Organic light-emitting diode displays

A display may have an array of pixels. Each pixel may have a light-emitting diode such as an organic light-emitting diode. The organic light-emitting diodes may each have a reflective electrode such as a metal anode and a partially reflective electrode such as a metal cathode. Emissive material may be formed between the electrodes. The electrodes of each organic light-emitting diode may form an optical cavity. A wrinkled layer may be formed over the optical cavity to reduce sensitivity to process variations associated with forming encapsulation structures for the display. The wrinkled layer may include annealed organic layers. The organic layers may wrinkle during an annealing process at an annealing temperature. The annealed organic layers may include a first organic layer with a glass transition temperature below the annealing temperature and a second organic layer with a glass transition temperature above the annealing temperature.

FIELD

BACKGROUND

Electronic devices often include displays. Displays such as organic light-emitting diode displays have pixels with light-emitting diodes. The light emitting diodes each have electrodes (i.e., an anode and a cathode). Emissive material is interposed between the electrodes. During operation, current passes through the emissive material between the electrodes, generating light.

The pixels in organic light-emitting diode displays may include optical cavities. The presence of an optical cavity may enhance color performance and efficiency, but may make the performance of each pixel sensitive to process variations. For example, variations in encapsulation layer thickness may result in undesired color variations.

SUMMARY

A display may have an array of pixels. Each pixel may have a light-emitting diode such as an organic light-emitting diode. The organic light-emitting diodes may each have a reflective electrode such as a metal anode and a partially reflective electrode such as a metal cathode. Emissive material may be formed between the electrodes. The electrodes of each organic light-emitting diode may form an optical cavity.

A wrinkled layer may be formed on the partially reflective electrode to reduce sensitivity to process variations associated with forming encapsulation structures for the display. The wrinkled layer may include annealed organic layers. The organic layers may wrinkle during an annealing process at an annealing temperature. The annealed organic layers may include a first organic layer with a glass transition temperature below the annealing temperature and a second organic layer with a glass transition temperature above the annealing temperature.

DETAILED DESCRIPTION

Input-output circuitry in device10such as input-output devices12may be used to allow data to be supplied to device10and to allow data to be provided from device10to external devices. Input-output devices12may 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, and other electrical components. A user can control the operation of device10by supplying commands through input-output devices12and may receive status information and other output from device10using the output resources of input-output devices12.

Input-output devices12may include one or more displays such as display14. Display14may be a touch screen display that includes a touch sensor for gathering touch input from a user or display14may be insensitive to touch. A touch sensor for display14may be based on an array of capacitive touch sensor electrodes, acoustic touch sensor structures, resistive touch components, force-based touch sensor structures, a light-based touch sensor, or other suitable touch sensor arrangements.

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.

Device10may be a tablet computer, laptop computer, a desktop computer, a display, a cellular telephone, a media player, a wristwatch device or other wearable electronic equipment, or other suitable electronic device.

Display14may be an organic light-emitting diode display or may be a display based on other types of display technology. Configurations in which display14is an organic light-emitting diode display are sometimes described herein as an example. If desired, organic light-emitting diodes may be used in non-display organic light-emitting diode devices (e.g., lighting devices).

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. 2. As shown inFIG. 2, display14may have an array of pixels22formed on substrate36. Substrate36may be formed from glass, metal, plastic, ceramic, or other substrate materials. 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 light24under the control of a pixel circuit formed from thin-film transistor circuitry such as thin-film transistors28and thin-film capacitors). Thin-film transistors28may be polysilicon thin-film transistors, semiconducting-oxide thin-film transistors such as indium gallium zinc oxide transistors, or thin-film transistors formed from other semiconductors. Pixels22may contain light-emitting diodes of different colors (e.g., red, green, and blue diodes for red, green, and blue pixels, respectively) to provide display14with the ability to display color images.

Display driver circuitry may be used to control the operation of pixels22. The display driver circuitry may be formed from integrated circuits, thin-film transistor circuits, or other suitable circuitry. Display driver circuitry30ofFIG. 2may contain communications circuitry for communicating with system control circuitry such as control circuitry16ofFIG. 1over path32. Path32may be formed from traces on a flexible printed circuit or other cable. During operation, the control circuitry (e.g., control circuitry16ofFIG. 1) may supply circuitry30with information on images to be displayed on display14.

To display the images on display pixels22, display driver circuitry30may supply image data to data lines D while issuing clock signals and other control signals to supporting display driver circuitry such as gate driver circuitry34over path38. If desired, circuitry30may also supply clock signals and other control signals to gate driver circuitry on an opposing edge of display14.

Gate driver circuitry34(sometimes referred to as horizontal control line control circuitry) may be implemented as part of an integrated circuit and/or may be implemented using thin-film transistor circuitry. Horizontal control lines G in display14may carry gate line signals (scan line signals), emission enable control signals, and other horizontal control signals for controlling the pixels of each row. There may be any suitable number of horizontal control signals per row of pixels22(e.g., one or more, two or more, three or more, four or more, etc.).

A cross-sectional side view of a portion of an illustrative organic light-emitting diode display that includes a light-emitting diode (diode26) for a pixel and thin-film transistor circuitry for an associated pixel circuit (pixel circuit48) is shown inFIG. 3. As shown inFIG. 3, display14may include a substrate layer such as substrate layer36. Substrate36may be a planar layer or a non-planar layer and may be formed from plastic, glass, ceramic, sapphire, metal, or other suitable materials. The surface of substrate36may, if desired, be covered with one or more buffer layers (e.g., inorganic buffer layers such as layers of silicon oxide, silicon nitride, etc.).

Thin-film transistor circuitry for pixel circuit48may be formed on substrate36. The thin film transistor circuitry may include transistors, capacitors, and other thin-film structures. As shown inFIG. 3, a transistor such as thin-film transistor28may be formed from thin-film semiconductor layer60. Semiconductor layer60may be a polysilicon layer, a semiconducting-oxide layer such as a layer of indium gallium zinc oxide, or other semiconductor layer. Gate layer56may be a conductive layer such as a metal layer that is separated from semiconductor layer60by an intervening layer of dielectric such as dielectric58(e.g., an inorganic gate insulator layer such as a layer of silicon oxide). Dielectric62may also be used to separate semiconductor layer60from underlying structures such as shield layer64(e.g., a shield layer that helps shield the transistor formed from semiconductor layer60from charge in buffer layers on substrate36).

Semiconductor layer60of transistor28may be contacted by source and drain terminals formed from source-drain metal layer52. Dielectric layer54(e.g., an inorganic dielectric layer) may separate gate metal layer56from source-drain metal layer52. Pixel circuit48(e.g., source-drain metal layer52) may be shorted to anode42of light-emitting diode26using a metal via such as via53that passes through dielectric planarization layer50. Planarization layer50may be formed from an organic dielectric material such as a polymer.

Light-emitting diode26is formed from light-emitting diode layers40on the thin-film transistor layers of pixel circuit48. Each light-emitting diode has a lower electrode such as anode42and an upper electrode such as cathode46. Display14may be a top emission display. In a top emission display, the lower electrode may be formed from a reflective conductive material such as patterned metal to help reflect light that is produced by the light-emitting diode in the upwards direction out of the display. The lower electrode may, as an example, be a reflective metal electrode formed from a silver alloy having a reflectivity of at least 98%, at least 99%, etc. The upper electrode (sometimes referred to as the counter electrode) may be formed from a partially reflective metal layer that forms an optical cavity for diode26. The upper electrode may, as an example, be formed from a magnesium silver alloy and may have a reflectivity of 50-70%, at least 40%, at least 55%, less than 80%, less than 75%, or other suitable reflectivity.

The partial reflectivity (partial transparency) of the upper electrode allows the upper electrode to transmit light outwards that has been produced by emissive material in the diode. Layers such as a hole injection layer, hole transport layer, emissive material layer, electron transport layer, and electron injection layer may be formed above the lower electrode and below the upper electrode.

In the illustrative configuration ofFIG. 3, display14has a top emission configuration and lower electrode42is an anode and upper electrode46is a cathode. Layers40include a patterned metal layer that forms anodes such as anode42. Anode42is formed within an opening in pixel definition layer66. Pixel definition layer66may be formed from a patterned photoimageable polymer. The photoimageable polymer may be formed from an opaque material and/or a layer of opaque material such as black masking layer66′ may cover other material in layer66(e.g., opaque layer66′ may cover a layer of semitransparent polyimide or other polymer).

In each light-emitting diode, organic emissive material44is interposed between a respective anode42and cathode46. Anodes42may be patterned from a layer of metal on a planarization layer in the thin-film transistor layers of pixel circuit48such as planarization layer50. Cathode46may be formed from a common conductive layer that is deposited on top of pixel definition layer66. Cathode46is partially reflective and partially transparent so that light24may exit light emitting diode26as current is flowing through emissive material44between anode42and cathode46. In each diode, an optical cavity is formed from a partially reflective cathode46and reflective anode42.

To protect light-emitting diodes26and other circuitry in pixels22, pixels22may be covered with encapsulation structures (e.g., a glass layer or thin-film capping layer formed on the upper surface of display14). The capping layer can contribute to the optical cavity of each light-emitting diode and therefore has the potential for influencing the color of light produced by each pixel. If care is not taken, process variations that arise when forming encapsulation for display14can cause variations in the properties of the optical cavities of light-emitting diodes26and therefore the color performance of display14.

To reduce the impact of process variations on display color performance, a wrinkled layer (sometimes referred to as a spinodal wrinkling structure) can be used as a capping layer for diodes26. Across the surface of the optical cavity of each diode26, the wrinkled layer effectively creates multiple smaller cavities with a variety of different cavity lengths. Taken together, these random cavity length variations reduce the impact of process variations in the passivation layers above the diodes on the color performance of the diodes. As a result, display14exhibits less sensitivity to encapsulation process variations.

Any suitable arrangement may be used in forming a wrinkled layer on display14. With one illustrative configuration, first and second layers of different respective organic materials are formed above diodes26. The first layer may be deposited on the surface of display14before the second layer. The first and second layers may be formed from materials of the type that are sometimes used in forming organic light-emitting diode emissive layers (e.g., hole transport layer materials, etc.) or other materials compatible with the formation of diodes26. The first layer may have a first glass transition temperature and the second layer may have a second glass transition temperature. The first glass transition temperature may be lower than the second glass transition temperature. By annealing the first and second layers at an annealing temperature between the first and second glass transition temperatures, the first and second layers may be caused to wrinkle.

FIG. 4is a cross-sectional side view of a portion of display14after first layer80and second layer82of wrinkled layer84have been deposited prior to annealing. With one illustrative arrangement, first layer80may be an organic material such as TPD (N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine, which is sometimes used as a hole transport layer in organic light-emitting diodes, and may have a glass transition temperature of about 60° C. Second layer82may, as an example, be NPB (N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine) and may have a glass transition temperature of about 100° C. The thicknesses of the first and second layers may be about 10-200 nm, 2-1000 nm, at least 20 nm, at least 30 nm, at least 50 nm, at least 100 nm, less than 150 nm, less than 120 nm, less than 100 nm, less than 80 nm, less than 40 nm, or other suitable thicknesses.

After annealing at a temperature T between 60° C. and 100° C. (e.g., for 5-10 minutes), stresses are produced that cause first layer80and second layer82to wrinkle and form wrinkled layer84ofFIG. 5. The period (peak-to-peak wrinkle size) of the wrinkles in layer84may be such that there are multiple wrinkles within each pixel area. For example, if each diode26has a lateral size of about 20-50 microns, the size of each wrinkle in layer84may be about 2-10 microns. The wrinkles may form a sinuous randomly-oriented pattern when viewed from above. Diode26may have a maximum lateral dimension of 20-50 microns, at least 15 microns, less than 75 microns, or other suitable size and the period of the wrinkles in wrinkled layer84may be such that the maximum lateral dimension divided by the period is 3-30, at least 2, at least 5, at least 10, less than 100, less than 50, less than 25, less than 20, less than 10, or other suitable amount. In general, it may be desirable for the period of the wrinkles in layer84to be greater than about one tenth of a wavelength of light (e.g., 0.05 microns, which is one tenth of 0.5 microns for green light) to ten times the wavelength of light (e.g., 5 microns for green light). The wrinkles of layer84may, for example, have a period of at least 0.1 micron, at least 0.5 microns, at least 1 micron, at least 2 microns, at least 4 microns, at least 5 microns, at least 10 microns, less than 20 microns, less than 15 microns, less than 8 microns, less than 3 microns, or other suitable size. The thickness of the wrinkles (crest-to-trough height difference) may be 0.05 microns to 10 microns, at least 0.1 microns, at least 0.5 microns, at least 1 micron, less than 20 microns, or other suitable size. If desired, wrinkled layer84may be used in larger light-emitting diodes used for lighting. For large light-emitting diodes such as diodes used in lighting, the dimensions of the wrinkles in wrinkled layer84can be enlarged accordingly.

The thickness variations imposed by the wrinkles in wrinkled layer84create optical cavity length variations for the optical cavities of diodes26. These optical cavity length variations are random and therefore help homogenize color variations that might otherwise arise from optical cavity variations in diodes26. As a result, the optical impact of process variations associated with forming encapsulation structures for display14(e.g., process variations leading to encapsulation thickness variations, etc.) may be reduced.

In the example ofFIG. 5, wrinkled layer is covered with encapsulation structures such as glass layer88. A layer such as layer86(e.g., an air gap or a layer of polymer) may be interposed between glass encapsulation layer88and wrinkled layer84.

In the example ofFIG. 6, encapsulation structures90have been formed from thin-film encapsulation layers such as passivation layer92, planarization layer94, and passivation layer96. First passivation layer92may be formed on the surface of wrinkled layer84. First passivation layer92may be formed from an inorganic dielectric layer such as a layer of silicon nitride or silicon oxynitride (as examples). The thickness of layer92may be about 0.5-2 microns, at least 0.3 microns, at least 0.8 microns, less than 3 microns, less than 1.5 microns, or other suitable thickness. Planarization layer94may be formed on passivation layer92. Planarization layer94may be formed from an organic material (e.g., a photo-cured or thermally cured liquid polymer). The thickness of planarization layer94may be 5-20 microns, at least 1.5 microns, at least 3 microns, at least 4 microns, less than 45 microns, less than 25 microns, less than 10 microns, or other suitable thickness. Second passivation layer96may be formed on layer94from an inorganic dielectric layer such as a layer of silicon nitride or silicon oxynitride (as examples). The thickness of layer96may be about 0.5-2 microns, at least 0.3 microns, at least 0.8 microns, less than 3 microns, less than 1.5 microns, or other suitable thickness.