TW202217785A - Stacked oled microdisplay with low-voltage silicon backplane - Google Patents

Abstract

A microdisplay comprising a light emitting OLED stack on top of a silicon-based backplane with 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 comprises at least two transistors with their channels connected in series between an external power source V _DD, and the bottom electrode of the OLED stack. The light-emitting OLED stack preferably has a Vth of at least 7.5 V or more. The control circuit can include a protection circuit comprised of a p-n diode, preferably a bipolar junction transistor.

Description

Stacked Organic Light Emitting Diode Microdisplay with Low Voltage Silicon Backplane

This application relates to OLED microdisplays, and more specifically to stacked OLED microdisplays with low voltage silicon backplanes.

Typically, a microdisplay is less than 2 inches diagonal (about 5 cm), and even an ultra-small display is less than 0.25" diagonal. In most cases, the resolution of the microdisplay is high and the pixel pitch is usually 5 to 15 microns. When first commercialized in the late 1990s, microdisplays were commonly used in rear projection TVs, head-mounted displays, and digital camera viewfinders. In recent years, devices such as smart watches have taken advantage of the high resolution of these displays Microdisplays are expected to proliferate in the next few years, with the global market expected to grow at a CAGR of 20%. One of the trends driving this growth will be the increasing adoption of near-eye displays, increased Reality and virtual reality devices such as Head Mounted Displays (HMDs), Heads Up Displays (HUDs) and Electronic Viewfinders (EVFs).

There are two main categories of microdisplays. The first is a projection microdisplay, which involves projecting a highly magnified image onto a surface. Types of projection microdisplays include rear projection TVs and compact data projectors. The second type is a near-eye display (NED), which consists of viewing a highly magnified virtual image through an eyepiece (eg, a virtual reality headset or camcorder viewfinder). Such displays are increasingly used for HMDs and HUDs, especially in the military and medical industries.

Both types of microdisplays offer significant advantages over conventional intuitive displays (eg, flat panel LCDs). Compared to other display types, the advantages of microdisplays include the ability to generate a large image from a very small lightweight source display unit, making it easy to integrate into space-constrained technologies such as wearable equipment; Perform high resolution and clarity; and achieve greater power efficiency. The higher the resolution and brightness and the lower the power consumption, the better the quality of the microdisplay. However, the challenges faced by microdisplay manufacturers are the relatively high cost of production and the need for high brightness and contrast as well as long operating life.

Microdisplays can be fabricated by a variety of display technologies, including liquid crystal on silicon (LCoS), liquid crystal displays (LCD), digital micromirror devices (DMD), digital light processing (DLP); polar body) and organic light emitting diode (OLED).

LCDs have occupied the microdisplay market in recent years. The advantages of LCD technology are high brightness, relatively low cost, and a relatively simple manufacturing process. The use of LCDs has enabled device manufacturers to reduce the size of microdisplay components over time. 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 together with a liquid crystal array for modulating light to form an image. There are limitations to this technology, such as polarization, color space, maximum illuminance limitations, LC temperature sensitivity, viewing angle, LCD transmission and extinction ratios, system constrained size, and others, which do not provide all desired performance characteristics.

Microdisplays based on microLED technology may have advantages over LCD microdisplays such as self-emission, a larger color gamut, wide viewing angle, better contrast ratio, faster refresh rate, lower power consumption (related to images) and Wide operating temperature range. Currently, microLED microdisplays are based on one of the standard gallium nitride (GaN) wafers used in standard LEDs. This method can provide a high-illuminance display device without lifetime issues at a relatively low price. Typically, standard GaN wafers are patterned into microLED arrays. Then, a micro LED display is produced by integrating the micro LED array with the transistor. However, due to color and illumination differences between individual micro-LEDs, there are several manufacturing concerns with this approach, including micro-LEDs being formed monolithically over transistors, pixel spacing, color rendering, and spatial uniformity.

OLED technology also has many attractive features of microLED technology for microdisplays. It is self-luminous, has excellent image quality, is very efficient compared to LCD or LCoS and has an ultra-high color rendering and wide color space. An important advantage of self-emissive OLED devices over backlight devices, such as LCDs, is that each pixel produces only the intensity required for the image, whereas the backlight pixels produce maximum intensity and subsequently absorb unwanted light. Furthermore, forming an OLED over a transistor is easier and less expensive than forming a micro-LED because the OLED layer can be vacuum deposited or coated directly on the transistor backplane. On the other hand, the illuminance and lifetime of OLEDs may be limited.

Addressing motion blur is also important for control circuitry in OLED microdisplays, which are sample-and-hold displays (see https://www.blurbusters.com/faq/oled-motion-blur/; Medium 2018 "Why Do Some OLEDs Have Motion Blur?" on December 28, 2015 and "Is Motion Resolution an on January 15, 2015" in https://www.soundandvision.com/content/motion-resolution-issue-oled-tvs Issue with OLED TVs").

The only way to reduce motion blur caused by sample-and-hold is to shorten the amount of time a frame is displayed. This can be achieved by using additional refreshes (higher Hz) or through dark periods between refreshes (flickering). For OLED microdisplays, the best solution is to "turn on and off" the display image by turning off the entire active area at the same time or by a "scrolling" technique, in which only a portion of the display image is turned off sequentially at a time. The "scroll" technique is preferred. The pixel off-time is very short and well below the threshold perceptible to the human eye to avoid perceptible flicker. This is accomplished by including an on-off transistor in the control circuitry that prevents current flow through the OLED when the on-off transistor is activated through a select line and switches the emission by turning the OLED pixels "off" for a desired period of time Purpose. In other words, the switch transistor system is a switch transistor in that it only "turns" the pixel on or "off" without adjusting the voltage or current. However, this solution of turning off pixels for a portion of an image display time (often referred to as frame time) only increases the OLED's need for increased illumination each time the OLED is "on" because the It is the average illuminance perceived by the eye within the frame. The motion blur reduction switch can be applied to any method of powering the OLED stack, such as current control or PWM.

OLED microdisplays utilizing silicon backplanes are very attractive from a cost and manufacturability perspective. See, for example, "Recent advances in small molecule OLED-on-Silicon microdisplays" by Ali et al, 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; US2019/0259337; Prache , Displays, 22(2), 49 (2001); Vogel et al. 2018 48 ^th European Solid-State Device Research Conference, p. 90, Sept. 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), pp. 40-5, 514 (2018).

Microdisplays require very high levels of illumination to be useful in all environmental conditions, such as outdoors in bright sunlight. Even under controlled environmental conditions, such as in VR Google, very high levels of illumination are required to create an immersive visual experience. The very high illumination from the display allows the use of lower efficiency optics that are smaller, lighter weight and lower cost, resulting in a more competitive headset. Currently, state-of-the-art OLED microdisplays do not provide the desired illuminance.

For example, a press release from one of the manufacturers of tandem OLED microdisplays stated that it might be able to deliver a full color product of 2.5k nits, but admitted that 5k nits would be 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 Jan 7, 2020). Some manufacturers have proposed a target of 10k nits or higher (see https://hdguru.com/calibration-expert-is-10000-nits-of-brightness-enough/, dated Jul 26, 2018 ). One of the most recent news releases on June 20, 2020 (https://www.businesswire.com/news/home/20200630005205/en/Kopin-Announces-Breakthrough-ColorMax%E2%84%A2-Technology-Unparalleled-Color) A tandem (2-stacked) OLED display emitting >1000 nits is described. It also announced, "It is expected to further improve brightness (>2000 nits) and color fidelity by optimizing OLED deposition conditions. By including a structure for enhancing out-coupling efficiency, OLED microdisplays can be implemented within a few years. brightness increased to >5000 nits".

One solution to increasing the total amount of light emitted by an OLED device is to stack multiple OLED units on top of each other, so 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 these OLED stacks increases based on the total number of individual OLED light emitting cells, the voltage required to drive the OLED stacks also increases based on the voltage to drive each individual OLED cell. For example, if a light emitting OLED cell requires 3V to produce 250 nits at a given current, then at the same current a stack of two such cells will require 6V to deliver 500 nits, a stack of 3 cells will require 9V to deliver 750 nits, and so on.

OLED stacks are well known; for example, US7273663, US9379346, US9741957, US9281487 and US2020/0013978 all describe OLED stacks of multiple stacks of light emitting OLED cells, each OLED cell separated by an intermediate connecting layer or a charge generating layer open. Springer et al. Optics Express, 24(24), 28131 (2016) report OLED stacks with 2 and 3 light emitting cells, where each cell has a different color. OLED stacks with up to six light emitting units have been reported ("High Brightness OLED Lighting" by Spindler et al., SID Display Week 2016, San Francisco, CA, May 23-27, 2016).

However, such a multi-stack OLED approach, which would require higher driving voltages, is difficult to apply to microdisplay applications. One problem is that microdisplays also need to have high resolution, which requires that the size of individual pixels must be as small as possible and that the active (light emitting) area of the microdisplay contains as many pixels as possible. This requires that the transistor system in the control circuitry of the backplane be small, but large enough to handle the required voltages and currents without causing permanent damage or current leakage.

Typically, as transistors get smaller, they cannot handle higher power due to leakage currents and other failure mechanisms, so voltage ratings get lower. Smaller lower voltage transistors have thinner insulating layers at the gate, so they also have more quiescent current leakage. Therefore, OLED devices that require higher voltages typically use larger transistors capable of handling higher voltages. WO2008/057372 discusses the problems associated with pixel circuit size reduction in microdisplays and the prior art. See also, for example, O. Prache , Journal of the Society for Information Display, 10(2), 133 (2002); O. Prache , "Active Matrix Molecular OLED Microdisplays, Displays, 22, 49-56 (2001); And "Microdisplays based upon organic lighat emitting diodes" by Howard et al., IBM J. of Res. & Dev., 45 (1), 15 (2001) discusses the need for silicon that provides high illumination and a large contrast ratio at low voltages Microdisplay on the back panel.

Additionally, when using MOSFET p-channel transistors to supply a constant current from a power supply at V _DD to an OLED with a cathode voltage of V _CATHODE , 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, when an attempt is made to turn off the OLED to form a black pixel, the current leakage of the transistor will be high and the OLED will continue to emit due to (V _anode – V _cathode ) will still be greater than the OLED threshold voltage. In an OLED microdisplay, since the OLED pixels will continue to emit light when it should be dark, current leakage through the drive transistor will reduce contrast. This effect will make pure black (not emissive) gray and reduce the magnitude of the tone level between pure black and pure white. This is not expected.

There is a need to increase the performance of OLED microdisplays on silicon backsides to emit light by utilizing OLED stacks with high illuminance and high voltage requirements. However, the control circuitry on the silicon backplane must be able to handle the increased voltage and current demands without a significant increase in size to maintain resolution and pixel pitch within the active area of the OLED. Rather, the control circuitry should maintain contrast by preventing or minimizing current leakage through the TFT. Contrast ratio between when a pixel should be "off," "black," or not emitting (usually, video code value (CV) = 0) versus when a pixel should be fully "on," "white," or at maximum emission (Typically, image signal CV = 255).

Typically, analog transistors with an operating range of 5 V or less are considered standard "low voltage" (LV) transistors in the semiconductor foundry industry that manufactures backplanes. Also common is that the voltage rating usually has a 10% safety limit, allowing reliable operation without degrading the lifetime of a "5 V transistor" or even a 5.5 V transistor; this is high enough to allow dynamic There is a certain degree of overvoltage in the voltage range and the extra burden voltage of the driving circuit. While the voltage limit is generally applied between any pair of contacts (gate, source, drain, body (also known as bulk or well)) leading to a transistor, it is particularly applicable to the largest gate- The drain voltage is such that the performance of the transistor under these conditions is typically within the specified range of 43,000 hours of operation. Sometimes, depending on the design of the transistor, the voltage limit for other contact pairs can be higher (eg, 7 V) but the transistor is still referred to as a LV or 5 V transistor. 5 V analog transistors are widely available in the industry because they are compatible with traditional TTL logic voltage levels for communication between integrated circuit (IC) chips. These 5 V transistors are also sometimes referred to as medium voltage (MV) transistors as the voltages for input-output communication continue to drop (eg, 3.3 V vs. 1.8 V standards), making the LV label a newer "more" Low-voltage" analog transistors. Although relative labels such as LV and MV may change over time, in this patent application the term LV or "low voltage" refers to a transistor rated at 5 V or less, and the term MV or "medium voltage" refers to transistors with voltage ratings higher than 5 V. Higher voltage analog transistors are also commonly used, but the exact voltage is not standardized in the IC manufacturing industry as it is with 5 V transistors. For example, higher voltage transistors are often required in industries such as automotive.

Currently, silicon backplanes with low voltage 5 V drive transistors can be used, which use tandem (two light emitting OLED cells separated by a CGL) OLED stacks to emit light. 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 2018 ; Xiao, “Recent Developments in Tandem White Organic Light-Emitting Diodes”, Molecules, 24, 151 (2019). The illuminance of these instances is not sufficient to meet technical requirements.

Han et al. "Advanced Technologies for Large-Sized OLED Displays" Chapter 3, 10.5772/intechopen.74869 (2018). This review article describes triple-stacked white OLED schemes and advances in backplane technology including technology with two transistors connected in series, but separately rather than in combination. This reference also states that the two transistor backplanes are "difficult to use in large high-resolution panels due to high line loads and short charging times" and thus employ a different type of backplane circuitry in their devices.

J. Cent. South Univ., 19, 1276−1282 (2012) by Liu et al. disclose a 3T1C circuit with two series-connected p-channel transistors in an OLED on a Si wafer microdisplay.

"A Novel Pixel Circuit with Threshold Voltage Variation Compensation in Three-Dimensional AMOLED on Silicon Microdisplays" by Zeng et al., P-27, SID 2019 Digest, p. 1313 describes a method for driving one of the AMOLED displays on a silicon backplane 4T1C pixel circuit. It discloses the use of two p-channel low voltage transistors connected in series between a power supply and an OLED. The first transistor is used to drive the OLED and control the current supplied to the OLED. The second transistor system is used to turn off one of the switching transistors of the OLED during program operation. The above circuit also contains additional transistors to determine and compensate for V _th changes.

US 9,066,379 describes control circuitry suitable for an OLED microdisplay. It discloses the use of two p-channel low voltage transistors connected in series between a power supply and an OLED. The first transistor system is used to drive the OLED and control the current supplied to the OLED. The second transistor system is used to switch (turn off) one of the switching transistors of the OLED. The circuit also requires a third transistor, the third transistor system being a medium voltage or high voltage p-channel transistor between the switching transistor and the OLED. The purpose of this third transistor is to prevent current flow to the OLED whenever the OLED's "off" period (V _ss -V _cathode ) is greater than the OLED threshold voltage.

"High-voltage circuits for power management on 65nm CMOS" by Pashmineh et al., Adv. Radio Sci., 13, 109–120, 2015, describe stacked low-voltage circuits based on series connections (also commonly referred to as "stacked" transistors) High-voltage circuits for voltage CMOS transistors.

"The Impact of the Transient Response of Organic Light Emitting Diodes on the Design of Active Matrix OLED Displays" by Dawson et al., International Electronic Devices Mtg 1998, 875-878 describes a pixel circuit with two series connected p-channel transistors.

"Organic Light-Emitting Diode-on-Silicon Pixel Circuit Using the Source Follower Structure with Active Load for Microdisplays" by Kwak et al., Japanese Journal of Applied Physics, 50, 03CC05 (2011), describes having three series-connected p-channel transistors and a pixel circuit of an overvoltage protection circuit. This reference states that since the operating voltage of OLEDs is higher than that of MOSFETs, "an overvoltage protection circuit" is required to prevent metal oxide semiconductor field effect transistors (MOSFETs) from breaking down. U5760477 and US9066379 also describe the use of a protection circuit.

Vogel et al. SID 2017 DIGEST Article 77-1 pp. 1125-1128 discloses the use of a protection circuit in a low voltage OLED Si microdisplay to extend the OLED voltage operating range.

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

揭示兩個串聯連接電晶體之額外專利參考包含：CN109166525、US20140125717、US7196682、US6229506、US9262960、US7443367、US6930680、US10614758、WO2019019590、US6946801、US7755585、US8547372、US8786591、US8797314、US9818344、US2018/0211592及US10269296。 The following references disclose pixel circuits with three transistors connected in series: US9324266, US9384692, US10600366, US7180486, US9858863 and US20190279567.

US9059123 and US9489886 disclose the use of n-p junction diodes, such as bipolar junction transistors, in pixel control circuits of OLED displays.

A microdisplay including a light-emitting OLED stack on top of a silicon-based backplane with individually addressable pixels and control circuitry is described, wherein the light-emitting OLED stack has a top electrode and a bottom electrode three or more OLED units in between; and the control circuitry of the silicon-based backplane includes at least two transistors for each individually addressable pixel, the channels of the at least two transistors being connected in series in a Between the external power supply V _DD and the bottom electrode of the OLED stack.

A microdisplay as above, wherein the threshold voltage V _th of the light emitting OLED stack is at least 7.5 V or greater than 7.5 V, or preferably at least 10 V or greater.

A microdisplay as in any above, wherein the OLED stack includes four or more OLED light emitting units.

A microdisplay as in any of the above, wherein the OLED light emitting cells are each separated from each other by a charge generating layer (CGL), or wherein the bottom electrode is segmented and each segment and control in the backplane The circuitry is in electrical contact, or where the OLED stack is top-emitting.

A microdisplay as in any of the above, wherein the OLED stack forms a microcavity, wherein the physical distance between the segmented bottom electrode and the top electrode is constant across all pixels.

A microdisplay as in any of the above, wherein the transistors with their equal channels connected in series are both rated at 5 V or less, or the transistor system closest to the power supply is the drive transistor and is rated at 5 V V or less than 5 V, and the transistor system closest to the bottom electrode of the OLED is a switching transistor and is rated for greater than 5 V.

A microdisplay as in any of the above, wherein the two transistors with their equi-channel series connection are both p-channel transistors, or wherein the two transistors with their equi-channel series connection are each in separate wells .

A microdisplay as in any above, wherein the control circuitry additionally includes a protection circuit having a p-channel transistor or diode. The protection circuit may include a pn diode, and wherein the cathode of the pn junction diode is connected to the node of the bottom electrode of the OLED stack, and the anode is connected to a voltage reference V _REF or a current reference I _REF .

The protection circuit as set forth above, wherein the diode system is a bipolar junction transistor. The bipolar junction transistor may be an NPN transistor with the base connected to a voltage source V _PROTECT or a current source I _PROTECT , the emitter connected to a node connected to the bottom electrode of the OLED stack, and the collector connected to the voltage source V _DD . The base of the BJT can be isolated.

A protection circuit as above, wherein the bipolar junction transistor is located in a well separate from the two transistors with their equi-channel series connection, or wherein the two transistors with their equi-channel series connection are both p-channel transistors and an NPN transistor each in a separate n-well and the bipolar junction transistor system in a separate p-well.

Any of the microdisplays as set forth above, additionally comprising an RGB color filter array (CFA) on top of the OLED stack, wherein the individual red filters, green filters, blue The filters are registered with the segmented bottom electrodes to form R, G, B pixels; or it additionally includes an RGBW color filter array (CFA) on top of the OLED stack, with individual red filters, green filters Optical filters, blue filters, and transparent or colorless filters are registered with the segmented bottom electrodes to form R, G, B, and W pixels.

Microdisplays provide very high luminance and contrast with a small pixel pitch size.

Cross-references to related applications

This application claims priority to: U.S. Provisional U.S. Application Titled "STACKED OLED MICRODISPLAY WITH LOW-VOLTAGE SILICON BACKPLANE" filed January 28, 2020, Attorney Docket No. OLWK-0021-USP Case 62/966,757 and U.S. Provisional U.S. Application 63/054387, also titled "STACKED OLED MICRODISPLAY WITH LOW-VOLTAGE SILICON BACKPLANE" and filed on July 21, 2020, with attorney case number OLWK-0021-USP2.

For the purposes of the present invention, the terms "on" or "over" mean that the structure in question is located above another structure, ie, on a side opposite the substrate. "Top", "uppermost" or "upper" refers to the side or surface remote from the substrate, while "bottom", "bottommost" or "bottom" refers to the side or surface closest to the substrate. Unless otherwise indicated, "on" should be interpreted to mean that two structures may be in direct contact or that an intervening layer may be present between the two structures. Reference to "layer" should be understood to have a single layer having two sides or two surfaces (topmost and bottommost); in some instances "layer" may refer to multiple layers considered as a whole and not limited to just one single layer.

In terms of light emitting units or layers, R indicates a layer that emits predominantly red light (>600 nm, desirably in the range 620 to 660 nm), and G denotes a layer that emits predominantly green light (500 to 600 nm, desirably 540 to 565 nm) range), and B indicates a layer that emits predominantly blue light (<500 nm, desirably in the 440 to 485 nm range). It is important to note that the R, G, and B layers can produce some degree of light outside the indicated ranges, but always less than the primary color. Y (yellow) indicates that a layer emits a lot of R and G light, but a smaller amount of B light. "LEL" means light emitting layer. Unless otherwise specified, wavelengths are in vacuum and not in situ.

An individual OLED light-emitting unit can produce a single "color" of light (ie, R, G, B, or a combination of 2 or more primary colors such as Y, C (blue-green) or W (white)). A single color of light can be generated within an OLED unit from a single layer or layers of one or more emitters of the same color, each layer having the same or different emitters emitting primarily within the same color. A single OLED unit can also provide a combination of two colors (ie, R+G, R+B, G+B) within a single OLED unit by having the following layers: a single emitter that emits light of both colors A layer, a layer with two different emitters, or a combination of individual layers each emitting one but a different color. A single OLED unit can also provide white light (a combination of R, G, and B) by having layers that emit light of all three colors, or a combination of individual layers that each emit a single (but different) colors, the sum of which is white. Individual OLED light-emitting units may have a single light-emitting layer or may have more than one light-emitting layer (either directly adjacent to each other or separated from each other by an intermediate layer). Individual light emitting cells may also contain various non-emissive layers such as hole transport layers, electron transport layers, blocking layers and known in the art to provide desired effects (eg, to facilitate emission and manage charge transfer across light emitting cells) other layers.

Because OLED light emitting cells can include multiple layers, an individual cell is sometimes referred to as a "stack," which can be confused with an OLED device having multiple cells. In this application, a "stacked" OLED has at least two OLED light emitting units stacked on top of each other on a substrate, so 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) rather than by separate and independently controlled intermediate electrodes. For an OLED light-emitting unit to be considered, it must be separated from another light-generating unit by a CGL. Therefore, a light-emitting layer adjacent to one of the OLED light-emitting units but not separated from the OLED light-emitting unit by a CGL is not considered as a separate unit. Within the stack, all or some of the individual OLED light emitting units may be the same or all may be different from each other. Within the OLED stack, other OLED light emitting cells can be placed between the top and bottom cathodes in any order. Stacked OLEDs can be monochromatic (each pixel of the OLED stack primarily emits the same color of light, such as green light) or can have multi-mode emission (where each pixel emits 2 or more colors of light (eg, yellow or white) or where different pixels emit different colors of light such that the overall emission contains 2 or more colors of light).

In some cases, the threshold voltage ( _Vth ) of the OLED stack can be estimated by straight-line extrapolation of the IV curve after significant light emission begins to return to the voltage axis. Since this method is inaccurate because the IV response curve of an OLED cannot be perfectly linear within its response range, the values calculated in this way are inaccurate. An approximate range for this measure is +/- 10%. More precisely, a threshold voltage can be defined as a voltage at which the current density of the exposed anode layer does not exceed 0.2 mA/cm ² and there is at least some reliably detectable illuminance (ie, at least 5 cd/A). The method used in this application.

Hereinafter, a transistor may be referred to as "on" or "off." In an "off" transistor, a signal is expected to be sent to the gate such that no current will pass through the terminal; in other words, the signal (usually, CV = 0) indicates that the signal requires no current to pass through the transistor so that no current will pass through the transistor. The transistor is turned off, but the transistor is regulated "off". In such cases, even though a transistor may be "off," there may still be some current leakage. Likewise, in an "on" transistor, a signal is expected to be sent to the gate such that at least some current will pass through the terminal; in other words, the signal (typically, CV = greater than 0 but less than 255) indicates that the image requires a pixel The emission is done to some extent so that instead of turning the transistor "on", the transistor is adjusted "on". Similarly, a pixel or OLED can be called "on" or "off" depending on the requirements of the image, and thus send the appropriate signal to the pixel or OLED.

Silicon backplanes are derived from a silicon wafer (also known as a block or substrate). It is used to fabricate a semiconductor wafer of integrated circuits, such as a crystalline silicon (c-Si). Wafers are used as substrates for microelectronic devices placed in and on wafers. It undergoes numerous microfabrication procedures such as doping, ion implantation, etching, thin film deposition and photolithography patterning of various materials. Finally, the individual microcircuits are separated and packaged into an integrated circuit by wafer dicing. Wafers are grown from crystals with a regular crystal structure in which the silicon is in a rhombic cubic structure with a lattice spacing. When diced into wafers, the surfaces are aligned in one of several opposing directions, known as crystal orientation. Silicon wafers are usually not 100% pure silicon, but are initially formed with doping concentrations of boron, phosphorus, arsenic or antimony impurities that are added to the melt and define the wafer as bulk n-type or p type. See Chapter 7 in "Flat Panel Display Manufacturing", Souk, L., Ed., 2018 for internal reference. The silicon backplane is expected to be a single crystal Si wafer.

In order to provide control circuitry for the operation of the stacked OLED, thin film transistors (TFTs) and other components (eg, capacitors, resistors, connecting wires or bus bars, etc.) are disposed on the surface of the silicon wafer. See, for example, "High Performance TFT Technologies for the AM-OLED Display manufacturing" by T. Arai, Thesis, Nara Institute of Science and Technology, 2016, M.K. Han's Proc. of ASID '06, 8-12 Oct, New Delhi, US9066379 and US10163998. It should be understood that a TFT may or may not include a silicon wafer as part of the TFT structure, or may be fabricated from a separate material deposited on the surface.

TFTs can be made using various semiconductor materials. The properties of a silicon-based TFT depend on the crystalline state of the silicon; that is, the semiconductor layer can be amorphous silicon, microcrystalline silicon, or the semiconductor layer can be annealed to polysilicon (including low temperature polysilicon (LTPS) and laser annealing).

The fabrication of silicon backplanes with suitable control circuitry is a well known, understood and predictable technique. However, due to the cost and complexity of the manufacturing process and equipment, it is often impractical to build a facility to manufacture a particular backplane. Instead, a foundry model is widely adopted in the industry, in which the functional characteristics of microelectronic devices have become more standardized. This standardization allows design to be separated from manufacturing. A design can be easier and cheaper to manufacture by different companies with compatible manufacturing methods following one of the proper design rules. Therefore, control circuitry on silicon backplanes is usually limited to using standard components selected from various options provided by backplane manufacturers. For example, a silicon backplane manufacturer may offer the option to include various standard transistor designs rated at 1.8 V, 2.5 V, 3.3 V, 5 V, 8 V, and 12 V into a customer design, but will not Transistors that are not included in the provided designs can be provided (at no significant cost).

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

Complementary metal-oxide-semiconductor (CMOS) technology uses p-type and n-type metal-oxide-semiconductor field effect transistors (MOSFETs) to implement complex integrated circuits. Depending on the manufacturing process, different voltage domains (ie, 1.8 V, 2.5 V, 3.3 V, 5 V, 12 V, etc.) may be used. In all voltage domains, MOSFET transistors have a drain, source, gate and bulk/well. The base of a MOSFET is the substrate; for an n-channel FET, the substrate is a p-type doped substrate or well with a low doping rate; for a p-channel FET, the substrate is a p-doped substrate or well with a low doping rate type doped well. The source and drain regions are formed by highly doped regions of n-type or p-type of the n-channel FET or p-channel FET. The controlled channel is formed between the source and drain, isolated from a thin oxide and usually covered with a layer of polysilicon to serve as a gate. All four terminals of the FET (source, drain, gate, substrate/well) are connected by metal contacts to the metal interconnect layer, which is finally connected to the OLED.

The emission area of a microdisplay is small and to achieve the necessary pixel pitch, the space for the control circuitry of each pixel is limited. For a full-color microdisplay, the space occupied by the control circuitry of each individual pixel should not exceed 100 square microns and preferably not exceed 50 square microns. For a monochromatic microdisplay in which all pixels emit the same color, the space for control circuitry can be 3 to 4 times larger because fewer pixels are required.

In a suitable low voltage 5 V transistor, the maximum voltage between any pair of terminals of all terminals must not exceed 5 V to avoid damage to the device. A typical safety margin of 10% overvoltage is acceptable for a short period of time. A medium voltage transistor rated greater than 5 V (eg, a medium voltage transistor rated at 7.5 V) has roughly the same settings as a 5 V transistor, but with a thicker gate oxide and larger geometry (channel width and length) to withstand higher voltages. Therefore, an MV transistor will typically be larger and take up more space than a corresponding LV transistor.

Although transistors in any size range can be made regardless of voltage rating, a low voltage MOS transistor rated at 5 V suitable for microdisplay applications has no more than 20 square microns and preferably no more than 10 square microns of the total area. A channel area (channel length x channel width) of a 5 V transistor suitable for microdisplay applications should not exceed 1 square micrometer and preferably not exceed 0.30 square micrometer. The two transistor contacts should each not exceed 1 square micrometer and preferably not exceed 0.30 square micrometer.

For purposes of the application, a BJT suitable for a protection circuit is a common setting for both types of NPN and PNP is vertical. For an NPN BJT, the collector is formed as a low-doped deep n-well with a low p-doped common silicon substrate (bulk). The base is formed as a p-well inside the deep n-well and is connected by a highly doped p-region. The emitter of the BJT is formed by a highly doped n-region inside the p-well. A typical size of the emitter region is about 500 nm x 500 nm and preferably does not exceed 0.30 square microns to keep the pixel size small. The maximum voltage between any pair of terminals of all BJT terminals (bulk, base, emitter, collector) should not exceed 5 V to avoid damage to the device. A typical safety margin of 10% overvoltage is acceptable for a short period of time.

OLED microdisplays (commonly referred to as "AMOLEDs") consist of an active matrix of OLED pixels that generate light (luminescence) when electrically activated, deposited or integrated onto an array of transistors on a silicon wafer , where the array acts as a series of switches to control the current flowing to each individual pixel. Typically, this continuous current flow is controlled by at least two transistors located at each pixel (to trigger luminescence), where one transistor starts and stops charging a storage capacitor and controls the other transistor, where the other transistor starts and stops charging a storage capacitor. The crystal provides a voltage source at the level required to create a constant current flow to the pixel. In some embodiments, the transistor (often referred to as a scan transistor or select transistor) that controls the current supplied by another transistor (often referred to as a drive transistor) is controlled by a select line , the selection line is shared by all pixels in a column. The signals transmitted by the scanning transistors are supplied by a data line that is shared by all the pixels in a row.

This is illustrated in Figure 1, which represents the simplest form of a prior art AMOLED pixel design. The simplest AMOLED pixel with pixel memory uses two transistors and one capacitor. The current drive transistor MP2 is typically connected to the anode of the OLED from the supply voltage V _DD . One transistor (MP2) is the OLED drive current, and the other transistor MP1 (also known as a scan transistor) acts as a switch to sample and hold a voltage on the storage capacitor C1 shown. A data line (supplying V _DATA ) controls the current V _DD through the driving transistor MP2. A select line controls MP1 and thus the charging of capacitor C1. Typically, transistors have intrinsic capacitance, so depending on the intrinsic capacitance of the transistor and the leakage current through the transistor, no additional capacitance may be required.

In the figures following FIG. 1, any capacitors present are omitted from the figures for clarity. The presence of a capacitor is optional and the circuit may have no capacitors or any number of capacitors as desired. The reference voltage of the capacitor can be V _DD or some other voltage.

Note that changes in threshold voltage (V _th ), carrier mobility or series resistance will directly affect the uniformity of the OLED current driving the TFT and thus the brightness of the image display. One of the main factors affecting the non-uniform current is the threshold voltage (V _th ) variation of the OLED drive transistor. Pixels may require other types of compensation from control circuitry, such as aging, degradation or burn-in of OLED material over time, unevenness or non-uniformity over the active area, or voltage drop across metal connection lines. Additionally, the control circuitry provided by the TFT may be required to control the timing of the current delivered to the pixel, eg by PWM. The design of control circuitry for OLEDs including various types of compensation and driving schemes has become a subject of much concern and many approaches have been proposed. Such compensation circuits may additionally be present in the control circuitry of stacked OLEDs with three or more stacks.

It has surprisingly been found that the high voltage/current required for OLED stacks with three or more stacks can be handled by a control circuit comprising at least two transistors with their equal channels connected in series. This is sometimes referred to as a "stacked transistor". Desirably, one of the series connected transistors is a low voltage drive transistor and the other is a switching transistor. This configuration allows the OLED pixels to be driven without significant current leakage from the TFT so that high brightness is achieved without loss of contrast.

Specifically, the driving TFT is connected in series with the switching transistor system, so that a first terminal of the driving transistor is electrically connected to (and closer to) an external power supply, and a second terminal of the driving transistor is electrically connected to the switching transistor. A first terminal of the crystal and a second terminal of the switching transistor are electrically connected to (and closer to) the bottom electrode of the OLED stack. This configuration allows the OLED stack to operate at a higher voltage than individual transistors that are designed to not experience substantial current leakage or physically damage the low voltage transistors.

This basic control circuit configuration is shown in Figure 2, where a p-channel drive transistor T1 is connected to V _DD (external power supply) on a first terminal (source in the case of p-channel transistors) and a second terminal (in a p-channel transistor, the drain) is connected to a switching transistor T2. The gate of the driving transistor T1 is controlled through the data line and an in-line selection transistor T3, and the gate of the in-line selection transistor T3 is controlled through a selection line SELECT1. Switching transistor T2 receives current (when T1 is turned on) from the second terminal of T1 on a first terminal (source) and is connected to the bottom electrode of the OLED on a second terminal (drain). The gate of the switching transistor T2 is directly controlled by the select line SELECT2. The driving transistor T1 and the switching transistor T2 are connected in series. When T1 and T2 are selected to be "on", current flows to the bottom electrode of the OLED stack and the OLED stack emits light. If T2 is selected "off", no current flows to the OLED stack regardless of whether T1 is "on" or "off". In some embodiments, the switching transistor T2 can provide an on-off function to prevent motion blur. If T1 is "off", no matter whether T2 is "on" or "off", the OLED will not emit light.

In Figure 2, the control circuitry of the backplane with at least two series-connected transistors between V _DD and the bottom electrode of the OLED electrode is a drive transistor and a switch transistor. In this application, a driving transistor has the function of regulating the voltage and current flowing to the OLED pixels to an appropriate level according to the image being displayed, and a switching transistor has the function of turning the current flowing to the OLED pixels on and off To provide an opening and closing function. For example, during operation of the microdisplay, switching transistor T2 interrupts the flow of power from drive transistor T1 to the bottom electrode of the OLED stack, and therefore does not emit light (black) for a portion of the frame time. This differs from scan transistor T3, which is only turned on for a very small fraction of the frame time (eg, 1/1200 for a display with 1200 columns) to charge a storage capacitor (Fig. 2 is not shown). Typically, a switching transistor can be smaller in size than a driving transistor at the same maximum operating voltage. However, if the switching transistor is required to be handled as in control circuitry with an OLED in a stack of three or more, it can be larger than the drive transistor and rated for a higher voltage.

Desirably, the drive transistor (T1) is closer to the power supply V _DD than the switching transistor (T2), which is closer to the bottom electrode of the OLED. Preferably at least one p-channel MOSFET transistor of the drive transistor system, and more preferably, both the drive transistor and the switching transistor are p-channel MOSFET transistors. The two series connected transistors can be both LV, both MV, or one LV transistor and one MV transistor. Preferably they are all LV transistors to reduce the oversize of the pixel circuit. However, in some embodiments, the transistor system LV is driven and the transistor system MV is switched.

Although only p-channel transistors are illustrated in FIG. 2, n-channel transistors or a mixture of n-channel transistors and one of p-channel transistors may be used. In such cases, the circuitry will need to be properly reconfigured in view of the polarity difference between the channel transistors and the p-channel transistors. In addition, other microelectronic components such as other transistors, capacitors, and resistors may be included in the control circuit as desired.

It is known in the art that MOSFET transistors require an intrinsic body diode connection to function as desired. Due to the structure of a MOSFET transistor, there is inherently a parasitic diode and this parasitic diode can affect the operation of the transistor. Typically, intrinsic body diodes are internally or externally connected to a power supply to apply a bias voltage. These bulk connections are also referred to as "bulk connections" or "transistor wells" among other terms. Such a connection is shown in Figure 3, where IBD1, IBD2 and IBD3 are connected to the intrinsic body diodes (for T1, T2 and T3) of a single voltage source V _DD2 . These transistors can share the same well for intrinsic body diode connection. However, the same power supply V _DD used to power T1 and T2 can also be used for IBD; that is, V _DD and V _DD2 are a common source.

However, it may be desirable to float one or more of the transistors in their separate wells on the Si backplane to avoid exceeding the operating voltage range of either of the components. Specifically, when both the first drive transistor and the switching transistor are p-channel transistors, each transistor may be located in its own separate n-well. This allows a larger dynamic voltage range to achieve control of the OLED than can be achieved with both transistors located in the same n-well. The use of isolated, floating or distinct wells for connecting transistors in series is known in the art, see eg US9066379, US5764077, US7768299, US9728528 and JP2016200828.

This is illustrated in Figure 4, where T1 occupies its own well, as indicated by the dashed line, and is biased via IBD1 through a connection to V _DD2 ; and T2 occupies a different well, as indicated by the dashed line Indicated, biased by connecting IBD2 separately from one of the transistor sources (V _DD ). In the embodiment of Figure 4, the bias applied to each IBD is different and thus each n-well is independent of each other.

The embodiments shown in FIGS. 2-4 have several advantages in design and operation for driving an OLED with three or more stacks. In the design, this circuit can be very compact since all the transistors are relatively small LV transistors, as commonly found in most foundries. All transistors can be p-channel transistors, and all n-wells are biased to V _DD , which eliminates the need for isolation or floating wells. These features may allow very compact pixel circuit designs for very high resolution and small size microdisplay designs.

Figure 5A shows a control circuit with three transistors in series. The circuit shown in FIG. 5A is similar to that shown in FIG. 2, but in FIG. 5A a third transistor T4 is connected in series between the power supply _VDD and the source of the drive transistor T1. As shown, the gate of T4 is controlled by the data line and an inline select transistor T5, whose gate is controlled by a select line SELECT3. Lines SELECT2 and SELECT3 in FIG. 5A may supply a signal at the same or different voltages, may switch at the same time or at different times, or may remain unswitched depending on operating conditions.

Desirably, the added transistor T4 may be a second drive transistor such that between the power supply _VDD and the bottom electrode of the OLED are two drive transistors (T4 and T1) and a switching transistor (T2), all of the above The transistors are connected in series. In this case, the signal supplied to the gate of T4 can also be the same as the signal supplied to T1 and can be supplied via T3/SELECT1. Although FIG. 5A shows a design with one data line and two select lines for data control, a similar design with two data lines and one select line can be used.

In embodiments where there are three or more transistors in series, the transistors may be a combination of one of LV transistors and MV transistors. Preferably, all the transistors are LV transistors to reduce the size of the pixel circuit. The plurality of transistors can be p-channel transistors, n-channel transistors, or a mixture. Preferably all transistors are p-channel transistors. If there is an n-channel transistor mixed with one of the p-channel transistors, preferably at least one of the drive transistors is a p-channel transistor and more preferably an LV transistor. The switching transistors are preferably LV transistors; however, it may be desirable to use one or more MV switching transistors when it is necessary to extend the operating range of the backplane circuit while complying with the voltage limitations of individual transistors.

In situations where any of the multiple series-connected transistors are of one type, multiple common transistors of the same type can share the same well to reduce design size. However, to expand the operating range of the backplane circuit while complying with the voltage limitations of individual transistors, it may be necessary to place transistors of the same type in separate wells.

5B shows a schematic of a control circuit with one of two drive transistors similar to the drive transistor shown in FIG. 5A, where the two drive transistor systems are controlled by a level shift circuit (LSC). A level shift circuit is a circuit used to convert a signal from one logic level or voltage domain to another logic level or voltage domain, typically used to resolve voltage incompatibilities between various parts of a system. For clarity, internal components and all connections to the LSC (eg, V _DD , ground, and other possible input connections) are not shown. In this embodiment, the LSC sets the gate voltages of the drive transistors T4 and T1 based on the signal SELECT1 and the signal from the data line so that the total voltage across T4 and T1 is always approximately equally separated between the two drive transistors . Small logic transistors are known to be useful for this function.

OLED-type microdisplays usually include a protection circuit in the MOSFET-type control circuit system of the backplane to limit the amount of power flowing through the transistor to prevent damage. It is desirable to include a protection circuit in the control circuitry of the backplane of an OLED microdisplay with three or more stacked cells for this purpose, since the power required to enable these devices to emit is relatively high. A protection circuit should maintain or "clamp" the voltage at the bottom electrode of the OLED so that the voltage does not drop below a desired voltage level when the OLED is not emitting. Such protection circuits may also be referred to as "voltage maintenance" circuits.

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

Figure 6 shows a schematic control circuit similar to the control circuit shown in Figure 2 but with the addition of a protection circuit. In this particular embodiment, the protection circuit includes a p-channel transistor connected on one end to a node between the drain of T2 and the bottom electrode of the OLED and connected to a power supply on the other end (In this example, a reference voltage V _REF (common to all pixels)). The gate of T6 is also connected to the node. Therefore, the diode-connected transistor T6 limits the minimum drain voltage of T2 to prevent it from collapsing. There may be other electronic components that are not shown as part of this circuit. This configuration is similar to that described by Kwak et al., but with one end of the p-channel transistor connected to ground instead of _VREF .

However, the problems caused by the higher voltage requirements of using an OLED with three or more cells remain when using a protection circuit with one of the MOSFET transistor examples shown in FIG. 6 . In this embodiment, using an LV or MV MOSFET transistor (ie, T6) achieves protection of the LV drive transistor and switching transistor and removes the protection circuitry, as compared to using one MV drive transistor and switching transistor and removing the protection circuitry Does not make the pixel design significantly smaller.

FIG. 7 shows a protection circuit similar to that shown in FIG. 6, but in FIG. 7 p-channel transistor T6 is replaced by diode D4. Desirably, diode D4 in the protection circuit is a pn junction diode. A pn diode system is a circuit element that allows electricity to flow in one direction but not the other (opposite) direction. The Pn diode can have a forward bias in the direction of easy current flow or a reverse bias with little or no current flow. This pn diode can be formed in CMOS designs in various ways; for example, as a vertical diode with a highly doped p region in an n-well. D4 is connected on one end to a node between the drain of T2 and the bottom electrode of the OLED, and on the other end is connected to a power source 45 which is a reference voltage V _REF or a reference current I _REF .

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 BJT1, a BJT (bipolar junction transistor), the collector (C) of the BJT is connected to V _DD , the emitter (E) is connected to T2 and A node between the bottom electrodes of the OLED stack and the base (B) is connected to a power source 50, which is a voltage source V _PROTECT or a current source I _PROTECT . One of the benefits of using a BJT in a protection circuit is that the current is amplified from the base current to the collector current. If desired, the collector of the BJT can be connected to a separate power supply than V _DD . The power supply V _PROTECT or I _PROTECT can be shared by all pixels. It should be noted that V _PROTECT or I _PROTECT may or may not be constant, but may vary depending on whether the OLED is desired to emit or not. This can help reduce the durability of the display by switching on and off when used in conjunction with a switching transistor. In FIG. 8A, BJT1 is a "NPN" type BJT transistor (preferably), but it can also be a "PNP" transistor with appropriate changes in design.

For an NPN type BJT, whenever the emitter voltage V _E at the bottom electrode of the OLED is greater than the base voltage V _B and V _B is less than the collector voltage V _C (V _E > V _B < V _C ), the BJT will is in off mode and no current flows. However, whenever the voltage _VE at the bottom electrode of the OLED is less than the voltage _VB and _VB is less than _VC ( _VE < _VB < _VC ), the BJT will be in forward acting mode. In this mode, the base-emitter junction is forward biased and the base-collector junction is reverse biased, and thus the collector-emitter current will be approximately proportional to the base current .

Therefore, in Figure 8A, if the _VC of BJT1 is _VDD and V _PROTECT (which is _VB ) is set to be less than the V _th of the OLED, then whenever the voltage _VE at the bottom electrode is higher than V _th + V _CATHODE , BJT1 is "off", but whenever _VE drops below V _th + V _CATHODE (ie, whenever T1 or T2 is "off"), the voltage at the bottom electrode is maintained at about V _th . Therefore, the protection circuit is designed to provide sufficient current whenever needed to protect the drive and switching transistors from having on either side of their terminals whenever the cathode voltage decreases (more negative) below a certain value voltage beyond its rated value. Furthermore, by setting the base voltage B (V _PROTECT ) lower than the turn-on voltage of the OLED (V _th higher than V _cathode ), power dissipation is achieved whenever the voltage delivered to the bottom electrode of the OLED is greater than or equal to V _th minimize.

In certain embodiments utilizing the protection circuit shown in Figure 8, the base voltage (V _B ) of the BJT is isolated from any external power supply. That is, there is no electrical connection between the power source V _PROTECT or I _PROTECT and the base of the BJT. The BJT is physically present with the existing collector and emitter connections shown in Figure 8, but the base connection that still exists is not connected to any external source. In such cases, _VB is not intentionally maintained at any particular value, nor is any voltage or current intentionally applied to _VB . V _B is allowed to "float" independently of voltages _{V C} and V _E , which are part of the functional control circuitry that operates the OLED _. However, there may be parasitic current paths within the backplane that internally bias the base under high blocking conditions. Note that in such embodiments, V _B may vary during operation of the OLED due to parasitic currents generated within the circuitry.

In other embodiments, the _VB of the BJT may be self-biased (sometimes referred to as having a "base bias"). When the BJT is self-biased, no external control input signal is applied to the base of the BJT, but the signal applied to the base of the BJT is determined by the value of a constant supply voltage (ie, V _DD ) and any bias connected to the transistor The value of the piezoresistor is set. One method of achieving BJT base self-biasing forms a "fixed base bias circuit". In this configuration, the base of the BJT is connected to a constant power supply (ie, V _DD ) with a single current limiting resistor. This will allow the base current (I _B ) of the BJT to remain constant at a given value V _DD , and therefore the operating point of the BJT also remains fixed. Alternatively, a simple voltage divider network can provide the desired bias voltage. Note that in such embodiments, the bias voltage and resulting parasitic currents within each pixel will respond to the operation of the OLED. This response can provide effective protection for the transistor.

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

Figure 9 also indicates IBD5, which is the intrinsic body diode of V _DD connected to the deep n-well of the Si substrate in which all transistors are located, and IBD6, which is the intrinsic body diode connected to the ground of the entire Si substrate body diode. It should be noted that the BJT in the protection circuit can be an NPN transistor or a PNP transistor. If the BJT is an NPN transistor, the transistor is in a p-well; if the BJT is a PNP transistor, it will be in an n-well.

It is preferred to use a protection circuit including a BJT. When used as part of a control circuit comprising at least two transistors having their equal channels connected in series, the pixel size of the BJT transistor design is small. In addition, the intrinsic amplification factor of a forward active mode BJT means that a relatively small parasitic current can produce a larger protection current. Therefore, the current from the reference/protection voltage source is smaller and the voltage drop across the reference voltage interconnect is greatly reduced.

In addition to maintaining a minimum voltage at the bottom electrode below the threshold voltage of the OLED, the protection circuit can also be designed to prevent other undesired effects such as short circuit protection, transient spikes, etc. by including other suitable circuit components. However, in some instances, the stacked OLEDs of the present invention can still be used to incorporate such known types of protection circuitry into pixel circuits when using them.

For many OLED stacks, especially OLED stacks comprising three light emitting units, it is desirable to have no intervening transistors in the electrical connection between the second terminal of the switching transistor and the bottom electrode of the OLED in the control circuitry, As shown in Figures 2-5. The reference to intervention means that in the electrical connection between the switching transistor and the OLED, the current does not pass directly through the other transistor (ie, one terminal of the transistor is connected to the switching transistor and the other terminal is connected to the OLED so that current will pass through the intervening transistors at certain points during operation of the OLED). There may be other (non-transistor) microelectronic components in-line between the switching transistor and the OLED.

There may be a stub (meaning not directly in-line) connection between the switching transistor OLED connection and a first terminal of a microelectronic component (ie, a non-intrusive transistor or diode), where the OLED operating current is not Directly through non-intrusive transistors or diodes. For example, this configuration is illustrated in Figures 4-6, where a non-intrusive component is connected on one side to the T2-OLED connection and the other side is connected to a power supply.

Desirably, the drive transistor system is a low voltage (LV) transistor. That is, the drive transistors are designed and sized to operate safely and efficiently at 5 V or below, regardless or regardless of the actual load in the circuit. This necessarily helps to minimize pixel size to maximize microdisplay resolution. It should be noted that an OLED stack with three or more OLED cells and driven by a drive transistor will typically require voltages in excess of 7.5 V. If desired, two or more series connected low voltage drive transistors in separate wells (as shown in Figure 2c) can be used to further mitigate the effects of higher voltages.

It is desirable to drive the transistor system a p-channel thin film transistor (also known as a p-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor)). The structure, properties and fabrication of p-channel transistors are well known. When the driving transistor system is a p-channel transistor, its source is electrically connected to an external power source, and its drain is electrically connected to a first terminal of the switching transistor. The gates of the drive electrodes are controlled by a data line separate from the power supply.

When the OLED stack includes three OLED light-emitting units, the switching transistor system, a low voltage transistor, is desired. That is, the switching transistors are designed and sized to operate safely and efficiently at 5 V or less, regardless or regardless of the actual load in the circuit. The current required for OLED operation flows through this transistor and the drive transistor.

However, if the OLED stack includes four or more OLED light-emitting units, the operating voltage can be well in excess of 7.5 V and the switching transistor can be medium voltage (for example, designed to be 7.5 V to 12 V) or high voltage ( designed for 18 V to 25 V). Alternatively, two or more series-connected switching transistors, selected together or independently, can be used to mitigate the effects of higher voltages. Connecting multiple switching transistors in series may all be LV or a mix of LV and MV transistors.

The switching transistor system is expected to be a p-channel transistor. When the switching transistor is a p-channel transistor, its source is connected to the drain of the drive transistor and its drain is electrically connected to the bottom electrode of the OLED. The gates of the switch electrodes are controlled by a select line, which is separated from the power supply and the data and select lines that control the driving transistors.

In certain embodiments, it may be desirable to drive transistors and switch transistor systems p-channel transistors, where both drive transistors and switch transistors are low voltage transistors, or drive transistor systems low voltage transistors and switch transistors The crystals are medium voltage or high voltage transistors. In some embodiments, it is also desirable that all transistors in the control circuitry be p-channel transistors.

Although the drive transistor and switching transistor system are electrically connected in series, there may be other intervening microelectronic components between the two transistors. The intervening components may be in-line, with the current between the drive transistor and the switching transistor passing directly through the in-line component. 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 to this additional component and to the switch transistor.

In its most basic form of operation, a voltage is supplied to the drive transistor from an external power source. The select line is set to allow the correct data voltage to be applied from the data line to the gate of the drive transistor, thereby allowing current from the power supply to pass through the drive transistor to the switch transistor. The switch transistor's select line is set to allow the correct charge to be applied to the switch transistor, allowing current to pass through the switch transistor to the bottom anode of the OLED stack in each individual pixel. This turns the pixel "on" and emits light according to the current it receives. When the displayed image requires an individual pixel to not emit or to reduce emission, the data voltage of the drive transistor can be changed to prevent or limit current flow through the drive pixel.

Other components in the control circuitry can be used to control the current supplied by the drive transistor, the current expected to power the OLED when the OLED is "on" or the current leakage through the drive transistor when the OLED is expected to be "off."

To prevent or minimize motion blur by providing an on-off function, regardless of the operation of the drive transistor (which controls the power supplied to its pixels) (exactly when the drive transistor is "on" (passes current)) , the switching transistors prevent current flow to the OLED. At the appropriate time for the pixel, the switching transistor can be selected so that the OLED pixel is "off" even if the image to be displayed requires the OLED pixel to be "on". This allows all segments or rotated segments of pixels in the display to be "turned off" (not emitting) to minimize the perception of motion blur in microdisplays.

In a microdisplay, power can be delivered in the form of a variable current or voltage from an external power source to the drive transistor to drive the OLED stack to deliver the desired illumination level. This is typically stored by charging a storage capacitor during write operations. The power level can be controlled at the external power source or if the power delivered is constant, the power can be set at the appropriate level by other microelectronic circuits within the backplane. This is called "current control" and is commonly used to power most OLED devices. Alternatively, the power supplied to the OLED stack can be constant and controlled by the full "on" time of the OLED pixels compared to the OLED pixel "off" time for a set period of time (frame ) in the total amount of light emission. This is called pulse width modulation or PWM control.

Since the described control circuit is capable of handling higher voltage and current demands than at least the design or voltage rating of the drive transistor without significant leakage or damage, it is possible to use OLEDs with increased emission (and higher voltages) stack. A tandem OLED device with 2 light-emitting units and relatively low _Vth requirements does not emit as much light as an OLED stack with three or more light-emitting units and relatively high _Vth requirements. The circuit described can be used with a stacked light emitting OLED having a threshold voltage (V _th ) greater than 7.5 V; more desirably, the V _th of the light emitting OLED stack is at least 10 V or greater. Alternatively, the circuit can be used with a stacked OLED that provides a full color microdisplay with a light emission of at least 2500 nits, or preferably at least 5000 nits.

There are two basic approaches to making a pixelated OLED microdisplay, in which the brightness of each individual pixel must be controlled by supplying power to one of the pixel electrodes via control circuitry on a backplane. The first method involves making each pixel individually produce red, green, or blue light (R, G, B, respectively), or the same color if a monochrome display is used. In this case, the light emitting OLED stack can be configured such that all stacked light emitting cells above an individual bottom electrode segment emit the same color of light (selected from R light, G light, or B light) to form R, G and B pixels. In certain embodiments with this feature, each color pixel forms a microcavity in which the distance between the segment bottom electrode and the top electrode is a function of the color of the emitted light. In this case, the length of the microcavity will depend on the emitted color, and the length of the microcavity will be different when the pixels are red, green and blue.

The second method is that all pixels of a color filter array (CFA) have a common multi-mode (white) emitting OLED layer to form individual RGB pixels. The advantage of the second method over the first method is that individual OLED pixels do not have to be formed for different schemes and thus will reduce manufacturing costs.

The number of individual OLED light-emitting units within a stacked OLED is limited only by the overall thickness of the OLED and the ability of the control circuitry to handle the power required to operate the OLED. As the number of OLED cells increases, the total amount of light emitted increases, but the package thickness, manufacturing process complexity, and threshold voltage all increase. An OLED with at least three stacked light emitting units will increase the illuminance through a cascade (two OLED units) of OLEDs. However, preferably the OLED has at least four stacked OLED light emitting units, and more preferably the OLED has at least five stacked OLED light emitting units. One OLED with six or even ten or more stacked OLED light emitting units is conceivable.

To minimize the increase in the voltage required to drive an OLED stack, a charge generation layer (CGL; also sometimes referred to as a connector or interlayer) is positioned between individual OLED light-emitting cells. This is because the CGL is structured such that electrons and holes are generated and injected into the adjacent organic emissive layer when a voltage is applied. Therefore, one injected electron can be converted into multiple photons using a CGL to allow higher illumination. Specifically, a CGL is expected to be located between each light emitting cell within the stack. However, a light generating unit need not have an adjacent CGL on both sides. The OLED light generating units on the top and bottom of the stack will typically have only one adjacent CGL. It is generally not necessary to use a CGL between a light emitting cell and one of the top or bottom electrodes, but a CGL may be used if desired.

Many different kinds of CGLs have been proposed and can be used in OLED stacks. See, for example, US 7728517 and US 2007/0046189. To form a CGL, an np semiconductor heterojunction at the interface of the n-type layer and the p-type layer is typically required to generate charge. So 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 Report. One of the CGLs expects the metal oxide system MoO ₃ . In some instances, the n-layer and p-layer may be separated by a thin intermediate layer. Typically, the CGL is configured such that the n-layer is closer to the anode and the p-layer is closer to the cathode.

One desired version of a CGL has three layers; an electron transport material doped with an n-dopant (eg, Li), a thin intermediate layer of the same (but undoped) electron transport material, and a A p-dopant is doped with a hole transport material. Suitable electron transport and hole transport materials and n- and p-dopants suitable for use in a CGL are well known and commonly used. Materials can be organic or inorganic. Selection of the appropriate material is not critical and any material may be selected based on its efficacy. The thickness of the CGL should be expected to be in the range of 200 Å to 450 Å. In many instances, the CGL will have an ETL on the anode side and an HTL on its cathode side to help improve charge transport and to help link the charge-generating dopants (if present) with the light emitting cells LEL separation.

Although the use of CGLs when OLED light emitting cells are stacked on top of each other helps to minimize the voltage increase, the overall voltage required for the stack still increases by about the voltage required for each individual cell individually. It is expected that an OLED with at least three stacked OLED light emitting units will require voltages beyond the recommended operating range of a 5 V drive transistor.

In one embodiment, all OLED light emitting cells within an OLED stack above an individual bottom electrode segment emit the same color, eg, red, green, or blue. This forms a pixelated RGB microdisplay. 10 illustrates a microdisplay 100 using three different stacks of OLED sub-pixels to form R pixels, G pixels, and B pixels. Each OLED subpixel stack contains three OLED light emitting cells emitting the same color, with a CGL separating each cell vertically from the other.

In the microdisplay 100 there is a silicon backplane 3 that includes, for example, an array of control circuits as shown in FIGS. 2-8 and other necessary components that will supply power to the sub-pixels according to an input signal. Over layer 3 with transistors and control circuitry, there may be an optional planarization layer 5 . Above layer 5 (if present) are individual first electrode segments 9 connected by electrical contacts 7 that extend through an optional planarization layer to form the one between the individual bottom electrode segments 9 and layer 3 Electrical contact between control circuitry. The individual bottom electrode segments 9 are electrically isolated from each other laterally by a pixel defining layer 1 . A non-emissive OLED layer 11 (eg, an electron or hole injection (EIL or HIL) or electron or hole transport (ETL or HTL) layer) is located over the segmented bottom electrode section 9 . A first light-emitting OLED unit 13 is located on the OLED layer 11 . Layer 15 is a charge generating layer which is located between the first light emitting layer 13 and a second light emitting OLED unit 17 and separates the first light emitting layer 13 and a second light emitting OLED unit 17 . There is a second charge generation layer 19 on the second light emitting OLED unit 17, the second charge generation layer 19 is located between the second light emitting OLED unit 17 and a third light emitting OLED unit 21, and connects the second light emitting OLED unit 17 and the A third light-emitting OLED unit 21 is separated. Above the third light emitting layer 21 is a non-emitting OLED layer 23, such as an electron or hole transport layer or an electron or hole injection layer and a transparent top electrode 25 through which light can transmit. The OLED microcavity is protected from environmental influences by an encapsulation layer 27 . In the embodiment shown, all organic layers within a single OLED stack are separated horizontally from adjacent stacks by a pixel-defining layer 1, but the top electrode 25 and encapsulation body 27 are common and extend across the entire active area . However, the top electrode 25 need not be continuous and can be segmented if desired. In the microdisplay 100, all OLED light emitting cells 13, 17, 21 located above the same bottom electrode section 9 emit the same color of light, B, G or R. This particular microdisplay is not a microcavity device; however, a similar pixelated RGB design using a microcavity effect could be used.

A well-known method of increasing the luminance and color purity of OLED emission utilizes the optical microcavity effect. This effect is based on the formation of an optical resonator between an emitting surface and a semi-reflective surface that allows some light to pass through. Multiple reflections between two surfaces will form standing waves depending on the optical distance between the two surfaces, which will strengthen some wavelengths of light and reduce others due to constructive and destructive interference effects. and destructive interference effects will occur depending on whether the emission occurs at the antinodes or nodes of the standing wave, respectively. Antinodes appear at different locations depending on the total space between the reflectors and depending on the wavelength being optimized. However, light emitted from a microcavity can exhibit strong angular dependence, where color shift and loss of illumination can occur when the viewing angle deviates from the perpendicular to the viewing surface. This is generally not a problem for NED applications due to the limited entry angle of the projection optics.

It would be desirable to use the microcavity effect to further increase the illumination of the OLED stack. For example, the microdisplay 100 shown in FIG. 10 can be redesigned to form the microcavity effect by using a reflective bottom electrode or a reflective layer below the bottom electrode, making the top electrode translucent so that it Have some degree of reflectivity and adjust the distance between the uppermost surface of the reflective element (bottom electrode 9 or underlying reflective layer) and the bottommost surface of the top electrode to form a microcavity suitable for that particular color of light.

11 illustrates a microdisplay 200 using a multimode (white) OLED microcavity shared by all pixels and a color filter array (CFA) to form R pixels, G pixels and B pixels. Multi-mode OLEDs produce a color of light. Ideally, a multi-mode OLED produces a white light that is approximately equal to the amount of R, G, and B light. Typically, this would correspond to _CIEx , _CIEy values of about 0.33, 0.33. However, depending on the characteristics of the color filters used to form the RGB pixels, some variation in these values may still be acceptable or even desirable. Microdisplay 200 also includes microcavity effects. In this embodiment, the multi-mode OLED stack contains three OLED light-emitting cells emitting different colors, where each cell is vertically separated from the other by a CGL, where the distance between a reflective surface and the top electrode is It is constant over the area of action.

In the microdisplay 200, there is a silicon backplane 3 comprising, for example, an array of control circuits as shown in Figures 2-8 and the necessary components that will supply power to the sub-pixels according to an input signal. There may be an optional planarization layer 5 over the layer 3 with the transistors and control circuitry. Above layer 5 (if present) are individual first electrode segments 9 connected by electrical contacts 7 that extend through an optional planarization layer to form individual bottom electrode segments 9 and controls in layer 3 Electrical contact between electrical systems. In this embodiment, the bottom electrode section 9 has two layers: a reflective layer 9b closer to the substrate 1 and an electrode layer 9a closer to the OLED layer. The individual bottom electrode segments 9 are electrically isolated from each other in the lateral direction. Above the segmented bottom electrode section 9 is a non-emissive OLED layer 11, such as an electron or hole injection or electron or hole transport layer. A red OLED light generating unit 13A is located on the OLED layer 11 . Layer 15 is a first charge generation layer, the first charge generation layer is located between the red OLED light generating unit 13A and a green OLED light generating unit 17A and separates the red OLED light generating unit 13A and a green OLED light generating unit 17A separated. There is a second charge generating layer 19 on the green light emitting layer 17A, the second charge generating layer 19 is located between the green OLED light generating unit 17A and a blue OLED light generating unit 21A and connects the green OLED light generating unit 17A and a The blue OLED light generating units 21A are separated. Above the blue OLED light generating unit 21A are a non-emissive OLED layer 23 (eg, an electron or hole transport layer or an electron or hole injection layer) and a translucent top electrode 25 . This forms an OLED microcavity 30 extending from the uppermost surface of the reflective surface 9B to the bottommost surface of the translucent top electrode 25, which is also a semi-reflective electrode. An encapsulation layer 27 protects the OLED microcavity from environmental influences. In this embodiment, there is a color filter array having color filters 29B, 29G, and 29R that filter the multi-mode emission produced by the OLED microcavity 30 such that B light, G light, and R light are emitted according to the power supplied to the underlying electrode section 9 .

12 shows a microdisplay 300, which is a variation of the microdisplay 200, having an additional blue OLED light-emitting layer 22 between the blue OLED light-emitting unit 21A and the OLED layer 23. FIG. In this embodiment, there is no CGL between the blue OLED unit 21A and the additional blue light emitting layer 22 . As with microdisplay 200, since blue cell 21A and blue layer 22 are separated by a CGL (and thus layer 22 is not considered a separate light-emitting OLED cell and should be considered part of blue light-emitting cell 21A), microdisplay Display 300 has three (instead of four) OLED light generating units. Even though the blue cell 21A is separated from the blue LEL 22 by a non-charge generating interlayer, there will still be three OLED cells in the stack. However, if an intermediate CGL is located between the blue emitting unit 21A and the additional blue emitting layer 22, the OLED stack will contain four OLED units.

13 shows another embodiment of a multi-mode OLED microdisplay 400 with a microcavity effect, which is similar to microdisplay 200, but has five OLED light emitting cells. In this case, some of the OLED cells emit light of the same color and other cells emit light of a different color. Specifically, the multi-mode 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 divided by CGL. separated.

In the microdisplay 400, the OLED unit 13A emits red light. The OLED unit 13A is separated from the yellow light emitting OLED unit 16 by the CGL 15 . The yellow light emitting OLED unit 16 is separated from the green light emitting OLED unit 17A by the CGL 14 . The green light emitting OLED unit 17A is separated from a first blue light emitting OLED unit 21A by the CGL 19 . A second blue light emitting unit 32 is separated from the first blue light emitting OLED unit 21A by the CGL 24 . All light emitting OLED cells of the other layer are the same as in FIG. 12 .

As previously described, OLED microdisplays are constructed on a silicon backplane that is used as a substrate. Typically, the backing plate will be flat with a uniform thickness. Since silicon backplanes are generally opaque, the OLED stack is preferably top-emitting. However, transparent backplanes are known; in these cases, the OLED stack can be top-emitting or bottom-emitting. The top surface of the substrate faces the OLED. Silicon backplanes may have various types of replacement layers (ie, planarization layers, light management layers, light blocking layers, etc.), which may or may not be patterned and may be on the top or bottom surface.

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

In microcavity applications where the first electrode is over a reflective layer, the first electrode should be transparent. However, in other applications, the first electrode layers 9a and 9b may collapse into a single reflective electrode such that its uppermost reflective surface forms one side of the optical microcavity (ie, 30 in Figure 10).

When the OLED stack is a top emitting microcavity and the bottom electrode is transparent, there should be an emissive layer below the bottom electrode defining a first side of the microcavity 30 . When a transparent anode is placed over a reflective surface, it is part of the optical cavity. The reflective layer 9b can be a reflective metal such as Al, Au, Ag, Mg, Cu or Rh or alloys thereof, a dielectric mirror or a high reflective coating. Dielectric mirrors are constructed from multiple layers of thin materials (eg, magnesium fluoride, calcium fluoride, and various metal oxides) deposited onto a substrate. High reflective coatings consist of multiple layers of two materials, one with a high refractive index (eg, zinc sulfide (n=2.32) or titanium dioxide (n=2.4)) and one with a low refractive index (eg, fluorine) magnesium oxide (n=1.38) or silicon dioxide (n=1.49)). The thickness of the layer is usually a quarter wave in terms of the wavelength of the reflected light. The reflective layer is expected to reflect at least 80% of incident light and optimally at least 90%. The preferred reflective layer is Al or Ag, a thickness of 300 Å to 2000 Å, and most preferably 800 Å to 1500 Å.

Desirably, when the OLED stack is bottom emitting, the bottom electrode is a transparent anode and should transmit as much visible light as possible, preferably having a transmittance of at least 70% or desirably at least 80%. Although the bottom transparent electrode can be made of any conductive material, a metal oxide (eg ITO or AZO) or thin metal (eg Ag) layer is preferred. Weak conductive materials may be used since they can become very thin (eg, TiN).

Electron transport and hole transport materials suitable for use in non-emissive layers (ie, 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 . The layers may be mixtures of these materials and may contain dopants for modifying their properties. Since it is non-luminescent, it contains no emissive material and is transparent. Selection of the appropriate material is not critical and any material may be selected based on its efficacy.

In embodiments utilizing the microcavity effect, since the spacing between the various OLED units within the microcavity and the size of the microcavity are important to maximize efficiency, the thicknesses of the various non-emissive layers typically need to be selected to provide the desired desired interval. It is desirable to tune the spacing between OLED cells and the size of the microcavity by using an appropriate thickness of the organic non-emissive layer (eg, hole transport layer).

The light-emitting layer usually has a host material (or a mixture of host materials) and a light-emitting compound, and the host material is the main component of the layer. Desirably, the light-emitting compound is phosphorescent due to its higher efficiency. However, in some instances, some LELs may use fluorescent or TADF (hot start delayed fluorescence) compounds as light emitting materials, while other LELs use phosphorescent materials. Specifically, the blue light emitting layer may use fluorescent or TADF compounds or combinations thereof, while the non-blue light emitting layers may use green, yellow, orange or red phosphorescent compounds or combinations thereof. The light-emitting layer may use a combination of light-emitting materials. The selection of suitable materials for LELs is well known and not critical, and any material may be selected based on its efficacy and emission characteristics. When a phosphor emitter is used, it is sometimes desirable that the excitons generated by the phosphor emitter be confined within the layer. Therefore, exciton blocking layers on either or both sides of the phosphorescent LEL can be used as desired. Such materials and their applications are well known. In addition, it may be desirable to add an HBL (hole blocking layer) and an EBL (electron blocking layer) around the light-emitting layer (especially the blue light-emitting layer) to improve lifetime and illuminance efficiency.

The top electrode (ie, 25 in Figure 10) should be transparent if the OLED stack is top-emitting; reflective if the OLED stack is bottom-emitting; and in the case of a microcavity , it is semi-transparent and semi-reflective: that is, it reflects part of the light and transmits the rest. In a microcavity, the bottommost interior surface of the top electrode defines a second side of the microcavity 30 . Desirably, the translucent top electrode reflects at least 5% and more desirably at least 10% of the light emitted by the LEL to create a microcavity effect. The thickness of the semi-transparent second electrode is important because it controls the amount of reflected and transmitted light. However, it cannot be too thin as it cannot efficiently transfer charge into the OLED or withstand pinning holes or other defects. One of the thicknesses of the upper electrode layers is desirably 100 Å to 200 Å, and more desirably 125 Å to 175 Å.

The top electrode is desirably a thin metal layer or a thin metal alloy layer. Suitable metals include Ag, Mg, Al and Ca or alloys thereof. Of course, Ag is preferred because of its relatively low blue absorptivity. To aid in electron transport and stabilization, an adjacent transparent metal oxide (eg, ITO, InZnO, or MoO3 ₎ layer may be present on the electrode surface. Alternatively, metal halides (eg, LiCl), organometallic oxides (eg, lithium quinolate), or other organic materials may be used.

A protective layer or spacer layer (not shown in Figures 10-13) may be present over the upper electrode to prevent damage during encapsulation.

An encapsulant 27 is deposited or placed over the top electrode 25 and any optional protective layer (if present). At a minimum, the encapsulation should completely cover the light emitting areas on the top and sides and directly contact the substrate. The encapsulation should not be penetrated by air and water. It can be transparent or opaque. It should not conduct electricity. It can be formed in situ or added as a separate pre-formed sheet and provide sealing of the side edges. An example of in situ formation would be thin film encapsulation. Thin film encapsulation involves depositing multiple layers with alternating layers of inorganic materials and polymeric layers until a desired degree of protection is achieved. Protocols and methods for forming thin film encapsulations are well known and any may be used as desired. Alternatively, the encapsulation may be provided using a pre-formed sheet or cover sheet attached over at least the sealing area and the enclosed area. The preformed sheets can be rigid or flexible. It can be made of glass (including flexible glass), metal or organic/inorganic barrier layers. It should have a thermal expansion coefficient close to that of the substrate to achieve a more robust connection. It may be desirable to attach the pre-formed encapsulation sheet over the sealing area using air- and water-resistant adhesives (eg, silicon or epoxy adhesives) or by thermal means (eg, ultrasonic welding or glass frit welding), This may require additional encapsulant, such as solder or glass frits. The side and bottom edges of the cover sheet can be specially designed to better mate with the sealing area or to promote a better seal. The cover sheet and sealing area can be designed together so that they fit or partially lock in place prior to forming the seal. Additionally, the cover sheet can be pretreated to promote better adhesion to the sealing area.

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

Crosstalk in a display is when the emitted illumination provided by one pixel is unintentionally affected by another pixel. This is undesirable because the affected pixels no longer provide accurate illumination according to the image signal and thus the quality of the image may degrade. Depending on the amount and nature of crosstalk, important factors in a display such as color reproduction, contrast (difference between maximum and minimum illuminance), grayscale, resolution, and "ghosting" can all be negatively affected. In general, crosstalk in microdisplays is a particularly large problem with small pixel pitches. Desirably, the amount of crosstalk between pixels is 15% or less, preferably 5% or less, and most preferably 3% or less.

The described stacked microcavity OLED device must be thick, wherein the spacing between the internal light emitting units is defined. This system must provide a microcavity effect to enhance R, G and B emission within a single microcavity. This can lead to increased crosstalk due to optical and chemical/electrical mechanisms. Some optical procedures that can increase the amount of crosstalk include light scattering and waveguides within OLED devices. Certain chemical/electrical processes that can increase crosstalk include lateral carrier migration within the same layer from an active pixel region to an adjacent inactive pixel region.

It is believed that multiple modes can cause crosstalk. The short range (0.2 µm to 0.7 µm) appears to be a combination of lateral charge carriers and optical mechanisms. The mid-range (3 µm to 7 µm) interactions appear to be mainly due to lateral charge carrier transport but may be partly due to optical mechanisms. Long-range (50 µm to 200 µm) interactions appear to be mainly due to light scattering from an active pixel region to an inactive region. It is also believed that, depending on the pixel pitch, there is a longer range optical contribution to the crosstalk based on the waveguide.

Some useful methods to minimize crosstalk problems due to carrier mobility in OLED devices include: - By varying the layer thickness of layers with high carrier mobility (eg HIL, HTL, CGL, ETL and EIL) and composition (to increase "sheet resistance") to reduce lateral charge carrier transport. Rather, charge carriers (holes or electrons) are generated within an active region and can move laterally across the gap between the illuminated and non-illuminated regions. This problem seems to occur mainly in layers next to or close to one of the electrodes. It is believed that the common HIL and HTL layers over the anode can be the biggest contributor to this problem. It appears that once a hole is created in an illuminated area of the HIL on one anode pad, it can migrate to a non-illuminated area of an adjacent anode pad, and the resulting voltage due to the hole can exceed the _Vth of the OLED and thus (nominally) non-illuminated) pixels will emit light anyway. Additionally, holes can follow the electrons into the conductive anode pads and flow laterally through the anode with very little lateral resistance. At the far side of the non-illuminated anode pad, current can return into the HIL (with the hole) to jump to the next non-illuminated anode pad. Therefore, the carrier migration problem cannot be limited to a short distance between adjacent anode pads, but can also have a longer distance component. Therefore, careful attention should be paid to the thickness and composition of both electrodes and, in particular, the anode. Thinner organic layers with less carrier mobility can help minimize these undesired carrier mobility programs. - Material selection for organic layers with high carrier mobility. Specifically, materials can be selected to minimize their contribution to crosstalk. In this regard, the type and level of dopants added to the HIL (eg, F4-TCNQ, F6-TCNNQ, or HAT-CN) and the HTM in the HIL or HTL (eg, aromatic amine compounds such as NPB or spiro TTB) The choice can be important. Only P-dopants or undoped HILs may also be effective. In some cases, an undoped HIL and a p-doped HTL can be used. Inorganic HIL materials such as MoO3 ₍ which can be mixed with organic materials) may also have advantages.

Some useful methods of minimizing crosstalk problems caused by optical processes within OLED devices include: - Optimizing the color filter to reduce the optical waveguide between the air/glass interface and the reflective anode, including the use of an optical filter layer specially designed to absorb light traveling at high angles from the normal direction of the substrate. - Reduce light scattering by reducing scattering points. Specifically, the amount of small particle debris on or near the anode should be minimized. Scattering can also occur due to the roughness of the cathode, which can depend on the composition and procedure used for deposition. (See, for example, "Efficient Upper-Excited State Fluorescence in an Organic Hyperbolic Metamaterial" by Shen et al., Nano Lett. 2018, 18, 3, 1693-1698). - The total anode surface should be as flat and smooth as possible over the active pixel area and between the pixels. Specifically, it is known that protrusions, bumps, or other structures that form a PDL (pixel defining layer) between pixels and extend over the surface of the anode within the pixel area can be used to scatter light back into the pixel area and prevent light from entering a Neighboring (non-illuminated) pixels. However, this approach is ineffective when there is a thicker OLED layer overlying the structure. Light trapped within a thicker layer is more likely to be internally reflected within the layer so that it can travel across the structure to the other side. If the anode and overlying OLED layers are uniformly flat, light guided within the layers of the display is more likely to continue uninterrupted until it is absorbed or reaches the edges of the display. - Use of absorbers for waveguide light. - Absorption of light by the dielectric of the backplane. - Designing the HIL and anode to form a barrier for charge from the HIL to the anode.

To control crosstalk, it may be useful to add an additional separate pixel control circuit to a node between the second switching transistor and the anode of the OLED to control the voltage at the anode when the OLED is driven such that nominally "off""off" (no power and therefore no transmission). Specifically, the pixel control circuit should reduce the voltage below the _Vth of the OLED only when the driver circuitry requires the pixel to emit no or no weak emission. In this way, any voltage developed by lateral carrier migration can be minimized. Experimental results

In the following examples, the numbers preceding each material (eg, 200 HIL) are the solid layer thickness in Angstroms unless otherwise indicated. All % are by weight. All devices were encapsulated using the same process after deposition of the cathode. All materials are available.

The backplane used in the following experimental setup was a monocrystalline silicon based backplane and was used as-is with the schematic control circuitry shown in Figures 6a and 6b. The backplane contains: a driving transistor (T1) and a switching transistor (T2) connected in series with low voltage (5 V rated) p-channel transistors; and a protection circuit with a An NPN type bipolar junction transistor (BJT1). In all experimental examples, the base of BJT1 was isolated; that is, the base was not connected to an external power source so that the BJT's base voltage was not intentionally controlled or set to any particular voltage. The backplane contains a reflective metal layer as the bottom electrode/anode.

A comparative 2-cell stacked OLED device A-1 was fabricated as follows: Layer 1 (reflective bottom electrode/anode): as delivered 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 main GH1 and 3% green phosphorescent dopant GPD1 Layer 6 (yellow LEL): 100 GH1 + 10% GPD1 + 3% red phosphorescent dopant RPD1 Layer 7 (Electron Transport Layer ETL1): 200 Electron Transport Material ETM1 Layer 8 to Layer 10 (charge generation layer CGL1): 270 three layers Tier 11A (HTL3): 300 HTM1 Tier 11B (HTL4): 280 HTM1 Layer 12 (Blue LEL2): 200 blue main BH1 + 4% fluorescent blue dopant BFD1 Tier 13 (ETL2): 150 ETM2 Layer 14 (Electron Injection Layer EIL): 150 ETM1 + 2% Li Layer 15 (translucent cathode): 105 75% Ag / 25% Mg

A second comparative OLED device A-2 was prepared in a similar manner, but the thicknesses of layers 11A and 11B were increased to 1090 and 4090, respectively. Both A1 and A-2 are two-cell stacked OLEDs, where layer 5 (G LEL) and layer 6 (red LEL) together represent one cell (a G-Y stack) and layer 11 (blue LEL) represents a second cell (a B stack), the second cell is separated from the first cell by a CGL. Layers 5 and 6 are a single unit because they are adjacent LELs that are 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 2320A and the microcavity thickness of A-2 being 6920A. These two-cell stacked OLED devices represent the current state of the art.

An inventive 3-cell stacked OLED B-1 was prepared in the same manner as A-1, but with the following additional layers interposed between layers 11A (whose thickness was adjusted to 470) and 11B (system 4090): Layer 16 (BLEL1) : 200 BH1 + 4% BFD1 Layer 17 (ETL3): 150 ETM2 Layer 18-20 (CGL2): 270 Tri-layer (same scheme as CGL1) OLED device B-1 is a 3-cell stacked OLED, in which layer 5 and layer 6 is the first GY unit (same as in A-1), layer 16 is an additional second B unit and layer 12 (same as in A-1) is a third B unit. The second unit (layer 16) is separated from the first unit (layers 5 and 6) and the third unit (layer 12) by a CGL. That is, B-1 contains an extra B cell compared to A-1 and thus will have higher illumination but increased voltage requirements. OLED B-1 is a microcavity device with the same microcavity thickness (6920 A) as device A-2. Figure 14 shows characteristic IV curves (current density versus voltage in mA/cm) for OLEDs A-1, A- ² (comparative two-cell devices), and B-1 (inventive 3-cell devices), which also are listed in Table 1. Table 1 - List IV data of OLED device I* V* L* A-1 Threshold value 5.5 1 6.0 30.4 5 6.7 40.0 10 7.1 41.5 100 10.8 35.5 A-2 Threshold value 5.3 1 6.3 29.0 5 7.3 30.3 10 8.0 30.3 100 13.5 24.5 B-1 Threshold value 7.6 1 9.0 28.5 5 9.9 32.8 10 10.7 33.7 100 16.0 28.6 *I = current density on anode pad in mA/cm ² V = voltage L = illuminance efficiency in cd/A In Figure 14 and Table 1, OLEDs A-1 and A-2 have about the same the V _th . However, OLED B-1 with an extra B cell has a significantly higher _Vth and requires a higher voltage (about 2.5 V higher) to produce the same current density as A-2.

The advantages of having a microcavity 3-cell stacked OLED on top of a 2-cell stacked OLED can be seen in Figure 15, which compares (intensity vs wavelength in nm) without A-1, A-2, and B- Spectral output in the case of a color filter of 1. OLED A-1 with a thinner microcavity has relatively smaller B and R emission compared to G emission. OLED A-2 with a microcavity suitable for all 3 colors has more B and R emission than G, but B emission is still limited. Compared to A-2, the blue emission of OLED B-2 with additional B cells is greatly increased.

The above results refer to a white OLED device before adding the color filters that will form the R, G, and B pixels for a microdisplay. It is well known in the art that, regardless of any inherent imbalance in the relative amounts of R, G, and B light produced by W OLEDs, the desired R, G in one of the displays can be achieved by driving individual color pixels to the appropriate luminance for that color and B balance (usually referred to as white illuminance (eg, a D65 white point)). For example, if a W OLED produces insufficient blue light and too much red light compared to green light, the B pixel can be driven at a higher voltage to generate more blue light, and the R pixel can be driven at a lower voltage so that all The amount of red light produced is reduced.

16 shows the peak current required to produce a D65 white point for each color of OLED devices A-1, A-2, and B-1 using color filters with equal areas of the R, G, and B subpixels A graph of density. The current density in B required for the two-unit devices (A-1 and A-2) is much greater than that for the three-unit device B-1. The current densities in R and G were about the same as in all 3 devices. The 3-unit device B-1 requires less current density and therefore less power to achieve the same target illumination at the same white point. The lifetime of a display is determined by the rate of efficiency decline of an OLED in continuous operation, which is primarily a function of current density and temperature. Therefore, the lifetime of display B-1 will be 2 times longer than that of A-1 or A-2.

In microdisplays that are required to generate relatively high luminance due to their small size, it is desirable that the total current (and current density) for each color be about the same (or as close as possible) when displaying white at maximum display luminance. This will achieve several advantages of the following inventions: neutral fading (where the luminance efficiency of each color of a sub-pixel degrades at about the same rate over the life of the device), high luminance (each pixel will be driven at its maximum brightness for resulting in improved efficiency), high contrast ratio (full dynamic range of each pixel is available), and longevity (each color of a pixel shares the current load equally).

However, the advantages of using OLEDs with three or more cells to increase illumination can be offset if the required voltage increases. In a display, power for each pixel is supplied and controlled by control circuitry in the backplane. However, in a microdisplay that is reduced in area but requires a high number of pixels, the physical space for transistors and other circuit components is limited. Typically, higher voltage and current requirements must be handled by larger, thicker, or more robust electronic circuitry. Currently, the size of OLED microdisplays dictates that electronic circuitry is rated to handle 5 V or less to achieve the desired pixel size and density (pixel pitch).

Electronic circuitry, such as transistors, can fail catastrophically if the power supplied to the gates of the transistors is too high for an extended period of time. However, not all levels or times of overvoltage will cause permanent transistor failure; in many cases, transistors will fail due to leakage. Increased leakage is a common failure mode caused by non-destructive overstressing of a semiconductor device when the junction or gate oxide suffers permanent damage that is insufficient to cause a catastrophic failure. Overstressing the gate can result in stress-induced current leakage.

Solving motion blur in microdisplays for virtual reality and similar products requires OLEDs to provide very high illumination for a very short period of time and then turn off for another period of time (the total period of on/off is less than a human can detection range to avoid flickering). This means that the control circuitry in the backplane must supply a high voltage/current to generate high illumination in one cycle and then shut off power to the OLED in another cycle. If the power applied to the transistors in the control circuitry is too high during the period that the OLED is on, current can leak through the transistor when the transistor should be "off" and the OLED does not turn off completely. The difference between the illuminance of an OLED when it is on and the illuminance when it should be off is often referred to as "contrast". Microdisplays are expected to have high contrast because they make images appear sharp and bright; low contrast makes images appear dark and flat.

Figure 17 shows sets of (illuminance versus cathode voltage) curves for a 2-cell comparative device and an inventive 3-cell device. The upper set of curves is when the device is driven at full illumination (white = CV255) and the drive transistor is "on". The lower set of curves are when the device is driven at zero illumination (black = CV0) and the drive transistor is nominally "off". However, the leakage of current through the drive transistor results in some small amount of illumination and the voltage at the cathode being non-zero. The difference between the two sets of curves represents the "contrast" of the device and it is desirable to make the "contrast" as high as possible by minimizing the illumination caused by current leakage through the drive transistors. As can be seen from Figure 17, the contrast ratios for all three devices are about the same (slightly less than ¹⁰⁵ ) and relatively constant across various cathode voltages.

Note that the inventive 3-cell device maintains approximately the same overall contrast ratio as the comparative 2-cell device, but requires higher operating voltages that exceed the design constraints of the drive transistors. Although B-1 requires less current density to achieve full white than A-1 or A-2 shown in Figure 16, B-1 still requires a larger voltage (about 2.5 V, see Figure 14). There is no evidence of increased leakage for higher voltage OLEDs with backplanes used in the present invention. Furthermore, there is no evidence of any catastrophic transistor failure in B-1 due to the higher voltage requirements (see Table 1), nor evidence that the device life appears to be affected from the control circuitry. These results are unexpected because the backplane mid-transistors of A-1, A-2, and B-1 are designed to be suitable for driving 1-cell OLEDs or dual-cell OLEDs where low voltage transistors are acceptable and commonly used. Transistor.

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

OLED B-2 is a 3-cell device similar to B-1 in that it contains a G-Y cell (layers 5 and 6) that is formed by a CGL (layers 8-10) and a second blue The color unit (layer 12) is separated, the second blue unit is then separated from a first blue unit (layer 18) by a second CGL (layers 14-16), (from the backplane ) presents the order of Y-G/B/B. Similar to B-1, the B-2 series has a microcavity of 6910 Å for a microcavity white light generating OLED.

Similarly, an inventive 4-cell OLED device C-1 was prepared as follows: Tiers 1 to 11: Same as Tiers 1-11 in B-2 Layer 12 (Blue LEL2): 200 BH1 + 4% BFD1 (50 Å reduction) Layer 13 (ETL2): 100 ETM2 (50 Å reduction) Layers 14 to 16: Same as Layers 14 to 16 in B-2 Layer 17 (HTL4): 420 HTM1 (3420 Å reduction) Layer 18 (BLEL1): 200 BH1 + 4% BFD1 (50 Å reduction) Layer 19 (ETL3): 100 ETM2 (50 Å reduction) Layers 20 to 22 (CGL3): 270 three layers (same scheme as CGL1 and CGL2) Layer 23 (HTL5): 2620 HTM1 Layer 24 (HTL6): 50 HTM2 Tier 25 (Yellow LEL): 200 GH1 + 10% GPD1 + 3% RPD1 Tier 26 (ETL4): 480 ETM1 Layers 27 to 28: Same as Layers 20 to 21 in B-2

OLED C-1 is a 4 cell device because it contains an extra Y cell (layer 25) and the same G-Y cell and two B cells as in B-2, in a Y-G/B/B form (from the backplane) /Y order. In C-1, the additional Y unit is separated from the first B unit (layer 18) by CGL3 (layers 20-22). The C-1 series has a microcavity of 6910 Å for a microcavity white light generating OLED.

Similarly, an inventive 5-cell OLED device D-1 was prepared as follows: Layers 1 to 10: Same as Layers 1 to 11 in C-1 Layer 11 (HTL3): 370 HTM1 (200 Å reduction) Layers 12 to 16: Same as Layers 12 to 17 in C-1 Layer 18 (ETL2): 100 ETM2 (50 Å reduction) Layers 14 to 16: Same as Layers 14 to 16 in C-1 Layer 17 (HTL4): 620 HTM1 (200 Å increase) Layers 18 to 22: Same as Layers 18 to 22 in C-1 Layer 23 (HTL5): 610 HTM1 (2010 Å reduction) Tier 24 (BLEL3): 200 BH1 + 4% BFD1 Tier 25 (ETL3): 100 ETM2 Layer 26 to Layer 28 (CGL4): 270 three layers (same scheme as CGL1, CGL2 and CGL3) Layer 29 (HTL6): 1440 HTM1 Layers 30 to 34: Same as Layers 24 to 28 in C-1

OLED D-1 is a 5-cell device because it contains an additional B-cell (layer 24) and the same Y-cell, G-Y-cell, and two B-cells as in C-1. It has a total of 3 B cells, one Y cell and one G-Y cell, all separated by a CGL, in Y-G/B/B/B/Y order (starting from the backplane). The C-1 series has a microcavity of 6910 Å for a microcavity white light generating OLED.

18 shows characteristic IV curves of OLEDs B-2, C-1 and D-1, which are also shown in Table 2. Table 2 - List IV data of OLED device I* V* L* B-2 Threshold value 8.2 - 1 9.1 30.9 5 10.2 35.4 10 10.9 35.8 100 15.8 30.2 C-1 Threshold value 10.5 - 1 11.7 52.6 5 13.0 53.6 10 13.9 52.7 100 19.0 42.2 D-1 Threshold value 12.8 - 1 14.5 50.5 5 16.0 51.9 10 17.1 51.6 100 22.1 42.1 * I = current in mA/cm ² V = voltage L = illuminance in cd/A

As shown in Figure 18 and Table 2, adding each cell within the OLED provides greater illumination but also increases _Vth and operating voltage by about 2.5 V.

The advantage of adding additional cells to a microcavity 3-cell OLED can be seen in Figure 19, which compares the spectral output of B-2 (3 cells), C-1 (4 cells) and D-1 (5 cells) ( no color filter). OLED B-2 has one G/Y unit and 2 B units. Adding another Y unit, the same as in C-1, increases the G and R emissions relative to B-2 but the increase in B emissions is not much. Adding a third B unit as in D-1 further increases blue emission while maintaining higher G and R emission.

Similar to the results shown in Figure 16, Figure 20 shows each color required for OLEDs B- ² , C-1 and D-1 to form a balanced white illumination of 1500 cd/m (using a color filter) The relative amount of peak current density for a pixel. Adding an extra Y cell in C-1 reduces the current required to produce the necessary R and G illumination compared to B-2. Adding an extra B cell in D-1 enables a further reduction in the voltage required to generate the necessary amount of B light compared to C-1. Since these OLEDs with three or more cells require less current density to achieve the same illuminance, their operating lifetimes will be significantly greater than those with fewer cells. The LT70 (time when transmit output degrades by 70%) of these devices is expected to be at least 18,000 hours of average video content.

Figure 21 (similar to Figure 17) shows several sets of (illuminance versus cathode voltage) curves for OLEDs B-2, C-1 and D-1. In all three of these examples, the contrast ratio was about the same (slightly less than ¹⁰⁵ ) and relatively constant across various cathode voltages. It is worth noting that the contrast ratio remains unchanged even when the operating voltage of these devices is 2 or 3 times the design limit of the drive transistor.

Without being bound by any particular theory or speculation, the reason why stacked OLEDs requiring a relatively high _Vth to maintain high contrast without burning out or destroying when using backplanes with low voltage transistors may be related to using a series connection Crystal related. Current leakage in transistors is a well-known problem and in general, the higher the voltages and currents involved, the greater the leakage. As discussed previously, this leakage can cause the OLED to emit light above Vth. Since the total leakage through the series connected transistors will be (leakage of the first transistor) x (leakage of the second transistor), it is possible that this multiplication effect is sufficient to reduce leakage at the bottom electrode of the OLED significantly enough to The OLED is made not to emit due to current leakage and thus the contrast ratio is maintained.

In operation, the control circuit located in the backplane of the experimental microdisplay and comprising at least two transistors with their equal channels connected in series is one example of a compact circuit that protects the drive transistor T1 from conditions outside the specified range (even when driving an OLED with three or more stacks at a switching voltage range greater than the specified operating range of LV transistors). If the switching voltage range of the OLED exceeds the LV operating range, only the switching transistor is exposed to these conditions. This damage to image quality is less than that of another pixel circuit design in which the drive transistors necessarily operate outside the specified operating range.

During operation, MOSFET devices age, which causes slow transitions in some of the characteristics such as threshold voltage, sub-threshold ramp, and transconductance at saturation. These are all symptoms of negative bias temperature instability (NBTI) and hot carrier injection (HCI) found in p-channel transistors. By limiting the operation of a transistor to the specified voltage range of the transistor, it is ensured that the transistor characteristics will remain within a narrow specified range over a long period of time (eg, 5 years). Operating the transistor outside the specified voltage range increases the rate at which the transistor's characteristics change, thereby shortening the period during which the characteristics are within the specified range.

It is important that the characteristics of the drive transistor remain within specified limits, otherwise image degradation (often referred to as imbalance) may result. The operation of the switching transistor is more robust even with changes in its performance characteristics, and can be driven with a sufficiently wide gate voltage to ensure satisfactory operation even when the transistor characteristics have transitioned outside the specified range. Thus, the circuit shown in Figures 2-6, including the circuit used in the backplane of the inventive experimental stacked OLED microdisplay, is an example of a compact circuit that is The higher switching voltages associated with three or more OLED cells are handled with the expected degradation in display quality for the specified operating range.

It is believed that the circuits of Figures 2-6 accomplish this by selecting the SELECT2 voltage to transmit "on" and "off" the transmit. To "turn off" the emission, one of the ideal choices for the SELECT2 voltage is V _DD , which will place T2 in the sub-threshold region of operation, effectively stopping the current flow. To transmit "on", the SELECT2 voltage can be selected as low as 5 V, which is below V _DD .

If the switching voltage swing of the OLED is less than about 4V, then when the pixel displays "black", the black level current will be set by driving the transistor in a normal manner and the drain of T1 will be higher than the SELECT2 "on" voltage and the switch Transistor T2 is still on. However, when the switching voltage swing of the OLED is greater than about 4V, as in the case of a multi-cell stacked OLED with more than 4 cells, when the black level current is developed, the drain voltage of T1 (also the source of T2) voltage) is close to the SELECT2 "on" voltage and turns off switching transistor T2 when the low drain voltage of T1 reduces the overvoltage (Vgs-Vth) of T2 to zero or slightly negative. This drives T2 into the sub-threshold range, resulting in the black level. Under these conditions formed by a multi-cell stacked OLED, the drive transistor T1 controls T2 to form the black level current.

Therefore, it is believed that the simple series-connected transistor design shown in FIGS. 2-6 is capable of driving a multi-cell stacked OLED with a switching voltage range exceeding that of low-voltage transistors, which is similar to, for example, one shown in FIG. 1 . Age-related imbalances are reduced compared to single-transistor drive circuits.

The protection circuit is designed to maintain the voltage at the anode of the OLED below a certain level whenever the OLED is intended to be "off" or not emitting and can help maintain high contrast. When the cathode voltage decreases (more negative), the protection circuit is designed to provide additional current to protect the drive and switching transistors so that the voltage levels do not violate the device's maximum ratings.

However, the protective effect was still observed in the experimental example where the base of the BJT was isolated and not connected to an external power source, so there was no need to deliberately control the base voltage. It has been observed that the exponential ramp of black current versus voltage (0.75 tens/volt) of the OLED display in the device without the stacked OLED is similar to the example with the OLED present. This indicates that the protection circuit still provides some current and voltage control at the anode of the OLED when the base of the BJT is isolated. Without being bound by any particular theory or speculation, it is possible that an adjacent n-well (eg, the n-well of switching transistor T2) may be a source of holes that migrate to the p-well (base) of BJT BJT1. Once these holes are in the base, they will diffuse across the depletion region to the n-type emitter contact (OLED anode pad), traveling through the base-emitter diode in the forward direction. This will be complemented by the diffusion of thermally excited electrons from the emitter into the base and their transport through the base and depletion regions into the collector, which is facilitated by the large electric field potential between the base and collector. In this case, holes from the adjacent n-well (of the drive transistor or switching transistor) provide charge (holes) into the base which would normally come from an external base connection. When the OLED anode voltage drops to a very low level (eg, to display black with three or more OLED cells), then one of the drive circuit transistors (which will be at V _DD ) is adjacent to the n-well and BJT base The potential difference between the electrodes (at the OLED anode voltage) is very large, increasing the hole current from the well of the drive transistor into the base of the BJT. This increase in base current increases the emitter current due to the amplification of the BJT.

However, the protection effect provided by the protection circuit is different for each frame and must be properly reset for each new frame of the image. This is not a problem when the base of the BJT is connected to an external power supply and actively controlled for each frame. For embodiments where the base of the BJT is isolated and not intentionally connected, a reset per frame can be provided when the switching transistor turns off the pixel because the switching transistor provides an on-off effect. Therefore, this aspect of guard effect control motion depends on having the drive and switching transistors connected in series because the emission must be turned off during data loading (or during some portion of the frame time) for the reset.

OLED schemes with three or more cells can be designed such that the voltage range from the black level (below Vth; eg, 2uA/cm2) to the white level (20mA/cm2) is relatively constant and less than about 6V. As shown in Figures 17 and 21, this can result in a contrast ratio of about 10,000:1 and can be slightly smaller due to the current efficiency drop at the high end of the current density. This voltage range is approximately within the allowable operating range for LV transistors only if the protection circuitry functions at the bottom end of the current range when the drive and/or switching transistors stop current. Thus, at the low current end of the range, the protection circuit additionally prevents the current density through the OLED from decreasing below about 2 uA/cm ² . While this effect is provided by the additional protection provided by having at least two transistors connected in series as discussed above, it limits the ability to achieve higher contrast. In exchange for this slightly higher black level and reduced contrast, the protection circuit allows the pixel driver circuit to achieve higher peak brightness by lowering the cathode voltage or to compensate for efficiency losses due to OLED aging, ensuring that the LV transistors are within their specified limits. operate within the voltage range.

The above description sets forth several different embodiments, which may involve different combinations of different individual features. The individual features of any embodiment may be combined in any order or degree as desired, without limitation, except when incompatible.

In the above description, reference is made to the accompanying drawings, which form a part hereof and in which there are shown, 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 present invention, and it is to be understood that other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. Therefore, the description of any example embodiment should not be construed in a limiting sense. Although the present invention has been described for purposes of illustration, it is to be understood that such details are for purposes of illustration only and changes may be made by those skilled in the art without departing from the spirit and scope of the invention.

1: pixel defining layer 3: silicon backplane 5: optional planarization layer 7: electrical contacts 9: first electrode segment 9A: first electrode layer 9B: reflective layer 11: non-emissive organic light emitting diode layer 13: First light-emitting organic light-emitting diode unit 13A: red light-emitting organic light-emitting diode unit 15: charge generating layer 16: yellow light-emitting OLED unit 17: second light-emitting organic light-emitting diode unit 17A: green light-emitting organic light-emitting diode Cell 19: Charge Generation Layer 21: Third Light Emitting Organic Light Emitting Diode Cell 21A: Blue Light Emitting Organic Light Emitting Diode Cell 22: Blue Light Emitting Layer 23: Non-emitting Organic Light Emitting Diode Layer 24: Charge Generation Layer 25: Top electrode 27: Capsule body 29: Color filter array 29B: Blue filter 29G: Green filter 29R: Red filter 30: Microcavity 32: Second blue light emitting unit 45: Power supply 50: Power Supply 100: RGB Microdisplay/Microdisplay 200: Multimode Microcavity Organic Light Emitting Diode Stacked Microdisplay/Microdisplay 300: Multimode Microcavity Organic Light Emitting Diode Stacked Microdisplay/Microdisplay 400: Multimode Micro Cavity OLED Stacked Microdisplay/Multi-Mode OLED Microdisplay/Microdisplay A-1: 2 Unit Stacked OLED Device/Organic Light Emitting Diode A-2: Second Comparison Organic Light Emitting Diode Device/Device/Organic Light Emitting Diode B-1: Inventive 3-Cell Stacked OLED/Organic Light Emitting Diode Device/Display/Organic Light Emitting Diode B-2: Invention Inventive 3-Cell OLED Device/Organic Light Emitting Diode BJT1: Bipolar Junction Transistor C1: Storage Capacitor C-1: Inventive 4-Cell OLED Device/Organic Light Emitting Diode D-1 : Inventive 5-Cell OLED Device/Organic Light Emitting Diode D4: Diode GND: Ground IBD1: Intrinsic Host Diode IBD2: Intrinsic Host Diode IBD3: Intrinsic Host Diode IBD5 : Intrinsic Body Diode IBD6: Intrinsic Body Diode LSC: Level Shift Circuit MP1: Transistor MP2: Current Drive Transistor/Transistor SELECT1: Select Line SELECT2: Select Line SELECT3: Select Line T1:p Channel drive transistor/drive transistor T2: switching transistor T3: inline selection transistor/scanning transistor T4: third transistor/drive transistor T5: inline selection transistor T6: protection circuit p-channel transistor V _B : Base voltage V _C : Collector voltage V _CATHODE : Cathode voltage V _DD : External power supply/voltage source/constant supply voltage V _DD2 : Voltage source V _E : Emitter voltage/voltage V _REF : Voltage reference/reference voltage

Figure 1 shows a simple prior art control circuit for an OLED. Figure 2 shows a basic control circuit with two transistors with their equal channels connected in series, the basic control circuit being suitable for an OLED stack with at least three OLED cells. FIG. 3 shows one embodiment of the intrinsic body diode connection of FIG. 2 . FIG. 4 shows another embodiment of the intrinsic body diode connection of FIG. 2 . Figure 5A shows a basic control circuit with three transistors with their equal channels connected in series, the basic control circuit being suitable for an OLED stack with at least three OLED cells. FIG. 5B is similar to FIG. 5A, except that the driving transistor system is controlled by a level shift circuit. Figure 6 shows a basic control circuit suitable for an OLED stack having at least three OLED cells and an additional protection circuit including a diode-connected p-channel MOSFET transistor. Figure 7 shows a similar protection circuit including one of the diodes. 8 shows a basic control circuit suitable for an OLED stack having at least three OLED cells and a protection circuit with a bipolar junction transistor (BJT). FIG. 9 shows a schematic cross-section through the Si backplane illustrating the connection locations, connections and transistor wells of the circuit shown in FIG. 8 . 10 shows a cross-sectional view of an RGB microdisplay 100 with three adjacent monochrome RGB OLED stacks, each individual stack having three OLED light emitting cells. 11 shows a cross-sectional view of a microdisplay 200 with a multimode microcavity OLED stack having three OLED light emitting units and an array of RGB color filters. 12 shows a cross-sectional view of a microdisplay 300 having a multimode microcavity OLED stack with three OLED light emitting cells including an additional blue light emitting layer and an array of RGB color filters. 13 shows a cross-sectional view of a microdisplay 400 with a multimode microcavity OLED stack having five OLED light emitting units and an array of RGB color filters. Figure 14 shows a plot of current density versus voltage for certain comparative and inventive OLED devices. Figure 15 shows spectra (plotted as intensity versus wavelength) for certain comparative and inventive OLED devices. 16 shows a graph of the peak current density required to produce a D65 white point for each color of a comparative OLED device with color filters and an inventive OLED device. 17 shows a plot of illumination versus voltage for the comparative and inventive OLED devices when driven to produce black and at peak white illumination. Figure 18 shows a plot of current density versus voltage for certain inventive OLED devices. Figure 19 shows the spectra (plotted as intensity versus wavelength) of certain inventive OLED devices. 20 shows a graph of the peak current density required to produce a D65 white point for each color of certain inventive OLED devices with color filters. 21 shows a graph of illumination versus voltage for certain inventive OLED devices when driven to produce black and at peak white illumination.

SELECT1: select line

SELECT2: select line

T1: p-channel drive transistor/drive transistor

T2: switching transistor

T3: Inline selection transistor/scanning transistor

V _CATHODE : cathode voltage

V _DD : External Power/Voltage Source/Constant Supply Voltage

Claims (20)

A microdisplay comprising a light-emitting OLED stack on top of a silicon-based backplane having individually addressable pixels and control circuitry, wherein: the light-emitting OLED stack has a top electrode and a bottom electrode three or more OLED units in between; and the control circuitry of the silicon-based backplane includes at least two transistors for each individually addressable pixel, the channels of the at least two transistors being connected in series at between an external power supply V _DD and the bottom electrode of the OLED stack. The microdisplay of claim 1, wherein the V _th of the light-emitting OLED stack is at least 7.5 V or greater than 7.5 V. The microdisplay of claim 1, wherein the V _th of the light-emitting OLED stack is at least 10 V or greater. The microdisplay of any one of claims 1 to 3, wherein the OLED stack comprises four or more OLED light emitting units. The microdisplay of any one of claims 1 to 3, wherein the OLED light-emitting units are each separated from each other by a charge generating layer (CGL). The microdisplay of claim 5, wherein the bottom electrode is segmented, and each segment is in electrical contact with the control circuitry in the backplane. The microdisplay of claim 6, wherein the OLED stack is top-emitting. The microdisplay of claim 7, wherein the OLED stack forms a microcavity in which the physical distance between the segmented bottom electrode and the top electrode is constant across all pixels. The microdisplay of any one of claims 1 to 3, wherein the transistors with their equal channels connected in series are all rated at 5 V or less. The microdisplay of any one of claims 1 to 3, wherein the transistor system closest to the power supply is a drive transistor and rated at 5 V or less, and the transistor closest to the bottom electrode of the OLED Transistor System A switching transistor and rated for greater than 5 V. The microdisplay of any one of claims 1 to 3, wherein the two transistors having their equal channels connected in series are both p-channel transistors. The microdisplay of claim 11, wherein the two transistors with their equal channels connected in series are each located in separate wells. The microdisplay of any one of claims 1 to 3, wherein the control circuit system additionally includes a protection circuit, and the protection circuit includes a p-channel transistor. The microdisplay of any one of claims 1 to 3, wherein the control circuit system additionally includes a protection circuit, and the protection circuit includes a p-n diode. The microdisplay of claim 14, wherein the cathode of the pn junction diode is connected to the node of the bottom electrode of the OLED stack, and the anode is connected to a voltage reference V _REF or a current reference I _REF . The microdisplay of any one of claims 1 to 3, wherein the control circuit system further includes a protection circuit, and the protection circuit includes a bipolar junction transistor. The microdisplay of claim 16, wherein the bipolar junction transistor system is an NPN transistor, in which the base is connected to a voltage source V _PROTECT or a current source I _PROTECT , and the emitter is connected to the NPN transistor The bottom electrode of the OLED stack is connected to a node and the collector is connected to a voltage source V _DD . The microdisplay of claim 16, wherein the base of the bipolar junction transistor is isolated, the emitter is connected to a node connected to the bottom electrode of the OLED stack and the collector is connected to a voltage source V _DD . The microdisplay of claim 16, wherein the bipolar junction transistor is located in a well separate from the two transistors having their equi-channel connected in series. The microdisplay of claim 19, wherein the two transistors with their equi-channel series connection are both p-channel transistors and are each located in a separate n-well, and the bipolar junction transistor system is located in a separate p-well One of the NPN transistors.

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