Patent Application
20110291992
Recent years have seen very substantial growth in the market for displays as the quality of displays improves, their cost falls, and the range of applications for displays increases. This includes both large area displays such as for TVs or computer monitors and smaller displays for portable devices.
The most common classes of display presently on the market are liquid crystal displays and plasma displays although displays based on organic light-emitting diodes (OLEDs) are now increasingly attracting attention due to their many advantages including low power consumption, light weight, wide viewing angle, excellent contrast and potential for flexible displays.
The basic structure of an OLED is a light emissive organic layer, for instance a film of a poly (p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In WO90/13148 the organic light-emissive material is a conjugated polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device.
A typical organic light-emissive device (“OLED”) is fabricated on a glass or plastic substrate coated with a transparent anode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material covers the first electrode. Finally, a cathode covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminium, or a plurality of layers such as calcium and aluminium. In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an exciton which then undergoes radiative decay to give light. The device may be pixilated with red, green and blue electroluminescent subpixels in order to provide a full colour display (for the avoidance of doubt, “pixel” as used herein may refer to a pixel that emits only a single colour or a pixel comprising a plurality of individually addressable subpixels that together enable the pixel to emit a range of colours).
Full colour liquid crystal displays typically comprise a white-emitting backlight, and light emitted from the device is filtered through red, green and blue colour filters after passing through the LC layer to provide the desired colour image.
A full colour display may be made in the same way by using a white or blue OLED in combination with colour filters. Moreover, it has been demonstrated that use of colour filters with OLEDs even when the pixels of the device already comprises red, green and blue subpixels can be beneficial. In particular, aligning red colour filters with red electroluminescent subpixels and doing the same for green and blue subpixels and colour filters can improve colour purity of the display (for the avoidance of doubt, “pixel” as used herein may refer to a pixel that emits only a single colour or a pixel comprising a plurality of individually addressable subpixels that together enable the pixel to emit a range of colours).
Downconversion, by means of colour change media (CCMs) for absorption of emitted light and reemission at a desired longer wavelength or band of wavelengths, can be used as an alternative to, or in addition to, colour filters.
One way of addressing displays such as LCDs and OLEDs is by use of an “active matrix” arrangement in which individual pixel elements of a display are activated by an associated thin-film transistor. The active matrix backplane for such displays can be made with amorphous silicon (a-Si) or low temperature polysilicon (LTPS). LIPS has high mobility but can be non-uniform and requires high processing temperatures which limits the range of substrates that it can be used with. Amorphous silicon does not require such high processing temperatures, however its mobility is relatively low, and can suffer from non-uniformities during use due to aging effects. Moreover, backplanes formed from either LIPS or a-Si both require processing steps such as photolithography, cleaning and annealing that can damage the underlying substrate. In the case of LIPS, in particular, a substrate that is resistant to these high-energy processes must be selected.
An alternative approach to patterning is disclosed in, for example, Rogers et al, Appl. Phys. Lett. 2004, 84(26), 5398-5400; Rogers et al Appl. Phys. Lett. 2006, 88, 213101; and Benkendorfer et al, Compound Semiconductor, June 2007, in which silicon on an insulator is patterned using conventional methods such as photolithography into a plurality of elements (hereinafter referred to as “chiplets”) which are then transferred to a device substrate. The transfer printing process takes place by bringing the plurality of chiplets into contact with an elastomeric stamp which has surface chemical functionality that causes the chiplets to bind to the stamp, and then transferring the chiplets to the device substrate. In this way, chiplets carrying micro- and nano-scale structures such as display driving circuitry can be transferred with good registration onto an end substrate which does not have to tolerate the demanding processes involved in silicon patterning.
In one aspect, the invention provides a display comprising one or more chiplet sensors for sensing light incident on the chiplet
In one embodiment, the sensor is configured to generate a response to external light sources. The response may be an adjustment to compensate pixel brightness for ambient light conditions.
Alternatively, or additionally, the sensor is configured to generate a response to light emitted by the display.
The display may be a touch-screen display, and the display may be capable of receiving a digital communication such as an infra-red signal originating from an infra red controller or pointer.
In a second aspect, the invention provides an optical displacement sensor for a circuit comprising a plurality of chiplets, the sensor comprising a photo-sensitive area formed by an array of individual light-sensitive elements, each element configured to produce a signal or signals in response to incident light, and wherein the displacement of a chiplet from a predetermined position is derivable from the output signal or signals.
The sensor preferably comprises control circuitry for compensating positional variation derived from the displacement of the chiplet.
The plurality of individual light sensitive elements may be photodiodes and/or phototransistors.
The incident photons may originate from organic light emitting diodes (OLEDs).
The sensor may be integrated with the chiplet.
A single chiplet sensor may serve multiple subpixels.
In another aspect, the invention provides a method of measuring the displacement of at least one chiplet in an active display, the method comprising:
detecting photons from one or more light sources and producing output signals based on the detection;
comparing the relative output signals to determine the position of the one or more light sources with respect to the chiplet.
In a further aspect, the invention provides method of compensating for variation of pixel emission brightness over time, wherein emission from a pixel or subpixel is detected by a chiplet and any variation in detected pixel emission brightness is adjusted.
Preferably, one chiplet sensor detects light emitted from a plurality of pixels or subpixels.
The chiplet may both drive one or more pixels or subpixels of the display and sense emission from these pixels or subpixels.
The light emitted from the display according to any of the above aspects of the invention may be coupled to the chiplet via an optical structure selected from one of a waveguide or a grating structure.
In a yet further aspect the invention provides a method of compensating for positional variations in chiplet drive circuitry arising during manufacture of a display comprising a plurality of chiplets and light sources driven by the chiplets, the method comprising:
providing a photon detection array positioned so as to detect positional output in light from the light sources and produce an output signal based on the detection;
comparing the output signal with a predetermined value representing the expected position of the light source to calculate the positional deviation;
controlling drive circuitry so as to drive the light sources in a manner which compensates for the detected deviation.
According to one embodiment of the present invention, an optical sensor is included in at least some chiplets. According to one embodiment, an array of photodiodes is used as the optical sensor to detect the position of the emitting OLED with respect to the chiplet through examination of the relative signals on photodiodes. According to one embodiment photodiodes are used together to detect the emission from the photodiode, correctly compensating for the relative amount of light falling on the sensors due to pixel to chiplet misalignment, and use the corrected signal to program the OLED for a particular light output.
Further advantages and novel features can be found in the appended claims.
For a better understanding of the invention and as to how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:
The chiplets may be formed from semiconductor wafer sources, including bulk semiconductor wafers such as single crystalline silicon wafers, polycrystalline silicon wafers, germanium wafers; ultra thin semiconductor wafers such as ultra thin silicon wafers; doped semiconductor wafers such as p-type or n-type doped wafers and wafers with selected spatial distributions of dopants (semiconductor on insulator wafers such as silicon on insulator (e.g. Si—SiO2, SiGe); and semiconductor on substrate wafers such as silicon on substrate wafers and silicon on insulator. In addition, printable semiconductor elements of the present invention may be fabricated from a variety of nonwafer sources, such as a thin films of amorphous, polycrystalline and single crystal semiconductor materials (e.g. polycrystalline silicon, amorphous silicon, polycrystalline GaAs and amorphous GaAs) that is deposited on a sacrificial layer or substrate (e.g. SiN or SiO2) and subsequently annealed, and other bulk crystals, including, but not limited to, graphite, MoSe2 and other transition metal chalcogenides, and yttrium barium copper oxide.
The chiplets may be formed by conventional processing means known to the skilled person.
Preferably, each driver or LED chiplet is up to 500 microns in length, preferably between about 15-250 microns, and preferably about 5-50 microns in width, more preferably 5-10 microns.
The stamp used in transfer printing is preferably a PDMS stamp.
The surface of the stamp may have a chemical functionality that causes the chiplets to reversibly bind to the stamp and lift off the donor substrate, or may bind by virtue of, for example, van der Waals force. Likewise upon transfer to the end substrate, the chiplets adhere to the end substrate by van der Waals force and/or by an interaction with a chemical functionality on the surface of the end substrate, and as a result the stamp may be delaminated from the chiplets.
The chiplets patterned with drive circuitry for addressing pixels or subpixels of a display may be transfer-printed onto a substrate carrying tracking for connection of the chiplets to a power source and, if required, drivers outside the display area for programming the chiplets.
To ensure accurate transfer onto a prepared end substrate, the stamp and end substrate may be registered by means known to the skilled person, for example by providing alignment marks on the substrate.
Alternatively, tracking for connection of the chiplets may be applied after the chiplets have been transfer printed.
In the case where the chiplets drive a display such as an LCD or OLED display, the backplane comprising the chiplets is preferably coated with a layer of insulating material to form a planarisation layer onto which the display is constructed. Electrodes of the display device are connected to the output of the chiplets by means of conducting through-vias formed in the planarisation layer.
In the case where the display is an OLED, the device according to the invention comprises a glass or plastic substrate 1 onto which the backplane (not shown) has been formed, an anode 2 and a cathode 4. An electroluminescent layer 3 is provided between anode 2 and cathode 4.
In a practical device, at least one of the electrodes is semi-transparent in order that light may be emitted. Where the anode is transparent, it typically comprises indium tin oxide. Preferably, the cathode is transparent in order to avoid the problem of light emitted from electroluminescent layer 3 being absorbed by the chiplets and other associated drive circuitry in the case where light is emitted through the anode. A transparent cathode typically comprises a layer of an electron injecting material that is sufficiently thin to be transparent. Typically, the lateral conductivity of this layer will be low as a result of its thinness. In this case, the layer of electron injecting material is used in combination with a thicker layer of transparent conducting material such as indium tin oxide.
It will be appreciated that a transparent cathode device need not have a transparent anode (unless, of course, a fully transparent device is desired), and so the transparent anode used for bottom-emitting devices may be replaced or supplemented with a layer of reflective material such as a layer of aluminium. Examples of transparent cathode devices are disclosed in, for example, GB 2348316.
Suitable materials for use in layer 3 include small molecule, polymeric and dendrimeric materials, and compositions thereof. Suitable electroluminescent polymers for use in layer 3 include poly(arylene vinylenes) such as poly(p-phenylene vinylenes) and polyarylenes such as: polyfluorenes, particularly 2,7-linked 9,9 dialkyl polyfluorenes or 2,7-linked 9,9 diaryl polyfluorenes; polyspirofluorenes, particularly 2,7-linked poly-9,9-spirofluorene; polyindenofluorenes, particularly 2,7-linked polyindenofluorenes; polyphenylenes, particularly alkyl or alkoxy substituted poly-1,4-phenylene. Such polymers as disclosed in, for example, Adv. Mater. 2000 12(23) 1737-1750 and references therein. Suitable electroluminescent dendrimers for use in layer 3 include electroluminescent metal complexes bearing dendrimeric groups as disclosed in, for example, WO 02/066552.
Further layers may be located between anode 2 and cathode 3, such as charge transporting, charge injecting or charge blocking layers.
The device is preferably encapsulated with an encapsulant (not shown) to prevent ingress of moisture and oxygen. Suitable encapsulants include a sheet of glass, films having suitable barrier properties such as alternating stacks of polymer and dielectric as disclosed in, for example, WO 01/81649 or an airtight container as disclosed in, for example, WO 01/19142. A getter material for absorption of any atmospheric moisture and/or oxygen that may permeate through the substrate or encapsulant may be disposed between the substrate and the encapsulant.
Throughout this specification, the term “control circuit” is used to refer to circuitry for programming the drive circuitry; “drive circuitry” is used to refer to circuitry for directly driving pixels of the display; and “display area” is used to refer to area defined by pixels of the display and associated drive circuitry.
Those skilled in the art will appreciate that while this disclosure has described what is considered to be the best mode and, where appropriate, other modes of performing the invention, the invention should not be limited to the specific configurations and methods disclosed in this description of the preferred embodiment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 08194474 | Oct 2008 | GB | national |
| 09006172 | Jan 2009 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2009/002509 | 10/21/2009 | WO | 00 | 8/18/2011 |
