Organic light-emitting diode displays with reflectors

A display may have an array of pixels formed from organic light-emitting diodes and thin-film transistor circuitry. Each pixel may include organic layers interposed between an anode and a cathode. The organic layers may emit out-coupled light that escapes the display and waveguided light that is waveguided within the organic layers. A reflector may be placed at the edge of the organic layers to reflect the waveguided light out of the display. The reflector may be located within a pixel definition layer and may be formed from metal or may be formed from one or more interfaces between high-refractive-index material and low-refractive-index material. The reflector may be formed from an extended portion of the pixel anode. The reflector may be formed from light-reflecting particles that are suspended in the pixel definition layer.

BACKGROUND

This relates generally to electronic devices with displays and, more particularly, to electronic devices with organic light-emitting diode displays.

Electronic devices often include displays. Displays such as organic light-emitting diode displays include arrays of pixels that emit light to display images for a user. The pixels of a display may include subpixels of different colors to provide the display with the ability to display color images. The organic light-emitting diodes are controlled by thin-film transistor circuitry.

It can be challenging to achieve high efficiency from an organic light-emitting diode display without sacrificing off-axis viewing quality. Typically, higher on-axis efficiency results in worse off-axis color variation. Off-axis color variation can be reduced, but usually this is at the expense of on-axis efficiency.

It would therefore be desirable to be able to provide improved organic light-emitting diode displays.

SUMMARY

A display may have an array of pixels on a substrate. The display may be an organic light-emitting diode display and the pixels may include organic light-emitting diodes of different colors. The display may include thin-film transistor circuitry that controls the organic light-emitting diode pixels.

Each organic light-emitting diode may have an anode, a cathode, and organic layers between the anode and cathode. The organic layers may emit out-coupled light that escapes the display and waveguided light that is waveguided within the organic layers. A reflector may be placed at the edge of the organic layers to reflect the waveguided light out of the display. The reflector may completely or partially surround each organic light-emitting diode in the display.

The reflector may be located within a pixel definition layer or may replace a pixel definition layer in the display. The reflector may be formed from metal or may be formed from one or more interfaces between high-refractive-index material and low-refractive-index material. The reflector may be formed from an extended portion of the pixel anode. The reflector may be formed from light-reflecting particles that are suspended in the pixel definition layer. An opaque portion of the pixel definition layer may be used to prevent color mixing between adjacent pixels.

By reflecting the waveguided light out of the organic layers, the reflector may help increase the overall extraction efficiency of the display. Additionally, the reflector may help mitigate cavity effects at wide viewing angles by increasing off-axis efficiency and reducing off-axis color variation.

DETAILED DESCRIPTION

An illustrative electronic device of the type that may be provided with a display is shown inFIG. 1. Electronic device10may be a computing device such as a laptop computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wrist-watch device, a pendant device, a headphone or earpiece device, a device embedded in eyeglasses or other equipment worn on a user's head, or other wearable or miniature device, a computer monitor or other display containing an embedded computer or other electronic equipment, a computer display or other monitor that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, or other electronic equipment. In the illustrative configuration ofFIG. 1, device10is a portable device such as a cellular telephone, media player, tablet computer, wrist device, or other portable computing device. Other configurations may be used for device10if desired. The example ofFIG. 1is merely illustrative.

In the example ofFIG. 1, device10includes a display such as display14mounted in housing12. Housing12, which may sometimes be referred to as an enclosure or case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. Housing12may be formed using a unibody configuration in which some or all of housing12is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.).

Display14may be a touch screen display that incorporates a layer of conductive capacitive touch sensor electrodes or other touch sensor components (e.g., resistive touch sensor components, acoustic touch sensor components, force-based touch sensor components, light-based touch sensor components, etc.) or may be a display that is not touch-sensitive. Capacitive touch screen electrodes may be formed from an array of indium tin oxide pads or other transparent conductive structures. A touch sensor may be formed using electrodes or other structures on a display layer that contains a pixel array or on a separate touch panel layer that is attached to the pixel array (e.g., using adhesive).

Display14may include an array of pixels formed from liquid crystal display (LCD) components, an array of electrophoretic pixels, an array of plasma pixels, an array of organic light-emitting diode pixels or other light-emitting diodes, an array of electrowetting pixels, or pixels based on other display technologies. Configurations in which display14is an organic light-emitting diode display are sometimes described herein as an example. The use of organic light-emitting diode pixels to form display14is merely illustrative. Display14may, in general, be formed using any suitable type of pixels.

Display14may be protected using a display cover layer such as a layer of transparent glass or clear plastic. Openings may be formed in the display cover layer. For example, an opening may be formed in the display cover layer to accommodate a button, a speaker port, or other component. Openings may be formed in housing12to form communications ports (e.g., an audio jack port, a digital data port, etc.), to form openings for buttons, etc.

Input-output circuitry in device10such as input-output devices18may be used to allow data to be supplied to device10and to allow data to be provided from device10to external devices. Input-output devices18may include buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, vibrators, cameras, sensors, light-emitting diodes and other status indicators, data ports, etc. A user can control the operation of device10by supplying commands through input-output devices18and may receive status information and other output from device10using the output resources of input-output devices18. Input-output devices18may include one or more displays such as display14.

Control circuitry16may be used to run software on device10such as operating system code and applications. During operation of device10, the software running on control circuitry16may display images on display14using an array of pixels in display14.

Display14may have a rectangular shape (i.e., display14may have a rectangular footprint and a rectangular peripheral edge that runs around the rectangular footprint) or may have other suitable shapes. Display14may be planar or may have a curved profile.

A top view of a portion of display14is shown inFIG. 3. As shown inFIG. 3, display14may have an array of pixels22. Pixels22may receive data signals over signal paths such as data lines D and may receive one or more control signals over control signal paths such as horizontal control lines G (sometimes referred to as gate lines, scan lines, emission control lines, etc.). There may be any suitable number of rows and columns of pixels22in display14(e.g., tens or more, hundreds or more, or thousands or more). Each pixel22may have a light-emitting diode26that emits light80under the control of a pixel control circuit formed from transistor circuitry such as thin-film transistors58and thin-film capacitors). Transistors58may be polysilicon thin-film transistors, semiconducting-oxide thin-film transistors such as indium gallium zinc oxide transistors, or transistors formed from other semiconductors.

A cross-sectional side view of a portion of an illustrative organic light-emitting diode display in the vicinity of one of light-emitting diodes26is shown inFIG. 4. As shown inFIG. 4, display14may include a substrate layer such as substrate layer30. Substrate30may be formed from polymer, glass, sapphire, a semiconductor material such as silicon, or other suitable materials.

Thin-film transistor circuitry44may be formed on substrate30. Thin-film transistor circuitry44may include layers32. Layers32may include inorganic layers such as inorganic buffer layers, barrier layers (e.g., barrier layers to block moisture and impurities), gate insulator, passivation, interlayer dielectric, and other inorganic dielectric layers. Layers32may also include organic dielectric layers such as a polymer planarization layer. Metal layers and semiconductor layers may also be included within layers32. For example, semiconductors such as silicon, semiconducting-oxide semiconductors, or other semiconductor materials may be used in forming semiconductor channel regions for thin-film transistors58(FIG. 3). Metal in layers32such as metal traces74may be used in forming transistor gate terminals, transistor source-drain terminals, capacitor electrodes, and metal interconnects.

As shown inFIG. 4, thin-film transistor circuitry44may include diode anode structures such as anode36. Anode36may be formed from a layer of conductive material such as metal on the surface of layers32(e.g., on the surface of a planarization layer that covers underlying thin-film transistor structures). Light-emitting diode26may be formed within an opening in pixel definition layer60. Pixel definition layer60may be formed from a patterned photoimageable polymer such as polyimide and/or may be formed from one or more inorganic layers such as silicon nitride, silicon dioxide, or other suitable materials.

In each light-emitting diode, layers of organic material38may be interposed between a respective anode36and cathode42. Anodes36may be patterned from a layer of metal (e.g., silver, aluminum, or other suitable metal) and/or one or more other conductive layers such as a layer of indium tin oxide, molybdenum oxide (MoOx), titanium nitride (TiNx), or other transparent conductive material. Cathode42may be formed from a common conductive layer that is deposited on top of pixel definition layer60. Cathode42may be formed from a thin metal layer (e.g., a layer of metal such as a magnesium silver layer) and/or indium tin oxide or other transparent conductive material. Cathode42is preferably sufficiently transparent to allow light80to exit light-emitting diode26.

Anode36may be formed from one or more layers of non-conducting materials (e.g., silicon oxide (SiOx), silicon nitride (SiNx), or polymers) with a top layer of conductive transparent material (e.g., indium tin oxide, indium gallium zinc oxide, etc.) and a bottom layer of reflective metal (e.g., silver, aluminum, a compound of reflective metals, etc.).

The example ofFIG. 4in which the anode of diode26is formed from a patterned conductive layer and the cathode of diode26is formed from a blanket conductive layer is merely illustrative. If desired, anode36may be formed from a blanket conductive layer and cathode42may be formed from a blanket conductive layer. The configuration of display14in which a transparent blanket cathode layer42covers diodes that have individually patterned anodes36allows light80to be emitted from the top of display14(i.e., the example ofFIG. 4in which display14is a “top emission” organic light-emitting diode display) is also merely illustrative. Display14may be implemented using a bottom emission configuration if desired. Layers such as layers36,38, and42are used in forming organic light-emitting diodes such as diode26ofFIG. 4, so this portion of display14is sometimes referred to as an organic light-emitting diode layer (see, e.g., layer130ofFIG. 4).

Metal interconnect structures may be used to interconnect transistors and other components in circuitry44. Metal interconnect lines may also be used to route signals to capacitors, to data lines D and gate lines G, to contact pads (e.g., contact pads coupled to gate driver circuitry), and to other circuitry in display14. As shown inFIG. 4, layers32may include one or more layers of patterned metal for forming interconnects such as metal traces74(e.g., traces74may be used in forming data lines D, gate lines G, power supply lines, clock signal lines, and other signal lines).

If desired, display14may have a protective outer display layer such as cover layer70. The outer display layer may be formed from a material such as sapphire, glass, plastic, clear ceramic, or other transparent material. Protective layer46may cover cathode42. Layer46, which may sometimes be referred to as a thin film encapsulation layer, may include moisture barrier structures, encapsulant materials such as polymers, adhesive, and/or other materials to help protect thin-film transistor circuitry.

Functional layers68may be interposed between layer46and cover layer70. Functional layers68may include a touch sensor layer, a circular polarizer layer, and other layers. A circular polarizer layer may help reduce light reflections from reflective structures such as anodes36and cathode42. A touch sensor layer may be formed from an array of capacitive touch sensor electrodes on a flexible polymer substrate. The touch sensor layer may be used to gather touch input from the fingers of a user, from a stylus, or from other external objects. Layers of optically clear adhesive may be used to attach cover layer70(e.g., a layer of glass, sapphire, polymer, or other suitable material) and functional layers68to underlying display layers such as layer46, thin-film transistor circuitry44, and substrate30.

Organic layers38may include an organic emissive layer (e.g., a red emissive layer in red diodes26that emits red light, a green emissive layer in green diodes26that emits green light, a blue emissive layer in blue diodes26that emits blue light, a combination of red, green, and blue emissive materials that emit white light, etc.). The emissive material may be a material such as a phosphorescent material or fluorescent material that emits light during diode operation. The emissive material in layer38may be sandwiched between additional diode layers such as hole injection layers, hole transport layers, electron injection layers, and electron transport layers.

During operation, holes and electrons are injected into organic layers38from anode36and cathode42, respectively. When an electron meets a hole, an electron-hole pair is formed and photons are released. The photons generated in organic layers38may be emitted at different angles. Some of the light generated in organic layers38such as light80will be emitted parallel to (or within a given angle of) the z-axis ofFIG. 4and will escape display14. This type of light is sometimes referred to as “air mode” light or “out-coupled mode” light. Other light generated in organic layers38such as light80′ may be emitted at a non-zero angle relative to the z-axis ofFIG. 4and may be reflected when it strikes an interface between a low-refractive-index layer and a high-refractive-index layer at a certain angle. Light that bounces back and forth within the organic layers of an organic light-emitting diode is sometimes referred to as “waveguide mode” light because the light is waveguided within the organic layers.

If care is not taken, waveguide mode light may become trapped and absorbed within the light-emitting diode device and may be unable to escape the display, thereby sacrificing light extraction efficiency (i.e., the ratio of photons generated in a light-emitting diode to photons escaping the display).

To increase the light extraction efficiency of display14, reflective structures may be used to help extract the waveguide mode light from display14.FIG. 5is a cross-sectional side view of a portion of display14illustrating how a reflective structure may be used to help extract waveguide mode light from display14. As shown inFIG. 5, light-emitting diode26may include organic layers38interposed between cathode42and anode36. Light-emitting diode26may emit light in different directions. Light80that is emitted within a given angle of the z-axis is extracted from display14as air mode light. Light80′ that is emitted at wider angles, however, may be reflected at interfaces between low-refractive-index materials and a high-refractive-index materials and may be guided within device26via total internal reflection. To help extract waveguide mode light80′, a reflector such as reflector72may be placed at one or more edges of organic layers38. Reflector72may have an angled surface relative to the z-axis (i.e., angled relative to the surface normal of substrate30ofFIG. 4) such that waveguide mode light80′ is directed out of display14towards viewer82. Reflector72may, for example, be angled between 25 and 35 degrees from the z-axis, between 20 degrees and 45 degrees from the z-axis, between 0 and 90 degrees from the z-axis, about 30 degrees from the z-axis, less than 30 degrees from the z-axis, or more than 30 degrees from the z-axis.

Reflector72may be formed from metal that has been deposited and patterned, sheet metal (e.g., stamped sheet metal), metallized polymer film, a thin-film metal on a plastic carrier, a dielectric thin-film stack that forms a dielectric mirror (a thin-film interference mirror) on a polymer film or molded plastic carrier, an interface between a high-refractive-index material and a low-refractive index material, a white reflective film (e.g., a glossy white polymer sheet formed from a white ink layer or other white layer on a polymer carrier covered with a glossy coating such as a glossy polymer coating), or other suitable reflector structure. Reflector72may have a rectangular ring-shaped outline (e.g., a rectangular footprint when viewed from above) or may have other suitable shapes.

By reflecting waveguide mode light80′ out of organic layers38, reflector72may help increase the overall extraction efficiency of display14. Additionally, whereas air mode light80may be subject to spectrum narrowing and color variation at wide viewing angles (e.g., due to cavity effects), waveguide mode light80′ has a broader spectrum and less sensitivity to viewing angle. As such, not only does reflector72increase the overall extraction efficiency of display14, but it also mitigates cavity effects at wide viewing angles (sometimes referred to as off-axis viewing angles) by increasing off-axis efficiency and reducing off-axis color variation.

FIG. 6is a top view of illustrative pixels in display14. Pixels in display14may include color pixels (sometimes referred to as subpixels) such as red pixel22R, blue pixel22B, and green pixel22G. As shown inFIG. 6, each pixel may include an organic light-emitting diode surrounded by an associated reflector. For example, red pixel22R may include light-emitting diode26R surrounded by reflector72R, blue pixel22B may include light-emitting diode26B surrounded by reflector72B, and green pixel22G may include light-emitting diode26G surrounded by reflector72G. The example ofFIG. 6in which reflectors72completely surround light-emitting diodes26is merely illustrative. If desired, reflectors72may only partially surround light-emitting diodes26, reflectors72may be located on one, two, three, or four sides of light-emitting diodes26, a reflector72on one side of light-emitting diode26need not contact a reflector72on an adjacent side of light-emitting diode26, reflectors72in one pixel need not contact reflectors72in an adjacent pixel, etc. Reflectors72may be located in the non-emitting portion of each pixel22(sometimes referred to as the inactive area of pixels22). Reflectors72may have the same structure for all pixels in display14or reflectors72may have different structures for different pixels. For example, reflectors72may have a different shape, size, and/or structure for different color pixels, if desired.

FIG. 7is a graph showing how off-axis viewing of display14may be improved using a reflector such as reflector72to extract waveguide mode light from organic layers38. Curve84represents air mode light (e.g., light80ofFIG. 5) emitted from display14, and curve86represents waveguide mode light (e.g., light80′ ofFIG. 5) emitted from display14. For on-axis viewing (e.g., around 0°), air mode light is highly efficient and therefore achieves relatively high luminance. In arrangements where the light-emitting diodes of display14include micro-cavities (e.g., in which light is reflected up and down within the pixel layers before exiting display14), a spectral narrowing can occur that causes off-axis efficiency to decrease and color variation to increase at wide viewing angles. The recycling of waveguide mode light80′ (curve86) mitigates these affects by adding broader spectrum light with less color variation at wide viewing angles.

FIGS. 8-15show cross-sectional side views of illustrative reflectors72that may be used to recycle waveguide mode light80′ as discussed in connection withFIGS. 5-7. In these examples, reflectors72are located between red pixel22R and blue pixel72B. However, it should be understood that this is merely illustrative and that the reflectors ofFIGS. 8-15may be used between any suitable pair of pixels in display14. It should also be understood that any one of the embodiments shown inFIGS. 8-15may be combined with any one or more of the other embodiments shown inFIGS. 8-15, if desired.

In the example ofFIG. 8, reflectors72R and72B are located in pixel definition layer60. Reflector72R may be used to reflect waveguide mode light within organic layers38R out of display14, and reflector72B may be used to reflect waveguide mode light within organic layers38B out of display14. As discussed in connection withFIG. 5, reflector72may be formed from sheet metal (e.g., stamped sheet metal), metallized polymer film, a thin-film metal on a plastic carrier, a dielectric thin-film stack that forms a dielectric mirror (a thin-film interference mirror) on a polymer film or molded plastic carrier, an interface between a high-refractive-index material and a low-refractive index material, a white reflective film (e.g., a glossy white polymer sheet formed from a white ink layer or other white layer on a polymer carrier covered with a glossy coating such as a glossy polymer coating), or other suitable reflector structure. In arrangements where reflector72is formed from an interface between a high-refractive-index material and a low-refractive index material, reflector72may be formed from a material that has a lower or higher refractive index than surrounding pixel definition layer60. For example, if pixel definition layer60has a refractive index of about 1.7, reflectors72R and72B may have a lower refractive index at about 1.4, or may have a higher refractive index at about 2.3. Materials having other refractive indices may be used, if desired. These examples are merely illustrative.

In the example ofFIG. 9, reflector72between adjacent pixels22R and22B is formed from a single structure (rather than the two separate structures shown inFIG. 8). The structure may have a first angled surface that reflects waveguide mode light from organic layers38R out of display14and a second opposing angled surface that reflects waveguide mode light from organic layers38B out of display14.

In the example ofFIG. 10, reflectors72are formed from extended portions of the anodes in pixels22. Anode36R of red pixel22R has a first portion36R1that is parallel to substrate30and a second portion36R2that is angled relative to substrate30. Anode36B of blue pixel22B has a first portion36B1that is parallel to substrate30and a second portion36B2that is angled relative to substrate30. Because anodes36R and36B are reflective (e.g., around 95% reflective), extended portion36R2may reflect waveguide mode light from organic layers38R out of display14and extended portion36B2may reflect waveguide mode light from organic layers38B out of display14.

In the example ofFIG. 11, reflector72is formed from light-reflective particles88within pixel definition layer60. Particles88may be titanium dioxide particles, silica composite particles, other ceramic particles, or other reflective particles. In the example ofFIG. 11, particles88are suspended in pixel definition layer60. If desired, particles88may be suspended in other materials (e.g., particles88may be suspended within a matrix such as a matrix formed from epoxy, acrylic, silicone, or other polymer materials within or on layer60).

In the example ofFIG. 12, reflector72is formed from light-reflective particles88on opposing sides of a light-blocking structure such as light-blocking structure90. Light-blocking structure90may be an opaque (e.g., black) portion of pixel definition layer60or may be other suitable light-blocking structure or opaque coating. Light-blocking structure90may help prevent color mixing between adjacent pixels.

In the example ofFIG. 13, reflector72is formed from a low-refractive-index material96. The difference in refractive index between organic layers38and low-refractive index material96may cause waveguide mode light out of diode26. Low-refractive-index material96may have a refractive index that is less than 1.6, for example, or may have other suitable refractive index that is lower than the refractive index of organic layers38R and38B. If desired, low-refractive index material96may replace pixel definition layer60.

In the example ofFIG. 14, reflector72is formed from reflective coatings76on structure78. Structure78may be formed from polymer, metal, or other suitable material. If desired, structure78may be formed from the same material as pixel definition layer60. Coating76may be metallized polymer film, a thin-film metal, a dielectric thin-film stack that forms a dielectric mirror (a thin-film interference mirror) on a polymer film or molded plastic carrier, an interface between a high-refractive-index material and a low-refractive index material, a white reflective film (e.g., a glossy white polymer sheet formed from a white ink layer or other white layer on a polymer carrier covered with a glossy coating such as a glossy polymer coating), or may be a coating of reflective particles suspended in a matrix.

In the example ofFIG. 15, reflectors72are formed from reflective coatings76that are formed on opposing outer surfaces of pixel definition layer60(rather than being formed within pixel definition layer60as in the example ofFIG. 14).