Multi-mode microcavity oled with multiple blue emission layers

Abstract

A multi-mode light emitting OLED microcavity device comprising: an opaque substrate; a layer with a reflective surface above the substrate; a first electrode above the reflective surface; an organic layer for light emission comprising a second blue light emitting layer proximate to the reflective surface, a first blue light emitting layer further from the reflective layer than the second blue light emitting layer, and at least one non-blue light emitting layer, wherein the distance between the midpoint of the second blue light emitting layer and the midpoint of the first blue light emitting layer is L1; a semi-transparent second electrode with an innermost surface through which the light emission passes; wherein the distance L0 between the reflective surface and the innermost surface of the semi-transparent second electrode is constant across the light emission area; and the ratio L1 / L0 is in the range of 0.30 to 0.40. The multi-mode microcavity OLED has enhanced blue light emission and can be particularly useful as a light source in micro-displays.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims the benefit of U.S. Non-Provisional Utility Application No. 16 / 695,391, filed November 11, 2020, with attorney docket number OLWK-0020-US. BACKGROUND

[0003] Generally, microdisplays have a diagonal less than two inches (about 5 cm), and pico displays have a diagonal less than one inch. In most cases, microdisplays have high resolution and pixel sizes typically range from 4 to 15 microns. Microdisplays were first introduced commercially in the late 1990s, and they are commonly used in rear-projection televisions, head-mounted displays, and digital cameras. In recent years, devices such as smartwatches have taken advantage of the high resolution and low power consumption of these displays. Microdisplays are expected to grow at a compound annual growth rate of 20% of the global market in the next few years. One of the reasons for this growth trend is the increasing adoption of near-eye displays and augmented reality devices and virtual reality devices, such as head-mounted displays (HMDs), heads-up displays (HUDs), and electronic viewfinders (EVFs).

[0004] There are two main categories of microdisplays. The first is a projection microdisplay, which includes a highly magnified image projected onto a surface. Types of projection microdisplays include rear-projection televisions and compact data projectors. The second is a near-eye display (NED), which includes a highly magnified virtual image viewed through an eyepiece, such as a virtual reality headset or a camera viewfinder. These displays are increasingly used in HMDs and HUDs, especially in the military and medical industries.

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

[0006] Microdisplays can be made from a range of display technologies, including liquid crystal on silicon (LCoS), liquid crystal display (LCD), digital micromirror device (DMD), digital light processor (DLP), and more recently, MicroLED (light-emitting diode) and organic light-emitting diode (OLED).

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

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

[0009] OLED technology has many attractive features for microdisplays over microLED technology. It has a self-emission function, has excellent image quality, is very efficient compared to LCDs or LCoS, and has ultra-high color reproducibility and a wider color space. Self-emitting OLED devices have important advantages over backlit devices such as LCDs, as each pixel only produces the intensity needed for the image, while backlit pixels produce maximum intensity and then absorb the unwanted light. Furthermore, since OLED layers can be vacuum deposited or directly coated, it is easier and less costly to form OLEDs on transistors compared to the formation of microLEDs. On the other hand, OLEDs can have limited brightness and limited lifetime.

[0010] There are two basic methods for manufacturing pixelated OLED displays (including microdisplays), where the brightness of each individual pixel must be controlled by powering one of the pixel electrodes via control circuitry (transistors). The first method involves making each pixel individually emit red, green, or blue light (R, G, B, respectively). The second method uses a color filter array (CFA) with a common multi-peak (white light) emitting OLED layer on all pixels to produce individual RGB pixels. The advantage of the second method over the first is that it eliminates the need to produce individual OLED pixels with different schemes, thus reducing manufacturing costs.

[0011] OLED-based microdisplays will require very high brightness from the OLED light-generating layer. A known method to improve the brightness and color purity of OLED emission is to utilize the optical microcavity effect. This effect is based on creating an optical resonator between a reflective surface and a semi-reflective surface, allowing some light to pass through. Depending on the optical distance between the two surfaces, multiple reflections between them generate standing waves, which will enhance some wavelengths of light and attenuate others, depending on whether the emission occurs at the anti-node or the node of the standing wave, resulting in constructive and destructive interference effects. The anti-node appears at different locations depending on the total space between the reflectors and the wavelength to be optimized. Mathematically calculated optical models are helpful in determining the ideal emitter location for a given structure.

[0012] Factors influencing the location of the antinode and subsequently the optimal emitter location include: the total optical distance between reflectors, the known phase shift that occurs when light is reflected from the absorber or reflector, the refractive index of the organic layer, and the reflective interface outside the translucent reflector.

[0013] As the optical thickness of the microcavity increases relative to the wavelength of light in the organic medium, light of a specific color can have multiple antinodes within the microcavity. This allows for the use of multiple emitters with the same spectrum to meet desired output requirements for a specific application. Multiple emitters can be used individually to create very bright monochrome displays, or they can be used in combination with other emitters to generate balanced white light displays.

[0014] However, light emitted from a microcavity can exhibit significant angle dependence, where color shifts and brightness losses can occur as the viewing angle deviates from the angle perpendicular to the viewing surface. This is generally not a problem for NED applications due to the limited incident angle of projection optics.

[0015] It is desirable to use multi-mode light-emitting OLED microcavities to make OLED-based microdisplays. That is, microdisplays in which light emission is from a single light-emitting OLED unit, the light-emitting OLED unit is multi-mode (more than one color of light) and common to all pixels, and the emission color of the individual pixels is controlled by a color filter array to produce RGB (or RGBW when some pixels are not color filtered).

[0016] Multi-mode (or white light) light-emitting OLEDs that exploit the microcavity effect are known. Examples include: US 6133692; US 7102282; EP 0683623; US 9385338; JP 2015130319; US 7888860; WO 2014039615; Lu et al, App Phys Let, 92, 123303 (2008); Young-Gu Ju (2011). "Micro-cavity in organic light-emitting diode", from Organic Light Emitting Diode - Material, Process and Devices, Prof. Seung Hwan Ko (Ed.), ISBN: 978-953-307-273-9; Chen et al, Organic Electronics, 12, 2065 (2011); and Park et al, ACS Photonics, 5, 655-662 (2018).

[0017] US 7098590 describes a passive matrix display using a white light OLED microcavity in which the thickness of the microcavity is an integer multiple of the sum of half of the peak wavelength of the light emitting layer within the microcavity.

[0018] However, none of the methods described in these references can provide the necessary high brightness required, especially in the blue. This is because the efficiency of OLED blue emitters is generally lower than the best G or R emitters. For example, phosphorescent G, Y and R emitters can have very high efficiency and excellent stability. However, even the best examples of phosphorescent B emitters will have lower efficiency than G or R emitters and the stability will be significantly reduced. Blue-emitting TADF (thermally activated delayed fluorescence) compounds can be more stable but still cannot approach the efficiency of phosphorescent G and R emitters. Fluorescent B emitters can be more stable but have significantly lower efficiency than phosphorescent emitters. In practice, many white-emitting OLEDs have low B emission relative to G and R emission.

[0019] This imbalance in RGB emission can be addressed in a number of ways. The efficiency of the G and R emitters can be reduced, but this approach reduces the overall efficiency of the device. In displays, the blue pixels can be run at a higher current relative to the G and R pixels to balance the overall emission, but this can shorten the overall device lifetime due to the inverse relationship between the stability of OLED materials and the applied current. The number or size of the B pixels can be increased relative to the G and R pixels, but this can impact the resolution as the effective distance between adjacent G or R pixels (which carry most of the resolution information) will increase. None of these potential solutions are ideal for micro-displays.

[0020] Another approach to increase the amount of B emission in a white light emitting OLED is to use multiple blue light emitting layers. Examples of this can be found in US8877350; US9655199; US9577221; US20110297922; US20120012820; US20130320837; US20080224602; US20060273714; and US10290825. However, none of these relate to microcavity OLEDs, and none of these arrangements are suitable for microcavity applications.

[0021] US2014 / 0183496 describes a microcavity OLED with a green light emitting layer located between two blue light emitting layers, where the thickness ratios of the various hole transport layers within the OLED are within defined ranges. However, the overall thickness of the microcavity and the spacing between the two blue light emitting layers are not disclosed.

[0022] US20120248971 and US9685622 both describe white light emitting OLEDs where the light emitting layers are located at a specific distance from the reflective cathode. US20160181560 describes an OLED with three light emitting layers, which are directly adjacent to each other, where the middle light emitting layer emits a different colour to the two outer layers, which can emit the same colour of light. However, none of these relate to microcavity OLEDs, and none of these arrangements are suitable for microcavity applications.

[0023] US7273663 describes a number of stacked OLEDs with white light emitting units in a non-microcavity application, each white light emitting unit separated by an intermediate connecting layer. In this approach, each individual white light emitting unit has a blue light emitting layer, so there are multiple B light emitting layers within the device, separated by light emitting layers which emit different colours of light. However, this device also has multiple G and R layers, and will also have the problem of imbalance between B light and G and R light. This arrangement can not be suitable for microcavity applications with multiple stacks of light emitting layers.

[0024] Schemes for white light emitting OLEDs with increased blue light emission suitable for microdisplays are needed. White light emitting microcavity OLEDs with at least two blue light emitting layers can provide increased blue light emission. Depending on the size of the microcavity, the spacing of the blue light emitting layers relative to each other and to the reflective surfaces of the microcavity can be important to achieve high blue light emission. SUMMARY

[0025] A multi-mode light emitting OLED microcavity device that can be used as a light source for a microdisplay includes: an opaque substrate; a layer with a reflective surface above the substrate; a first electrode above the reflective surface; organic layers for light emission including a first blue light emitting layer, a second blue light emitting layer, and at least one non-blue light emitting layer, the second blue light emitting layer being proximate to the reflective surface, and the first blue light emitting layer being further from the reflective layer than the second blue light emitting layer, wherein the distance between the midpoint of the second blue light emitting layer and the midpoint of the first blue light emitting layer is LI; a semi-transparent second electrode with an innermost surface, the light emission passing through the semi-transparent second electrode; wherein the distance L0 between the reflective surface of the semi-transparent second electrode and the innermost surface is constant across the light emission area; the ratio LI / L0 is in the range of 0.30 to 0.40.

[0026] In some embodiments, the multi-mode light emitting microcavity can have a ratio L2 / L0 in the range of 0.90 to 0.98, where L2 is the distance between the midpoint of the first blue light emitting layer and the reflective surface. Such embodiments can also have a ratio L3 / L0 in the range of 0.52 to 0.64, where L3 is the distance between the midpoint of the second blue light emitting layer and the reflective surface.

[0027] The multi-mode light emitting microcavity OLED can have two different ranges of distance L0; a first range of distance L0 in the range of 6500 A to 7800 A, and a second range of distance L0 in the range of 8000 A to 9000 A.

[0028] In some embodiments, a non-blue light emitting layer is between the first blue light emitting layer and the second blue light emitting layer, and can emit green light. There can be a second non-blue light emitting layer between the second blue light emitting layer and the reflective surface, which can emit red light. There can be a third blue light emitting layer between the red light emitting layer and the reflective surface. There can also be a fourth blue light emitting layer between the third blue light emitting layer and the reflective surface.

[0029] In some embodiments, the upper surface of the anode is the reflective surface. That is, the functionality provided by the layer with a reflective surface above the substrate and the first electrode (preferably an anode) above the reflective surface are combined into a single layer, such that the first electrode (anode) is reflective, and its upper surface defines one side of the microcavity.

[0030] The multi-mode light emitting OLEDs described above can be used in microdisplays, where the substrate of the multi-mode light emitting microcavity OLED includes a backplane with control circuitry for the individual operation of a plurality of sub-pixels, and has an array of color filters aligned with the individually controlled sub-pixels over a semi-transparent cathode, forming at least R, G, B sub-pixels. In some embodiments, the microdisplay is an RGBW microdisplay. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 A spectrum (intensity vs. wavelength) of a typical OLED emitter without any proximate interfaces in a non-microcavity environment is shown.

[0032] Figure 2 A graph showing the calculated relative efficiency of R, G and B colors vs. distance from the electrode in a 695 nm microcavity is shown.

[0033] Figure 3 A graph showing the calculated emission intensity vs. wavelength over the emitter position range of a 6960 A microcavity is shown.

[0034] Figure 4 A schematic cross-section of a microdisplay using OLED microcavities according to the present application is shown. 100

[0035] Figure 5 A cross-section of a first OLED microcavity device is shown. 200

[0036] Figure 6 A cross-section of a second OLED microcavity device is shown. 250

[0037] Figure 7 A cross-section of a third OLED microcavity device is shown. 300

[0038] Figure 8 A cross-section of a fourth OLED microcavity device is shown. 400

[0039] Figure 9 Spectra of experimental examples Al-A4 in the blue emission region are shown.

[0040] Figure 10 Spectra of experimental examples Bl and B2 in the blue emission region are shown.

[0041] Figure 11 Spectra of experimental examples Cl-C6 in the blue emission region are shown. DETAILED DESCRIPTION

[0042] ​​​​​For purposes of the present disclosure, the term "over" or "above" means that the structure involved is positioned above another structure, i.e., on the side opposite the substrate. "Uppermost" or "upper" refers to the side or surface furthest from the substrate, while "lowermost" or "bottom" refers to the side or surface closest to the substrate. "Interior" or "inner" refers to the side or surface of a layer that is closest to the organic layer within the microcavity. Unless otherwise noted, "over" is to be interpreted that the two structures can be in direct contact or that intervening layers can be present between them. A "layer" is to be understood as a layer having two sides or surfaces (uppermost and lowermost), and multiple layers can be present without being limited to a single layer.

[0043] R represents a layer that emits primarily red light (> 600 nm, ideally in the range of 620-660 nm), G represents a layer that emits primarily green light (500-600 nm, ideally in the range of 540-565 nm), and B represents a layer that emits primarily blue light (< 500 nm, ideally in the range of 440-485 nm). The selection of these ranges depends on the ideal color gamut, visual response, and available emitters, and not on the optics of the microcavity. The microcavity is selected to optimally support the ideal wavelength range.

[0044] Notably, the R, G, and B layers can produce some light outside the indicated ranges, but the amount produced is always less than the primary color. Y (yellow) represents a layer that emits a substantial amount of R and G light and much less B light. "LEL" means light emitting layer. Unless otherwise noted, wavelengths are expressed in vacuum values, not in situ values.

[0045] A multi-mode OLED produces more than one color of light. Ideally, a multi-mode OLED produces white light with approximately equal amounts of R, G, and B light. Typically, this corresponds to a CIE x value of 0.33 and a CIE y value of 0.33. Even if equal amounts of R, G, B light are not included, white light can typically be produced in an OLED by having three separate R, G, and B light emitting layers, two separate light emitting layers such as blue and yellow, or even a single white light emitting layer. However, using a light emitting layer with a mixed emission (e.g., a Y layer) typically results in an inherent efficiency loss since the total emission is divided into different channels. Using different color emitters in a single layer can be problematic because the emitter with the lower emission energy will dominate.

[0046] The theory behind the microcavity effect is well known and can be modeled to predict the optimal location within the microcavity to maximize constructive interference for any particular wavelength. See, for example, Grant R. Fowles, Holt Reinhart and Winston, Introduction to Modern Optics, Inc., 1968, ISBN 0-03-065365-7; O. H. Crawford, J. Chem. Phys. 89(10), 6017-6027 (1988); Dodabalapur et al., App Phys Let, 64(19) 2486 (1994); and Dodabalapur et al., App Phys Let, 65(18) 2308 (1994).

[0047] In general, modeling of OLED microcavities shows that the maximum emission efficiency will be found if the light is produced at an anti-node at an odd multiple of a quarter wavelength between the reflective and semi-transparent surfaces of the microcavity as any phase shift of the two reflectors is adjusted. Since the wavelength is different, the anti-nodes for different "colors" will be at different locations within the microcavity. In this context, a quarter wavelength refers to the wavelength within the medium of the device, not the vacuum wavelength. In other words, theory predicts that the emission layer should be located at a particular distance between the defining surfaces of the microcavity (i.e., at an anti-node) to maximize the microcavity effect for efficiency depending on the emission wavelength.

[0048] It is important to understand that microcavity theory relies on optical distance, which can be different from physical distance. Optical distance is the product of physical distance and refractive index, which can be wavelength dependent. For example, for typical organic non-emissive OLED materials, the refractive index is about 1.906 @ 400 nm, 1.85 @ 450 nm, 1.80 @ 500 nm, 1.77 @ 550 nm, 1.75 @ 600 nm, and 1.73 @ 650 nm. However, since all the OLED materials within the microcavity will have similar refractive indices, and have similar wavelength dependence, the physical distance will correspond proportionally to the optical distance within a tolerable range. For the purposes of this application, physical distance will be used in place of optical distance for all microcavity distances, unless otherwise indicated.

[0049] OLED emitters tend to be relatively wide and have a large amount of emission at wavelengths near the maximum wavelength. Figure 1 The spectra of typical B, G, and R OLED emitters in a non-microcavity environment are shown. For these particular emitters, B λ max = 456 nm, G λ max = 540 nm, and R λ max= 620 nm. Based on these examples, the integration ranges used in the calculations are 562-682 nm for red light, 486-594 nm for green light, and 410-498 nm for blue light, which represent a center wavelength of 620 nm, 540 nm, and 456 nm, respectively, with a bandwidth of + / - 10%.

[0050] The total thickness of the microcavity will have an effect on the wavelength of the emitted light. For example, Figure 2 The calculated positions of the anti-nodes (higher intensity) and nodes (lower intensity) of a single theoretical white light emitter in a 695 nm microcavity are shown (as a function of distance from the cathode to the midpoint of the emissive layer), where the cathode and anode are the reflective and semi-reflective electrodes, respectively. The output has been normalized to the normal output of a white light emitter embedded in glass to air. Within a microcavity of this size, there are 4 high intensity positions for red light, 5 high intensity peaks for green light, and 6 high intensity peaks for blue light. In addition, the model shows that the preferred intensity is approximately similar for red and green light in the microcavity, but a higher intensity is predicted closer to the anode for blue light.

[0051] Since the light within the microcavity forms a standing wave based on the optical distance of the microcavity, there are multiple nodes where the optimal enhancement occurs. Figure 3 The wavelengths of light that are enhanced within a 6950 A microcavity are shown. Figure 3 Each curve (not individually identified) represents the predicted emission of a theoretical white light emitter placed at a distance of a multiple of 40 nm from the reflective electrode at each wavelength. Figure 3 The peaks in the envelope of the curves of Figure 2 show the wavelengths that can be enhanced if the appropriate emitter position is chosen. The wavelengths at the bottom of the envelope of the curves cannot be enhanced for any emitter position.

[0052] The ideal overall size or width of a microcavity is determined by four factors: the ideal emission output (which can be modulated by the use of color filters), the spectrum (intensity versus wavelength) of the light emitter within the LEL, the thickness of the LEL, and the driving voltage (which increases with the thickness of the device and the number of LELs present). It is important to note that even if all of the internal components within a microcavity are the same, microcavities of different sizes will have different emission outputs. This is due to the fact that the microcavity effect depends not only on the overall size of the microcavity, but also on the location within the microcavity where the photons of a given wavelength are generated. All of these factors, as well as the necessary tradeoffs in their combination, must be considered when designing an efficient OLED microcavity.

[0053] However, it is not always possible to position the emission layers at the ideal locations according to the microcavity theory. This is because one also needs to manage hole / electron recombination within each light-emitting layer, manage charge transfer through the organic layer between the electrodes, and prevent undesirable interaction between the various light-emitting layers. All of this is managed by using different materials in the different layers; these layers need to be of a certain thickness to achieve their purpose. This can be particularly difficult when using two separate blue light layers, as they will occupy different locations, such that the microcavity effect is maximized for each. Compromises can need to be made to balance these requirements.

[0054] Since the multi-mode OLED microcavity will include at least two blue light LELs and at least one LEL of another color of light (non-blue), the dimensions of the microcavity need to be large enough to accommodate multiple anti-nodes of light for each color, such that all of the necessary LELs can be spaced as needed to maximize intensity. This also leaves room for non-light-emitting layers to help maintain charge flow through the device and prevent undesirable cross-talk between the different color light LELs or between different lateral areas in the device.

[0055] There are two different desired ranges for the thickness L0 of the multi-mode microcavity (the distance between the upper surface of the reflective layer and the lower surface of the semi-transparent electrode). This is because, according to the microcavity theory, each range can accommodate a different number of anti-nodes in an effective manner for each of the R, G, and B colors. The first desired range for L0 is 6000-8000 A, and a more desirable range is 6500-7800 A. The second desired range for L0 is 7500-9500 A, and a more desirable range is 8000-9000 A. Of these two ranges, the first range is preferred.

[0056] Figure 4 A microdisplay using a multi-mode OLED microcavity according to the present application is shown 100 . Microdisplay 100 comprising a substrate 1 and a control circuit layer 3 comprising transistors, connecting traces, and other necessary components forming a control circuit that will supply power to the sub-pixels according to input signals. Above the layer 3 with transistors and control circuit, there can be an optional planarization layer 5. The substrate 1 / control circuit layer 3 with transistors / optional planarization layer 5 is often referred to as the backplane.

[0057] The individual first electrode portions (also called deposition layers) 9, connected by electrical contacts (also called layers) 7, are above layer 5 (if present), the electrical contacts 7 extending through the optional planarization layer to make the individual electrode portions 9 electrically contact the control circuit in layer 3. In this embodiment, the first electrode portions 9 have two layers: a reflective layer 9B closer to the substrate 1 and a first electrode layer (also called first electrode portion) 9A closer to the OLED layers. The individual first electrode portions 9 are laterally electrically insulated from each other. Non-light-emitting OLED layers 11, such as electron or hole injection layers or electron or hole transport layers, are above the segmented first electrode portions 9. This embodiment includes a first blue light-emitting layer and a second blue light-emitting layer. The second blue light-emitting layer 13 is above the OLED layers 11. Layer 15 is a charge generation layer, between and separating the second blue light-emitting layer 13 and a non-blue light-emitting layer 17. A charge generation layer 19 is above the non-blue light-emitting layer 17, between and separating the non-blue light-emitting layer 17 and a first blue light-emitting layer 21. The physical distance between the midpoint of the second blue light-emitting layer 13 and the first blue light-emitting layer 21 is a distance LI. Non-light-emitting OLED layers 23, such as electron or hole transport layers or electron or hole injection layers, and a semi-transparent second electrode 25 are above the first blue light-emitting layer 21. This forms an OLED microcavity 30 with a distance L0, extending from the upper surface of the reflective layer 9B to the lower surface of the semi-transparent second electrode 25, which is also a semi-reflective electrode. The OLED microcavity is protected from the environment by an encapsulation layer 27. In this embodiment, there is a color filter array with color filters 29B, 29G, and 29R, which filters the multi-mode emission produced by the OLED microcavity 30 so that B, G, and R light is emitted depending on the electrical power supplied to the underlying electrode portions 9.

[0058] In an OLED micro-display 100 , the first electrode layer 9A and the reflective layer 9B are segmented, i.e., divided into electrically insulated individual portions corresponding to sub-pixels and are not continuous over the active light-emitting surface. However, in some embodiments where the reflective layer is not electrically conductive, the reflective layer does not have to be segmented, but can extend continuously under the isolated portions of the first electrode layer 9A across the active light-emitting area. In this application, the electrical contacts 7 will go through the reflective layer 9B to connect the first electrode portions 9A with the control circuit in layer 3. When the reflective layer 9B is electrically conductive and segmented, the electrical contacts 7 only need to directly contact the reflective layer 9B segments. The sub-pixel layout or segmented electrode layout is not critical, but all known layouts can be used.

[0059] A micro-display such as a micro-display 100 should have a size of no more than 100 cm 2(i.e. a rectangle of 20cm x 5cm or a square of 10cm x 10cm) and is expected to be no more than 24cm 2 (i.e. a rectangle of 6cm x 4cm). Typically the viewing distance is expected to be no more than 20cm and is expected to be no more than 15cm. The micro display can have any shape, including square, rectangular, circular, oval and irregular.

[0060] A micro display such as micro display 100 A micro display such as micro display

[0061] A micro display such as micro display 100 is opaque and is top emitting, i.e. light is emitted from the side opposite to the substrate 1 without passing through the substrate 1. This is due to the presence of the control circuit layer 3 which is opaque, located above the substrate and below the OLED microcavity. Therefore, the OLED microcavity has to be arranged to emit light by top emission as there is an opaque layer between the light emitting layer and the substrate. However, by creating transparent areas between the opaque control circuit areas, a micro display such as micro display 100 may be made at least partially transparent so that at least some light can pass through the micro display.

[0062] In Figure 4 , the physical distance L0 of the microcavity 30 is constant throughout the active light emitting area. In particular, the thickness of the microcavity 30 is not adjusted based on the colour of the light of the sub-pixel. In a micro display using the OLED microcavity of the present application, all sub-pixels will have the same microcavity thickness L0.

[0063] Figure 5 A cross-section of a top-emitting OLED microcavity device according to the present application is shown 200 200 includes two non-blue light emitting layers. On a substrate 50, there are, in order, an unsegmented reflective layer 52, an unsegmented first electrode layer 54, a hole injection layer 56, a hole transport layer 58, a second non-blue light emitting layer 60, an electron transport layer 62, a charge generation layer 64, a hole transport layer 66, a second blue light emitting layer 68, an electron transport layer 70, a charge generation layer 72, a hole transport layer 74, a first non-blue light emitting layer 76, an electron transport layer 78, a charge generation layer 80, a hole transport layer 82, a first blue light emitting layer 84, an electron transport layer 86, an electron injection layer 88, a semi-transparent (and semi-reflective) second electrode 90, and a final encapsulation 92. There can be other layers not shown between the various identified layers. The distance L0 of the microcavity 95 is defined by the upper surface of the reflective layer 52 and the lower surface of the semi-transparent second electrode 90. L1 is the physical distance between the midpoint of the second blue light emitting layer 68 and the midpoint of the first blue light emitting layer 84. L2 is the physical distance from the midpoint of the first blue light emitting layer 84 to the upper surface of the reflective layer 52. L3 is the physical distance between the midpoint of the second blue light emitting layer 68 and the upper surface of the reflective layer 52.

[0064] microdisplay 100 200 The substrate 1 of a microdisplay or the substrate 50 of an OLED microcavity device can be silicon, glass (including flexible glass), metal, or a polymeric material. In general, it will be flat and of uniform thickness. For a bottom-emitting OLED, the substrate should be transparent. For a top-emitting OLED, the substrate can be opaque or transparent (allowing for dual-sided emission) as desired. The top surface of the substrate faces the OLED. Since the substrate will be part of the overall encapsulation of the OLED, it should be sufficiently air and water tight so that the OLED will have a desirable lifetime. The substrate can be rigid or flexible. The substrate can have various types of sub-layers (i.e., planarization layers, light management layers, etc.), which can be patterned or unpatterned, and which can be on the top surface or the bottom surface.

[0065] ​​The uppermost internal reflective surface of the reflective layer 52 defines the first side of the microcavity 95. The reflective layer 52 can be a reflective metal such as Al, Au, Ag, Mg, Cu or Rh or alloys thereof, a dielectric mirror or a high-reflectance coating. A dielectric mirror is composed of multiple thin layers of materials such as magnesium fluoride, calcium fluoride and various metal oxides deposited on a substrate. A high-reflectance coating is composed of multiple layers of two materials, one with a high refractive index such as zinc sulfide (n = 2.32) or titanium dioxide (n = 2.4) and the other with a low refractive index such as magnesium fluoride (n = 1.38) or silicon dioxide (n = 1.49). The thickness of these layers is typically a quarter of the wavelength of the light being reflected. Ideally, the reflective layer reflects at least 80% of the incident light, and most preferably, at least 90% of the incident light. The preferred reflective layers are Al or Ag, with a thickness of 300-2000 A, and most preferably 800-1500 A.

[0066] In an OLED microcavity device 200 the first (bottom) electrode layer 54 is unsegmented, i.e., it is continuous and unpartitioned throughout the active light-emitting area. Thus, the OLED microcavity device 200 is a large-area light emitter suitable for illumination and backlight applications rather than displays. In the OLED microcavity 95 in Figure 5 the physical distance Lo from the reflective surface to the semi-transparent second electrode is constant throughout the active light-emitting area.

[0067] In a microdisplay 100 and an OLED microcavity device 200 the first electrode portion 9A or first electrode layer 54 can be an anode or a cathode and can be transparent, reflective, opaque or semi-transparent. In applications where the first electrode is above the reflective layer, it should be transparent. However, in other applications, the first electrode layer 9A, 54 is reflective such that its uppermost reflective surface forms one side of the optical microcavity 95.

[0068] Ideally, the first electrode is a transparent anode and should transmit as much visible light as possible, preferably with a transmittance of at least 70% or more ideally at least 80%. Although the first transparent electrode can be made of any conductive material, a thin layer of a metal oxide such as ITO or AZO or a metal such as Ag is preferred. A poorly conductive material such as TiN can be used for the first electrode layers 9A and 54 as long as it is thin.

[0069] The minimum thickness of the reflective layer and / or the first electrode is determined by the desired reflectivity and conductivity. For small pixels (nm), conductivity is not an issue. For large-area devices (mm to cm), there must be no voltage drop across the electrodes, as this will cause variations in device brightness. Another factor to consider is having a layer with properties similar to a bulk material. In the case of Ag, uniformity must be ensured because it will aggregate when it is too thin. This would be problematic if the layer were only a few atomic layers (20 to 50 Å). When the anode and reflective layer are the same layer, it is ideally made of a metal comprising Al, Au, Ag, or Mg or alloys thereof and having a thickness of at least 30 nm, ideally at least 60 nm.

[0070] Reflective anodes may require a thin additional coating of inorganic material to help improve hole transport across the reflective anode / organic layer interface, or to protect the reflective metal surface during handling and cleaning. When the transparent anode is located above the reflective surface, it is part of the optical cavity.

[0071] Electron and hole transport materials suitable for non-emitting layers (such as hole injection layer 56, hole transport layers 58, 66, 74, 82, electron injection layer 88, or electron transport layers 62, 70, 78, 86) are well-known and commonly used. These layers can be mixtures of such materials and can include dopants to modify their properties. Because they are non-luminescent, they do not contain luminescent materials and are transparent. Choosing a suitable material is not critical, and any material can be selected based on its properties.

[0072] Since the spacing between the individual LELs within a microcavity and the size of the microcavity are important for maximizing efficiency, it is typically necessary to select the thickness of each non-light-emitting layer to provide ideal spacing. Ideally, the spacing between LELs and the size of the microcavity are adjusted by using hole transport layers of appropriate thickness (such as 58, 66, 74, or 82).

[0073] exist Figure 5 In the illustrated embodiment, a second non-blue light emitting layer 60 is present, located closer to the reflective surface 52 than the second blue light emitting layer 68. In some embodiments, the second non-blue LEL 60 may not be included, and it is optional. However, it is ideal to include the second non-blue LEL 60.

[0074] The light emitting layer typically has a host material (or mixture of host materials) and a light emitting compound, the host material being the main component of the layer. In this case, the non-blue light emitting compound emits light having a main emission > 500 nm. There can also be a small amount of blue light emission, ideally less than 20% of the maximum emission > 500 nm. The disadvantage of blue light emission in the non-blue light layer is that the blue light emission is not as efficiently enhanced as the non-blue light wavelengths because the position of the emitter is chosen for maximum efficiency of non-blue light emission. Ideally, the non-blue light emitting compound is phosphorescent because they have higher efficiency.

[0075] In many embodiments, ideally the second non-blue LEL closest to the reflective layer primarily emits red light. Suitable host materials and non-blue light emitting materials (such as R emitting phosphorescent compounds) for the light emitting layer (such as 60) are well known and commonly used. It is not critical to select the appropriate materials and any material can be selected based on their performance and emission characteristics. When using phosphorescent emitters, it is sometimes necessary to confine the excitons produced by the phosphorescent emitters within the layer. Thus, if desired, an exciton blocking layer can be used on one or both sides of the phosphorescent LEL. Such materials and their use are well known.

[0076] The second blue light emitting layer 68 is over the second non-blue light emitting layer 60 and is separated from the second non-blue light emitting layer 60 by the ETL 62 / CGL 64 / HTL 66. In Figure 5 In embodiments of the application, the second blue LEL 68 is on the side of the second non-blue light emitting layer 60 opposite the reflective surface. The midpoint of the second blue light emitting layer 68 is located at a distance L3 from the upper surface of the reflective surface. The ratio L3 / L0 should be in the range 0.52 - 0.64, or more preferably in the range 0.56 - 0.60.

[0077] The blue light emitting layer typically includes a host material (or mixture of host materials) and a blue light emitter. As noted previously, a blue phosphorescent emitter would be highly suitable, but the examples known to date do not have sufficient stability. Even if phosphorescent blue light emitters with sufficient stability were available, their efficiency can be lower than that of the R and B phosphorescent emitters, so their availability can not by itself solve the problem of insufficient blue light from multi-mode OLED microcavities. For this reason, fluorescent and TADF (thermally activated delayed fluorescence) blue light emitters are particularly useful for the present invention. Although B emission is typically < 500 nm, the preferred range of blue light emission from blue light emitting materials in a non-microcavity environment is 440-485 nm, more preferably 445-475 nm, and most preferably 450-470 nm. These preferred wavelength ranges are determined by the available blue light emitters, the visual response of the eye (longer wavelengths appear brighter), and the color gamut requirements of the device (shorter wavelengths give a larger color gamut). Host materials and fluorescent and TADF blue light emitting materials suitable for use in light emitting layers such as 68 and 84 are well known and commonly used. Selection of suitable materials is not critical, and any material can be selected based on their performance and emission characteristics.

[0078] The first non-blue light emitting layer 76 is over the second blue light emitting layer 68 and separated from the second blue light emitting layer 68 by the ETL 70 / CGL 72 / HTL 74. Figure 5 In embodiments of the application, the second blue light emitting layer 68 is between the second non-blue light emitting layer 60 and the first non-blue light emitting layer 76 and separated from each other by HTL / CGL / ETL layers.

[0079] In terms of color emission or scheme, the first non-blue LEL 76 can be the same as or different from the second non-blue LEL 60 (if present). For example, the LELs 60 and 76 can both emit R light or both emit G light, where the internal components and materials can be the same or different. For example, they can use the same composition to have the same G emission, or different compositions to emit green light, so the maximum wavelength of the G emission is not the same. Alternatively, the LELs 60 and 76 can emit different colors of light, for example, one emits G light and the other emits R light. Other combinations of light emission are possible, for example, the first LEL 76 emits G light and the second LEL 60 emits Y light.

[0080] In many embodiments, it is desirable that the first non-blue LEL (such as 76) located between the first and second blue light emitting layers primarily emits green light. More preferably, the first LEL 76 emits G light and the second LEL 60 (if present) emits R light. Host materials and non-blue light emitting materials (such as G light emitting phosphor compounds suitable for use in light emitting layers such as 76) are well known and commonly used. It is not critical to select the appropriate materials, and any material can be selected based on their performance and emission characteristics.

[0081] The first blue light emitting layer 84 is over the first non-blue light emitting layer 76 and is separated from the first non-blue light emitting layer 76 by the ETL 78 / CGL 80 / HTL 82. In Figure 5 In embodiments of the application, the first blue LEL 84 is located on the side of the first non-blue light emitting layer 76 opposite the second blue LEL 68. In this arrangement, the second blue LEL 68 is closer to the reflective surface 52 and the first blue LEL 84 is further from the reflective surface and closer to the semi-reflective second electrode, with the first non-blue LEL 76 in between.

[0082] Ideally, the ratio L1 / L0 should be in the range of 0.30-0.40 to maximize the efficiency of both BLELs. More ideally, the ratio L1 / L0 should be in the range of 0.32-0.38. The spacing between the first blue LEL 84 and the second blue LEL 68 is important as it maximizes the efficiency of both layers in the microcavity environment. According to theory, the two BLELs should be located at different anti-nodes (spaced an odd multiple of a quarter wavelength, assuming a blue wavelength of 460 nm, an odd multiple of 115 nm) to maximize their respective efficiencies. However, as discussed previously, the size of the microcavity is important in order to be able to accommodate the different color LELs according to the preferred locations. Thus, the appropriate spacing between the two BLELs will depend on the size of the microcavity and thus will correspond to the ratio L1 / L0.

[0083] The ratio L2 / L0 should be in the range of 0.92-0.98, or more preferably, in the range of 0.93-0.96. This ensures that the first blue LEL is located near the electrode, which is desirable for high blue light emission (see Figure 2 ).

[0084] In terms of color emission or scheme, the first blue LEL 84 can be the same or different from the second blue LEL 68. For example, they can use the same composition to have the same B emission, or different compositions to emit blue light such that the maximum wavelength or half bandwidth of the B emission is not the same.

[0085] In many embodiments, it is desirable for the first B LEL 84 and the second B LEL 68 to have the same scheme and to emit at the same maximum blue wavelength. For the second blue LEL 68, host materials and blue light emitting materials such as fluorescent and TADF compounds suitable for light emitting layers such as 84 are well known and commonly used. It is not critical to select the appropriate materials, and any material can be selected based on their performance and emission characteristics.

[0086] It is desirable to position the first non-blue LEL 76 between the second blue light emitting LEL 84 and the first blue light emitting 68, as this allows for space to be preserved in the multi-mode OLED microcavity. Positioning a non-blue LEL that emits light very different from the blue LELs between them can effectively utilize this space.

[0087] Although not shown in Figure 5 , it is optionally possible to include an additional non-blue light emitting layer closer to the semi-reflective electrode than the first B LEL 84. This optional non-blue light emitting layer can emit G, R, or Y light as desired. Such a layer can have any of the same schemes as previously described for the first non-blue LEL or the second non-blue LEL. It is also possible to include one or more additional blue light emitting layers closer to the semi-reflective electrode than the first B LEL 84. Such layers can have any of the same schemes as previously described for the first B LEL or the second B LEL.

[0088] The second electrode 90 is semi-transparent and semi-reflective, i.e., it reflects some of the light and transmits the rest. The lowermost inner surface of the second electrode 90 defines the second side of the microcavity 95 with a physical distance of L0. Desirably, the semi-transparent upper electrode 90 reflects at least 5% of the light emitted by the LELs, more desirably, at least 10% of the light emitted by the LELs in order to establish the microcavity effect.

[0089] The thickness of the semi-transparent second electrode is important as it controls the amount of reflected light and the amount of transmitted light. However, the semi-transparent second electrode cannot be too thin as it can not effectively transport charge into the OLED or be affected by pinholes or other defects. The thickness of the ideal upper electrode layer is 100-200 A, and more desirably 125-175 A.

[0090] The ideal top electrode is a thin layer of metal or metal alloy. Suitable metals include Ag, Mg, Al, and Ca or alloys thereof. Of these, Ag is preferred because of its relatively low absorption of blue light. One problem with a semi-transparent electrode absorbing blue light is that many OLED materials absorb some visible light, particularly blue light, and each reflection within the microcavity permanently removes some photons. As an example, a 125 A semi-transparent top electrode of Ag reflects 15% of blue light (420 nm) and 39% of red light (650 nm), and 250 A of Ag reflects 43% of 420 nm light and 72% of 650 nm light. An ideal alloy is Ag / Mg (ratio 9:1 to 1 :9). To aid electron transport and stabilization, there can be an adjacent transparent metal oxide layer on the electrode surface, such as ITO, InZnO, or MoO3. Optionally, a metal halide such as LiCl, an organometallic oxide such as lithium quinolate, or other organic material can be used.

[0091] There can be a protective or spacer layer (not shown in FIG. 1) over the top electrode to prevent damage during encapsulation. Figure 5

[0092] An encapsulation 92 is deposited or placed over the top electrode 90 and any optional protective layer, if present. The encapsulation should at least completely cover the top and side light emitting areas and be in direct contact with the substrate. The encapsulation should be air and water tight. It can be transparent or opaque. It should be electrically non-conductive. It can be formed in place or added as a separate pre-formed sheet with a means of sealing the side edges.

[0093] An example of in-place formation is a thin film encapsulation. Thin film encapsulation involves the deposition of multiple layers of replacement layers of inorganic materials and polymer layers until the desired degree of protection is achieved. Formulations and methods of forming thin film encapsulations are well known and any formulation and method can be used as desired.

[0094] ​Optionally, a pre-formed sheet or cover can be used to provide encapsulation, which is attached at least over the sealing and enclosed areas. The pre-formed sheet can be rigid or flexible. It can be made of glass (including flexible glass), metal, or an organic / inorganic barrier layer. It should have a coefficient of thermal expansion close to that of the substrate for a stronger bond. Air- and water-resistant adhesives such as silicone or epoxy adhesives may be required, or the pre-formed encapsulation sheet may be attached over the sealing area by thermal means such as ultrasonic welding or glass fusing, which may require additional sealants such as solder or glass fusing. The side and bottom edges of the cover can be specially designed to better fit the sealing area or promote a better seal. The cover and sealing area can be designed together so that they partially fit or lock into place before forming a seal. Furthermore, the cover can be pretreated to promote better adhesion to the sealing area.

[0095] Can Figure 5 The most preferred embodiment shown is designated as a top-emission device (according to the LEL sequence within the microcavity): substrate / reflective layer / anode (first electrode) / R (second non-blue LEL) / BLEL2 (second BLEL) / G (first non-blue LEL) / BLEL1 (first BLEL) / cathode (second electrode) device. However, other top-emission embodiments will include (between the first and second electrodes): G / BLEL2 / R / BLEL1, Y / BLEL2 / G / BLEL1, G / BLEL2 / Y / BLEL1, G / BLEL2 / G / BLEL1, R / BLEL2 / R / BLEL1, Y / BLEL2 / Y / BLEL1, Y / BLEL2 / R / BLEL1, R / BLEL2 / Y / BLEL1, BLEL2 / G / BLEL1, BLEL2 / R / BLEL1, BLEL2 / YBLEL1, Y / BLEL2 / BLEL1, and BLEL2 / BLEL1 / Y.

[0096] However, as Figure 5As shown, when the first electrode of the OLED microcavity is unsegmented, and therefore the device is a large-area light emitter, the OLED microcavity can also be a bottom light emitter, i.e., light is emitted through the substrate. In some embodiments, the ideal first electrode is a semi-transparent electrode (especially a cathode) and the second electrode (especially an anode), wherein the reflective surface is above the top of the OLED microcavity. Those skilled in the art will understand that for this arrangement, the order of the hole transport layer and the electron transport layer will necessarily be reversed. The ideal bottom-emitting device according to the invention can be designated as (according to the LEL order within the microcavity): (transparent) substrate / cathode (second electrode) / BLEL1 (first BLEL) / G (first non-blue LEL) / BLEL2 (second BLEL) / R (second non-blue LEL) / anode (first electrode) / reflective layer device. However, other bottom-emission implementations will include (between the first and second electrodes): BLEL1 / R / BLEL2 / G, BLEL1 / G / BLEL2 / Y, BLEL1 / Y / BLEL2 / G, BLEL1 / R / BLEL2 / Y, BLEL1 / Y / BLEL2 / R, R / BLEL1 / R / BLEL2, G / BLEL1 / G / BLEL2, Y / BLEL1 / Y / BLEL2, BLEL1 / G / BLEL2, BLEL1 / R / BLEL2, Y / BLEL2 / BLEL1, BLEL2 / BLEL1 / Y, and BLEL1 / Y / BLEL2.

[0097] Figure 6 A top-emitting OLED microcavity device according to the present invention is shown. 250 A cross-section of one embodiment. Except for the relative positions of the second blue light emitting layer 68 and the first non-blue light emitting layer 76 being interchanged, it is similar to... Figure 5 The implementation shown is similar. Furthermore, there are no other changes. In this implementation, the two blue light emitting layers are not separated by a non-light emitting layer. However, the distance L1 between the midpoint of the second blue light emitting layer 68 and the midpoint of the first blue light emitting layer 84 is adjusted to be similar to... Figure 5 The same implementation method ensures that the ratio L1 / L0 is the same in both implementations. For example, L1 can be increased by increasing the thickness of the hole transport layer 82 by an appropriate amount, and L3 can be decreased by decreasing the thickness of the hole transport layers 66 and 58 by the same amount, to maintain a constant microcavity distance L0. L2 in 200 and 250 The two are the same.

[0098] Figure 7 The image shows an OLED microcavity device. 300 (similar to) Figure 5). In this embodiment, an additional HTL 99 / CGL 98 / ETL 96 / third blue light emitting layer 94 stack is added between layers 58 (HTL) and 60 (second non-blue LEL). The other layers are unchanged and can be formulated in the same manner as described for the similar layers in Figure 5 The additional HTL, CGL, ETL, and third blue LEL can be formulated in the same manner as described for the similar layers in 200 . The top emitting OLED microcavity has three blue LELs and two non-blue LELs. The first non-blue LEL 76 is located between the second blue LEL 68 and the first blue LEL 84, and the second non-blue LEL 60 is located between the second blue LEL 68 and the third blue LEL 94. As described for the OLED microcavity device 300 , the G and R LELs can be interchanged or changed to Y LELs. Further, the OLED microcavity device 200 can be reformulated as a bottom emitting embodiment as described for the OLED microcavity device . In other embodiments, the third blue LEL can be located between the second non-blue light emitting layer (if present) and the second blue light emitting layer. However, in embodiments where the second non-blue LEL is not present, the second and third blue LELs will not be separated by a light emitting layer.

[0099]

[0100] Figure 8 An OLED microcavity device 400 is shown (similar to Figure 7 ). In this embodiment, an additional HTL 107 / CGL 105 / ETL 103 / fourth blue light emitting layer 101 stack is added between layers 58 (HTL) and 94 (3 BLEL). The other layers are unchanged and can be formulated in the same manner as described for the similar layers in Figure 5 . This top emitting OLED microcavity has four blue LELs and two non-blue LELs, with the first non-blue LEL 76 located between the second blue LEL 68 and the first blue LEL 84, and the second non-blue LEL 60 located between the second blue LEL 68 and the third blue LEL 94. As shown in this embodiment, there are no non-blue LELs between the 3 BLEL (99) and the 4 BLEL (101). However, a third non-blue LEL can optionally be added between the third BLEL and the 4 BLEL. For example, the OLED microcavity can have the following structure: substrate / reflective layer / BLEL4 / Y / BLEL3 / R / BLEL2 / G / BLEL1 / cathode. As described for the OLED microcavity device 300As described, in the OLED microcavity device 400 G and RLEL can be interchanged or changed to YLEL. Further, the OLED microcavity device 400 may be reformulated as described for the bottom emission embodiment of the OLED microcavity device 200 described.

[0101] L5 is the physical distance between the midpoint of the third blue light emission layer 94 and the midpoint of the fourth blue light emission layer 101. Ideally, L5 / L0 is in the range of 0.10-0.20, more preferably in the range of 0.13-0.16.

[0102] Due to the number of stacked light emission layers and the need to minimize voltage in OLEDs having multiple light emission layers, it is often desirable to include a charge generation layer (CGL) (sometimes also referred to as a connector or interlayer) to minimize the voltage increase when using multiple LEL stacks in embodiments such as Figures 5-8 . In particular, it is desirable for the CGL to be located between each light emission layer within the stack, and more ideally, the CGL has an HTL on one side and an ETL on the opposite side. It is generally not necessary to position a CGL between a light emission layer and an electrode, but in special cases, the application of such a CGL can be advantageous.

[0103] The charge generation layer (which can consist of more than one layer) has the ability to generate both holes and electrons. Typically, the CGL has an n-layer (which can be doped with an n-dopant that generates electrons) and a p-layer (which can be doped with a p-dopant that lacks electrons), the n-layer can generate electrons and the p-layer can accept electrons when subjected to an electrical bias. In some cases, the n-layer and the p-layer can be separated by a thin interlayer. Typically, the CGL is arranged so that the n-layer is closer to the anode and the p-layer is closer to the cathode.

[0104] An ideal solution for a CGL has three layers: an electron transport material doped with an n-dopant (e.g., Li), a thin interlayer of the same electron transport material, and a hole transport material doped with a p-dopant. Suitable electron transport materials and hole transport materials, as well as n-dopants and p-dopants suitable for use in CGLs such as 64, 72, and 80, are well known and commonly used. The above-mentioned materials can be organic or inorganic. It is not critical to select the appropriate materials, and any material can be selected based on their performance.

[0105] Ideally, the thickness of the CGL should be in the range of 200-450 A. In many cases, the CGL will have an ETL on the anode side and an HTL on its cathode side to help improve charge transport and help separate the charge generation dopants (if present) from the LELs.

[0106] Simulation results

[0107] It is an object of the present invention to increase the amount of blue light emission in a multi-mode OLED microcavity, where the microcavity has a single and constant thickness throughout the light emitting area. Not only is the relative distance between the B LELs to each other and the relative distance of the B LELs to the reflective layers important for obtaining high blue light efficiency, but also other factors such as the relative order of the LELs and the properties of the semi-transparent electrodes are important for maximizing the blue light emission.

[0108] The total thickness of the microcavity will have an effect on the wavelength of the emitted light. As shown in Figure 1 , OLED emitters tend to be relatively broad and have a large amount of emission at wavelengths around the maximum wavelength. However, in a microcavity environment, the wavelength of the maximum emission can be different from the maximum emission in a non-microcavity. This is because the microcavity effect acts at every wavelength and thus, depending on the dimensions of the microcavity, the emission is enhanced at some wavelengths and diminished at others. This can result in a shift of the wavelength of the maximum emission and cause a change in the overall spectral shape. Since a single multi-mode microcavity cannot be individually optimized for each color of light, some shift in the wavelength of the maximum emission of each color can be expected.

[0109] Table A shows the simulated effect of microcavity thickness on the wavelength of the enhanced emission.

[0110] Table A - predicted enhanced wavelengths

[0111]

[0112] 1 in A

[0113] 2 in nm, referring to color regions

[0114] In this range of dimensions of the microcavity, the preferred wavelengths of blue, green and red light (as shown in Figure 1 ) are enhanced, while less desirable wavelengths such as 480 nm and 580 nm (not close to the main saturated color wavelengths of a display) are not enhanced. As the microcavity length increases, the peak of the enhanced wavelengths will continuously shift to longer wavelengths and there can be additional peaks. Based on the simulations, there are two different ranges of microcavity dimensions that are particularly suitable for enhancing the wavelengths of the desired R, G and B light, the first desired range being about 6500-7800 A and the second desired range being about 8000-9000 A.

[0115] Therefore, in order to maximize the overall efficiency, the appropriate cavity length must first be determined, where the peak of the enhanced wavelengths is in the desired R, G and B range, and then the positions of the individual emitters within the microcavity are determined.

[0116] Table B shows the predicted optimal positions for blue LELs for maximum enhancement in a 6950 A microcavity. Since the LELs have finite thickness, these positions are based on the distance of the midpoint of the LEL to the upper surface of the reflective layer. In the calculations, physical distances are used to describe the thickness of the multi-mode microcavity and the relative position of the LELs. Note that the quarter wavelength for a typical blue light emitter (460 nm) is about 115 nm.

[0117] Table B - Predicted optimal positions for blue LELs in a 6950 A microcavity

[0118]

[0119] 1 Distance in nm from the partially transparent surface

[0120] 2 Distance in nm from the reflective surface

[0121] 3 Normalized

[0122] As shown in Table B, the calculated positions for maximum enhancement are about 121-125 nm apart, and the ratio of (distance between nodes) / (microcavity length) is between 0.17-0.18.

[0123] Table C shows the predicted optimal positions for blue LELs for maximum enhancement in a 8450 A microcavity.

[0124] Table C - Predicted optimal positions for blue LELs in a 8450 A microcavity

[0125]

[0126] 1 Distance in nm from the partially transparent surface

[0127] 2 Distance in nm from the reflective surface

[0128] 3 Normalized

[0129] As shown in Table C, the calculated positions for maximum enhancement are about 114-121 nm apart, and the ratio of (distance between nodes) / (microcavity length) is between 0.13-0.14.

[0130] The following OLED microcavity structures with two or more BLELs were simulated to determine the optimal positions of the BLELs within the microcavity. These calculations are based on B = 456 nm, G = 540 nm, and R = 620 nm for λ maxEmission (non-microcavity). The thickness of the microcavity (between the uppermost reflective surface and the lower surface of the semi-transparent electrode) was 6950 A, the LEL thicknesses were all 200 A, and there was a non-light-emitting spacer layer between each LEL in all cases. The devices were bottom-emitting. Of these, Model Formats B and D had a BLEL separated by at least one non-blue LEL, and Model Formats A and C had adjacent BLELs.

[0131] Model Format A: B2LEL / B1LEL / RLEL / GLEL

[0132] Model Format B: B2LEL / GLEL / RLEL / B1LEL

[0133] Model Format C: GLEL / B2LEL / B1LEL / RLEL

[0134] Model Format D: RLEL / B2LEL / GLEL / B1LEL

[0135] The calculated results for blue light intensity are shown in Table D.

[0136] Table D - Calculated B intensity for Model BLEL positions

[0137]

[0138] 1 Distance between midpoints of blue LELs (in nm) (distance between LELs / 695 nm microcavity distance)

[0139] Similar to Tables B and C, Table D indicates that the optimal distance between blue light emitting layers should be about 120 nm or about 0.16 of the microcavity distance. It is predicted that further increases in the distance between blue light emitting layers (i.e., a half- wavelength or more, but not an odd multiple of a quarter- wavelength) will give worse results. In this regard, the examples with an intermediate non-blue LEL between the blue LELs (Formats B and D) are similar to Format C, which has no non-blue LELs between the two blue LELs.

[0140] Experimental results

[0141] In the following examples, the number before each material (e.g., 130 ITO) is the physical layer thickness in Angstroms, unless otherwise noted. All devices were encapsulated using the same procedure after deposition of the cathode. In OLED Series A and B, all examples had a microcavity thickness of 7750 A between the reflective surface and the semi-transparent cathode.

[0142] OLED series A

[0143] Example Al (comparative): An OLED microcavity device with a conventional R / G / B BLEL1 internal structure was prepared on a glass substrate as follows.

[0144] Layer 1 (reflective surface): 1000 Al

[0145] Layer 2 (anode): 130 ITO

[0146] Layer 3 (HIL): 250 hole transport material (HTM) A with 8% p-dopant A

[0147] Layer 4 (HTL): 2450 HTM A

[0148] Layer 5 (red LEL): 200 mixed host material A (ratio 9:1) / B with 3% phosphorescent red dopant, 10% stabilizer

[0149] Layer 6 (ETL): 100 electron transport material (ETM) A

[0150] Layer 7 (charge generation layer (CGL) 1): 3 layers (total thickness 370) consisting of 100 ETM B / 20 ETM B / 250 HTM A with 10% p-dopant A with 2% Li

[0151] Layer 8 (HTL): 2050 HTM A

[0152] Layer 9 (green LEL): 200 host A with 10% phosphorescent green dopant

[0153] Layer 10: ETL: 100 ETM A

[0154] Layer 11 (CGL2): 370 (same scheme as CGL1 in layer 7)

[0155] Layer 12 (HTM): 930 HTM A

[0156] Layer 13 (blue LEL1): 200 host C with 4% fluorescent blue dopant A

[0157] Layer 14 (ETL): 300 ETM A

[0158] Layer 15 (EIL): 100 ETM B with 2% Li

[0159] Layer 16 (semi-transparent cathode): 125 co-deposited Ag (75%) and Mg (25%)

[0160] In Example Al, the distance between the midpoint of the BLEL1 and the front inner surface of the reflective layer was 7250 A, and the distance between the front inner surface of the semi-transparent cathode was 500 A.

[0161] Example A2 (Invention): An OLED microcavity device with the R / BLEL2 / G / BLEL1 internal structure of the Invention was prepared in the same manner as Comparative Example Al, except for the following changes:

[0162] Layer 8 (HTL): The thickness of 2050 HTMA was reduced to 950, and an additional B light-emitting layer and CGL3 were added before deposition of Layer 9; and the following layers were added over Layer 8 and before Layer 9 (in order):

[0163] Layer 8A (blue LEL2): 200 (same as the scheme for Layer 13)

[0164] Layer 8B (ETL) 100 ETMA

[0165] Layer 8C (CGL3): 370 (same as the scheme for CGL1)

[0166] Layer 8D (HTL): 430 HTMA

[0167] The blue LEL2 is located between the green and red LELs, and is separated from each other by the ETL / CGL / HTL layers. The distance from the midpoint of BLEL1 to the reflective surface is 7250 A, the distance from the midpoint of BLEL2 to the reflective surface is 4550 A, and the distance between the midpoint of BLEL1 and the midpoint of BLEL2 is 2700 A.

[0168] Example A3 (Invention): An OLED microcavity device similar to Invention Example A2 was prepared in the same manner as Example A2 (R / BLEL2 / G / BLEL1), except that the thickness of Layer 8 was increased from 950 to 1150, and the thickness of Layer 8D was reduced from 430 to 230. This effectively moves the midpoint of the BLEL further away from the reflective surface by 200 A, and further closer to the BLEL1 and cathode by 200 A. In Example 3, the distance from the midpoint of BLEL2 to the reflective surface is 4750 A, the distance from the midpoint of BLEL2 to the semi-transparent cathode is 3000 A, and the distance between the midpoint of BLEL1 and the midpoint of BLEL2 is also reduced from 2700 A to 2500 A.

[0169] Example A4 (Invention): An OLED microcavity device similar to Inventive Example A2 (R / BLEL2 / G / BLEL1) was prepared in the same manner, except that the thickness of layer 8 (from 950) was reduced to 750, and the thickness of layer 8D (from 430) was increased to 630. This effectively moved the midpoint of BLEL2 further toward the reflective surface by 200 A, and further away from BLEL1 and the cathode by 200 A. In Example A4, the distance from the midpoint of BLEL2 to the reflective surface was 4350 A, the distance from the midpoint of BLEL2 to the semi-transparent cathode was 3400 A, and the distance between the midpoint of BLEL1 and the midpoint of BLEL2 was also increased from 2700 A to 2900 A.

[0170] Tables 1 and 2 show the results for OLED Series A.

[0171] Table 1 - Performance data for OLED Series A

[0172]

[0173] 1 At 10 mA / cm 2 measured

[0174] As Figure 9 shown, Inventive Examples A2-A4 show that adding a BLEL1 on the side of the G-LEL opposite BLEL2 results in emitting more blue light compared to Comparative Example Al, as shown by the increased CIEx, CIEy values (closer to 0.33, 0.33) and increased blue emission intensity.

[0175] Table 2 shows the distance LI between BLEL1 and BLEL2, and the ratio LI / LO.

[0176] Table 2 - Distance between B light emitting layers within the microcavity for OLED Series A

[0177]

[0178] 1 Distance (in nm), measured from the midpoint of the LEL

[0179] Tables 1 and 2 also show that the optimal position of BLEL1 and BLEL2 relative to the reflective layer in the microcavity is unexpectedly significantly different from the position predicted by microcavity theory, and still an increased blue emission is observed, as Figure 9 shown. For example, Inventive Examples A2-A4 have a similar structure to Model Format D with a non-blue LEL between the two BLELs, but as shown in Table D, the distance between the two BLELs is not the distance predicted by microcavity theory to be optimal.

[0180] OLED series B

[0181] Example Bl: OLED microcavity device (R / BLEL2 / G / BLELl) is prepared in the same manner as Inventive Example A2, except that the thickness of the cathode (layer 16) is increased from 125 to 145.

[0182] Example B2: OLED microcavity device with the inventive internal structure of BLEL3 / R / BLEL2 / G / BLELl is prepared in the same manner as Inventive Example Bl, except that:

[0183] Layer 4 (HTL): The thickness of 2450 HTMA is reduced to 1240, and an additional B light emitting layer and CGL4 are added before deposition of layer 5 as follows:

[0184] Layer 4A (blue LEL3): 200 (same as for layer 13)

[0185] Layer 4B (ETL): 200 ETM A

[0186] Layer 4C (CGL4): 370 (same as for CGLl)

[0187] Layer 4D (HTL): 260 HTMA

[0188] and the following layers are modified:

[0189] Layer 8 (HTL): thickness is increased from 950 to 1070

[0190] Layer 8B (ETL): thickness is increased from 100 to 200

[0191] Layer 8D (HTL): thickness is reduced from 430 to 310

[0192] Layer 12 (HTM): thickness is reduced from 930 to 910

[0193] Layer 14 (ETL): thickness is reduced from 300 to 200

[0194] In Example B2, the midpoint of BLELl is located 7350 A from the front interior surface of the reflective layer and 400 A from the front interior surface of the semitransparent cathode, the midpoint of BLEL2 is located 4590 A from the front interior surface of the reflective layer and 3160 A from the front interior surface of the semitransparent cathode, the distance between the midpoint of BLELl and the midpoint of BLEL2 is 2760 A, the midpoint of BLEL3 is located 1720 A from the front interior surface of the reflective layer and 6030 A from the front interior surface of the semitransparent cathode, and the distance between the midpoint of BLEL2 and the midpoint of BLEL3 is 2870 A.

[0195] Tables 3 and 4 show the results for OLED series B.

[0196] Table 3 - Performance data for OLED series B

[0197]

[0198] 1 measured at 10 mA / cm 2 measured at 10 mA / cm

[0199] Table 4 - Distance between B light emitting layers within the microcavity for OLED series B

[0200]

[0201] 1 Distance (in nm) measured from the midpoint of the LEL

[0202] Inventive Example B1 has a similar R / BLEL2 / G / BLEL1 structure as Inventive Example A2. Inventive Example B2 has a BLEL3 / R / BLEL2 / G / BLEL1 structure. As shown, the additional BLEL located between the RLEL and the reflective surface shows further improvement in CIEx, CIEy values (closer to 0.33, 0.33) and increased blue light emission intensity. Figure 10

[0203] Tables 3 and 4 also show that the optimal position of BLEL1, BLEL2 and BLEL3 in the microcavity relative to the reflective layer is unexpectedly significantly different from the position according to microcavity theory and still an increased blue light emission is observed. For example, Inventive Example B2 has a similar structure as Model Format E with a non-blue LEL between the two BLELs and a third BLEL closest to the reflective surface, but as shown in Table D, the distance between the two B LELs is not the distance predicted to be optimal.

[0204] OLED series C

[0205] Example C1 : OLED microcavity device (BLEL3 / R / BLEL2 / G / BLEL1) is prepared in the same manner as Inventive Example B2.

[0206] Example C2: OLED microcavity device (BLEL4 / BLEL3 / R / BLEL2 / G / BLEL1) is prepared in the same manner as Inventive Example C1, except that an additional B light emitting layer as follows is added prior to deposition of layer 4A, as well as CGL4:

[0207] Layer 4A' (blue LEL4): 200 (same as the scheme for layer 13)

[0208] ​Layer 4B' (ETL): 200 ETL A

[0209] Layer 4C' (CGL4): 370 (same scheme as CGL1)

[0210] Layer 4D' (HTL): 370 HTMA

[0211] Example C3: An OLED microcavity device (B4 / B3 / R / B2 / G / B1) is prepared in the same manner as Inventive Example C2, except that:

[0212] Layer 4D (HTL): thickness reduced from 260 to 210

[0213] Layer 8 (HTL): thickness reduced from 1070 to 1020

[0214] Layer 8D (HTL): thickness reduced from 310 to 260

[0215] Layer 12 (HTL): thickness reduced from 910 to 860

[0216] Example C4: An OLED microcavity device (B4 / B3 / R / B2 / G / B1) is prepared in the same manner as Inventive Example C2, except that:

[0217] Layer 4D (HTL): thickness reduced from 260 to 160

[0218] Layer 8 (HTL): thickness reduced from 1070 to 970

[0219] Layer 8D (HTL): thickness reduced from 310 to 210

[0220] Layer 12 (HTL): thickness reduced from 910 to 810

[0221] Example C5: An OLED microcavity device (BLEL4 / BLEL3 / R / BLEL2 / G / BLEL1) is prepared in the same manner as Inventive Example C2, except that the thickness of layer 4 (HTL) is increased from 100 to 300.

[0222] Example C6: An OLED microcavity device (BLEL4 / BLEL3 / R / BLEL2 / G / BLEL1) is prepared in the same manner as Inventive Example C2, except that the thickness of layer 4 (HTL) is increased from 100 to 500.

[0223] Tables 5, 6 and 7 show the performance data for the OLED series C.

[0224] Table 5 - Performance data for OLED series B

[0225]

[0226] 1 at 10 mA / cm 2

[0227] Table 6 - OLED Series C 1 Distance of B light emitting layer within microcavity

[0228]

[0229] 1 Units are A, measured from midpoint of LEL

[0230] Table 7 - Relative distance of BLEL within microcavity for OLED Series C

[0231]

[0232] Inventive Example CI has a BLEL3 / R / BLEL2 / G / BLEL1 structure, similar to Inventive Example B2. The remaining OLED Series C has a BLEL4 / BLEL3 / R / BLEL2 / G / BLEL1 structure, and another blue light emitting layer (BLEL4) is added between the RLEL and the reflective surface. As shown, this increases the blue light emission, but this improvement is sensitive to the relative position of BLEL2 within the microcavity. In addition, the results show that increasing the thickness of the microcavity of this scheme affects the wavelength of the blue light emission. Figure 11

[0233] Tables 5-7 also show that the optimal position of BLEL1, BLEL2, BLEL3, and BLEL4 within the microcavity relative to the reflective layer is unexpectedly significantly different from the position according to microcavity theory, and still an increased blue light emission is observed. For example, Inventive Example C3 has a structure similar to Model Format F with a non-blue LEL between two BLELs and with a third and fourth BLEL closest to the reflective surface, but as shown in Table D, the distance between the two BLELs is not the distance predicted to be optimal.

[0234] In the foregoing description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the application can be practiced. These embodiments are described in detail to enable those skilled in the art to practice the application, and it is to be understood that other embodiments can be utilized and that structural, logical, and electrical changes can be made without departing from the scope of the present application. Thus, the description of any exemplary embodiment is not to be considered limiting in any regard. While the present application has been described with respect to a limited number of embodiments, it will be apparent that many modifications and variations thereof will be suggested to those skilled in the art in view of this description. It is therefore contemplated that the use of such modifications and variations can be made without departing from the scope of the present application. Accordingly, no limitation is placed on the scope of the present application by the description of the exemplary embodiments which follow. ​​

[0235] List of fittings

[0236] 1 substrate

[0237] 3 control circuit layer

[0238] 5 optional planarization layer

[0239] 7 electrical contact

[0240] 9 first electrode portion

[0241] 9A first electrode layer

[0242] 9B reflective layer

[0243] 11 non-light emitting OLED layer

[0244] 13 second blue light emitting layer

[0245] 15, 64, 72, 80, 96, 105 charge generation layer

[0246] 17 non-blue light emitting layer

[0247] 19 charge generation layer

[0248] 21 first blue light emitting layer

[0249] 25 semi-transparent second electrode

[0250] 27, 92 encapsulation

[0251] 29 color filter array

[0252] 29B blue light filter

[0253] 29G green light filter

[0254] 29R red light filter

[0255] 30, 95 microcavity

[0256] 50 substrate

[0257] 52 reflective layer

[0258] 54 first electrode layer

[0259] 56 hole injection layer

[0260] 58, 66, 74, 82, 99, 107 hole transport layer

[0261] 60 second non-blue light emitting layer

[0262] 62, 78, 86, 96, 103 electron transport layer

[0263] 68 second blue light emitting layer

[0264] 70 electron transport layer

[0265] 76 first non-blue light emitting layer

[0266] 84 first blue light emitting layer

[0267] 88 electron injection layer

[0268] 90 semi-transparent second electrode

[0269] 94 third blue light emitting layer

[0270] 101 fourth blue light emitting layer

[0271] 100 Microdisplay with multi-mode OLED microcavity

[0272] 200 Multi-mode OLED microcavity device

[0273] 300 Multi-mode OLED microcavity device

[0274] 400 Multi-mode OLED microcavity device

[0275] L0 microcavity distance

[0276] L1 distance between midpoint of first BLEL and midpoint of second BLEL

[0277] L2 distance between midpoint of first BLEL and front surface of reflective surface

[0278] L3 distance between midpoint of second BLEL and front surface of reflective surface

[0279] L4 distance between midpoint of second BLEL and midpoint of third BLEL

[0280] L5 distance between midpoint of third BLEL and midpoint of fourth BLEL

Claims

1. A multi-mode light emitting OLED microcavity device comprising: an opaque substrate; a first electrode over the substrate, a layer having a reflective surface disposed between the substrate and the first electrode or an upper surface of the first electrode is a reflective surface; organic layers for light emission including a first blue light emitting layer, a second blue light emitting layer, and at least one non-blue light emitting layer, the second blue light emitting layer is proximate to the reflective surface and the first blue light emitting layer is further from the reflective surface than the second blue light emitting layer, wherein a distance between a midpoint of the second blue light emitting layer and a midpoint of the first blue light emitting layer is LI; a semi-transparent second electrode having an innermost surface through which light emission passes; wherein a distance L0 between the reflective surface and the innermost surface of the semi-transparent second electrode is constant across a light emission area; a ratio LI / L0 is in a range of 0.30 to 0.40; and the distance L0 is in a range of 6500 A to 7800 A or in a range of 8000 A to 9000 A.

2. The multi-mode light emitting OLED microcavity device of claim 1, wherein, a ratio L2 / L0 is in a range of 0.90 to 0.98, where L2 is a distance between the midpoint of the first blue light emitting layer and the reflective surface.

3. The multimode light emitting OLED microcavity device of claim 2, wherein, a ratio L3 / L0 is in a range of 0.52 to 0.64, where L3 is a distance between the midpoint of the second blue light emitting layer and the reflective surface.

4. The multimode light emitting OLED microcavity device of claim 1, wherein, the non-blue light emitting layer is between the first blue light emitting layer and the second blue light emitting layer.

5. The multimode light emitting OLED microcavity device of claim 4, wherein, the non-blue light emitting layer emits green light.

6. The multimode light emitting OLED microcavity device of claim 5, wherein, including a second non-blue light emitting layer between the second blue light emitting layer and the reflective surface.

7. The multimode light emitting OLED microcavity device of claim 6, wherein, the second non-blue light emitting layer emits red light.

8. The multimode light emitting OLED microcavity device of claim 7, wherein, including a third blue light emitting layer between a red light emitting layer and the reflective surface.

9. The multimode light emitting OLED microcavity device of claim 8, wherein, including a fourth blue light emitting layer between the third blue light emitting layer and the reflective surface.

10. The multimode light emitting OLED microcavity device of claim 1, wherein, an upper surface of the first electrode is the reflective surface.

11. A microdisplay comprising the multi-mode light emitting OLED microcavity device of claim 1.

12. The microdisplay of claim 11, wherein, an upper surface of the first electrode is the reflective surface.

13. The microdisplay of claim 11, wherein: the substrate of the multi-mode light emitting OLED microcavity device comprises a backplane having control circuitry for respective operation of a plurality of sub-pixels defined by segmented first electrodes, the segmented first electrodes connected to the control circuitry; and having a color filter array over the semi-transparent second electrode aligned with the respective controlled sub-pixels to form at least R, G, B sub-pixels.

14. The microdisplay of claim 13, wherein, some color filters in the color filter array are transparent or missing to form R, G, B, and W sub-pixels.

Images