types of compensation by the control circuit may be required for the pixel; for example, aging, degradation, or burn-in of the OLED material over time, unevenness or non-uniformity across the active area, or voltage drop in the metal connecting lines. In addition, the control circuit provided by the TFT may need to control the timing of the current delivered to the pixel, for example, by PWM. The design of control circuits for OLEDs, including various types of compensation and driving schemes, has been a subject of intense interest, and many approaches have been proposed. Such compensation circuits may additionally be present in the control circuit for stacked OLEDs having three or more stacks.

[0066] It has been unexpectedly discovered that the high voltages / currents required by OLED stacks having three or more stacks can be handled by a control circuit comprising at least two transistors with their channels connected in series. This is sometimes referred to as a "stacked transistor." Preferably, one of the series-connected transistors is a low-voltage drive transistor and the other is a switch transistor. This arrangement allows the OLED pixel to be driven without significant leakage current through the TFT, resulting in high brightness without loss of contrast.

[0067] In particular, the drive TFT and switch transistor are connected in series such that a first terminal of the drive transistor is electrically connected to (and therefore close to) an external power supply, a second terminal of the drive transistor is electrically connected to the first terminal of the switch transistor, and a second terminal of the switch transistor is electrically connected to (and therefore close to) the bottom electrode of the OLED stack. Such an arrangement allows the OLED stack to be operated at higher voltages than the individual transistors are designed for without significant leakage current or physical damage to the low-voltage transistors.

[0068] This basic control circuit arrangement is shown in FIG. 2, where a p-channel drive transistor T1 has V at its first terminal (for a p-channel transistor, the source). DD The drive transistor T1 is connected to an external power source (an external power supply) and at its second terminal (for a p-channel transistor, the drain) to the switch transistor T2. The gate of the drive transistor T1 is controlled via a data line and an in-line select transistor T3, the gate of which is controlled by the select line SELECT1. The switch transistor T2 receives current at its first terminal (source) from the second terminal of T1 (when T1 is "on") and is connected at its second terminal (drain) to the bottom electrode of the OLED. The gate of the switch transistor T2 is controlled directly by the select line SELECT2. Both the drive transistor T1 and the switch transistor T2 are controlled in series. When both T1 and T2 are selected "on," current flows to the bottom electrode of the OLED stack, which emits light. When T2 is selected "off," no current flows to the OLED stack, regardless of whether T1 is "on" or "off." In some embodiments, the switch transistor T2 can provide a shutter function to prevent motion blur. If T1 is "off", it doesn't matter if T2 is "on" or "off", the OLED will not emit light.

[0069] In Figure 2, V DDThe backplane's control circuitry includes at least two series-connected transistors between the OLED stack and the lower electrode: a drive transistor and a switch transistor. In this application, the drive transistor regulates the voltage and current flowing to the OLED pixel at appropriate levels according to the image being displayed, while the switch transistor switches the current flowing to the OLED pixel on and off to provide a shutter function. For example, during operation of the microdisplay, the switch transistor T2 blocks power from the drive transistor T1 to the lower electrode of the OLED stack, resulting in no light emission (black) for a portion of the frame time. This differs from the scan transistor T3, which is on for only a small portion of the frame time (e.g., 1 / 1200 for a display with 1200 rows) to charge a reservoir capacitor (not shown in Figure 2). Often, for the same maximum operating voltage, the switch transistor can be smaller in size than the drive transistor. However, if the switch transistor is required to handle higher voltages, such as in the case of a control circuit for an OLED with three or more stacks, it can be larger and rated for a higher voltage than the drive transistor.

[0070] Preferably, the drive transistor (T1) is closer to the power supply V than the switch transistor (T2) which is located closer to the bottom electrode of the OLED. DD is located near the gate of the pixel circuit. It is preferred that at least the drive transistor is a p-channel MOSFET transistor, and more preferably that both the drive and switch transistors are p-channel MOSFET transistors. The two series-connected transistors can both be LV, both MV, or one LV and one MV transistor. Preferably, they are both LV transistors to reduce the oversize of the pixel circuit. However, in some embodiments, the drive transistor is LV while the switch transistor is MV.

[0071] Although only p-channel transistors are illustrated in Figure 2, n-channel transistors, or a mixture of n-channel and p-channel transistors, may be used. In such cases, the circuit must be appropriately rearranged to account for the polarity differences between n-channel and p-channel transistors. Additionally, other microelectronic components such as transistors, capacitors, and resistors may be included in the control circuit as needed.

[0072] As is known in the prior art, MOSFET transistors require intrinsic body diode connections to perform as desired. Due to the structure of MOSFET transistors, parasitic diodes are inherently present, which can affect the operation of the transistor. Typically, the intrinsic body diodes are connected to a power supply, either internally or externally, to apply bias. These body connections are referred to as "bulk connections" or "transistor wells," as well as other terms. This is shown in FIG. 3, where IBD1, IBD2, and IBD3 are connected to separate voltage sources V DD2 These transistors can share the same well for the intrinsic body diode connections. However, they can be connected to the same power supply V that is used to power T1 and T2. DD can be used for IBD as well, i.e., V DD and V DD2 is a common source.

[0073] However, to avoid exceeding the operating voltage range of any of the components, it may be desirable for one or more of the transistors to be floating in their own separate wells on the Si backplane. In particular, when both the first drive transistor and the switch transistor are p-channel transistors, each transistor can be located in its own separate n-well. This allows for a greater dynamic voltage range for controlling the OLED than could be achieved with both transistors in the same n-well. The use of isolated, floating, or different wells for series-connected transistors is known in the prior art; see, for example, U.S. Pat. Nos. 9,066,379, 5,764,077, 7,768,299, 9,728,528, and JP2016200828.

[0074] This is illustrated in Figure 4, where T1 occupies its own well as indicated by the dotted line and is connected to V through IBD1. DD2 and T2 occupies a different well as indicated by the dotted line, which is connected to the transistor source (V DD ) In the embodiment of FIG. 4, the bias applied to each IBD is different, so that each n-well is independent of the others.

[0075] The embodiment shown in Figures 2-4 has several advantages in its design and operation for driving OLEDs with three or more stacks. In design, the circuit can be very compact since all the transistors are relatively small LV transistors commonly found in most foundries. All transistors are connected to a V DD The pixel circuitry can be a p-channel transistor biased to 0 V, thereby eliminating the need for an isolated or floating well. These features can enable ultra-compact pixel circuit designs for microdisplay designs with very high resolution at small sizes.

[0076] Figure 5A shows a control circuit with three transistors in series. The circuit shown in Figure 5A is connected to a power supply V DD 5A is similar to that shown in FIG. 2 except that there is a third transistor T4 connected in series between the source of drive transistor T1 and the source of drive transistor T1. As shown, the gate of T4 is controlled via a data line and in-line select transistor T5, and the gate of in-line select transistor T3 is controlled by select line SELECT3. Lines SELECT2 and SELECT3 in FIG. 5A may provide signals at the same or different voltages, may be switched simultaneously or at different times, or may be left unswitched depending on operating conditions.

[0077] Preferably, the additional transistor T4 can be a second drive transistor, so that the power supply V DD Between the OLED and the bottom electrode are two drive transistors (T4 and T1) and one switch transistor (T2), all in series. In this case, the signal supplied to the gate of T4 can also be the same as that supplied to T1 and can be supplied via T3 / SELECT1. While Figure 5A shows a design with one data line and two select lines for data control, a similar design is possible with two data lines and one select line.

[0078] In embodiments where there are three or more series transistors, they may be a combination of both LV and MV transistors. Preferably, they are all LV transistors to reduce the size of the pixel circuit. These multiple transistors may be p-channel, n-channel, or mixed. Preferably, they are all p-channel. When there is a mixture of n-channel and p-channel transistors, there is preferably at least one drive transistor that is a p-channel transistor, and more preferably it is an LV transistor. While the switch transistors are preferably LV transistors, it may be desirable to use one or more MV switch transistors when it is necessary to extend the operating range of the backplane circuit while observing the voltage limitations of the individual transistors.

[0079] If any of the series transistors is of one type, multiple common transistors of the same type may 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 limits of the individual transistors, it may be necessary to place transistors of the same type in separate well regions.

[0080] FIG. 5B shows a schematic of a control circuit with two drive transistors similar to that shown in FIG. 5A, where the two drive transistors are controlled by a level shift circuit (LSC). A level shift circuit is a circuit used to translate a signal from one logic level or voltage domain to another, often to resolve voltage incompatibilities between various parts of a system. For clarity, the internal components are also shown, along with all of the connections to the LSC (e.g., V DD, ground, and other possible input connections) are also not shown. In this embodiment, the LSC sets the gate voltages for drive transistors T4 and T1 based on signal SELECT1 and signals from the data lines so that the total voltage across T4 and T1 is always approximately evenly divided between the two drive transistors. As is known, small logic transistors can be used for this function.

[0081] OLED-based microdisplays often include protection circuitry within the MOSFET-based control circuitry of the backplane to limit the amount of power flowing through the transistors to prevent damage. Due to the high power required to induce light emission from such devices, it is desirable to include protection circuitry within the control circuitry of the backplane of OLED-based microdisplays with three or more stacked units for this purpose. The protection circuitry must maintain or "clamp" the voltage at the OLED's bottom electrode so that it does not fall below a desired voltage level when the OLED is not emitting light. Such protection circuitry may also be referred to as a "voltage maintenance" circuit.

[0082] To protect the low voltage transistors present in the control circuitry and to stay within the specified operating range of the transistors as set by the foundry, the protection circuitry must provide a current of 4 μA / cm 2 Less than 2 μA / cm for a three-unit stacked OLED with a Vth of approximately 7.5 V, or more preferably, 2 It is desirable to maintain a black level current (CV=0, or the pixel is "off") in the bottom electrode of a stacked OLED for the following pixels: For a four-unit stacked OLED device, a similar black level current is desired, with a typical Vth of approximately 10V.

[0083] 6 shows a schematic control circuit similar to that shown in FIG. 2 with the addition of a protection circuit. In this particular embodiment, the protection circuit is connected at one end to a node located 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 The gate of T6 is also connected to node T1. Thus, diode-connected transistor T6 limits the minimum drain voltage of T2, preventing its breakdown. There may be other electronic components as part of this circuit that are not shown. This arrangement allows one end of the p-channel transistor to be connected to V REF This is similar to that described by Kwak et al., except that the resistor R is connected to ground instead of R.

[0084] However, the problems caused by the higher voltage requirements of using an OLED with more than two units still apply to the use of a protection circuit using MOSFET transistors such as that shown in Figure 6. In this embodiment, using an LV or MV MOSFET transistor (i.e., T6) to achieve protection of the LV drive and switch transistors does not result in a significantly smaller pixel design compared to using MV drive and switch transistors and omitting the protection circuit.

[0085] FIG. 7 shows a protection circuit similar to that shown in FIG. 6, except that p-channel transistor T6 is replaced by diode D4. Preferably, diode D4 in the protection circuit is a p-n junction diode. A p-n diode is a circuit element that allows electrical flow in one direction but not the other (opposite) direction. A p-n diode can have a forward bias in the direction of easy current flow, or a reverse bias in which little or no current flows. Such a p-n diode can be formed inside a CMOS project in various ways, for example, as a vertical diode with a highly doped p-region in an n-well. At one end, D4 is connected to a node located between the drain of T2 and the bottom electrode of the OLED, and at the other end, a reference voltage V REF or the reference current I REF The power supply 45 is either

[0086] FIG. 8 shows a protection circuit similar to that shown in FIG. 7. In FIG. 8, the diode D4 of the protection circuit is specifically a BJT1, and the BJT (bipolar junction transistor) has a collector (C) connected to V DD , the emitter (E) is connected to the node between T2 and the bottom electrode of the OLED stack, and the base (B) is connected to a voltage source V PROTECT or current source I PROTECT One benefit of using a BJT in a protection circuit is current amplification from the base current to the collector current. If desired, the collector of the BJT can be connected to a power supply 50, which can be either V DD The power supply V PROTECT or I PROTECT can be common to all pixels. PROTECT or I PROTECTNote that θ may or may not be a constant, but can vary depending on whether the OLED is intended to be emitting light or not. This can be advantageous when used in conjunction with a switch transistor to reduce the persistence of the display through a shutter. In Figure 8, BJT1 is an "NPN" type BJT transistor (preferred), but could also be a "PNP" transistor with appropriate modifications in the design.

[0087] For an NPN type BJT, the emitter voltage V at the bottom electrode of the OLED E is the base voltage V B Larger than V B is the collector voltage V C Whenever it is smaller than (V E >V B <V C ), the BJT is in shutdown mode and no current is passed. However, the voltage V at the bottom electrode of the OLED E is the voltage V B is smaller than V B V C Whenever it is smaller than (V E <V B <V C ), the BJT is in its forward active mode. In this mode, the base-emitter junction is forward biased and the base-collector junction is reverse biased, so that the collector-emitter current is approximately proportional to the base current.

[0088] Therefore, in Figure 8, V of BJT1 C V DD and V PROTECT (V B ) is the V th If the voltage at the bottom electrode V is set to be less than E V th +V CATHODE Whenever V is higher than V, BJT1 is "off", but V E V th +V CATHODE(i.e., whenever T1 or T2 is "off"), the voltage at the bottom electrode is V th Therefore, the protection circuit is designed to provide sufficient current whenever needed to prevent the drive and switch transistors from having voltages across their terminals exceeding their ratings whenever the cathode voltage decreases below a certain value (more negative voltage). Furthermore, the base voltage B (V PROTECT ) below the OLED turn-on voltage (V above V th ), the voltage delivered to the bottom electrode of the OLED is V th Whenever it is greater than or equal to it, power loss is minimized.

[0089] In some embodiments utilizing the protection circuit shown in FIG. 8, the base voltage (V B ) is isolated from any external power source, i.e., power supply V PROTECT or I PROTECT There is no electrical connection between V and the base of the BJT. The BJT physically exists with existing collector and emitter connections as shown in Figure 8, but the base connection, although still present, is not connected to any external source. In such a case, V B is not intentionally maintained at any particular value, nor is any voltage or current intentionally applied to it. V B is the voltage V that remains as part of the active control circuitry that operates the OLED. C and V E The base is allowed to "float" independently of V. Nevertheless, there may be parasitic current paths in the backplane that internally bias the base with high impedance. In this type of embodiment, V B It should be noted that θ can vary during operation of the OLED due to the generation of parasitic currents in the circuit.

[0090] In another embodiment, the V BA BJT can be self-biased (sometimes referred to as having a "base bias"). When a BJT is self-biased, there is no externally controlled input signal applied to the base of the BJT, but rather the signal applied to the base of the BJT is a constant supply voltage (i.e., V DD ) and the value of any bias resistors connected to the transistor. One way to achieve self-biasing of the base of a BJT is to create a "fixed base bias circuit." In this arrangement, the base of the BJT is connected to a constant power supply (i.e., V) by a single current-limiting resistor. DD ) which is connected to the base current (I B ) is V DD For a given value of , the bias voltage remains constant, thus allowing the operating point of the BJT to also remain fixed. Alternatively, a simple voltage divider network can provide the required bias voltage. Note 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. Such response can provide effective protection for the transistor.

[0091] 9 is a schematic cross-sectional view of a transistor well for the circuit shown in FIG. 8. T1, T2, and T3 are V DD Note that the p-wells of BJT1 (NPN BJTs) may be located in separate, but not isolated or floating, n-wells connected to V, all separated from the p-well of BJT1 (NPN BJT). For convenience, the source (s), gate (g), and drain (d) regions are marked for T1, T2, and T3. Similarly, the emitter (e), base (b), and collector (c) regions are marked for BJT1. The collector (c) region of BJT1 is connected to V through a deep n-well. DD Although shown as being connected to V, in some embodiments it may alternatively be connected to V DD can be directly connected to

[0092] Figure 9 also shows the V of the deep n-well in the Si substrate where all of the transistors reside. DD The figure shows IBD5, an intrinsic body diode connection to ground, as well as IBD6, an intrinsic body diode connection to ground for the entire Si substrate. Note that the BJTs in the protection circuit can be either NPN or PNP transistors. If the BJT is an NPN transistor, the transistor is in a p-well, while if the BJT is a PNP transistor, it is in an n-well.

[0093] The use of protection circuits with BJTs is preferred. When used as part of a control circuit with at least two transistors whose channels are connected in series, BJT transistor designs allow for smaller pixel sizes. In addition, the inherent amplification factor of forward-active BJTs means that relatively small parasitic currents can generate much larger protection currents. Therefore, the current drawn from the reference / protection voltage source is much smaller, dramatically reducing the voltage drop across the reference voltage interconnect.

[0094] The protection circuit may also be designed to prevent other undesirable effects such as short circuit protection, transient spikes, etc., in addition to maintaining the minimum voltage on the bottom electrode below the threshold voltage of the OLED by including other appropriate circuit components. However, in some cases, it may still be useful to include such known types of protection circuitry in the pixel circuit when using the stacked OLED of the present invention.

[0095] For many OLED stacks, especially those with three light-emitting units, it is desirable to have no intervening transistor in the electrical connection between the second terminal of the switch transistor and the bottom electrode of the OLED in the control circuit, as shown in Figures 2-5. Intervening means that the electrical connection between the switch transistor and the OLED does not pass current directly through another transistor (i.e., one terminal of the transistor is connected to the switch transistor while the other terminal is connected to the OLED, so that current passes through the intervening transistor at some point during operation of the OLED). There may be other (non-transistor) microelectronic components in-line between the switch transistor and the OLED.

[0096] There may be a branched (meaning not directly in-line) connection between the switch transistor-OLED connection and the first terminal of the microelectronic component (i.e., a non-intervening transistor or diode), where the OLED operating current does not pass directly through the non-intervening transistor or diode. For example, this arrangement is illustrated in Figures 4-6, where one side of the non-intervening component is connected to the T2-OLED connection and the other side is connected to a power source.

[0097] Desirably, the drive transistor is a low-voltage (LV) transistor. That is, it is designed and sized for safe and effective operation at 5V or less, regardless of or without consideration of the actual load in the circuit. This is necessary to help minimize pixel size to maximize microdisplay resolution. Note that OLED stacks with three or more OLED units driven by a drive transistor typically require voltages in excess of 7.5V. If necessary, two or more series-connected low-voltage drive transistors in separate wells (as shown in Figure 2) can be used to further mitigate the effects of higher voltages.

[0098] The drive transistor is preferably a p-channel thin film transistor (also referred to 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 switch transistor. The gate of the drive electrode is controlled by a separate data line separate from the power supply.

[0099] When an OLED stack includes three OLED light-emitting units, the switch transistor is preferably a low-voltage transistor. That is, it is designed and sized for safe and effective operation at 5V or less, regardless of or without regard to the actual load in the circuit. The current required for OLED operation flows through this transistor as well as the drive transistor.

[0100] However, when the OLED stack includes four or more OLED light-emitting units, the operating voltage may be much greater than 7.5 V, and the switch transistor may be medium voltage (e.g., designed for 7.5 V to 12 V) or high voltage (18 to 25 V) as needed. Alternatively, two or more series-connected switch transistors, either commonly selected or independently selected, may be used to mitigate the effects of higher voltages. The series-connected switch transistors may all be LV or a mix of LV and MV transistors.

[0101] The switch transistor is preferably a p-channel transistor. When the switch transistor is a p-channel transistor, its source is connected to the drain of the drive transistor, which is electrically connected to the bottom electrode of the OLED. The gate of the switch electrode is controlled by a power supply and a select line separate from the data and select lines that control the drive transistor.

[0102] In some embodiments, it is desirable that both the drive and switch transistors are p-channel transistors, and that both are low voltage transistors, or that the drive transistors are low voltage and the switch transistors are medium or high voltage. In some embodiments, it is also desirable that all transistors in the control circuit are p-channel transistors.

[0103] The drive transistor and switch transistor are electrically connected in series, although there may be other intervening microelectronic components between the two transistors. The intervening components may be in-line, so that the current between the drive transistor and the switch transistor passes directly through the in-line components. There may also be other microelectronic components electrically connected to the connection between the drive transistor and the switch transistor, so that the current flowing from the drive transistor also flows through this additional component as well as the switch transistor.

[0104] In its most basic form of operation, a voltage is supplied to the drive transistor from an external power supply. 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, allowing current from the power supply to flow through the drive transistor to the switch transistor. The select line for the switch transistor is set to allow the correct charge to be applied to the switch transistor, allowing current to flow through the switch transistor to the bottom anode of the OLED stack in each individual pixel. This turns the pixel "on," and the pixel emits light in response to the current received. When the displayed image calls for no or reduced emission from an individual pixel, the data voltage for the drive transistor can be changed to prevent or limit current from flowing through the drive pixel.

[0105] Other components in the control circuit may contribute to controlling the current being supplied by the drive transistor, which is either the current intended to power the OLED when it is "on," or the leakage current through the drive transistor when the OLED is intended to be "off."

[0106] To prevent or minimize motion blur by providing a shutter function, the switch transistor can prevent current from flowing to the OLED, regardless of the operation of the drive transistor (which controls the power supplied to that pixel), particularly when the drive transistor is "on" (passing current). At the appropriate time for the pixel, the switch transistor can be selected so that the OLED pixel remains "off" even when the image to be displayed requires that pixel to be "on." This allows all or alternate areas of the pixels in the display to be "off" (non-emitting), thereby minimizing the perception of motion blur in the microdisplay.

[0107] In microdisplays, power from an external power source can be delivered as a variable current or voltage to the drive transistor to drive the OLED stack and deliver the desired level of brightness. This is often stored during write operations by charging a storage capacitor. The power level can be controlled in the external power source, or if the delivered power is constant, the power can be set to the appropriate level by other microelectronic circuitry in the backplane. This is called "current control" and is typically used to power most OLED devices. Alternatively, the power delivered to the OLED stack can be constant, and the total amount of light emitted over a set period of time (frame) is controlled by the amount of time the OLED pixel is fully "on" compared to the amount of time it is "off." This is called pulse-width modulation, or PWM control.

[0108] The control circuitry as described can handle higher voltage and current demands than the design or rated voltage of at least the drive transistor without significant leakage or damage, thereby enabling the use of OLED stacks with increased light output (and higher voltages). th OLED devices with two light-emitting units in series have the requirement of a relatively higher V th The OLED stacks with three or more light-emitting units do not emit as much light as the OLED stacks with three or more light-emitting units, which has the requirement that the threshold voltage (V th ) and more preferably, the V th is at least 10 V or greater. Alternatively, the circuit can be used with stacked OLEDs to provide full color microdisplays with an emission of at least 2500 nits, or preferably at least 5000 nits.

[0109] There are two basic approaches to creating pixelated OLED microdisplays, which require controlling the brightness of each individual pixel by supplying power to one of the pixel electrodes via control circuitry on the backplane. The first approach involves having each pixel individually generate red, green, or blue light (R, G, and B, respectively), or the same color in the case of a monochrome display. In this case, the light-emitting OLED stack can be arranged so that all of the stacked light-emitting units above an individual bottom electrode segment emit the same color light (selected from R, G, or B light) to create R, G, and B pixels. In some embodiments with this feature, each color pixel forms a microcavity whose distance between the segment bottom electrode and top electrode depends on the color of the emitted light. In this case, the length of the microcavity depends on the color emitted and is different for red, green, and blue pixels.

[0110] The second approach is to have a common multimodal (white) light-emitting OLED layer across all pixels with a color filter array (CFA) to create the individual RGB pixels. This approach has the advantage over the first approach in that it reduces manufacturing costs because there is no need to create individual OLED pixels with different formulations.

[0111] The number of individual OLED light-emitting units in 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 units increases, the total amount of light emitted increases, but so do the package thickness, manufacturing process complexity, and threshold voltage. OLEDs with at least three stacked light-emitting units offer increased brightness over series (two OLED units) OLEDs. However, OLEDs with at least four stacked OLED light-emitting units, and more preferably, OLEDs with at least five stacked OLED light-emitting units, are preferred. OLEDs with as many as six to ten or more stacked OLED light-emitting units are contemplated.

[0112] To minimize the increase in voltage required to drive the OLED stack, a charge generation layer (CGL; sometimes referred to as a connector or intermediate layer) is located between each OLED light-emitting unit. This is because the CGL is structured so that electrons and holes are generated upon application of a voltage and injected into the adjacent organic emissive layer. Therefore, the use of a CGL can potentially convert one injected electron into multiple photons, enabling higher brightness. In particular, it is desirable to locate a CGL between each light-generating unit in the stack. However, it is not necessary for a light-generating unit to have adjacent CGLs on both sides. OLED light-generating units at the top and bottom of the stack usually have only one adjacent CGL. Typically, it is not necessary to use a CGL between the light-emitting unit and either the top or bottom electrode, but a CGL may be used if desired.

[0113] Many different types of CGLs have been proposed and can be used in OLED stacks. See, for example, U.S. Patent No. 7,728,517 and U.S. Patent Application Publication No. 2007 / 0046189. To form a CGL, an np semiconductor heterojunction located at the interface of the n-type and p-type layers is typically required for charge generation. Therefore, a 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. A preferred metal oxide for a CGL is MoO3. In some cases, the n- and p-layers can be separated by a thin interlayer. Often, the CGL is arranged so that the n-layer is closer to the anode and the p-layer is closer to the cathode.

[0114] One desirable formulation for a CGL has three layers: an electron-transporting material doped with an n-dopant (e.g., Li), a thin intermediate layer of the same (but undoped) electron-transporting material, and a hole-transporting material doped with a p-dopant. Suitable electron-transporting and hole-transporting materials, along with suitable n-dopants and p-dopants for use in CGLs, are well known and commonly used. The materials can be organic or inorganic. The selection of appropriate materials is not critical; any can be selected based on their performance. The thickness of the CGL should desirably be in the range of 200 to 450 Å. In many cases, the CGL has an ETL on the anode side and an HTL on the cathode side to help improve charge transport and separate the charge-generating dopant (if present) from the LEL in the light-emitting unit.

[0115] Although the use of a CGL helps minimize the voltage increase when OLED light-emitting units are stacked on top of each other, the total voltage required by the stack still increases solely by approximately the voltage required by each individual unit. OLEDs with at least three stacked OLED light-emitting units are expected to require voltages that exceed the recommended operating range of a 5V drive transistor.

[0116] In one embodiment, all of the OLED light-emitting units in an OLED stack above an individual bottom electrode segment emit the same color, for example, red, green, or blue. This results in a pixelated RGB microdisplay. Figure 10 illustrates a microdisplay 100 that uses three different OLED subpixel stacks to create R, G, and B pixels. Each OLED subpixel stack includes three OLED light-emitting units that emit the same color, with each unit vertically separated from the others by a CGL.

[0117] Within microdisplay 100 is a silicon backplane 3 that includes an array of control circuits, such as those shown in Figures 2-8, and other necessary components that power the subpixels according to an input signal. An optional planarization layer 5 may be present above layer 3 with the transistors and control circuits. Above layer 5 (if present) are individual first electrode segments 9 connected by electrical contacts 7 that extend through the optional planarization layer to make electrical contact between the individual bottom electrode segments 9 and the control circuits in layer 3. The individual bottom electrode segments 9 are electrically isolated laterally from each other by pixel-defining layer 1. Above the segmented bottom electrode segments 9 are non-emissive OLED layers 11, such as electron- or hole-injecting (EIL or HIL) or electron- or hole-transporting (ETL or HTL) layers. A first light-emitting OLED unit 13 is above OLED layer 11. Layer 15 is a charge-generating layer that lies between and separates first light-emitting layer 13 and second light-emitting OLED unit 17. Above the second light-emitting OLED unit 17 is a second charge-generating layer 19, separating and separating the second light-emitting OLED unit 17 from the third light-emitting OLED unit 21. Above the third light-emitting layer 21 is a non-emissive OLED layer 23, such as an electron- or hole-transporting layer or an electron- or hole-injecting layer, 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-defining layer 1, while the top electrode 25 and encapsulation layer 27 are common and extend across the entire active area. However, the top electrode 25 need not be continuous and may be segmented if desired. In the microdisplay 100, all of the OLED light-emitting units 13, 17, and 21 above the same bottom electrode segment 9 emit light of the same color: either B, G, or R. This particular microdisplay is not a microcavity device, but a similar pixelated RGB design using the microcavity effect can be used.

[0118] One well-known method for increasing the brightness and color purity of OLED emission is by utilizing the optical microcavity effect. This effect is based on creating an optical cavity between a reflective surface and a semi-reflective surface that allows some light to pass through. Multiple reflections between two surfaces create standing waves, depending on the optical distance between the two surfaces. This enhances some wavelengths of light and attenuates others due to constructive and destructive interference effects, depending on whether the emission occurs at an antinode or node of the standing wave, respectively. Antinodes occur at different locations depending on the total space between the reflectors and the optimized wavelength. However, light emitted from a microcavity can exhibit extreme angular dependence, with color shifts and brightness losses occurring when the viewing angle deviates from normal to the viewing plane. This is often not an issue in NED applications due to the limited angle of incidence of projection optics.

[0119] It may be desirable to further increase the brightness of an OLED stack by using a microcavity effect. For example, a microdisplay 100 such as that shown in Figure 10 may be redesigned to create a microcavity effect by using a reflective bottom electrode or reflective layer below the bottom electrode, making the top electrode semi-transparent so that the top electrode has some reflectivity, and adjusting the distance between the top surface of the reflective element (bottom electrode 9 or underlying reflective layer) and the bottom surface of the top electrode to create a microcavity suitable for that particular color of light.

[0120] Figure 11 illustrates a microdisplay 200 that uses a multimodal (white) OLED microcavity that is common across all pixels along with a color filter array (CFA) to create R, G, and B pixels. A multimodal OLED produces light of two or more colors. Ideally, a multimodal OLED produces white light with roughly equal amounts of R, G, and B light. Typically, this produces a CIE of approximately 0.33, 0.33. x , CIE yvalues. However, some variation from these values ​​may still be acceptable or even desirable depending on the characteristics of the color filters used to create the RGB pixels. Microdisplay 200 also incorporates a microcavity effect. In this embodiment, the multimodal OLED stack includes three OLED light-emitting units that emit different colors, each separated vertically from another by a CGL, and the distance between the reflective surface and the top electrode is constant across the active area.

[0121] Within microdisplay 200, there is a silicon backplane 3 that includes an array of control circuits, such as those shown in FIGS. 2-8, and the necessary components for powering the subpixels according to an input signal. An optional planarization layer 5 may be present above layer 3 with the transistors and control circuits. Above layer 5 (if present), there are individual first electrode segments 9 connected by electrical contacts 7 that extend through the optional planarization layer to make electrical contact between the individual bottom electrode segments 9 and the control circuits 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 laterally electrically isolated from each other. Above the segmented bottom electrode segments 9 is a non-emissive OLED layer 11, such as an electron or hole injection or electron or hole transport layer. Red OLED light-emitting unit 13A is above OLED layer 11. Layer 15 is a first charge-generation layer that is between and separates red OLED light-generating unit 13A and green OLED light-generating unit 17A. Above the green light-emitting layer 17A is a second charge-generation layer 19, which is 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 is a non-emissive OLED layer 23, such as an electron or hole transport layer or an electron or hole injection layer, and a semitransparent top electrode 25. This forms an OLED microcavity 30 that extends from the top surface of the reflective surface 9B to the bottom surface of the semitransparent top electrode 25, which is also a semi-reflective electrode. The OLED microcavity is protected from the environment by an encapsulation layer 27. In this embodiment, a color filter array is present, having color filters 29B, 29G, and 29R, which filter the multimodal emission generated by the OLED microcavity 30 so that B, G, and R light are emitted according to the power supplied to the underlying electrode segments 9.

[0122] FIG. 12 shows a variant of microdisplay 200, microdisplay 300, that has an additional blue OLED light-emitting layer 22 located between blue OLED unit 21A and OLED layer 23. In this embodiment, there is no CGL between blue OLED unit 21A and additional blue light-emitting layer 22. Like microdisplay 200, microdisplay 300 has three (not four) OLED light-generating units because blue unit 21A and blue layer 22 are not separated by a CGL (thus, layer 22 is not counted as a separate light-emitting OLED unit but should be considered part of blue light-emitting unit 21A). Even if blue unit 21A and blue LEL 22 were separated by a non-charge-generating interlayer, there would still be only three LED units in the stack. However, if there was an intermediate CGL located between blue light-emitting unit 21A and additional blue light-emitting layer 22, the OLED stack would include four OLED units.

[0123] 13 shows another embodiment of a multimodal OLED microdisplay 400 having a microcavity effect similar to microdisplay 200, except that there are five OLED light-emitting units. In this case, some of the OLED units emit light of the same color, while others emit light of different colors. In particular, multimodal microdisplay 400 has two blue OLED light-generating units, one green OLED light-generating unit, one yellow OLED light-generating unit, and one red OLED light-generating unit, all of which are separated by a CGL.

[0124] In microdisplay 400, OLED unit 13A is red-emitting. It is separated from yellow-emitting OLED unit 16 by CGL 15. Yellow-emitting OLED unit 16 is separated from green-emitting OLED unit 17A by CGL 14. Green-emitting OLED unit 17A is separated from first blue-emitting OLED unit 21A by CGL 19. Second blue-emitting unit 32 is separated from first blue-emitting OLED unit 21A by CGL 24. All other layers are the same as in FIG. 12.

[0125] As mentioned above, OLED microdisplays are constructed on a silicon backplane, which serves as a substrate. Generally speaking, the backplane is flat and of uniform thickness. Because silicon backplanes are typically opaque, the OLED stack is preferably top-emitting. However, transparent backplanes are known, and in such cases, the OLED stack can be top- or bottom-emitting. The top surface of the substrate faces the OLED. The silicon backplane can have various types of primer layers (i.e., planarization layers, light management layers, light-blocking layers, etc.), which can be patterned or unpatterned and can be on either the top or bottom surface.

[0126] The bottom electrode segment (9 or 9a) can be an anode or a cathode and can be transparent, reflective, opaque, or semi-transparent. 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 an alloy thereof, having a thickness of at least 30 nm, and desirably at least 60 nm.

[0127] In microcavity applications where the first electrode is above a reflective layer, the first electrode must be transparent. However, in other applications, the first electrode layers 9a and 9b may be combined into a single reflective electrode, whose top reflective surface then forms one side of the optical microcavity (i.e., 30 in FIG. 11).

[0128] When the OLED stack is a top-emitting microcavity and the bottom electrode is transparent, a reflective layer below the bottom electrode must be present, defining the first side of the microcavity 30. When a transparent anode is located on a reflective surface, it becomes 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. Dielectric mirrors are constructed from multiple thin layers of materials such as magnesium fluoride, calcium fluoride, and various metal oxides, deposited on a substrate. Highly reflective coatings consist 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 layer thickness is typically a quarter wavelength relative to the wavelength of the light being reflected. It is desirable for the reflective layer to reflect at least 80%, and most preferably at least 90%, of the incident light. A preferred reflective layer is Al or Ag having a thickness of 300 to 2000 Å, most preferably 800 to 1500 Å.

[0129] Desirably, when the OLED stack is bottom-emitting, the lower electrode is a transparent anode, preferably with a transmittance of at least 70%, or more desirably at least 80%, to transmit as much visible light as possible. The lower transparent electrode can be made of any conductive material, but is preferably a thin layer of a metal oxide such as ITO or AZO, or a metal such as Ag. Poorly conductive materials (e.g., TiN) may also be used, provided they are thinned.

[0130] Suitable electron-transport and hole-transport materials for use in non-emissive layers (i.e., 11 and 23 in Figure 10), such as hole-injection, hole-transport, or electron-injection or electron-transport layers, are well known and commonly used. These layers may be mixtures of such materials and may contain dopants to modify their properties. Because they are non-emissive, they do not contain emissive materials and are transparent. The selection of appropriate materials is not critical and any may be selected based on their performance.

[0131] In embodiments utilizing the microcavity effect, the spacing between the various OLED units within the microcavity, as well as the size of the microcavity, is important for maximizing efficiency, and it is typically necessary to select the thicknesses of the various non-emissive layers to provide the desired spacing. Desirably, control of the spacing between the OLED units and the size of the microcavity is achieved by using organic non-emissive layers, such as hole-transporting layers, of appropriate thicknesses.

[0132] The light-emitting layer typically comprises a host material (or a mixture of host materials) as the main component of the layer and a light-emitting compound. Preferably, the light-emitting compound is phosphorescent because it has higher efficiency. However, in some cases, some LELs use fluorescent or TADF (thermally activated delayed fluorescent) compounds as light-emitting materials, while others use phosphorescent materials. In particular, 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 appropriate materials for the LEL is well known and is not critical; each may be selected based on their performance and emission characteristics. When using a phosphorescent emitter, it is sometimes necessary to confine the excitons generated by the phosphorescent emitter within the layer. Therefore, an exciton-blocking layer may be used as needed on one or both sides of the phosphorescent LEL. Such materials and their applications are well known. Additionally, it may be desirable to add HBL (hole blocking layer) and EBL (electron blocking layer) layers around the light-emitting layers, especially the blue light-emitting layer, to improve lifetime and luminance efficiency.

[0133] The top electrode (i.e., 25 in Figure 10) must be transparent if the OLED stack is top-emitting, reflective if the OLED stack is bottom-emitting, or semi-transparent and semi-reflective if the OLED stack is a microcavity, i.e., it reflects some light and transmits the rest. In the case of a microcavity, the lowermost inner surface of the top electrode defines the second side of the microcavity 30. Desirably, the semi-transparent top electrode reflects at least 5%, and more desirably at least 10%, of the light emitted by the LEL to establish the microcavity effect. The thickness of the semi-transparent second electrode is important because it controls the amount of light reflected and how much is transmitted. However, it cannot be too thin because it may not efficiently transfer charge to the OLED or may suffer from pinholes or other defects. The thickness of the upper electrode layer is desirably 100-200 Å, and more desirably 125-175 Å.

[0134] The top electrode is preferably a thin layer of a metal or metal alloy. Suitable metals are Ag, Mg, Al, and Ca, or their alloys. Of these, Ag is preferred because it has relatively low blue absorption. To aid in electron transport and stabilization, there may be an adjacent layer of a transparent metal oxide on the electrode surface, such as ITO, InZnO, or MoO3. Alternatively, metal halides such as LiCl, organometallic oxides such as lithium quinolinate, or other organic materials may be used.

[0135] There may be a protective or spacing layer (not shown in Figures 10-13) over the upper electrode to prevent damage during encapsulation.

[0136] An encapsulation 27 is deposited or placed over the top electrode 25 and optional protective layer, if present. At a minimum, the encapsulation must completely cover the light-emitting region on the top and sides and be in direct contact with the substrate. The encapsulation must be resistant to air and water penetration. It can be transparent or opaque. It must not be conductive. It can be formed in situ or added as a separate preformed sheet with measures to seal the side edges. An example of in situ formation is thin-film encapsulation. Thin-film encapsulation involves the deposition of multiple layers, using alternating layers of inorganic material and polymeric layers, until the desired degree of protection is achieved. Formulations and methods for forming thin-film encapsulation are well known, and either can be used as desired. Alternatively, the encapsulation can be achieved using a preformed sheet or coverslip that is applied over at least the sealing and containment regions. The preformed sheet can be rigid or flexible. It can be made of glass (flexible glass), metal, or organic / inorganic barrier layers. It must have a thermal expansion coefficient closer to that of the substrate to achieve a more robust connection. The preformed encapsulation sheet may need to be attached to the sealing area using an airtight and waterproof adhesive, such as a silicone 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 and bottom edges of the coverslip may be specifically designed to have a better fit to the sealing area or to promote a better seal. The coverslip and sealing area may be designed together so that they partially fit or lock into place before the seal is formed. Furthermore, the coverslip may be pretreated to promote better adhesion to the sealing area.

[0137] 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 requires a relatively high voltage to emit light. The present invention is not limited to OLEDs, nor to any other display technology that requires more than 5 V, preferably more than 7.5 V, or even more than 10 V to provide an emission of at least 1000 nits, or preferably at least 5000 nits.

[0138] Crosstalk in a display occurs when the luminance emitted by one pixel is unintentionally affected by another pixel. This is undesirable because the affected pixel no longer provides the correct luminance according to the image signal, which can degrade image quality. Depending on the amount and nature of crosstalk, important factors such as color reproduction, contrast (the difference between maximum and minimum luminance), grayscale, resolution, and "ghosting" within the display can all be adversely affected. In general, crosstalk in microdisplays is particularly problematic when the pixel pitch is small. It is desirable for the amount of crosstalk between pixels to be 15% or less, or preferably 5% or less, and most preferably 3% or less.

[0139] The stacked microcavity OLED devices described are necessarily thick with a defined spacing between the internal light-emitting units. This is essential to provide the microcavity effect and enhance the R, G, and B emissions within a single microcavity. This can result in increased crosstalk due to 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 active pixel regions to neighboring inactive pixel regions within the same layer.

[0140] There appear to be multiple modes resulting in crosstalk. The short-range (0.2-0.7 μm) appears to be a combination of lateral charge carrier and optical mechanisms. The medium-range (3-7 μm) interactions appear to be primarily due to lateral charge carrier transport, but may also be partially due to optical mechanisms. The long-range (50-200 μm) interactions appear to be primarily due to light scattering from active pixel areas to inactive areas. There may also be an even longer-range optical contribution to crosstalk based on waveguiding according to the pixel pitch.

[0141] Some useful methods for minimizing the problem of crosstalk due to carrier transport in OLED devices include the following: Lateral charge carrier migration, which is reduced by varying the layer thickness and composition (to increase "sheet resistance") in layers with high carrier mobility (e.g., HIL, HTL, CGL, ETL, and EIL). In particular, charge carriers (either holes or electrons) can be generated in the active area and migrate laterally across the gap between the lit and unlit areas. This 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 largest source of this problem. When holes are generated in the lit area of ​​the HIL on one anode pad, they can migrate to the unlit area of ​​the neighboring anode pad, and the resulting voltage due to the holes increases the V th, the (nominally unlit) pixel will likely emit light anyway. In addition, holes can enter the conductive anode pads and flow laterally through the anode with very little lateral resistance. On the other side of the unlit anode pad, the current can return (as holes) into the HIL for a jump to the next unlit anode pad. Thus, carrier migration problems may not be limited to the shorter distances between adjacent anode pads, but may also have a longer-distance component. For this reason, careful attention must be paid to the thickness and composition of both electrodes, and sometimes the anode. Thinner organic layers with low carrier mobility can help minimize these undesirable carrier migration processes. - Material selection for the organic layer with high carrier mobility. In particular, materials can be selected to minimize their contribution to crosstalk. The type and level of p-dopant (e.g., F4-TCNQ, F6-TCNNQ, or HAT-CN) added to the HIL can be important in this regard, as can the choice of HTM (e.g., aromatic amine compounds such as NPB or Spiro-TTB) in the HIL or HTL. P-dopants alone, or undoped HILs, can also be effective. In some cases, undoped HILs and p-doped HTLs can be used. Inorganic HIL materials, such as MoO3, which can be mixed with organic materials, can also have advantages.

[0142] Some useful methods for minimizing the problem of crosstalk due to optical processes in OLED devices include the following: - Optimized color filters to reduce light guiding between the air / glass interface and the reflective anode, including the use of an optical filtering layer specifically designed to absorb light traveling at high angles from the substrate normal. -Light scattering is reduced by reducing scattering sites. In particular, the amount of small particle debris on or near the anode must be minimized. Scattering can also arise from roughness in 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, 1693-1698.) The entire anode surface must be as flat and smooth as possible, both in the active pixel area and between pixels. In particular, protrusions, ridges, or other structures that form the interpixel PDL (pixel-defining layer) and extend above the surface of the anode within the pixel area are known to be useful in scattering light back into the pixel area and can prevent it from entering neighboring (unlit) pixels. However, this approach is less effective when there is a thicker OLED layer overlying the structure. Light trapped within the thicker layer is more likely to be reflected internally within the layer, allowing the light to travel over the structure to the other side. If the anode and overlying OLED layer are uniformly flat, light waveguiding within the display's layers is likely to continue uninterrupted until it is absorbed or reaches the edge of the display. - Use of absorbers for guiding light. -Light absorption by the backplane insulator. -Design of the HIL and anode to create a barrier for charge from the HIL to enter the anode.

[0143] To control crosstalk, it may be useful to add a further separate pixel control circuit to a node located between the second switch transistor and the anode of the OLED, which controls the voltage at the anode whenever the OLED is driven so that it is nominally "off" (unpowered and therefore not emitting light). In particular, the pixel control circuit would only apply a voltage to the OLED's V thIn this way, any voltage created by lateral carrier movement can be minimized. Experimental results

[0144] In the following examples, the number preceding each material (e.g., 200 HIL) is the physical layer thickness in Angstroms unless otherwise stated. All percentages are by weight. All devices were encapsulated after cathode deposition using the same procedure. All materials were purchased commercially.

[0145] The backplane used in the following experimental devices was a single-crystal silicon-based backplane, and was used as received with the schematic control circuit shown in Figures 6a and 6b. The backplane included a drive transistor (T1) and a switch transistor (T2), which were series-connected low-voltage (rated at 5 V) p-channel transistors, and a protection circuit with an NPN-type bipolar junction transistor (BJT1) for each pixel. In all experimental examples, the base of BJT1 was isolated; i.e., the base was not connected to an external power source, so that the base voltage of the BJT could not be unintentionally controlled or set to any particular voltage. The backplane included a reflective metal layer as the bottom electrode / anode.

[0146] Comparative two-unit stacked OLED device A-1 was prepared as follows. Layer 1 (reflective bottom electrode / anode): As supplied Layer 2 (hole injection layer HIL): 200 hole transport material HTM1 + 6% p-dopant PD1 Layer 3 (hole transport layer HTL1): 410 hole transport material HTM1 Layer 4 (HTL2): 50 hole transport material HTM2 Layer 5 (green LEL): 100% green host GH1 and 3% green phosphorescent dopant GPD1 Layer 6 (yellow LEL): 100GH1 + 10%GPD1 + 3% red phosphorescent dopant RPD1 Layer 7 (electron transport layer ETL1): 200 electron transport material ETM1 Layers 8 to 10 (charge generating layer CGL1): 270 three layers Layer 11A(HTL3):300HTM1 Layer 11B(HTL4):280HTM1 Layer 12 (Blue LEL2): 200 blue host BH1 + 4% fluorescent blue dopant BFD1 Layer 13 (ETL2): 150ETM2 Layer 14 (electron injection layer EIL): 150ETM1+2%Li Layer 15 (translucent cathode): 105 75%Ag / 25%Mg

[0147] A second comparative OLED device, A-2, was prepared in a similar manner, except that the thicknesses of layers 11A and 11B were increased to 1090 and 4090 Å, respectively. Both A-1 and A-2 are two-unit stacked OLEDs, in which layer 5 (G LEL) and layer 6 (red LEL) together represent one unit (GY stack), and layer 11 (blue LEL) represents a second unit (B stack) separated from the first unit by a CGL. Layers 5 and 6 are a single unit because they are adjacent LELs not separated from each other by a CGL (layers 8 and 9). Both A-1 and A-2 are microcavity devices, with the microcavity thickness of A-1 being 2320 Å and in A-2 the microcavity is 6920 Å. These two-unit stacked OLED devices represent the current prior art.

[0148] The inventive three-unit stack OLEDB-1 was prepared in a similar manner, except that the following additional layers were inserted between layers 11A (whose thickness was adjusted to 470) and 11B (at 4090): Layer 16(BLEL1):200BH1+4%BFD1 Layer 17 (ETL3): 150ETM2 Layers 18-20 (CGL2): 270 three-layer (same composition as CGL1)

[0149] OLED Device B-1 is a three-unit stacked OLED, with layers 5 and 6 being the first GY unit (same as in A-1), layer 16 being an additional second B unit, and layer 12 (as in A-1) being the 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, B-1 contains an additional B unit compared to A-1, and therefore has higher luminance but increased voltage requirements. OLED B-1 is a microcavity device with the same microcavity thickness (6920 Å) as Device A-2. Figure 14 shows characteristic I-V curves (mA / cm vs. voltage) for OLEDs A-1, A-2 (comparative two-unit device), and B-1 (inventive three-unit device). 2 The current density at the time of the test is shown in Table 1. [Table 1]

[0150] In Figure 14 and Table 1, OLEDA-1 ​​and A-2 have almost the same V th However, OLED B-1 with additional B units has a significantly higher V th and requires an even higher voltage (about 2.5 V higher) to produce the same current density as A-2.

[0151] The advantage of having a microcavity three-unit stacked OLED over a two-unit stacked OLED can be seen in Figure 15, which compares the spectral output (intensity vs. wavelength in nm) of A-1, A-2, and B-1 without color filters. OLED A-1, with a thinner microcavity, has relatively little B and R emission compared to G emission. OLED A-2, with a microcavity suitable for all three colors, has more B and R emission compared to G, but is still limited in B emission. OLED B-2, with an additional B unit, has greatly increased blue emission compared to A-2.

[0152] The above results refer to a white light OLED device before the addition of color filters to create R, G, and B pixels for a microdisplay. As is commonly known in the prior art, regardless of any inherent imbalance in the relative amounts of R, G, and B light produced by a W OLED, a desired R, G, and B balance in a display (usually stated in terms of white brightness, such as the D65 white point) can be achieved by driving each individual color pixel to the appropriate brightness for that color. For example, if a W OLED produces insufficient blue light and excess red light compared to green light, the B pixels can be driven at a higher voltage to produce more blue light, while the R pixels can be driven at a lower voltage so that less red light is produced.

[0153] Figure 16 shows a plot of the peak current density required for each color for OLED devices A-1, A-2, and B-1 using color filters to generate a D65 white point, assuming equal areas for the R, G, and B subpixels. The current density in B required by the two-unit devices (A-1 and A-2) is much higher than that of the three-unit device B-1. The current densities in R and G are approximately the same in all three devices. The three-unit device B-1 requires a lower current density, and therefore less power, to achieve the same target luminance at the same white point. The lifetime of the display is determined primarily by the rate of OLED efficiency degradation with continued operation, which is a function of current density and temperature. Therefore, display B-1 will have a lifetime that is more than twice that of A-1 or A-2.

[0154] In microdisplays, which need to produce relatively high brightness given their smaller size, it is desirable to keep the total current (and current density) approximately the same (or as close as possible) for each color while displaying white at maximum display brightness. This results in several advantages in terms of neutral fade (each color of sub-pixel degrades in luminance efficiency at approximately the same rate over the device lifetime), high brightness (each pixel is driven at its maximum brightness, resulting in increased efficiency), high contrast (the full dynamic range of each pixel is available), and increased lifetime (the current load is shared equally by each color of pixel).

[0155] However, the advantage of using an OLED with three or more units to provide increased brightness may be offset if there is an increase in the required voltage. In a display, power to each pixel is supplied and controlled by control circuitry in the backplane. However, in microdisplays, which require a large number of pixels in a reduced area, there is 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 dictates that the electronic circuits be rated to handle 5V or less to achieve the desired pixel size and density (pixel pitch).

[0156] Electronic circuits, such as transistors, can fail catastrophically if excessive power is supplied to the transistor's gate for too long. However, not all levels or durations of overvoltage permanently damage the transistor; in many cases, the transistor fails due to leakage. Increased leakage is a common failure mode resulting from non-catastrophic overstress of semiconductor devices, when the junctions or gate oxides suffer permanent damage that is not sufficient to cause catastrophic failure. Overstress on the gate can result in stress-induced leakage current.

[0157] Solving motion blur in microdisplays used in virtual reality and similar products requires the OLED to provide very high brightness for a very short period of time and then be turned off for another period of time (the total on / off period is less than the range of human perception to avoid flicker). This means that the control circuitry in the backplane must supply voltage / current to generate high brightness for one period and then cut off power to the OLED for another period. If the power supplied to the transistors in the control circuitry is too high during the period when the OLED is turned on, current can leak through the transistors when they should be "off" and the OLED does not turn off completely. The difference between the OLED brightness when it is "on" and when it should be "off" is usually referred to as "contrast." High contrast is desirable for microdisplays because it makes images appear sharp and clear, while low contrast makes images appear dull and flat.

[0158] FIG. 17 shows a set of curves (luminance vs. cathode voltage) for a two-unit comparative device and an inventive three-unit device. The upper set of curves is when the device is driven at full luminance (white=CV255) and the drive transistor is "on." The lower set of curves is when the device is driven at no luminance (black=CV0) and the drive transistor is nominally "off." However, current leakage through the drive transistor produces some small amount of luminance, and the voltage at the cathode is non-zero. The difference between the two sets of curves represents the "contrast" of the device, and is desirably as high as possible by minimizing the luminance caused by leakage current through the drive transistor. As can be seen from FIG. 17, the contrast of all three devices is nearly the same (10 5 (slightly smaller than ) and is relatively constant over the range of cathode voltages.

[0159] The inventive three-unit device maintains approximately the same overall contrast as the comparative two-unit device, despite requiring a higher operating voltage beyond the design limits of the drive transistors. Although a lower current density is required to achieve full white in B-1 than in A-1 or A-2, as shown in Figure 16, B-1 still requires more voltage (approximately 2.5 V, see Figure 14). There is no evidence of increased leakage in the higher voltage OLEDs using the backplanes used in the present invention. Furthermore, there is no evidence of any catastrophic transistor failure due to the higher voltage requirement in B-1 (see Table 1), and device lifetime for the control circuitry appears unaffected. These results are unexpected because the transistors in the backplanes for A-1, A-2, and B-1 were designed using transistors that tolerate low voltage transistors and are suitable for driving commonly used one- or two-unit OLEDs.

[0160] Using the same low-voltage backplane as A-1, A-2, and B-1, another inventive three-unit OLED device, B-2, was prepared as follows. Layer 1 (bottom electrode / anode): Layer 2(HIL):200HTM1+6%PD1 Layer 3 (HTL1): 410HTM1 Layer 4 (HTL2): 50HTM2 Layer 5 (green LEL): 100GH1 and 3%GPD1 Layer 6 (yellow LEL): 100GH1+10%GPD1+3%RPD1 Layer 7 (ETL1):200ETM1 Layer 8~10 (CGL1): 270 three layers Layer 11 (HTL3): 570HTM1 Layer 12 (Blue LEL2): 250BH1+4%BFD1 Layer 13 (ETL2): 150ETM2 Layers 14-16 (CGL2): 270 three-layer (same composition as CGL1) Layer 17 (HTL4):3840HTM1 Layer 18(BLEL1):250BH1+4%BFD1 Layer 19 (ETL3): 150ETM2 Layer 20 (EIL): 100ETM2+2%Li Layer 21 (translucent cathode): 105 75%Ag / 25%Mg

[0161] OLED B-2 is a three-unit device like B-1 because it contains, in YG / B / B order (from the backplane), a GY unit (layers 5 and 6) separated by a CGL (layers 8-10) from a second blue unit (layer 12), which is separated from a first blue unit (layer 18) by a second CGL (layers 14-16). Like B-1, B-2 is a microcavity white light-generating OLED with a 6910 Å microcavity.

[0162] In a similar manner, an inventive four-unit OLED device C-1 was prepared as follows. Layers 1 to 11: Same as layers 1 to 11 in B-2 Layer 12 (blue LEL2): 200BH1 + 4%BFD1 (50 Å reduction) Layer 13 (ETL2): 100 ETM2 (50 Å reduction) Layers 14-16: Same as layers 14-16 in B-2 Layer 17 (HTL4): 420HTM1 (3420 Å reduction) Layer 18 (BLEL1): 200BH1 + 4%BFD1 (50 Å reduction) Layer 19 (ETL3): 100 ETM2 (50 Å reduction) Layers 20-22 (CGL3): 270 three-layer (same composition as CGL1 and CGL2) Layer 23(HTL5):2620HTM1 Layer 24 (HTL6):50HTM2 Layer 25 (yellow LEL): 200GH1+10%GPD1+3%RPD1 Layer 26 (ETL4): 480ETM1 Layers 27-28: Same as layers 20-21 in B-2

[0163] OLED C-1 is a four-unit device because it contains an additional Y unit (layer 25) along with the same GY and two B units in B-2, 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 20-22). C-1 is a microcavity white light-generating OLED with a microcavity of 6910 Å.

[0164] In a similar manner, an inventive five-unit OLED device D-1 was prepared as follows. Layers 1 to 10: Same as layers 1 to 11 in C-1 Layer 11 (HTL3): 370HTM1 (reduced by 200 Å) Layers 12-16: Same as layers 12-17 in C-1 Layer 18 (ETL2): 100 ETM2 (50 Å reduction) Layers 14-16: Same as layers 14-16 in C-1 Layer 17 (HTL4): 620HTM1 (200 Å increase) Layers 18-22: Same as layers 18-22 in C-1 Layer 23 (HTL5): 610HTM1 (2010 Å reduction) Layer 24(BLEL3):200BH1+4%BFD1 Layer 25 (ETL3): 100ETM2 Layers 26-28 (CGL4): 270 triple layer (same composition as CGL1, CGL2, and CGL3) Layer 29 (HTL6):1440HTM1 Layers 30-34: Same as layers 24-28 in C-1

[0165] OLED D-1 is a five-unit device because it contains an additional B unit (layer 24) along with the same Y, GY, and two B units in C-1. It has a total of three B units, one Y unit, and one GY unit, in the order YG / B / B / B / Y (from the backplane), all separated by a CGL. C-1 is a microcavity white-light-generating OLED with a 6910 Å microcavity.

[0166] FIG. 18 shows the characteristic IV curves for OLEDs B-2, C-1, and D-1, which are also tabulated in Table 2. [Table 2]

[0167] As shown in Figure 18 and Table 2, each additional unit in the OLED provides greater luminance, but V th The operating voltage is also increased by about 2.5V.

[0168] The benefit of adding additional units to a microcavity three-unit OLED can be seen in Figure 19, which compares the spectral output (without color filters) of B-2 (3 units), C-1 (4 units), and D-1 (5 units). OLED B-2 has two B units in addition to the G / Y units. Adding another Y unit, as in C-1, increases the amount of G and R emission relative to B-2, but to a lesser extent in terms of B emission. Adding a third B unit, as in D-1, further increases the blue emission while retaining higher G and R emissions.

[0169] Similar to the results shown in FIG. 16, FIG. 20 shows the results for OLEDs B-2, C-1, and D-1 at 1500 cd / m 2Figure 1 shows the relative amount of peak current density for each color pixel required to create a balanced white luminance (using color filters). The addition of an additional Y unit in C-1 reduces the current required to generate the required amount of R and G luminance compared to B-2. The addition of an additional B unit in D-1 further reduces the voltage required to generate the required amount of B light compared to C-1. Because these OLEDs with three or more units require a lower current density to achieve the same luminance, their operational lifetime is significantly longer than OLEDs with fewer units. The LT70 (time for luminous output to degrade by 70%) lifetime of these devices is predicted to be at least 18,000 hours for average video content.

[0170] Figure 21 (similar to Figure 17) shows a set of curves (luminance vs. cathode voltage) for OLEDs B-2, C-1, and D-1. In all three of these examples, the contrast is nearly the same (10 5 (slightly less than 0.01), and is relatively constant over a range of cathode voltages. It is noteworthy that contrast is maintained even when the operating voltage of these devices is two or three times the design limit of the drive transistor.

[0171] Why the relatively high V th Without being limited to any particular theory or speculation as to why stacked OLEDs requiring V maintain high contrast but do not experience burnout or destruction when using backplanes with low voltage transistors, which may be related to the use of series-connected transistors. Leakage current in transistors is a well-known problem, and generally speaking, the higher the voltages and currents involved, the greater the amount of leakage. As discussed above, this leakage occurs when it is V thBecause the total leakage through series-connected transistors is (leakage of the first transistor) × (leakage of the second transistor), this multiplicative effect may be sufficient to reduce leakage at the bottom electrode of the OLED significantly enough so that the OLED does not emit light due to leakage current, and therefore contrast is maintained.

[0172] In operation, a control circuit in the backplane of an experimental microdisplay, comprising at least two transistors with their channels connected in series, is an example of a small circuit that prevents the drive transistor T1 from operating under conditions outside its specified range, even when driving an OLED with three or more stacks with an on-off voltage range greater than the specified operating range of the LV transistor. When the OLED's on-off voltage range exceeds the LV operating range, only the switch 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.

[0173] Through operation, MOSFET devices age, resulting in gradual shifts 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. Restricting transistor operation within a specified voltage range for the transistor ensures that the transistor characteristics remain within a narrowly specified range for an extended period, such as five years. Operating the transistor outside the specified voltage range increases the rate of change of the transistor characteristics, thereby reducing the period over which the characteristics remain within the specified range.

[0174] It is important that the characteristics of the drive transistor remain within specified limits; otherwise, image degradation (often referred to as mura) can result. The operation of the switch transistor is more robust to such changes in performance characteristics and can be driven over a wide enough range of gate voltages to ensure satisfactory operation even when the transistor characteristics are shifted outside of the specified range. Thus, the circuits shown in Figures 2-6, including those used in the backplane for the experimental stacked OLED microdisplay of the present invention, are examples of compact circuits designed to handle the higher on-off voltages associated with three or more OLED units without the expected degradation in display quality due to exceeding the specified operating range of the transistors.

[0175] The circuits in Figures 2-6 are believed to accomplish this by selecting the SELECT2 voltage for illumination "on" and illumination "off." For illumination "off," a valid choice of SELECT2 voltage is V DD , which places T2 in the subthreshold region of operation, effectively stopping the current. For light "on," the SELECT2 voltage is V DD can be selected as low as 5V below.

[0176] When the OLED's on-off voltage swing is less than approximately 4V, the black level current is set by the drive transistor in the normal manner when the pixel displays "black," and the drain of T1 exceeds the SELECT2 "on" voltage, leaving the switch transistor T2 conductive. However, when the OLED's on-off voltage swing is greater than approximately 4V, as in the case of a multi-unit stacked OLED with five or more units, the drain voltage of T1 (which is also the source voltage of T2) approaches the SELECT2 "on" voltage when generating the black level current, and the switch transistor T2 shuts off when the low drain voltage of T1 reduces the overvoltage (Vgs-Vth) of T2 to zero or slightly negative. This puts T2 into the subthreshold mode, creating the black level. Under these conditions, created by a multi-unit stacked OLED, the drive transistor T1 controls T2 to generate the black level current.

[0177] For these reasons, the simple series-connected transistor designs shown in FIGS. 2-6 are believed to be capable of driving multi-unit stacked OLEDs with an on-off voltage range that exceeds that of low-voltage transistors, with less aging-related unevenness than, for example, the single transistor drive circuit shown in FIG. 1.

[0178] The protection circuitry is designed to maintain the voltage at the OLED's anode below a certain level whenever the OLED is intended to be "off" or non-emitting, which can help maintain high contrast. As the cathode voltage decreases (more negative voltages), the protection circuitry is designed to provide additional current to protect the drive and switch transistors from voltage levels that are contrary to the device's maximum rating.

[0179] However, the protection effect is still observed in experimental examples where the BJT base is isolated and not connected to an external power source so that the base voltage is not intentionally controlled. In devices lacking a stacked OLED, the exponential slope of the black current versus voltage for OLED displays (0.75 decades / volt) has been observed to be similar to that in cases where the OLED is present. This suggests that the protection circuit still provides some current and voltage control at the OLED anode, even when the BJT base is isolated. Without being limited to any particular theory or reasoning, a nearby n-well (e.g., the n-well of switch transistor T2) may be a source of holes migrating to the p-well (base) of BJT BJT1. Once these holes are in the base, they diffuse across the depletion region to the n-type emitter contact (OLED anode pad) and travel forward through the base-emitter diode. This is complemented by the diffusion of thermally excited electrons from the emitter into the base, and their transport into the collector through the base and depletion region, facilitated by the large field potential between the base and collector. In this scenario, holes from a nearby n-well (of the drive or switch transistor) donate charge (holes) into the base, usually coming from the external base connection. When the OLED anode voltage drops to a very low level (e.g., showing black with three or more OLED units), the nearby n-well (V DD The potential difference between the drive-transistor well and the BJT base (located at the OLED anode voltage) is large enough to increase the flow of holes from the drive-transistor well into the BJT base. This increase in base current increases the emitter current due to amplification by the BJT.

[0180] However, the protection effect provided by the protection circuitry varies from frame to frame and must be appropriately reset for each new frame of the image. This is not an issue when the base of the BJT is connected to an external power supply and is actively controlled every frame. For embodiments in which the base of the BJT is isolated and not intentionally connected, a frame-by-frame reset can be provided since it provides a shutter effect when the pixel is turned "off" by the switch transistor. Therefore, this aspect of protection effect control movement relies on having series-connected drive and switch transistors, since it is necessary to turn off light emission during data loading (or for some portion of the frame time) to have a reset.

[0181] OLED formulations with three or more units have a low black level (V th The voltage range from a current density of less than 10,000, e.g., 2 uA / cm², to a white level (20 mA / cm²) can be designed to be relatively constant and less than about 6 V. As shown in Figures 17 and 21, this can result in a contrast of about 10,000:1, which may be slightly less due to reduced current efficiency at the high end of the current density. This voltage range is roughly within the allowable operating range of the LV transistors, and the protection circuitry only becomes active at the low end of the current range, when the drive and / or switch transistors stop the current. Thus, at the low current end of the range, the protection circuitry additionally operates when the current density through the OLED is about 2 uA / cm². 2 This effect, in addition to the protection provided by having at least two series-connected transistors as discussed above, also limits the ability to achieve higher contrast. In exchange for this slightly increased black level and reduced contrast, the protection circuitry allows the pixel drive circuit to achieve higher peak brightness by lowering the cathode voltage, or to compensate for efficiency loss due to OLED aging, with the assurance that the LV transistors are operating within their specified voltage range.

[0182] The above description describes several different embodiments that may involve different combinations of different individual features. Individual features from any of the embodiments may be combined in any order or degree as desired, without limitation, except when incompatible.

[0183] In the above description, reference is made to the accompanying drawings, which form a part hereof, and which show by way of illustration specific embodiments that may be practiced. These embodiments are described in detail to enable those skilled in the art to practice the invention, and it should be understood that other embodiments may be utilized and that structural, logical, and electrical changes may be made without departing from the scope of the invention. Accordingly, the description of any example embodiment should not be taken in a limiting sense. While the invention has been described for purposes of illustration, it should be understood that such details are for that purpose only and that variations may be made by those skilled in the art without departing from the spirit and scope of the invention. [Explanation of symbols]

[0184] MP1 switch transistor, MP2 drive transistor, C1 capacitor, V DD External power supply, SELECT1~SELECT3 select lines, T1 first p-channel drive transistor, T2 switch transistor, T3 in-line select transistor, V CATHODE Cathode voltage, IBD1 to IBD6 Intrinsic body diode, V DD2 External power supply, T4 second p-channel switch transistor, T6 protection circuit p-channel transistor, D4 diode, V REFReference voltage, BJT1 Bipolar junction transistor, LSC Level shift circuit, 1 Pixel defining layer, 3 Silicon backplane, 5 Optional planarization layer, 7 Electrical contact, 9 First electrode segment, 9A First electrode layer, 9B Reflective layer, 11, 23 Non-emissive OLED layer, 13 First emissive OLED unit, 13A Red emissive OLED unit, 15, 19, 24 Charge generating layer, 16 Yellow emissive unit, 17 Second emissive OLED unit, 17A Green emissive OLED unit, 21 Third emissive OLED unit, 21A Blue emissive OLED unit, 22 Blue emissive layer, 25 Top electrode, 27 Encapsulation, 29 Color filter array, 29B Blue filter, 29G Green filter, 29R Red filter, 30 Microcavity, 32 Second blue emissive unit, 45,50 Power, GND Ground, 100 RGB Pixelated OLED, 200 Multimodal OLED Microcavity Device, 300 Multimodal OLED Microcavity Device, 400 Multimodal OLED Microcavity Device.

Claims

1. 1. A microdisplay comprising a light-emitting OLED stack on a silicon-based backplane having individually addressable pixels and control circuitry, the light-emitting OLED stack having three or more OLED units between a top electrode and a bottom electrode; The control circuitry of the silicon-based backplane supplies each individually addressable pixel with an external power supply V ＤＤ and at least two transistors having channels connected in series between the lower electrode of the light-emitting OLED stack and the lower electrode of the light-emitting OLED stack; the transistors having channels connected in series are both rated at 5V or less; each of the OLED units is separated from the others by a charge generation layer (CGL); A microdisplay, wherein the bottom electrode is segmented, each segment being in electrical contact with the control circuitry in the backplane.

2. 2. The microdisplay of claim 1, wherein the V of the light-emitting OLED stack ｔｈ is at least 7.5 V or more.

3. 2. The microdisplay of claim 1, wherein the V of the light-emitting OLED stack ｔｈ is at least 10 V or more.

4. 4. The microdisplay according to any one of claims 1 to 3, characterized in that the light-emitting OLED stack comprises four or more OLED units.

5. 10. The microdisplay of claim 1, wherein the light-emitting OLED stack is top-emitting.

6. 2. The microdisplay of claim 1, wherein the light-emitting OLED stack forms a microcavity in which the physical distance between the segmented bottom electrode and the top electrode is constant across all pixels.

7. 1. A microdisplay comprising a light-emitting OLED stack on a silicon-based backplane having individually addressable pixels and control circuitry, the light-emitting OLED stack having three or more OLED units between a top electrode and a bottom electrode; The control circuitry of the silicon-based backplane supplies each individually addressable pixel with an external power supply V ＤＤ and at least two transistors having channels connected in series between the lower electrode of the light-emitting OLED stack and the lower electrode of the light-emitting OLED stack; the transistors having channels connected in series are both rated at 5V or less; The two transistors having channels connected in series are both p-channel transistors; A microdisplay, characterized in that the two transistors whose channels are connected in series are each located in a separate well.

8. 1. A microdisplay comprising a light-emitting OLED stack on a silicon-based backplane having individually addressable pixels and control circuitry, the light-emitting OLED stack having three or more OLED units between a top electrode and a bottom electrode; The control circuitry of the silicon-based backplane supplies each individually addressable pixel with an external power supply V ＤＤ and at least two transistors having channels connected in series between the lower electrode of the light-emitting OLED stack and the lower electrode of the light-emitting OLED stack; the transistors having channels connected in series are both rated at 5V or less; The microdisplay, wherein the control circuit additionally comprises a protection circuit comprising a p-channel transistor.

9. 1. A microdisplay comprising a light-emitting OLED stack on a silicon-based backplane having individually addressable pixels and control circuitry, the light-emitting OLED stack having three or more OLED units between a top electrode and a bottom electrode; The control circuitry of the silicon-based backplane supplies each individually addressable pixel with an external power supply V ＤＤ and at least two transistors having channels connected in series between the lower electrode of the light-emitting OLED stack and the lower electrode of the light-emitting OLED stack; the transistors having channels connected in series are both rated at 5V or less; A microdisplay, wherein the control circuit additionally comprises a protection circuit comprising a pn diode.

10. 10. The microdisplay of claim 9, wherein the cathode of the pn diode is connected to a node of the bottom electrode of the light-emitting OLED stack and the anode is connected to a voltage reference V ＲＥＦ or current reference I ＲＥＦ A microdisplay connected to the

11. 1. A microdisplay comprising a light-emitting OLED stack on a silicon-based backplane having individually addressable pixels and control circuitry, the light-emitting OLED stack having three or more OLED units between a top electrode and a bottom electrode; The control circuitry of the silicon-based backplane supplies each individually addressable pixel with an external power supply V ＤＤ and at least two transistors having channels connected in series between the lower electrode of the light-emitting OLED stack and the lower electrode of the light-emitting OLED stack; the transistors having channels connected in series are both rated at 5V or less; The microdisplay, wherein the control circuit additionally comprises a protection circuit comprising a bipolar junction transistor.

12. 12. The microdisplay of claim 11, wherein the bipolar junction transistor has a base connected to a voltage source V ＰＲＯＴＥＣＴ or current source I ＰＲＯＴＥＣＴ , the emitter is connected to a node connected to the bottom electrode of the light-emitting OLED stack, and the collector is connected to a voltage source V ＤＤ a NPN transistor connected to said

13. 12. The microdisplay of claim 11, wherein the bipolar junction transistor has a base that is insulated, an emitter that is connected to a node that is connected to the bottom electrode of the light-emitting OLED stack, and a collector that is connected to a voltage source V ＤＤ A microdisplay connected to the

14. 12. The microdisplay of claim 11, wherein the bipolar junction transistor is located in a separate well from the two transistors whose channels are aligned in series.

15. 15. A microdisplay as claimed in claim 14, characterized in that the two transistors with their channels connected in series are both p-channel transistors, each located in a separate n-well, and the bipolar junction transistors are NPN transistors located in separate p-wells.

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