Light extraction apparatus and OLED displays

A light extraction apparatus for an organic light-emitting diode (OLED) includes an OLED emitter (100), a plurality of tapered reflectors (210), and a spacer layer (202). Each tapered reflector includes a first surface (212), a second surface (214) opposite to the first surface and comprising a surface area larger than a surface area of the first surface, and at least one side surface (216) extending between the first surface and the second surface. The spacer layer (202) includes a first surface coupled to the OLED emitter and a second surface coupled to the first surface of each of the plurality of tapered reflectors. Light emitted from the OLED passes through the spacer layer and into the plurality of tapered reflectors. The at least one side surface of each of the plurality of tapered reflectors includes a slope to redirect light into an escape cone and out of the second surface of the corresponding tapered reflector.

This application is a National Stage application under 35 U.S.C. § 371 of International Application No. PCT/RU2018/000370, filed on Jun. 6, 2018, the content of which is relied upon and incorporated herein by reference in its entirety.

BACKGROUND

Field

The present disclosure relates generally to organic light-emitting diode (OLED) displays. More particularly, it relates to OLED displays and apparatus and methods for light extraction from OLED displays.

Technical Background

OLEDs typically include a substrate, a first electrode, one or more OLED light-emitting layers, and a second electrode. OLEDs can be top emitting or bottom emitting. A top-emitting OLED may include a substrate, a first electrode, an OLED structure having one or more OLED layers, and a second transparent electrode. The one or more OLED layers of the OLED structure may include an emission layer and can also include electron and hole injection layers and electron and hole transport layers.

Light emitted by the OLED structure is trapped by total internal reflection (TIR) wherever it passes from a layer with a higher refractive index to a layer with a lower refractive index, for example from the OLED structure that typically has a refractive index in the 1.7-1.8 range to a glass substrate that typically has an index of approximately 1.5, or from a glass substrate to air that has an index of 1.0.

To form a display, the OLEDs may be arranged on a display substrate and covered with an encapsulation layer. However, the light emitted from the OLEDs will once again be subject to TIR from the upper surface of the encapsulation layer even if the space between the encapsulation layer and the OLEDs is filled with a solid material. This further reduces the amount of OLED-generated light available for use in the OLED display.

SUMMARY

Some embodiments of the present disclosure relate to a light extraction apparatus for an organic light-emitting diode (OLED). The light extraction apparatus includes an OLED emitter, a plurality of tapered reflectors, and a spacer layer. Each tapered reflector includes a first surface, a second surface opposite to the first surface and comprising a surface area larger than a surface area of the first surface, and at least one side surface extending between the first surface and the second surface. The spacer layer includes a first surface coupled to the OLED emitter and a second surface coupled to the first surface of each of the plurality of tapered reflectors. Light emitted from the OLED passes through the spacer layer and into the plurality of tapered reflectors. The at least one side surface of each of the plurality of tapered reflectors includes a slope to redirect light by reflection into an escape cone and out of the second surface of the corresponding tapered reflector.

Yet other embodiments of the present disclosure relate to a bottom-emitting OLED display. The display includes a spacer layer, an array of tapered reflectors, and a substrate. The spacer layer is coupled to an array of OLEDs. Each OLED of the array of OLEDs has a bottom surface through which light is emitted into the spacer layer. At least two tapered reflectors of the array of tapered reflectors overlap each OLED of the array of OLEDs. Each tapered reflector of the array of tapered reflectors includes a first surface, a second surface opposite to the first surface and comprising a surface area larger than a surface area of the first surface, and at least one side surface extending between the first surface and the second surface. The first surface of each tapered reflector of the array of tapered reflectors is coupled to the spacer layer and facing the array of OLEDs. The substrate is coupled to the second surface of each tapered reflector of the array of tapered reflectors.

Yet other embodiments of the present disclosure relate to a top-emitting OLED display. The display includes a substrate, a spacer layer, an array of tapered reflectors, and an encapsulation layer. The substrate supports an array of OLEDs. Each OLED of the array of OLEDs has a top surface through which light is emitted. The spacer layer is coupled to the top surface of each OLED of the array of OLEDS. At least two tapered reflectors of the array of tapered reflectors overlap each OLED of the array of OLEDs. Each tapered reflector of the array of tapered reflectors includes a first surface, a second surface opposite to the first surface and comprising a surface area larger than a surface area of the first surface, and at least one side surface extending between the first surface and the second surface. The first surface of each tapered reflector of the array of tapered reflectors is coupled to the spacer layer and facing the array of OLEDs. The encapsulation layer is coupled to the second surface of each tapered reflector of the array of tapered reflectors.

OLED displays including the light extraction apparatus disclosed herein significantly improve the out-coupling of light from the displays and increase the efficiency and peak brightness of the displays. The external efficiency of OLED displays may be increased by a factor of 100% compared to displays not including the light extraction apparatus. Due to the increased external efficiency, the pixels of the display may be driven with less current for the same brightness, which increases the useful lifetime of the display and reduces the “burn-in” effect. Alternatively, or in addition, the pixels of the display may generate a higher peak brightness, which enables a high dynamic range (HDR). These capabilities are achieved while increasing the overall thickness of the displays by a few tens of microns. The light extraction apparatus is color-neutral, and therefore is equally beneficial for red, green, and blue pixels. In addition, the light extraction apparatus does not introduce optical scattering (i.e., haze) that can reduce sharpness and contrast. Further, the light extraction apparatus does not scramble the polarization state of light and is therefore compatible with the use of circular polarizers to reduce ambient light reflection.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. However, this disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

Directional terms as used herein—for example up, down, right, left, front, back, top, bottom, vertical, horizontal—are made only with reference to the figures as drawn and are not intended to imply absolute orientation.

Cartesian coordinates are used in the Figures for the sake of reference and ease of discussion and are not intended to be limiting as to orientation or direction.

The term “light extraction” in connection with an OLED refers to apparatus and method for increasing the amount of light emitted from the OLED using features that do not reside within the actual OLED layered structure.

The refractive index no of the OLED is an effective refractive index that includes contributions from the various layers that make up the OLED structure and in an example is in the range from about 1.6 to 1.85, while in another example is in the range from about 1.7 to 1.8, and in another example is in the range from about 1.76 to 1.78.

Referring now toFIG.1A, an exemplary bottom-emitting OLED structure100is schematically depicted. Bottom-emitting OLED structure100includes a transparent substrate102(e.g., glass, plastic, etc.), a transparent anode104(e.g., ITO), and a cathode116. In certain exemplary embodiments, cathode116may be made of a highly reflective metal, such as silver or aluminum. Between anode104and cathode116, bottom-emitting OLED structure100may include a hole injection layer (HIL)106, a hole transport layer (HTL)108, an emission layer (EML)110, an electron transport layer (ETL)112, and an electron injection layer (EIL)114. Bottom-emitting OLED structure100emits light through the bottom surface of anode104and through substrate102.

Top-emitting OLED structure120includes a substrate122(e.g., glass, plastic, etc.), an anode124, and a transparent cathode136(e.g., an ultra-thin metal such as Al or Ag or an alloy such as Mg:Ag or Ba:Ag). In certain exemplary embodiments, top-emitting OLED structure120may also include a capping layer (e.g., WO3) over cathode136. Between anode124and cathode136, top-emitting OLED structure120may include an HTL126, an electron blocking layer (HBL)128, an EML130, a hole blocking layer (HBL)132, and an ETL134. Top-emitting OLED structure120emits light through the top surface of cathode136.

FIG.2Adepicts a cross-sectional view of a section of an exemplary bottom-emitting OLED display200. OLED display200includes a bottom-emitting OLED100, a spacer layer202, an array208of tapered reflectors210, and a substrate218. A plurality (i.e., at least two) of the tapered reflectors210overlap OLED100. In certain exemplary embodiments, at least two rows and at least two columns of tapered reflectors210(i.e.,4tapered reflectors) overlap OLED100. In other embodiments, at least three rows by five columns of tapered reflectors210(i.e.,15tapered reflectors) overlap OLED100. In yet other embodiments, at least ten rows by at least thirty columns of tapered reflectors210(i.e.,300tapered reflectors) overlap OLED100.

Each tapered reflector210includes a first surface212and a second surface214opposite to the first surface212. The second surface214is larger (i.e., has a greater surface area) than the first surface212. Each tapered reflector210also includes at least one side surface216extending between the first surface212and the second surface214. In certain exemplary embodiments, each tapered reflector210of the array208of tapered reflectors has the form of a truncated pyramid having a trapezoidal cross-section as will be described in more detail below. Tapered reflectors210may be coupled to each other at the second surface214of each tapered reflector as shown inFIG.2Asuch that the at least one side surface216of each tapered reflector does not extend completely to the second surface214.

Spacer layer202includes a first surface204optically coupled to the bottom of OLED100and a second surface206optically coupled to the first surface212of each tapered reflector210. In certain exemplary embodiments, spacer layer202has a thickness between about 10% and 100% of a length of the first surface212of each tapered reflector210. The refractive index of the spacer layer202may be greater than or equal to the refractive index of the array208of tapered reflectors210.

Substrate218is optically coupled to the second surface214of each tapered reflector210. Substrate218may be made of glass, plastic, or another suitable transparent material. The refractive index of the array208of tapered reflectors210may be greater than or equal to the refractive index of the substrate218. Light emitted from the OLED100passes through the spacer layer202and into the tapered reflectors210. The at least one side surface216of each tapered reflector210includes a slope to redirect light by reflection into an escape cone and out of the second surface214of the corresponding tapered reflector210. The light then passes through substrate218into an external environment222.

Three light rays224a,224b, and224care shown inFIG.2A. Light ray224apropagates from the bottom of OLED100through spacer layer202and through a tapered reflector210directly to the second surface214of the tapered reflector, where the light propagates into substrate218and passes therethrough to external environment222. Light ray224bpropagates from the bottom of OLED100through spacer layer202and through a tapered reflector210directly to a side surface216of the tapered reflector, where the light is reflected and redirected to the second surface214of the tapered reflector. Light ray224bthen propagates into substrate218and passes therethrough to external environment222. Light ray224cpropagates from the bottom of OLED100into spacer layer202and directly to a portion of the second surface206of the spacer layer that is between tapered reflectors210, where the light is reflected by TIR within the spacer layer. The light is reflected again by OLED100at the first surface204of spacer layer202. Light ray224cthen passes again through spacer layer202and this time through a tapered reflector210directly to a side surface216of the tapered reflector, where the light is reflected and redirected to the second surface214of the tapered reflector. Light ray224cthen propagates into substrate218and passes therethrough to external environment222. Additional details regarding the light propagation within OLED display200will be discussed below with reference toFIG.4.

FIG.2Bdepicts a cross-sectional view of a section of an exemplary top-emitting OLED display250. OLED display250includes a top-emitting OLED120, a metal contact252, a substrate218, a spacer layer202, an array208of tapered reflectors210, and an encapsulation layer254. A plurality (i.e., at least two) of the tapered reflectors210overlap OLED120. In certain exemplary embodiments, at least two rows and at least two columns of tapered reflectors210(i.e.,4tapered reflectors) overlap OLED120. In other embodiments, at least three rows by five columns of tapered reflectors210(i.e.,15tapered reflectors) overlap OLED120. In yet other embodiments, at least ten rows by at least thirty columns of tapered reflectors210(i.e.,300tapered reflectors) overlap OLED120.

Each tapered reflector210includes a first surface212and a second surface214opposite to the first surface212. The second surface214is larger (i.e., has a greater surface area) than the first surface212. Each tapered reflector210also includes at least one side surface216extending between the first surface212and the second surface214. In certain exemplary embodiments, each tapered reflector210of the array208of tapered reflectors has the form of a truncated pyramid having a trapezoidal cross-section as will be described in more detail below. Tapered reflectors210may be coupled to each other at the second surface214of each tapered reflector as shown inFIG.2Bsuch that the at least one side surface216of each tapered reflector does not extend completely to the second surface214.

Spacer layer202includes a first surface204optically coupled to the top of OLED120and a second surface206optically coupled to the first surface212of each tapered reflector210. In certain exemplary embodiments, spacer layer202has a thickness between about 10% and 100% of a length of the first surface212of each tapered reflector210. The refractive index of the spacer layer202may be greater than or equal to the refractive index of the array208of tapered reflectors210.

Substrate218(e.g., glass, plastic, etc.) is coupled to OLED120via metal contact252. Encapsulation layer254is optically coupled to the second surface214of each tapered reflector210. Encapsulation layer254may be made of glass, plastic, or another suitable transparent material. The refractive index of the array208of tapered reflectors210may be greater than or equal to the refractive index of the encapsulation layer254. Light emitted from the OLED120passes through the spacer layer202and into the tapered reflectors210. The at least one side surface216of each tapered reflector210includes a slope to redirect light by reflection into an escape cone and out of the second surface214of the corresponding tapered reflector210. The light then passes through encapsulation layer254into an external environment222.

Three light rays264a,264b, and264care shown inFIG.2B. Light ray264apropagates from the top of OLED120through spacer layer202and through a tapered reflector210directly to the second surface214of the tapered reflector, where the light propagates into encapsulation layer254and passes therethrough to external environment222. Light ray264bpropagates from the top of OLED120through spacer layer202and through a tapered reflector210directly to a side surface216of the tapered reflector, where the light is reflected and redirected to the second surface214of the tapered reflector. Light ray264bthen propagates into encapsulation layer254and passes therethrough to external environment222. Light ray264cpropagates from the top of OLED120into spacer layer202and directly to a portion of the second surface206of the spacer layer that is between tapered reflectors210, where the light is reflected by TIR within the spacer layer. The light is reflected again by OLED120at the first surface204of spacer layer202. Light ray264cthen passes again through spacer layer202and this time through a tapered reflector210directly to a side surface216of the tapered reflector, where the light is reflected and redirected to the second surface214of the tapered reflector. Light ray264cthen propagates into encapsulation layer254and passes therethrough to external environment222. Additional details regarding the light propagation within OLED display250will be discussed below with reference toFIG.4.

FIG.3Ais a top-down view of an exemplary OLED display300that employs the light-extraction apparatus and methods disclosed herein.FIG.3Bis a close-up top-down view of a section of OLED display300whileFIG.4is a close-up x-z cross-sectional view of a section of the OLED display300. In one embodiment, OLED display300is a bottom-emitting OLED display and includes the OLED display structure depicted inFIG.2A. In another embodiment, OLED display300is a top-emitting OLED display and includes the OLED display structure depicted inFIG.2B.

With reference toFIGS.3A,3B, and4, the OLED display300includes an array302of OLEDs304, a spacer layer202, an array208of tapered reflectors210, and a transparent layer306. The array302of OLEDs304resides on the first surface204of spacer layer202. In one embodiment, each OLED304is a bottom-emitting OLED, such as OLED100previously described and illustrated with reference toFIG.1A. In another embodiment, each OLED304is a top-emitting OLED, such as OLED120previously described and illustrated with reference toFIG.1B. In the case of each OLED304being a bottom-emitting OLED, transparent layer306is a substrate, such as substrate218previously described and illustrated with reference toFIG.2A. Note that for the case of each OLED304being a bottom-emitting OLED, the z-axis orientation of OLED display300ofFIG.4may be reversed to match the z-axis orientation of OLED display200ofFIG.2A. In the case of each OLED304being a top-emitting OLED, transparent layer306is an encapsulation layer, such as encapsulation layer254previously described and illustrated with reference toFIG.2B. Note that for the case of each OLED304being a top-emitting OLED, the metal contact252and the substrate218of OLED display250ofFIG.2Bhave been excluded from OLED display300ofFIG.4for simplicity.

As shown inFIG.3B, the OLEDs304have a length Lx in the x-direction and a length Ly in the y-direction. In one embodiment, Lx equals Ly. The OLEDs304in OLED array302are spaced apart from each other in the x-direction and the y-direction by side-to-side spacings Sx and Sy. In one embodiment, Sx equals Sy. In one embodiment, the OLEDs304are all the same size and are equally spaced apart. In other embodiments, the OLEDs304do not all have the same dimensions Lx, Ly and the spacings Sx, Sy are not all the same. OLEDs304may each emit the same color of light or different colors of light, such as red, green, blue, and/or white. While OLEDs304are illustrated as having a rectangular arrangement, in other embodiments, OLEDs304may have a non-rectangular arrangement (e.g., diamond).

Array208of tapered reflectors210is optically coupled to OLEDs304with at least two tapered reflectors210overlapping and optically coupled to each OLED304.FIG.5is an elevated exploded view of an example tapered reflector210. Each tapered reflector210includes a body215, a first surface212, at least one side surface216, and a second surface214. The first surface212includes at least one outer edge212E, and the second surface214includes at least one outer edge214E. The tapered reflector body215is made of a material having a refractive index nP.

The second surface214of tapered reflector210is larger (i.e., has a greater surface area) than the first surface212, i.e., the second surface is the “base” of the tapered reflector. In certain exemplary embodiments, the second surface214of tapered reflector210is at least 1.5 times as large in area as the first surface212of tapered reflector210. In one embodiment, the first and second surfaces212and214are rectangular (e.g., square) so that there are a total of four side surfaces216. In an example where tapered reflector210is rotationally symmetric, it can be said to have one side surface216. Side surfaces216can each be a single planar surface or made of multiple segmented planar surfaces, or be a continuously curved surface.

Thus, in one example, tapered reflector210has the form of a truncated pyramid comprising a trapezoidal cross-section, also called an incomplete or truncated rectangular-based pyramid. Other shapes for tapered reflector210can also be effectively employed, as discussed below. The tapered reflector210has a central axis AC that runs in the z-direction. In the example where second (top) surface214and first (bottom) surface212have a square shape, the second surface214has a width dimension WT and the first surface212has a width dimension WB. More generally, the second surface214has (x, y) width dimensions WTx and WTy and first surface212has (x, y) width dimensions WBx and WBy. The tapered reflector210also has a height HP defined as the axial distance between the first surface212and the second surface214.

In certain exemplary embodiments, tapered reflectors210are formed as a unitary, monolithic structure made of a single material. This can be accomplished using a molding process, imprinting process (e.g., ultraviolet or thermal imprinting), or like process, such as a microreplication process using a resin-based material.

As shown inFIG.4, the first surface212of tapered reflector210is arranged on the second surface206of spacer layer202. OLED304is arranged on the first surface204of spacer layer202. Spacer layer202may include an index-matching material having a refractive index nIMand is used to interface tapered reflectors210to OLEDs304. The tapered reflector refractive index nPis preferably, for example, as close as possible to the OLED refractive index no. In one embodiment, the difference between npand nOis no more than about 0.3, more preferably no more than about 0.2, more preferably no more than about 0.1, and most preferably no more than about 0.01. In another embodiment, the index-matching material refractive index nIMis no lower than the tapered reflector refractive index nP, and preferably has a value between nPand nO. In an example, the tapered reflector refractive index nPis between about 1.6 and 1.8.

In one embodiment, the index-matching material of spacer layer202has an adhesive property and serves to attach tapered reflectors210to OLEDs304. The index-matching material comprises, for example, an inorganic material, a glue, an optically clear adhesive, a bonding agent, or the like. The combination of each OLED304, tapered reflectors210, and spacer layer202define a light-emitting apparatus. The tapered reflectors210and spacer layer202define a light extraction apparatus.

The second surfaces214of tapered reflectors210are optically coupled to a first (lower) surface308of transparent layer306. The second surfaces214of tapered reflectors210may tile the first surface308of transparent layer306without any substantial space in between second edges214E. An external environment222exists immediately adjacent a second (upper) surface310of transparent layer306. The external environment222is typically air, although it can be another environment in which one might use a display, such as vacuum, inert gas, etc.

With reference again toFIGS.2A,2B, and4, the array208of tapered reflectors210defines confined spaces220between adjacent tapered reflectors and the second surface206of spacer layer202. In certain exemplary embodiments, spaces220are filled with a medium such as air, while in other embodiments, the spaces are filled with a medium in the form of a dielectric material, such as a light absorbing (i.e., black) material. The filling of spaces220with a given medium of refractive index nSis discussed in greater detail below.

The tapered reflectors210are typically made of a material that has a relatively high refractive index, i.e., preferably as high as that of the OLED emission layer110(FIGS.1A and1B). The tapered reflectors210are operably arranged upon and overlap corresponding OLEDs304in an inverted configuration with the aforementioned spacer layer202therebetween. Each OLED304can be considered a pixel in OLED array302, and each combination of OLED304, spacer layer202, and at least two tapered reflectors210is a light-emitting apparatus, with the combination of light-emitting apparatus defining an array of light-emitting apparatus for OLED display300.

The OLEDs304emit light towards the first surface212of tapered reflectors210. Because of the relatively high refractive index nPof the tapered reflectors210and the refractive index nIMof spacer layer202, light rays320generated in the emission layer of OLED304can propagate from the OLED either directly or upon being reflected by the cathode of the OLED without being trapped by TIR. Three light rays320a,320b, and320care shown inFIG.4. Light ray320apropagates from OLED304through spacer layer202and through a tapered reflector210directly to the second surface214of the tapered reflector, where the light propagates into transparent layer306and passes therethrough to external environment222. Light ray320bpropagates from the OLED304through spacer layer202and through a tapered reflector210directly to a side surface216of the tapered reflector, where the light is reflected and redirected to the second surface214of the tapered reflector. Light ray320bthen propagates into transparent layer306and passes therethrough to external environment222. Light ray320cpropagates from the OLED304into spacer layer202and directly to a portion of the second surface206of the spacer layer that is between tapered reflectors210, where the light is reflected by TIR within the spacer layer. The light is reflected again by OLED304at the first surface204of spacer layer202. Light ray320cthen passes again through spacer layer202and this time through a tapered reflector210directly to a side surface216of the tapered reflector, where the light is reflected and redirected to the second surface214of the tapered reflector. Light ray320cthen propagates into transparent layer306and passes therethrough to external environment222. In an example where the first surfaces112of tapered reflectors210cover about one fourth of the area of the second surface206of spacer layer202, it will take on average about four reflections for the light ray to be able to exit the display300.

In certain exemplary embodiments, side surfaces216have a slope defined by a slope angle θ relative to the vertical, e.g., relative to a vertical reference line RL that runs parallel to central axis AC, as shown. If the slope of sides216is not too steep (i.e., if the slope angle θ is sufficiently large), the TIR condition will be met for any point of origin of the light rays320entering tapered reflector210and no light rays will be lost by passing through sides216and into the spaces220immediately adjacent the sides of tapered reflector210.

Moreover, if the height HP of tapered reflector210is sufficiently large, all of the light rays320incident upon the second surface214will be within a TIR escape cone219(FIG.7B) defined by the refractive index nPof tapered reflector210and the refractive index nEof the transparent layer306and thus escape into the transparent layer306. In addition, light rays320will also be within the TIR escape cone defined by the refractive index nEof the material of transparent layer306and the refractive index neof the external environment222that resides immediately adjacent the second surface310of the transparent layer306.

Thus, neglecting light absorption of the otherwise transparent anode in the OLED structure of OLED304and light absorption in spacer layer202, 100% of light320generated by the OLED can in principle be communicated into the external environment222that resides above transparent layer306. In essence, the index-matched material that makes up body215of each tapered reflector210allows for the tapered reflectors210to act as perfect (or near-perfect) internal light extractors while the reflective properties of sides216allow for the tapered reflectors to be perfect (or near-perfect) external light extractors.

Without the array208of tapered reflectors210, the power coupled out of OLED display300would be about 30% of the source power for a well-designed OLED structure, and would not change significantly based on the thickness of spacer layer202. With array208of tapered reflectors210, the power coupled out of OLED display300increases as the thickness of spacer layer202increases until a maximum efficiency is reached. This change is due to how light rays320emitted from the OLEDs304propagate after they traverse the spacer layer202.

Light rays outside the escape cone (that would be trapped within the display without tapered reflectors210), such as light ray320b, are redirected by TIR at the side surface216of the tapered reflector210and are now inside the escape cone and able to exit the display300. Light rays, such as light ray320c, outside the escape cone that strike the second surface206of spacer layer202between first surfaces212of adjacent tapered reflectors210are reflected by TIR back towards the OLED304. The light rays are then reflected back towards the array208of tapered reflectors210, where again the light rays can be either transmitted or reflected, depending upon where they strike.

For a very small thickness of the spacer layer202, a light ray that on a first strike is mid-way between the first surfaces212of adjacent tapered reflectors210might need many bounces before the light ray strikes the first surface112of one of the tapered reflectors, and each time the light ray bounces back, some power is lost due to absorption in the OLED304and/or the spacer layer202. As the thickness of the spacer layer grows, fewer bounces are needed for such light ray to travel laterally far enough to encounter a first surface212of a tapered reflector210and subsequently exit the display300. Accordingly, the light extraction efficiency initially increases with an initial increase in the spacer layer thickness. The probability for the light ray to strike the first surface212of a tapered reflector210after a given number of bounces, however, does not change with a further increase in spacer layer thickness. Once the light extraction efficiency reaches a maximum, theoretically it should not change with a further increase in the thickness of the spacer layer202.

By the logic presented above, it might seem that any thickness of the spacer layer202above a certain value would serve the purposes of the present disclosure well, in terms of achieving maximum possible light extraction efficiency. There is, however, another consideration to be taken into account. Since the light rays that are initially outside of the escape cone may travel an appreciable distance laterally after each bounce, this can lead to a “washing out” of the pixel, and may result in “cross-talk” between display pixels, reducing sharpness and eventually also contrast. Therefore, in certain exemplary embodiments, the optimum thickness of the spacer layer202is just large enough to reach the maximum efficiency but not any larger.

In certain exemplary embodiments for bottom-emitting OLED displays, the increase in external efficiency is about 60% compared to a bottom-emitting OLED display without an array208of tapered reflectors210. In certain exemplary embodiments for top-emitting OLED displays, the increase in external efficiency is about 100% compared to a top-emitting OLED display without an array208of tapered reflectors210. The actual improvement achieved depends upon the sequence and composition of the OLED304and the resulting interplay between the OLED's waveguiding properties and radiation dynamics of the light emitting dipole molecules, possibly including cavity effects that may be used to increase the light output in OLED displays.

Explanation of TIR Conditions

At the boundary of any two dissimilar transparent materials such as air and glass having refractive indices n1 and n2, respectively, light rays incident upon the boundary from the direction of the higher-index material will experience 100% reflection at the boundary and will not be able to exit into a lower index material if they are incident at the boundary at an angle to the surface normal which is higher than a critical angle θ. The critical angle is defined by sin(θc)=n1/n2.

All light rays that are able to escape the higher-index material and not be subjected to TIR therein will lay within a cone having a cone angle of 2θc. This cone is referred to as the escape cone and discussed below in connection withFIG.7B.

It can be shown that for any sequence of layers with arbitrary refractive indices, the critical angle θcand the escape cone219are defined by the refractive index of the layer where the light ray originates, and the refractive index of the layer or medium into which it escapes. Thus, an anti-reflective coating cannot be used to modify the TIR condition and cannot be used to aid light extraction by overcoming TIR conditions.

For a point source with isotropic emission into a hemisphere and the same intensity for any angle, the amount of light able to escape the source material is equal to the ratio of the solid angle of the escape cone219is given by 2π(1−cos(θc)) and the full solid angle of the hemisphere (2π) is equal to 1−cos(θc). Taking an example of an OLED material with a refractive index n2=1.76 and air with refractive index n1=1.0, the critical angle is θc=arcsin(1/1.76)=34.62°.

The amount of light that will exit into the air for any sequence of different material layers on top of the OLED material (i.e., the light output as compared to the light input) is equal to 1−cos(34.62°)=17.7%. This is referred to as the external light extraction efficiency LE. This result assumes the OLED is an isotropic emitter, but the estimate of the light extraction efficiency based on this assumption is very close to the actual result obtained with more rigorous analysis and what is observed in practice.

Tapered Reflector Shape Considerations

FIG.6Ais a side view of an exemplary tapered reflector210that includes at least one curved side surface216.FIG.6Bis a side view of an embodiment of another tapered reflector210that includes at least one segmented planar side surface216. In certain exemplary embodiments, one or more side surfaces216can be defined by a single curved surface, e.g., cylindrical, parabolic, hyperbolic, or any other shape besides planar, as long as tapered reflector210is wider at second surface214than at first surface212. In one embodiment, tapered reflector210is rotationally symmetric and so includes a single side216.

Although not strictly required, the performance of the light-emitting apparatus is optimized if at any point on side surface216of a tapered reflector210the TIR condition is observed for any possible point of origin of light entering the tapered reflector through the first surface212of the tapered reflector.FIG.7Ais a plot of the z coordinate vs. x coordinate (relative units) for an example complex surface shape for side surface216calculated using a simple numerical model. The z-axis and x-axis represent normalized lengths in the respective directions. The light originating from an OLED304and passing through the spacer layer202is assumed to extend in the x-direction from [−1, 0] to [1, 0], and there is another side216that starts at [−1, 0] location but that is not shown in the plot ofFIG.7A. The shape of side216was calculated such that rays originating at [−1, 0] are always incident on the surface exactly at 45° to a surface normal. Any other ray originating at z=0 and x between −1 and 1 will have a higher incidence angle on side216than the ray originating at [−1, 0].

Performance of the light-emitting apparatus can be further improved if the height HP of tapered reflector210is such that all of the light rays emitted by OLED304exiting directly into the transparent layer306are within the escape cone219, as illustrated in the schematic diagram ofFIG.7B.FIG.7Bincludes a plane TP defined by the second surface214of tapered reflector210. The condition is met when second surface214of tapered reflector210is entirely within (i.e., not intersected by) the lines219L that define the limits of the escape cone219. The escape cone lines219L originate at the edges212E of first surface212and intersect plane TP at the critical angle θcwith respect to second surface214, where the value of θcis defined by the refractive index of the tapered reflector material npand air naas sin(θc)=na/np.

In a general case, there exists an optimum height HP of the tapered reflector210that depends on the geometry (size of and spacing between) OLEDs304and the refractive index npof tapered reflectors210. If the height HP is too small, all light rays emitted from the OLEDs304that fall at the side surfaces216of the tapered reflector210will undergo TIR, but some rays will go directly to the second surface214and be incident thereon at an angle larger than the critical angle and therefore will be trapped at the first boundary with air in the display. If the height HP is too large, all light rays going directly to the second surface214will be within the escape cone219, but some light rays falling on the side surfaces216will be within the escape cone for the side surfaces and thus exit the side surfaces. In certain exemplary embodiments, the optimum height HP of the tapered reflectors HP is typically between (0.5) WB and 2WT, more typically between WB and WT. Also in one embodiment, the local slope of the side walls216can be between about 2° and 50°, or even between about 10° and 45°. In certain exemplary embodiments, the ratio of the width WB of first surface212, the width WT of second surface214, and the height HP of each tapered reflector210to each other is between about 1:2:1.4 and 1:2:1.8. For example, each tapered reflector210may have a 28×28 μm first surface212, a 56×56 μm second surface214, and a 42 μm height.

Tapered Reflector Array

As noted above, the plurality of tapered reflectors210define a tapered reflector array208. The first surfaces212of the tapered reflectors210overlap with and are optically coupled to the emitting surfaces of OLEDs304. Since the second surfaces214of tapered reflectors210are larger than the first surfaces212, in one example (seeFIG.4) the second surfaces are sized to cover substantially the entire first surface308of transparent layer306, or as close as the specific manufacturing technique employed allows.

The example OLED display300can be thought of as having a solid material layer residing immediately above spacer layer202with a thickness equal to the height HP of tapered reflectors210and with a rectangular grid of intersecting V-groove spaces220cut into the solid material layer. Such a structure can be microreplicated in a layer of suitable resin or a photocurable or thermally curable material, with a master replication tool configured to define a rectangular grid of triangular cross-section ridges. Such a tool, for example, can be manufactured by first diamond machining the pattern that looks exactly like the tapered reflector array, and then making a master by replicating an inverse pattern. The master can be metalized for durability.

If the second surface214of each tapered reflector is twice as large as the first surface212, and the height HP of the tapered reflector is 1.5 times as tall as the bottom surface is wide, and the side walls are flat, then the slope angle θ of side surface216is arctan(⅓)=18.4°. Manufacturing tapered reflector210or an array208of tapered reflectors210having this slope angle is within the capability of diamond machining technology.

If the bottoms of the V-grooves are more rounded, then for the same slope angle θ, the height HP of tapered reflector210can be smaller than 1.5 times the size (dimension) of the first surface212. For a different configuration of OLED display300, or a different technique for making the replication masters, different restrictions on the geometry of the tapered reflectors may apply.

As explained above, to form a periodic array208of tapered reflectors210, the replication tool or mold is a negative replica of the structure, which might be considered to be an array of truncated depressions or “bowls”. When using such a tool for forming tapered reflector array208, it may be preferred to avoid trapping air in the bowls when the tool is pressed into a layer of liquid or moldable replication material. One technique to avoid such air trapping is to manufacture a replication tool or mold as an array of complete and not truncated pyramidal bowls. In this case, the height of the tapered reflectors can be controlled by the thickness of the replication material layer. The tool is pressed in the replication material until in comes in contact with transparent layer306. Air pockets will be left above each of the replicated tapered reflectors on purpose. Care can be taken to avoid rounding of the tapered reflector tops by surface tension.

The improved light-emission apparatus and methods disclosed herein rely entirely on light reflection and not light scattering. Thus, the polarization of ambient light reflected by a reflective cathode of an OLED is unchanged upon reflection, which means that the approach is perfectly compatible with the use of circular polarizers. Also, there is no haze in reflection and therefore no decrease of the display contrast ratio, which is a problem characteristic of almost all other approaches to improving light extraction using scattering techniques.

Resin Tapered Reflectors

As noted above, in one embodiment the array208of tapered reflectors210can be formed using a resin since resins are amenable to molding processes and like mass-replication techniques. When forming the array208using a resin, it is preferred that edges of transparent layer306be free of resin so that it can be coated by a frit for edge sealing. In addition, it is preferred that the resin be able to survive a 150° C. processing temperature typical of making touch sensors. Also, it is preferred that the resin exhibit no or extremely low outgassing within the operating temperature range, at least of the type most detrimental for OLED materials, namely oxygen and water.

Material for the Spaces Between the Tapered Reflectors

As noted above, the array208of tapered reflectors210and spacer layer202define confined spaces220filled with a medium having a refractive index nS. In certain exemplary embodiments, the confined spaces220are filled with air, which has a refractive index of nS=na=1. In other embodiments, spaces220can be filled with a solid material. It is generally preferred that the medium within spaces220has as low a refractive index as possible so that escape cone219stays as large as possible.

To achieve the best possible light extraction benefit, it is preferable that the index nSof the filler material be 1.2 or smaller. An example of a material with such a low refractive index is aerogel, which is porous organic or inorganic matrix filled with air or other suitable dry and oxygen-free gas. A silica-based aerogel can also serve an additional role of absorbing any residual water contamination, increasing the lifetime of the OLED materials. If the material making up the body215of each tapered reflector210has a refractive index nPof 1.7 and the refractive index of aerogel is 1.2, then the critical angle θcwill be about 45°, which is an acceptable critical angle.

Tapered Reflector Modifications

The tapered reflectors210can be modified in a number of ways to enhance the overall light extraction efficiency. For example, in one embodiment side surfaces216can include a reflective coating. This configuration allows for essentially any transparent material to fill spaces220since the tapered reflectors210no longer operate using TIR.

FIG.8Ais a top view of an exemplary hexagonal shape for tapered reflectors210. In this example, each tapered reflector210includes a hexagon shaped first surface212and six side surfaces216extending from the first surface212to a hexagon shaped second surface. The dashed lines inFIG.8Arepresent the side surfaces216of adjacent tapered reflectors210in an array of tapered reflectors.

FIG.8Bis a top view of an exemplary triangular shape for the tapered reflectors210. In this example, each tapered reflector210includes a triangle shaped first surface212and three side surfaces216extending from the first surface212to a triangle shaped second surface. The dashed lines inFIG.8Brepresent the side surfaces216of adjacent tapered reflectors210in an array of tapered reflectors.

Electronic Devices Utilizing the OLED Display

The OLED displays disclosed herein can be used for a variety of applications including, for example, in consumer or commercial electronic devices that utilize a display. Example electronic devices include computer monitors, automated teller machines (ATMs), and portable electronic devices including, for example, mobile telephones, personal media players, and tablet/laptop computers. Other electronic devices include automotive displays, appliance displays, machinery displays, etc. In various embodiments, the electronic devices can include consumer electronic devices such as smartphones, tablet/laptop computers, personal computers, computer displays, ultrabooks, televisions, and cameras.

FIG.9Ais a schematic diagram of a generalized electronic device340that includes OLED display300as disclosed herein. The generalized electronic device340also includes control electronics350electrically connected to OLED display300. The control electronics350can include a memory352, a processor354, and a chipset356. The control electronics350can also include other known components that are not shown for ease of illustration.

FIG.9Bis an elevated view of an example electronic device340in the form of a laptop computer. The laptop computer includes an OLED display300as disclosed herein.FIG.9Cis a front-on view of an example electronic device340in the form of a smart phone. The smart phone includes an OLED display300as disclosed herein.

It will be apparent to those skilled in the art that various modifications and variations can be made to embodiments of the present disclosure without departing from the spirit and scope of the disclosure. Thus it is intended that the present disclosure cover such modifications and variations provided they come within the scope of the appended claims and their equivalents.