Electroluminescent display and driving circuit with reduced photoluminescenceTechnical Field
The present invention relates generally to display driver circuits for electro-optic displays, and more particularly to a circuit and method for reducing the reemission of absorbed light, such as for increasing the color shift of organic light emitting diode displays.
Background
Organic Light Emitting Diodes (OLEDs) comprise a particularly advantageous form of electro-optic display. They are bright, colored, rapidly switchable, and provide a wide viewing angle, are easy to manufacture on a variety of substrates, and are inexpensive. Organic light emitting diodes can be fabricated using either polymers or small molecules in the color range (or in multicolor displays), depending on the materials used. Examples of polymer-based organic light-emitting diodes are described, for example, in WO90/13148, WO 95/06400, WO 99/48160, while examples based on so-called small molecules are described in U.S. Pat. No. 4,539,507.
Fig. 1a shows the basic structure 100 of a typical organic light emitting diode. A glass or plastic substrate 102 supports a transparent anode layer 104, the transparent anode layer 104 for example comprising Indium Tin Oxide (ITO) on which are deposited a hole transport layer 106, an electroluminescent layer 108 and a cathode 110. The electroluminescent layer 108 may for example comprise PPV (poly (p-phenylene vinylene)) and a hole transport layer 106, the hole transport layer 106 helping to match the hole energy levels of the anode layer 104 and the electroluminescent layer 108; the electroluminescent layer 108 may for example comprise PEDOT: PSS (polystyrene sulfonate doped polyethylene-dihydroxyphen). The cathode layer 110 typically comprises a low work function metal such as calcium and may include an additional layer, such as an aluminum layer, next to the electroluminescent layer 108 for improving the energy level matching of the electrons. Contact lines 114 and 116 to the anode and cathode, respectively, provide a connection to an energy source 118. For small molecule devices, the same basic structure may also be used.
Other examples of materials that may be used for the layer 108 include: poly (2-methoxy-5- (2' -ethyl) hexylhydroxyphenyl-vinylene) ("MEH-PPV"), derivatives of PPV (such as di-alkoxy or di-alkyl derivatives), polyfluorenes and/or copolymers incorporating polyfluorene segments, PPV and/or related copolymers, poly (2, 7- (9, 9-di-n-octafluorenes) - (1, 4-benzene- ((4-sec-butylphenyl) imino) -1, 4-benzene)) (TFB), (PFB) poly (2, 7- (9, 9-di-n-octafluorenes) - (1, 4-benzene- ((4-methoxyphenyl) imino) -1, 4-benzene)) (PFM), poly (2, 7- (9, 9-bis-n-octafluorene) - (1, 4-benzene- ((4-methoxyphenyl) imino) -1, 4-benzene- ((4-methoxyphenyl) imino-14-benzene)) (PFMO), poly (2, 7- (9, 9-bis-n-octafluorene) (F8), or poly (2, 7- (9, 9-bis-n-octafluorene) -3, 6-benzothiadiazole (F8 BT). alternatively, so-called small molecules as described in U.S. Pat. No. 4,539,507, such as hydroxyquinoline aluminate ("Alq 3"), may be used.
In the example shown in FIG. 1a, light 120 is emitted through the transparent pole 104 and the substrate 102, and such a device is referred to as a "bottom emitter". Devices that emit through the cathode can also be constructed, for example, by keeping the thickness of the cathode layer 110 less than 50-100nm so that the cathode is substantially transparent.
Organic light emitting diodes may be deposited in a matrix of pixels on a substrate to form a single or multi-colour pixel display. A multicolor display may be constructed using groups of red, green, and blue emitting pixels. In such displays, the display is typically produced by addressing the elements by actuating a row (or column) of selected pixels, and writing the row (or column) of pixels to the elements. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel, whereas passive matrix displays have no such memory element and instead scan repeatedly, approximating a television picture, giving the impression of a stable image.
Fig. 1b shows a cross section through a passive matrix organic light emitting diode display 150, in which the same elements as described in fig. 1a are indicated by the same reference numerals. In a passive matrix display 150 the electroluminescent layer 108 comprises a plurality of pixels 152 and the cathode layer 110 comprises a plurality of electrically insulated wires 154, the wires 154 being oriented into the page in fig. 1b, each wire 154 having an associated contact 156. Similarly, the indium tin oxide anode layer 104 also includes a plurality of anode lines 158, only one of which 158 is shown in FIG. 1b, the anode lines 158 being oriented at right angles to the cathode lines. A contact (not shown in fig. 1 b) is also provided for each anode line. Electroluminescent pixels 152 at the intersections of the cathode and anode lines can be addressed by applying a voltage between the associated anode and cathode lines.
Reference is now made to fig. 2a, which shows in principle a driving arrangement for a passive matrix organic light emitting diode display 150 of the type shown in fig. 1 b. A plurality of constant current generators 200 are provided, each constant current generator 200 being connected to a power supply line 202 and to one of a plurality of column lines 204, only one of which is shown for clarity. A plurality of row lines 206 (only one of which is shown) are also provided, one of which is selectively coupled to ground 208 via a switch connection 210. As shown, with respect to line 202 using a positive supply voltage, column line 204 includes an anode connection 158 and row line 206 includes a cathode connection 154, but if the supply line 202 is negative and is relative to ground line 208, the connection is reversed.
As shown, the pixels 212 of the display have been powered and are thus illuminated. To generate an image, the row connection 210 is maintained while each column line is activated in turn until the entire row has been addressed, then the next row is selected and the process is repeated. Alternatively, a row may be selected and all columns written in parallel, i.e., a row selected and current driven to each column line simultaneously, so that each pixel in a row may be illuminated at the desired brightness simultaneously. Although this latter arrangement requires more column driver circuitry, it is preferred because it enables each pixel to be updated more quickly. In another alternative arrangement, each pixel in a column may be addressed in turn, and then the next column addressed, although this is not perfect, since, in particular, there is also the effect of column capacitance as will be discussed below. It will be appreciated that the functions of the column driver and the row driver may be interchanged according to the arrangement of figure 2 a.
It is advantageous to provide a current controlled drive for the organic light emitting diode instead of a voltage controlled drive, because the brightness of the organic light emitting diode is determined by the current flowing through it, which determines the number of photons it outputs. In a voltage controlled configuration, the brightness may vary across the display area and may change with time, temperature and aging, making it difficult to predict how bright a pixel will be when driven with a given voltage. In a color display, the accuracy of the color representation may also be affected.
Figures 2b-2d show a drive current 220 applied to a pixel, a drive voltage 222 applied across the pixel, and a light output 224 from the pixel as a function of time 226 when the pixel is addressed, respectively. The row containing this pixel is addressed and a drive current is issued to the column in which this pixel is located at the time indicated by the dashed line 228. The column lines (and pixels) have an associated capacitance and thus rise gradually to a maximum value 230. Until point 232 is reached, the voltage across the pixel is greater than the diode voltage drop of the organic light emitting diode and the pixel does not begin to emit light. Similarly, when the drive current is turned off at time 234, the voltage and light output gradually decay as the column capacitance discharges. In the case where the pixels within a column are all written simultaneously, i.e., the column is driven in parallel, the time interval between times 228 and 234 corresponds to the row scan period.
It is desirable to be able to provide a display of the grayscale type, i.e. in which the apparent brightness of the individual pixels is variable, rather than simply being set on or off. In the context of the present invention, "gray scale" refers to such variable brightness display whether the pixel is white or in color.
The conventional method of changing the brightness of a pixel is to change the time that the image is on using "pulse width modulation" (PWM). In the context of fig. 2b above, the apparent brightness of the pixel can be varied by varying the percentage of the time interval between times 228 and 234 that is applied to the drive current. In the pulse width modulation scheme, the pixels are either fully on or fully off, but the apparent brightness of the pixels changes as the overall result is seen in the observer's eye.
The pulse width modulation scheme provides a good linear luminance response, but in order to overcome the effects associated with delaying the turning on of the pixel, such a scheme typically employs a pre-charge current pulse (not shown in fig. 2 b) at the leading edge 236 of the drive current waveform, while employing a discharge pulse at the trailing edge 238 of the drive current waveform. Thus, in displays incorporating such brightness control, it may take half the power consumption to charge (and discharge) the column capacitance. Applicants believe that other factors that contribute significantly to power consumption for a display plus driver combination include: the power consumption of the organic light emitting diode itself (related to the efficiency of the organic light emitting diode), resistive losses in the row and column lines, and the limited compliance effect of the current drivers, which are particularly important in practical circuits, will be described in detail later.
Fig. 3 shows a schematic diagram of a generic driver circuit for a passive matrix organic light emitting diode display. An organic light emitting diode display is indicated by dashed line 302 and comprises n row lines 304 and m column lines 308, each of the n row lines 304 having a corresponding row electrode contact 306 and each of the m column lines 308 having a corresponding column electrode contact 310. An organic light emitting diode is connected between each pair of row and column lines, and in the arrangement shown, the anode of the organic light emitting diode is connected to the column line. A y-driver 314 drives the column lines with a constant current and an x-driver 316 drives the row lines 304, selectively connecting the row lines to ground. Both the y-driver 314 and the x-driver 316 are typically under the control of a processor 318. The power supply 320 provides power to the circuitry, and in particular to the y-driver 314.
Fig. 4 shows a typical active matrix organic light emitting diode display circuit 400. One circuit 400 is provided for each pixel of the display and ground 402, VSS404, row select 414 and column data bus 416 are provided to interconnect the individual pixels. Thus, each pixel has a power and ground connection, each row of pixels has a common row select line 414, and each column of pixels has a common data line 416.
Each pixel has an organic light emitting diode 406. the organic light emitting diode 406 is connected in series with a driver transistor 408 between ground 402 and a power supply 404. The gate connection 409 of the drive transistor 408 is coupled to a storage capacitor 410 and the control transistor 412 is coupled to a column data line 416 under the control of a row select line 414. The control transistor 412 is a Field Effect Transistor (FET) switch that connects the column data line 416 to the gate 409 and the capacitor 410 when the row select line 414 is activated. Thus, when the switch 412 is turned on, the voltage on the column data line 416 may be stored on the capacitor 410. This voltage on the capacitor is able to hold at least the update period of the frame because the impedance to the gate connection of the drive transistor 408 is relatively high and the switching transistor 412 is in its off state.
The drive transistor 408 is typically a field effect transistor and the (drain-source) current passed depends on the transistor gate voltage being less than the threshold voltage. Thus, the voltage of the gate node 409 controls the current through the organic light emitting diode 406 and, thus, the brightness of the organic light emitting diode.
A voltage driven active matrix display is described in US5,684,365; a current driven active matrix display is described in WO 99/65012. Other specific examples of organic light emitting diode display drivers are described in US6,014,119, US6,201,520, US6,332,661, EP1,079,361a and EP1,091,339 a; organic light emitting diode display driver integrated circuits are sold by the ClareMicronix of Clare corporation (Beverly, MA, USA). The Clare micron driver provides a current control driver and realizes gray scale display by using a conventional pulse width modulation method; the driver circuit described in US6,014,119 uses pulse width modulation to control brightness; the column driver in the driver circuit described in US6,201,520 has a constant current generator to generate digital (on/off) pixel control; the reference current generator in the pixel driver circuit described in US6,332,661 sets the current output for a multi-column constant current driver, however, this arrangement is not suitable for variable brightness displays; EP1,079,361A and EP1,091,339 a both describe similar drivers for organic electroluminescent display elements in which voltage driving is employed rather than current driving.
Unlike LCDs, display technologies based on inherently emissive devices tend to have a bright, visually pleasing appearance. It is therefore always desirable to improve the visual contrast of emissive displays, especially for organic light emitting diode based displays, but it is not always clear what effect is the cause of the reduced contrast. The applicant has realised that electroluminescent materials commonly used in organic and non-organic light emitting diodes are also generally photoluminescent and that this photoluminescence may degrade contrast.
Broadly, photoluminescence is the phenomenon whereby a material absorbs light of one wavelength and re-emits light of a longer wavelength. Even under laboratory conditions, this photoluminescence is difficult to observe, but produces an effect: the appearance of the display is made less vivid, especially in bright ambient light conditions, especially in outdoor sunlight. The applicant has found that such contrast-reducing photoluminescence may either be stimulated by absorbed ambient light or by self-absorption, such as in particular in a display comprising a plurality of pixels, the light emitted by one pixel in the display may cause photoluminescence in an adjacent, usually off-pixel. In color displays, this effect may also cause color shifts, as will be described below.
Referring in more detail to fig. 1a and 1b, incident ambient light passes through the substrate 102, the transparent anode 104 and the hole transport layer 106 to the electroluminescent material layer 108 where it is absorbed, creating excitons, i.e., bound electron-hole pairs. Alternatively, excitons are generated by light from nearby illuminated pixels that propagates through the photoluminescent layer 108, and/or the transparent anode 104, and/or the hole transport layer 106, and/or the substrate 102.
Because there is no additional field, a substantial fraction of these optically excited excitons quickly decay in accordance with the laws of radioactivity, while emitting light substantially isotropically in accordance with the photoluminescence spectrum of the material(s) forming the layer 108. The fraction of excitons that decay in accordance with the regularity of radioactivity depends on the photoluminescent efficiency of the material, as well as the applied field. When the diode formed by the device is in the off state (i.e. the anode and cathode are at the same potential-typically but not necessarily) the layer 108 is in a static photoluminescent state. Thus, when viewing the display, the viewer sees a combination of the emitted photoluminescence and the reflected and/or scattered light from the display, both of which have a tendency to reduce the contrast of the display.
The prior art contrast improvement techniques have focused on the use of anti-reflective means such as filters, circular polarizers as described in US6,211,613 (WO97/38452) assigned to the present applicant, and black anti-reflective cathodes as described in US5,049,780. However, these techniques may be insufficient, for example, they also reduce the desired optical radiation. Moreover, none of these techniques reduces the intensity of self-excited photoluminescence.
Background art relating to color purity improvement in electroluminescent displays is described in EP1,087,444 relating to independent red, green and blue gamma correction and EP1,093,322 relating to organic light emitting diode device instructions.
The applicant has realised that in a photoluminescent diode based display, such as a passive matrix or active matrix based organic light emitting diode display, the contrast can be increased by reducing the photoluminescence which reduces the contrast. In case the display comprises light emitting diodes, in particular organic light emitting diodes, this photoluminescence may be reduced or quenched by reverse biasing selected ones of the light emitting diodes, i.e. light emitting diodes which do not emit light at any particular moment in time.
The possibility of improving the contrast of an organic light emitting diode display by reducing or quenching photoluminescence has not previously been recognized. The application of reverse bias to organic light emitting diodes is well known in the art but this is not required or suitable for improving contrast by reducing photoluminescence. Thus, these prior art reverse bias schemes differ from the schemes described below that improve contrast reduction due to photoluminescence.
The experimental observations of the basic phenomenon of photoluminescence quenching of the ITO/PPV/A1 structure are described in "synthetic metals" (67(1994)169-172) of Lemmer et al.
The application published in WO 98/41065 relates to the application of a drive voltage of any polarity to a display based on electroluminescent polymers, which will drive either red emission from the polymer interface, or green emission from most polymers. In both cases, however, the light-emitting semiconductor is forward-biased (this device effectively comprises two back-to-back diodes).
US6,201,520 describes the use of reverse biasing for non-selected pixels in a pixel-based organic light emitting diode display to prevent cross-talk that would otherwise be caused by the (electrically) semi-excited state of the non-selected pixels. However, US6,201,520 does not specify any particular value of reverse bias drive and does not provide any teaching regarding reverse biasing of the display sufficient to provide quenching photoluminescence to improve contrast. Furthermore, the mechanism of applying a reverse bias in US6,201,520 also limits the reverse bias voltage to the magnitude of the forward bias voltage, whereas in general it is preferred to apply a reverse bias voltage greater than the forward bias voltage to achieve sufficient photoluminescence reduction to improve contrast.
US5,965,901, assigned to the present applicant, describes the use of a pulsed drive scheme for organic light-emitting polymer devices to improve the lifetime of the device, in which positive going pulses are separated by negative going pulses (reverse bias). However, this document does not attempt to apply a reverse bias to some pixels while applying a forward bias to other pixels, and is therefore not suitable for reducing photoluminescence stimulated by the emission of pixels within the display. Furthermore, this document also does not provide any teaching regarding reverse biasing of the display sufficient to provide quenching of photoluminescence to improve contrast.
EP1,094,438A describes periodically applying a reverse bias (e.g. per frame) to reduce leakage currents due to short-circuits through the membrane.
Disclosure of Invention
According to the present invention, there is provided a driver for a display, the display comprising a plurality of light emitting diode display elements, the driver comprising: an addressing circuit for addressing the display elements; a first driver cooperating with said addressing circuitry to provide forward drive to at least one of said display elements to illuminate the display element; and a second driver to provide a reverse bias drive to the other display elements while illuminating the at least one display element to reduce photoluminescence intensity from the other display elements.
Reverse biasing some display elements while forward biasing other display elements helps to improve contrast by reducing photoluminescence due to absorption of ambient light and self-excitation. Providing two drivers, one providing forward bias and the other providing reverse bias, may simplify the drive circuitry and facilitate forward biasing some display elements while reverse biasing others. For example, providing two drivers for a passive matrix display may even allow some pixels of a column of pixels selected to be forward biased to be reverse biased.
Preferably, the first driver is a current driver, such as a controllable or adjustable or modulatable substantially constant current driver, and the second driver is a voltage driver. However, an accurate reverse bias voltage driver is not necessary, and thus, the voltage driver does not have to be a constant voltage driver. Thus, the first driver is preferably configured such that it can provide a positive output relative to ground potential, while the second driver provides a negative output; "positive" in this context means a forward bias direction. Providing forward current drive and reverse voltage drive is well suited for the function of both drivers, since forward current drive helps to provide a consistent and/or controllable output, whereas quenching of photoluminescence, although requiring a small current associated with light, is after all a voltage drive effect in a broad sense. To enable operation from a single-ended supply, the driver preferably includes means, such as an inverter or charge pump, to generate a negative supply for the second driver, providing reverse bias drive.
The driver may be configured such that pulse width modulation control of the brightness of the display element is provided, for example by modulating a substantially constant current drive. With such an arrangement, some of the benefits of the invention can be achieved without the need to forward bias additional display elements while not reverse biasing some display elements, since there is a period of time when none of the pixels are in forward drive under conditions where not all of the pixels are at their maximum brightness. During this clock period (or clock periods) a reverse bias may be applied to reduce photoluminescence from ambient lighting (rather than from self-excited photoluminescence), giving some improvement in contrast. In one embodiment, the driver is constructed so that a passive matrix display can be driven-the driver having row and column drivers for addressing the pixels, either individually or row (or column) at a time.
The degree of visual observation of the contrast improvement depends on the brightness of the illumination and also on its wavelength or spectral characteristics, since photoluminescence generates wavelengths that are larger than the wavelength of the incident illumination. Preferably, the reverse bias is sufficient to achieve an improvement in contrast that is visible and discernable in sunlight, typical illuminances of sunlight being 10000 (or more) lux (oblique sunlight) and 100000 (or more) lux (direct sunlight), the spectrum of sunlight being close to that of a black body at 5400K.
In another aspect of the invention, there is provided a display driver for a color display comprising at least two types of electroluminescent pixels, the emission of the first type of pixels peaking at a first wavelength and the emission of the second type of pixels peaking at a second, longer wavelength; the display driver is configured such that it can drive at least some of said first type of pixels at a time different from the time said second type of pixels are driven, wherein the display driver is further configured such that it reverse biases at least some of said second type of pixels during driving of at least some of said first type of pixels.
The pixels may be driven alternately or sequentially so that a second group of pixels of a second color (not the first) is reverse biased when driving a group of pixels of the first color. Forward biasing and reverse biasing may be carried out, for example, under control of a processor, which may give substantial obvious advantages to a display driver user. Control of the brightness and/or color of the pulse width modulation may also be added. The implementation of the reverse bias may be rather fast and unnoticeable to a viewer of the display. The display may be of a passive matrix type or an active matrix type, or may be of some other type, such as a segmented display having separate electrodes for each display element or display segment.
According to a related aspect of the invention, there is also provided a display driver circuit for providing an electroluminescent display with improved contrast, the electroluminescent display comprising: a plurality of Electroluminescent (EL) display elements; said display driver circuitry comprising a driver for applying a drive of a first polarity to at least one first display element of said electroluminescent display elements to cause said at least one first display element to electroluminescent and a means for applying a drive of a second polarity to at least one second display element of said electroluminescent display elements to at least partially quench photoluminescence from said at least one second display element; the first and second display elements comprise different display elements; the driving of the first and second polarities includes driving of opposite polarities; the driving of the first and second polarities at least partially overlap in time.
The driver is preferably an adjustable, controllable, or modulatable, substantially constant current driver. It may be desirable to quench photoluminescence, for example, 5%, 10%, 20%, 50% or more, to give a visible contrast improvement. The display driver circuit may provide contrast improvement, for example, of greater than 1%, greater than 5%, greater than 10%, or greater than 20%, as measured by integrating sphere, open frame, or sampling sphere methods, such as described in document NISTIR6738 of the american institute of standards and technology, the diffuse ambient contrast measurement method proposed for flat panel displays (Edward F Kelley, 4 months 2001).
The means for applying a drive of the second polarity may comprise a voltage drive means, for example, a voltage drive of at least 5 volts, preferably at least 10 volts, most preferably at least 20 volts, may be provided. Alternatively, the means for applying a drive of the second polarity may comprise means for connecting a drive of the first polarity back through the display element to the front.
The electroluminescent display may be a passive matrix display and the display driver circuitry then comprises driver circuitry for the row and column electrodes. The driver circuit is configured to reverse bias the pixels having a common row or column electrode with a forward driven display element or pixel.
According to a related aspect, the invention also provides a method of improving contrast ratio of a display using a display driver, the display comprising a plurality of light emitting diode display elements, the method comprising: operating a display driver to reverse bias non-emissive display elements so as to at least partially quench photoluminescence from the non-emissive display elements to improve the display contrast.
This approach gives similar advantages for the display drivers described above, can be used to improve contrast in multi-color displays, and can actually improve color shift. In the above method, it will be appreciated that if a display element that is normally non-emissive is emissive at other times, such as the display element being rapidly driven to turn on and off to give an illuminated appearance, then such a display element may appear to be being illuminated.
In another aspect, the invention provides a use of a display driver to improve the contrast ratio of a display comprising a plurality of light emitting diode display elements, the use comprising reverse biasing non-emissive display elements using the display driver to at least partially quench photoluminescence from the non-emissive display elements.
The invention provides an active matrix multicolour display comprising a plurality of light emitting diode display elements and a plurality of associated display element driver circuits arranged such that the display elements associated therewith are both provided with forward bias drive and reverse bias drive.
The present invention also provides a method of improving the contrast ratio of a multicolour organic electroluminescent display device comprising a plurality of organic electroluminescent elements and a drive means for selectively controlling the current through each display element and the bias voltage across each display element such that each organic electroluminescent element can be selectively forward biased to cause light emission from the element to be either unbiased or reverse biased, the method being characterised by: upon selecting the organic electroluminescent element to be forward biased, the additional selected organic electroluminescent element is reverse biased at a voltage sufficient to quench the photoluminescent emission from the additional selected electroluminescent element.
There is also provided a method of increasing the color shift of an emissive color display, the color display comprising at least two types of electroluminescent pixels, the emission of the first type of pixel peaking at a first wavelength and the emission of the second type of pixel peaking at a second, longer wavelength, the method comprising the steps of: when illuminating some of the first type of pixels, at least some of the second type of pixels are reverse biased.
Of all the drivers described above, the various driver circuits and display methods preferably include organic light emitting diode display elements. The display elements may be arranged in a matrix, either monochrome to provide a monochrome display, or including groups of different colour pixels in a matrix to provide a multicolour display. Alternatively, the organic light emitting diode display element may comprise separately individually drivable segments of one display, such as a 7-segment digital display or a multi-segment display dedicated to a particular application.
Drawings
These and other aspects of the invention will be further described, by way of example only, with reference to the accompanying drawings, in which:
FIGS. 1a and 1b show cross-sectional views taken through an organic light emitting diode and a passive matrix organic light emitting diode display, respectively;
FIGS. 2a-2d show a conceptual driver arrangement for a passive matrix organic light emitting diode display, a plot of drive current versus time for a pixel of the display, a plot of pixel voltage versus time, and a plot of light output versus time for a pixel, respectively;
FIG. 3 shows a general driver circuit schematic of a prior art passive matrix organic light emitting diode display;
FIG. 4 illustrates a typical active matrix current controlled OLED driver circuit;
FIG. 5 shows a driver for a pixel-based color organic light emitting diode display;
FIGS. 6a and 6b show spectra of electroluminescent materials illustrating quenching of electroluminescence;
FIG. 7 shows pixel drive waveforms for photoluminescence quenching for improved display contrast;
figures 8a and 8b show first and second driver circuits for reverse biasing a pixel of a passive matrix display;
FIGS. 9a-9f show non-illuminated, non-reverse biased cross-sectional views through the color display of FIG. 5, respectively; a blue illuminated pixel having red and green photoluminescence; a blue illuminated pixel having red and green photoluminescence and reverse bias quenching; a green illuminated pixel having red photoluminescence; a green illuminated pixel having red photoluminescence and reverse bias quenching; and a red illuminated pixel;
FIG. 10 is a CIE color space diagram illustrating the shrinkage of an OLED display in color shift due to photoluminescence;
FIG. 11 shows color pixel drive waveforms for photoluminescence quenching for improving color shift;
FIG. 12 shows an experimental setup for characterizing photoluminescence quenching;
FIGS. 13a and 13b show photoluminescence quenching signals of two devices measured using the apparatus of FIG. 12;
FIG. 14 shows photoluminescence intensity as a function of illumination wavelength for the device of FIG. 13 a;
FIG. 15 shows a possible theoretical mechanism for photoluminescence quenching.
Detailed Description
Applicants have recognized that the contrast ratio of light emitting diode based displays, for example passive or active matrix organic light emitting diode based displays, can be improved by reducing the contrast ratio-reducing photoluminescence. In case the display comprises light emitting diodes, especially organic light emitting diodes, this photoluminescence may be reduced or quenched by reverse biasing a selected batch of light emitting diodes, i.e. those that do not emit at any particular moment in time.
For example, consider a simple organic light emitting diode display, such as the display shown in fig. 1a and 1b, in which no forward or reverse bias is applied. The apparent color of the (unlit) display is a combination of the color of the photoluminescence from the electroluminescent layer 108 of the display and the intrinsic color of said layer 108 and other layers, in particular the color of the cathode layer. Thus, for example, if the layer 108 is inherently colorless and the photoluminescence in white ambient light is blue, the display (or pixels not being forward driven) will appear blue when unbiased. However, if a reverse bias is applied, the display (or non-illuminated pixel) will appear colorless, or have a cathode color, and thus the contrast between the pixel on and off states can be improved. The cathode is preferably absorptive or black. If the cathode is partially transparent so that when the display (or pixel) is off (no photoluminescence), a viewer can see what is behind it, an absorbing layer and an optical black layer can be formed behind the cathode.
Referring now to fig. 5, fig. 5 illustrates an example of a pixel-based display and driver architecture 500. Broadly, this structure corresponds to the display structure described above, except that the photoluminescent layer 108 is pixel-based, i.e., the layer 108 is divided into a plurality of separate display elements 502. Similarly, the cathode layer(s) is divided into a plurality of separate cathodes, each having its own contact. However, the substrate 102, the anode 104, and the hole transport layer 106 are common to all pixels. Thus, an individual pixel can be disconnected by applying a reverse bias between the common anode 104 and the appropriate cathode connection 156. In another pixel-based display, X-Y pixel addressing can be performed using row and column electrodes.
According to the arrangement of figure 5, the individual photoluminescent display elements have different colours to provide a colour display. For example, the use of a blue photoluminescent material can give a blue pixel; filtering out the white photoluminescence emission gives red and green pixels.
The display device includes a display driver circuit 504 and a power supply schematically represented by a battery 506. The display 502 comprises a plurality of red 508, green 510, and blue 512 pixels arranged in a pattern that provides the appearance of a variable color display at a distance. A wide variety of pixel patterns are possible, with the exception of one pattern, which helps reduce visual artifacts. For example, a repeating pattern of 4 pixels of red, green, and blue may be employed.
The display driver 504 receives a display signal input 514 and provides an output 516 to the drive electrodes 156. As shown in fig. 5, the common anode connection 104 and the negative terminal of the power supply (battery 506) are both grounded. The display driver applies the positive voltage of power supply 506 to the selected cathode connection in accordance with the display signal input on line 514. The display signal may comprise a single pixel on/off signal or may comprise an analog or digital pixel brightness signal representing a desired level of pixel brightness between on and off states. In the color display shown in the figure, separate signals are preferably provided for each red, green and blue pixel to give the appearance of a variable color pixel.
The display driver may also incorporate a means for providing a Pulse Width Modulated (PWM) drive signal with an adjustable duty cycle to each pixel in response to the display signal 514 input on line 514. The pulse width modulated drive signal may be 0, a first current or voltage drive level that is forward biased, and a second reverse bias voltage (or current) drive level. To reduce display flicker, the frequency of the pulse width modulated signal is preferably greater than 50 Hz; more preferably greater than 60 Hz, particularly preferably greater than 75 Hz. For example, the color of the pixel and the brightness of the pixel may be controlled by selecting one of a plurality of mark-space ratios (mark-spaces) provided by the pulse generator.
Referring now to fig. 6, typical spectra for two different types of electroluminescent material are shown in fig. 6a and 6 b. The y-axis in fig. 6a and 6b represents the intensity of light emitted from a device such as that shown in fig. 1a and 1 b.
The spectrum of figure 6a represents a material that, although having a relatively high photoluminescence efficiency, also has a strong intrinsic color. An example of such a material is polymer blend F8BT-TFB, which has a photoluminescence efficiency of greater than 80%, a photoluminescent yellow color under white light, but also has an intrinsic yellow color, so that the material appears yellow even if the photoluminescence is quenched. The reason for this residual or intrinsic color is that this material inherently absorbs a set of wavelengths that give it a yellow appearance. When depositing such materials as thin films, the yellow color is still apparent, since then the absorption of the material is still an important factor.
Fig. 6a shows 3 spectra 600 (not to scale) illustrating the variation of light intensity with wavelength for a material with an inherent color, such as F8 BT-TFB. Spectrum 604 represents the light emission spectrum of a material such as that in the device of fig. 1a or 1b when the applied bias is 0. For forward biasing, this spectrum shifts to spectrum 606 where the electroluminescent emission is enhanced and the peak shifts to the longer wavelength direction (reddish). When a device comprising such a material is reverse biased, the spectrum shifts to spectrum 602, indicating that the intensity of the photoluminescent light emission is reduced and the peak wavelength shifts in the blue direction.
In contrast, fig. 6b shows a set of spectra 610 (not graduated) for a material containing no intrinsic color. Spectrum 604 represents the photoluminescence of the device when the bias is 0, when the electroluminescent emission is enhanced for forward biased spectrum 616, and spectrum 612 substantially quenches the photoluminescence when a reverse bias is applied. As can be seen from fig. 6b, the peak positions of the spectra 612, 614, 616 are substantially unchanged, since the contribution to the color of the device is substantially only the emitted photoluminescence/electroluminescence, and not, as shown in fig. 6a, the contribution of the intrinsic color of the material.
Typically, the light from the display comprises two components. The first component includes intrinsic electroluminescence and photoluminescence, and the second component is derived from ambient illumination reflected and scattered by the display. The second component is reducible, for example by using a transparent or black cathode as described in US5,049,780; or by using a circular polarisation filter as described in US6,211,613 (WO 97/38452). In these devices, there is relatively little light scattering from the photoluminescent layer itself, in which case the spectrum of fig. 6a may be close to that of fig. 6 b.
Although these spectra and one possible photoluminescence quenching mechanism to be described below are discussed with reference to F8BT and TFB, these are given by way of example only and are intended to be illustrative. The application of the present invention is not limited to these materials and the present invention may be used with any electroluminescent/photoluminescent material, including inorganic materials, and is particularly applicable to any organic light emitting diode based device.
Referring now to fig. 7, a typical Pulse Width Modulation (PWM) waveform 700 is shown (not shown) that is known in the art for controlling pixel brightness, but additionally incorporates a photoluminescence quenching phase. This waveform represents the change in voltage applied to a pixel over time, which ranges between a first forward bias level (in the illustrated example +10 volts) and a second reverse bias level (in the illustrated example-20 volts). This reverse bias level may correspond to the reverse bias required to substantially complete photoluminescence quenching under typical operating conditions. Alternatively, partial photoluminescence quenching (e.g., 5%, 10%, 20%, 50% or more quenching) can be considered sufficient to provide a beneficial contrast improvement. The waveform portion of the voltage level 702 is referred to as a "mark" and the waveform portion of the voltage level 704 is referred to as a "space". The waveforms of FIG. 7 may be generated by the display driver circuit 504 of FIG. 5.
For simplicity of illustration, the waveforms are shown in fig. 7 as alternating between forward and reverse voltage drive levels. However, it is generally preferred to provide a substantially constant current forward drive arrangement, optionally adjustable or arbitrarily controllable, rather than a forward voltage drive. Reverse current drive may also be provided, but this is not preferred for voltage drive because the light emitting diodes of the pixels typically have a high impedance in the reverse direction. The forward drive should preferably be carefully controlled to provide uniform display brightness; unlike forward drive, the exact level of reverse drive is not critical and does not require strict adjustment.
During the mark portion of the waveform, the pixel emits light, while during the blank portion of the waveform, any photoluminescence caused by ambient light or illumination of other pixels is substantially quenched, thus increasing the apparent contrast. Pulse width modulated brightness control is particularly suitable for passive matrix displays. In such passive matrix displays, when a pixel is selected and forward biased, the other pixels in the display may be reverse biased as described above with reference to figures 2a, 3 and 5. Depending on the switching and driving arrangement used, the reverse biased pixels may comprise pixels in other rows and/or columns than the row and/or column of the selected pixel(s), or additional pixels in the same row and/or column as the selected pixel may also be reverse biased.
Those skilled in the art will also recognize that in utilizing a pulse width modulation based brightness control display driver, it is a simple matter to reverse bias all pixels in the display at intervals in the pulse width modulation waveform (e.g., interval 704 in fig. 7) when the selected pixel itself is reverse biased. To this end, all pixels may be selected, for example, at time intervals 704 when the selected pixels are off (and reverse biased), and driven from a common bias generator or multiple reverse bias generators or drivers.
The frequency of the pulse width modulated waveform is selected so that the pixel appears not to be rapidly switched on and off so that the emission of the pixel appears substantially continuous, but the brightness is proportional to the on period of the waveform or the period of the mark. To achieve this, a frequency of at least 25-50 Hz is typically required. It can be seen that at the mark-space transition position 706 shown, the pixel appears at about 25% of its total brightness. The transition locations 708, 710 correspond to 50% and 75% of the pixel brightness, respectively; a pixel brightness of 100% corresponds to a steady state of +10 volts (in this example), with a 100% mark: duty cycle of space ratio. Waveforms other than those shown in fig. 7 may be used, and the drive waveform does not require square edges.
As mentioned above, an advantage of using pulse width modulation is that there is a substantially linear relationship between duty cycle and apparent pixel brightness. Only byTo change the pixel brightness by changing the reverse bias voltage, the characteristics of the individual pixels change quite closely together, and some form of linear relationship, such as a look-up table, may be necessary. An additional or alternative form of brightness control is to subdivide each pixel into n sub-pixels, the area ratio of which is a power of 2 (2)0、21、22Etc.) so that 2 is given according to which sub-pixel is selected to be turned onnDifferent brightness.
In principle, each pixel in the display may have a different brightness relative to the other pixels, and thus the display driver 504 of FIG. 5 should be capable of driving each pixel with a pulse width modulated waveform appropriate for its selected brightness. One way to achieve this requirement is to: a separate, variable pulse width pulse generator is provided for each pixel in the display, or for each row or column of pixels. An integrated circuit for this purpose is available from the Clare Micronix subsidiary of the Clare company (California, USA), and comprises an MXED101 and an MXED 102. For example, the MXED102, which is a 240-channel cascaded column driver, provides 240 independently adjustable pulse width modulated outputs. Data for these devices is also available on the Clare Micronix website, and these data pages are referenced herein.
In operation, we believe that when a diode formed by an anode, a cathode and an electroluminescent layer is reverse biased, i.e. the anode is held at a lower potential than the cathode, a fraction of excitons generated by incident ambient or other illumination are split into their constituent holes and electrons. These holes and electrons will then leave the structure by means of an applied electric field. This prevents the radioactive decay of the fraction of excitons and hence the emission of photoluminescence. The fraction of excitons split and leaving in this way is determined by the reverse voltage applied to the device, so that the level of photoluminescence can be controlled from a maximum value without any voltage applied to it to a smaller value related to the degree of reverse bias.
It will be appreciated that the additional power consumption of such reverse biased devices is extremely low, since essentially the only power consumption required is that of holes and electrons leaving the split exciton. This power consumption varies with the degree of incident illumination and the photoluminescence efficiency. It will also be appreciated that this power consumption is dependent to some extent on the magnitude of the contrast required and the level of incident illumination, since a greater reverse bias is required for a greater photoluminescence reduction. For example, under high ambient light conditions, such as bright sunlight, the power consumption of the reverse bias is greater. The improvement in contrast is most pronounced in materials with high electroluminescent efficiency and high photoluminescence quenching efficiency. An example of such a material is F8 BT-TFB.
Referring now to fig. 8a, a driver circuit 800 is shown, which is similar to the circuit of fig. 3, but includes means for reverse biasing non-emitting pixels. The organic light emitting diode display 302 corresponds to the organic light emitting diode display of fig. 3 and like features are denoted by like reference numerals. In fig. 8a, a battery 802 supplies power to a switch mode power supply unit 804 to effectively provide a regulated dc power output. A separate inverter 808 is used to generate the negative supply voltage for reverse biasing. In a practical design, the inverter 808 may be combined with the switched mode power supply 804, or the inverter 808 may further comprise a second switched mode power supply, or a charge pump, or any other suitable means of generating a negative supply. Alternatively, the power supply driven by battery 802 may be split to provide positive, negative and ground reference voltages, or a conventional dual rail power supply may be used, for example, if the display is a mains power supply or the like.
A positive power supply from power supply 804 supplies power to, for example, a positive driver 806 that includes a constant current generator. The negative voltage from inverter 808 powers a reverse bias driver 810, which is typically a voltage driver; the reverse bias driver 810 is an adjustable or non-adjustable voltage source that is reversed from the constant current source. The drive outputs from the forward driver 806 and the reverse driver 810 are provided to a column driver 814 comprising a plurality of switches 814a, one switch 814a for each column electrode. Each switch is configured such that one column electrode can be connected to either the forward driver 806 or to the reverse driver 810. A data/control input of the processor 812 is used to provide data for the display to the display driver and a first output of the processor 812 is used to control the column driver 814, in particular the switch 814 a. A row driver 816 is also provided, the row driver 816 comprising a plurality of switches 816a, each for selectively connecting a row electrode 306 of the display 302 to ground. Switch 816a is under the control of processor 812 in a similar manner.
In operation, the processor 812 controls the row driver 816 to select a row of the passive matrix organic light emitting diode display 302, i.e., selectively connects a row to ground, and controls the column driver 814 to selectively connect one or more column electrodes to the forward driver 806. The pixel(s) connected between the forward driven column(s) and the selected row are thus forward biased and emit light. The "unselected" columns are connected to the inversion driver 810 and the "unselected" rows are also connected to ground, thus causing the "off" pixels to be reverse biased. Thus, it should be understood that in a simplified arrangement, the row driver 816 may be omitted. The processor 812 may be combined with hardware and/or software to perform pulse width modulated brightness control of "on" pixels.
Referring now to fig. 8b, a conceptual diagram of one type of optics 850 of the driver circuit shown in fig. 8a is shown. As represented for the driver circuit 800 of fig. 8a, a forward driver 806 and a reverse driver 810 are provided, the forward driver preferably comprising a substantially constant power supply and the reverse driver 810 preferably comprising a (negative) voltage drive, which is adjustable or controllable for brightness control. Column switches 852a, b are provided to connect one column electrode of the display 301 to the forward driver 806 or to ground. Similarly, row switches 854a, b are provided to selectively connect each row to either the inversion driver 812 or to ground.
As shown in fig. 8b, switches 852a and 854a connect to forward biased organic light emitting diode pixel 312a to cause it to emit light. Similarly, switch 852b also couples organic light emitting diode pixel 312b to forward driver 816, and pixel 312b in the same row as pixel 312a is also forward driven, in light emission. Switch 854 (and switches of other unselected rows not shown in fig. 8 b) is configured so that its unselected row(s) are connected to the recessive driver 810. The reverse driver 810 is configured such that the reverse driver 810 can provide a reverse bias drive large enough that the organic light emitting diode pixels 312c and 312d are reverse biased even though their anodes are connected to the forward driver 806. Thus, the output of the recessive driver 810 may be greater than the desired recessive bias by an amount approximately equal to the desired recessive bias voltage of the recessive driver 806. In the case where the forward driver 806 is a current driver, the forward driving voltage depends mainly on the characteristics of the organic light emitting diode, but generally, an approximate forward driving voltage output or output range can be estimated, and then the reverse bias voltage can be provided with a sufficient margin.
The arrangement of figure 8b is useful because it is common practice to select a row and then drive a plurality or all of the column electrodes simultaneously to provide an effective update to the display. Therefore, it is desirable to be able to apply a reverse bias to the unselected pixels in this case.
It will be appreciated that in a segmented display, or in a combined display where at least some of the display elements have separate drive electrodes, the reverse biasing of unselected or non-emissive display elements will simply be: the selection is made to apply either forward bias drive or reverse bias drive to the emissive and non-emissive display elements or display segments.
There is a particular problem with colour electroluminescent displays in that secondary emission of absorbed light occurs, as shown in figure 9. Ambient light and light of a wavelength shorter than the wavelength of a particular pixel color may cause photoluminescence of the pixel. Thus, for example, when a blue pixel is to be illuminated, nearby red and green pixels will be photoluminescent; when a green pixel is illuminated, a nearby red pixel will be photo-illuminated.
Fig. 9a schematically shows a red pixel 902, 908, a green pixel 904, 910 and a blue pixel 906 under the influence of ambient light 912. Ambient light 912 will cause low levels of photoluminescence 914 from all pixels.
In fig. 9b, the blue pixel 906 is driven forward, emitting photoluminescence 916, both the red pixel 908 on one side of the blue pixel 906 and the green pixels 904, 910 on the other side of the blue pixel 906 producing photoluminescence 918. Similarly, slightly more distant red 902 and green 910 pixels are also photoluminescent 920, but at a reduced intensity. In fig. 9d, the green pixel 904 is electroluminescent 922, causing the adjacent red pixels 902, 908 to photoluminescence 924, but the blue pixel 906 is not stimulated to emit photoluminescence because of the shorter wavelength. In fig. 9f, the red pixels 902, 908 are driven forward while electroluminescence 926, but the green and blue pixels 904, 906, and 910 are not photoluminescent because their wavelength of electroluminescence/photoluminescence is shorter than the wavelength of emission 926 of the red pixels 902, 908.
Fig. 10 shows a CIE chromaticity diagram with ideal red, green, and blue pixel colors marked at locations 1002, 1004, and 1006, respectively. The self-excited photoluminescence effect described above with reference to figure 9 is to move the blue pixel position 1006 towards green and red (see figure 9b) to position 1006 a. Similarly, the green pixel position 1004 is moved to red (see FIG. 9d) to position 1004 a. But the red pixel location 1002 is substantially unchanged (fig. 9 f). Thus, as can be immediately seen from fig. 10, the color of the display shifts, i.e., the range of colors produced by the display becomes smaller. It should also be understood that, in a broad sense, the effects of ambient lighting are: the color shift is shortened by moving the color location 1002, 1004, 1006 of each pixel inward toward white.
It will be appreciated from figure 10 that accordingly, the colour shift of a display incorporating photoluminescent display elements can be improved by at least partially quenching the photoluminescence, in particular the self-excited photoluminescence. FIG. 9c shows the effect of photoluminescence quenching applied to the pixels 902, 904, 908, 910 of FIG. 9b which have been reverse biased. Similarly, FIG. 9e shows the effect of photoluminescence quenching applied to the pixels 902, 908 of FIG. 9d that have been reverse biased. It can be seen that only selected pixels need to be reverse biased, particularly those pixels that emit light at a longer wavelength than the current drive wavelength. Thus, when the blue pixel is electroluminescent, the red and green pixels should be reverse biased; when the green pixels are electroluminescent, the red pixels should be reverse biased; when the red pixel is electroluminescent, no reverse bias is required. It will be appreciated that in order to improve the color shift of the display, the required photoluminescence is not fully quenched, as partial quenching will result in at least some improvement in color shift.
To be able to provide a color display where, for example, blue pixels emit light when quenching red and green, 3 sets of interleaved waveforms 800 may be used to ensure that only pixels of one color are forward driven at any one time. To do so, the cycle shown in FIG. 7 is extended, such as shown in FIG. 11, to increase the reverse bias or to increase the "null" period 704.
In fig. 11, the same reference numerals as in fig. 7 are used to denote the same features, and a, b, or c is added to denote drive waveforms for red, green, and blue pixels, respectively. As shown in fig. 11, the cycle period is extended by a factor of 3 for 3 colors in practice, and thus, for example, a red pixel can be reverse-biased when a green pixel is forward-driven and when a blue pixel is forward-driven. The result of this is a reduction 1/3 in the maximum brightness of the pulse width modulated pixel, at least for red pixels. It will be appreciated that the green pixels need not be reverse biased when the red pixels are on, and the blue pixels need not be reverse biased when the red and/or green pixels are on.
The waveforms of fig. 11 can be generated, for example, by the display driver circuit 504 of fig. 5, which operates, for example, along the lines discussed above with reference to fig. 8a and/or 8 b. Alternatively, an active matrix display may be used, with one circuit providing a forward bias and a second similar (but reversed) circuit providing a reverse bias, for each (colour) pixel, for example having two pixel driver circuits of the type shown in figure 4. Data may then be written to the pixel driver circuit, such as under control of the processor, to bias the pixels in either the forward or reverse direction.
Referring now to fig. 12, this figure shows an experimental set-up 1200 for measuring the intensity of photoluminescence emitted by an organic light emitting diode display device when reverse bias is applied.
The xenon lamp 1202 is coupled to a monochromator 1206 by a lens 1204 to enable selection of a narrow range of emission wavelengths. The output of the monochromator 1206 is focused through a pair of lenses 1208, 1210 onto a display device 1214 under test. The lenses 1208, 1210 cause the output of the monochromator to be modulated by a mechanical chopper wheel 1212, the chopper wheel 1212 being driven by a lock-in amplifier 1224. Photoluminescence from the test device 1214, excited by illumination from the monochromator 1206, is collected by a lens 1216 and directed onto a photodiode 1220 also connected to a lock-in amplifier 1224. The collected light is filtered by a low pass color filter 1218, rejecting scattered light from the monochromator 1206, while allowing photoluminescence to pass. A voltage source 1222 is used to provide a variable reverse bias voltage to display device under test 1214. The lock-in amplifier 1224 provides an output that is representative of the level of photoluminescence from the display device 1214.
Examples of the invention
The results for two typical display devices are given below. A first display device comprises a polymer blend F8 BT: TFB 80: 20 with two layers of calcium/aluminium cathode. The second display device comprised a polymer mixture of F8 BT: TFB: poly (2, 7- (9, 9-di-n-octylfluorene) -co- (2, 5-thiophene-3, 6-benzothiadiazole-2, 5-thiophene) 79: 20: 1 with a three layer lithium fluoride/calcium/aluminum cathode the photoluminescence of both displays was yellow and had an intrinsic yellow color.
Fig. 13a and 13b show the photoluminescence emission as a function of reverse bias for the first and second display devices, respectively. In each case, the display device is excited with light from the monochromator 1206 at a wavelength of 466nm, and the filter 1218 and photodiode 1220 are arranged to collect light at a wavelength of greater than 570 nm. The two curves are normalized to a maximum photoluminescence level of 100% at zero bias.
The two curves show that at a reverse bias voltage of about 20 volts, the photoluminescence is reduced to about half of its initial value. Once the reverse bias voltage is removed, the observed photoluminescence returns to its original intensity.
It should be understood that the reverse bias required to quench photoluminescence is related to the material(s) used in the associated organic light emitting diode device structure, as well as to ambient light conditions. Thus, in some cases, such as for polymer light emitting diode based displays, relatively low reverse bias voltages, such as-5 volts, -10 volts, -15 volts, or-20 volts, may be sufficient to quench photoluminescence or produce a visible improvement in display contrast. The optimum value of the reverse bias for each particular display can be determined experimentally, and for this purpose the above idea can be followed, or the simplest approach can be used, i.e. adjusting the reverse bias upwards starting from a low or zero level and displaying the contrast by visual inspection.
Fig. 14 shows the photoluminescence intensity from the monochromator 1206 as a function of the illumination wavelength for the first display device. Photoluminescence cut off when the excitation wavelength is greater than about 570 nm; the residual tail of the curve of fig. 14 is from the scattered light from the excitation source. It can be seen that maximum photoluminescence is observed when the excitation source has a wavelength between 400nm and 500 nm. This feature facilitates the selection of an appropriate illumination source. In the display device of fig. 14, the threshold 570nm for photoluminescence corresponds to the minimum photon energy in the photoluminescent material that still produces excitation. Thus, in a display device where it is desired to block photoluminescence excited by ambient or background light, placing a filter cut off at a wavelength of about 570nm in front of the display device will reduce ambient light excited photoluminescence while still allowing photoluminescence emissions at wavelengths greater than 570nm to pass.
FIG. 15 shows a theoretical mechanism that is believed to fully explain photoluminescence quenching. In one of the polymers F8BT in the photoluminescent polymer mixture, the incident illumination generates a pi-pi transition, generating an exciton, i.e. a bound hole-electron pair. The exciton can be excited by a binding energy greater than that of excitonbThe thermal energy of (a) is dissociated. In one electric field, the energy required to dissociate the excitons may be reduced to about EbXed, where X is the electric field, e is the electron charge, and d is the distance that must be separated for complete dissociation of the hole and electron.
Referring again to fig. 15, fig. 15 shows the vacuum energy level 1500 and the Lowest Unoccupied Molecular Orbital (LUMO) levels 1502 and 1504 of TFB and F8BT, respectively. Fig. 15 also shows the Highest Occupied Molecular Orbital (HOMO) energy levels 1506 and 1508 of TFB and F8BT, respectively. In a simple graph, if one hole transferred to the Highest Occupied Molecular Orbital (HOMO) (0.56 electron volts) of the TFB polymer gains energy that exceeds the binding energy of an exciton on the F8BT polymer, the exciton on F8BT will dissociate. Similarly, if the energy gained by the transfer of one electron to the Lowest Unoccupied Molecular Orbital (LUMO) of the F8BT polymer exceeds the binding energy of the exciton on the TFB polymer for the electron, the exciton on the TFB polymer will dissociate. It is believed that by applying a reverse bias electric field, the energy required to dissociate excitons on the F8BT polymer and on the TFB polymer is reduced and the energy required to excite the hole/electron transfer process, i.e., the transfer process, is less, and thus, the process is more likely to occur at a given temperature. Dissociation must occur faster than the radioactive recombination. The measurements lead to the conclusion that: the estimated reduction in binding energy should be consistent with or equal to the energy required to separate the hole-electron pairs by a distance approximately equal to the separation between the TFB and F8BT polymer chains.
The embodiments described above relate primarily to the application of the invention to passive matrix displays, but it will be appreciated by those skilled in the art that the invention is not limited to such displays. For example, contrast or color shift may be improved in a segmented display having a separate drive line for each segment, or in an active matrix display in which one or more transistors associated with each pixel maintain the drive level of the pixel after data is written to the pixel to set the drive level. Similar applications of the invention are not limited to organic light emitting diode based displays but also include other types of emissive displays such as inorganic light emitting diode based displays.
No doubt many effective alternatives will occur to the skilled person, and it will be understood that the invention is not limited to the described embodiments, but may include various modifications within the spirit and scope of the appended claims, as will be apparent to a person skilled in the art.