Patent Application
20110037791
The invention relates to a display device based on pixels with variable chromatic coordinates comprising a plurality of color sub-pixels each comprising a light emitter formed by an organic light-emitting diode and a color filter, the chromatic coordinates of the pixel being determined periodically and the light emitters being identical.
In color display systems, the color of each pixel is made up from three primary colors. The CIE 1931 standard can for example be used to define any color visible to the human eye from three standard primary colors constituted by a precise shade of blue (B), red (R) and green (G). With this standard, all the shades of color accessible to the human eye are defined by precise chromatic coordinates which each correspond to a particular distribution of the standard primary colors.
In conventional manner, a pixel is defined by its color and its luminance, i.e. by its visible light intensity. The luminance and chromatic coordinates of a pixel with variable chromatic coordinates are thus redefined periodically according to the image to be displayed.
In conventional manner, a high-definition display system is obtained by means of a very high density of sub-pixels, each pixel comprising a sub-pixel of each primary color.
However light-emitting materials, and in particular organic materials, are difficult to pattern. It is therefore generally chosen to use a continuous white light emitting layer for the emitters, i.e. an emitting layer which is common to all the sub-pixels. For each sub-pixel, the continuous white light emitting layer is associated with a specific color filter, which depends on the color to be obtained for the sub-pixel considered.
As illustrated in
In conventional manner, variation of the chromatic coordinates of the pixel is performed periodically by modulating its primary color distribution. This primary color distribution modulation results in practice in modulation of the light energy given off, i.e. in modulation of the luminance of each of the sub-pixels. This luminance modulation is conventionally achieved by varying the supply current intensity of the sub-pixel concerned. In this way, the luminance of the pixel is determined by the sum of the currents flowing in the light emitters, whereas the color of the pixel depends on the luminance of its sub-pixels and therefore on the current distribution between the different sub-pixels. It is therefore known to modulate the current between the sub-pixels to modulate the color and luminance of the pixel.
Another control technique exists—by modulating the polarization time (PWM for Pulse Width Modulation). This technique consists in keeping the current constant for each sub-pixel. Modulation of the pixel color and luminance is then obtained by modulating the application time of the current of each sub-pixel.
These two techniques give rise to large energy losses as the white light emitted by each light emitter passes through the corresponding color filter. If the white light has a homogeneous distribution in each of the primary colors, when it passes through the color filter, two thirds of the light energy is absorbed by the filter to only let the color corresponding to the filter pass. Operation of the pixel with an acceptable luminance thereby involves the use of very high-luminance light emitters. In practice, a high luminance is obtained by using a high current, which results in a high energy consumption and reduction of the lifetime of the light emitters.
The object of the invention is to provide a pixel control circuit that is easy to implement enabling the consumption of the pixel to be limited, its lifetime and/or luminance to be increased and a compact display system to be obtained.
This object is achieved by the appended claims and more particularly by the fact that the device comprises:
Other advantages and features will become more clearly apparent from the following description of particular embodiments of the invention given for non-restrictive example purposes only and represented in the appended drawings, in which:
In conventional manner, pixel 1 with variable chromatic coordinates comprises a plurality of color sub-pixels 2, for example three color sub-pixels, is made from a continuous layer in which diode 3 emitting white light is formed. The light emitters of the sub-pixels, organic light-emitting diodes, are thus identical. Each color sub-pixel 2 is associated with a color filter 4 which only lets one of the primary colors pass to the outside. Color sub-pixels 2 used are for example sub-pixels having precise shades of blue, green and red. Pixel 1 can advantageously comprise an additional sub-pixel, without a color filter, which emits a white light to facilitate the realization and luminance of white.
Pixel 1 is associated with a control circuit which in particular enables the power supply conditions (voltage, current and application time) of each of the sub-pixels to be fixed independently via two sets of electrodes arranged on each side of the emitting layer. The circuit control thus enables the luminance and chromatic coordinate of pixel 1 to be determined periodically.
The emission spectrum of emitting layer 3, i.e. the color emitted by this layer, can vary with the power supply conditions (voltage, current) to a more or less great extent according to the composition of this layer. In general manner, this phenomenon has to be limited. On the contrary, according to the invention, it is advantageous to choose a composition that generates a significant variation of the emission spectrum with the polarization.
Thus, as illustrated in
An organic diode light-emitting 3 in known manner comprises a light-emitting layer itself able to comprise at least two sub-layers made from emitting materials of different shades. The light-emitting layer advantageously presents one of the following schematic structures:
The latter structure in general procures the maximum variation of its emission spectrum with the polarization and will therefore be preferred for implementation of the invention.
Emission can be intrinsic to the materials chosen for making the sub-layers or be obtained by doping. Other stackings are possible based on multidoped layers, i.e. layers comprising at least two dopants which enable emission of the sub-layer considered in at least two colors. The following stackings can in particular be cited:
Diode 3 can conventionally comprise additional layers, in particular associated with transport and/or confinement of the charge carriers in the structure, such as hole and/or electron restraining layers, buffer layers and hole and/or electron transport layers, necessary for correct operation of the latter. These layers are not dealt with explicitly for the sake of clarity.
Furthermore, the additional sub-pixel, devoid of a filter, is supplied under operating conditions called nominal operating conditions, to emit a white light.
Each organic light-emitting diode 3 of pixel 1 is supplied (current and/or voltage) independently from the others by the control circuit for each one to emit in the color corresponding to its own color filter 4. The voltage and/or current applied to each sub-pixel, and therefore to each light emitter, is determined according to the color of the sub-pixel. What is involved for example is to make the organic light-emitting diode 3 associated with the red color filter emit in the red band, diode 3 associated with the blue filter emit in the blue band and diode 3 associated with the green filter emit in the green band. The emission spectrum of each light-emitting diode 3 is thus close to the transmission spectrum of its color filter. Most of the light energy emitted by an organic light-emitting diode 3 thus passes through the corresponding color filter 4, which results in a large increase of the light output of pixel 1. The control circuit therefore controls light emitters 3 separately, which emitters have an emission spectrum that can be modulated according to their supply voltage and/or current. The supply voltage and/or current applied to each sub-pixel is then determined according to its color for its emission spectrum to be close to the transmission spectrum of color filter 4 associated thereto. The organic light-emitting diodes described above are particularly suitable in so far as their color varies greatly with the supply voltage and/or current. The luminance of each pixel is modulated by adjusting the application time of this current and/or of this voltage.
Organic light-emitting diode 3 associated with red color filter 4 is advantageously supplied by a lower current IR than the diodes associated with the blue and green filters, which enables a deep red to be obtained. In similar manner, organic light-emitting diode 3 associated with blue color filter 4 is advantageously supplied by a higher current IB than the diodes associated with the red and green filters, which enables a deep blue to be obtained.
For example purposes, an emitting layer made up from the following Blue/Green/Red emission sub-layers is considered: SEB010 doped SEB020 (with a thickness of about 10 nm)/TMM004 doped TEG341 (with a thickness of about 7 nm)/TMM004 doped TER040 (with a thickness of about 20 nm), all these materials being marketed by Merck.
For each sub-pixel, the selection criteria of the currents to be used are dictated by the chromatic coordinates that are desired to be obtained for the sub-pixel in question and by the luminance obtained after filtering. The table below gives the luminance (in Cd/m2) obtained after filtering and the chromatic coordinates (X, Y) in a CIE1931 chromaticity diagram, for a frame time t of 20 ms, according to the polarization (voltage/current pair) for the same diode.
According to this table, if a luminance equal to 100 Cd/m2 is required for the pixel, and therefore for each sub-pixel, the following characteristics are privileged:
In this manner, each diode is supplied under conditions under which obtaining an emission spectrum close to the transmission spectrum of the associated color filter is enhanced. The differences of light intensity of the diode which result from these polarization differences are modulated by the specific power supply times for each sub-pixel. Each of the sub-pixels thus presents the same luminance, here for example 100 Cd/m2.
For example purposes,
Practically, for each organic light-emitting diode 3 of pixel 1, the control circuit fixes the supply conditions (current and/or voltage) which allow optimum light efficacy with the corresponding color filter 4. For each organic light-emitting diode 3 of pixel 1, the control circuit for example fixes a current which defines the color emitted by the diode and also the instantaneous luminance of the latter.
Polarization of the diode having been chosen to optimize the emitted color, the luminance obtained is adjusted to the required luminance by adjusting the application time of this polarization: the diode is no longer supplied throughout frame time t.
For this, the addressing circuit of diode 3 comprises control means of the supply voltage and/or current application time according to the color of the sub-pixel.
For example purposes, the diode addressing circuit comprises a control transistor T3, operating as a switch, connected between the control electrode (gate) of second transistor T2 and the power supply source terminal connected to the diode, preferably ground. The control electrode (gate) of control transistor T3 is connected to a reset line (RL) which, with control transistor T2, forms control means of the current application time through diode 3.
When transistor T3 is off, the voltage on the gate of transistor T2 is maintained by means of capacitor C and the required current flows in diode 3. When transistor T3 is on, capacitor C discharges and the potential of the power supply source terminal connected to the diode (preferably ground) is transposed onto the gate of transistor T2, turning transistor T2 off: there is then no longer any current flowing in the diode.
Reset line (RL) and control transistor T3 thereby enable a maximum time to be fixed during which the diode is supplied during each frame period Δt. In this way, the control means of the supply condition (voltage/current) application time enable the mean luminance of each sub-pixel to be adjusted on the frame time, i.e. they enable a predetermined mean luminance to be obtained during a predetermined period.
In conventional manner, the control circuit periodically fixes the chromatic coordinates of pixel 1 and its luminance by modulating the luminances of the sub-pixels, for a frame period Δt, for example of 20 ms. Thus, at each beginning of period Δt, the control circuit selects the sub-pixels 2 necessary for obtaining the chromatic coordinates of the pixel and controls the luminance of each of these sub-pixels 2.
For a given luminance, control transistors T3 associated with sub-pixels of different colors are on during times ton which are dependent on this color (ton≦Δt) during each period Δt. The time takes account of the differences of instantaneous luminance existing between the different sub-pixels of one and the same pixel due to the differences of their supply voltage and/or supply current. To obtain a given mean luminance over a period Δt, the control circuit controls supply time ton of each of sub-pixels 2. The application time ton of the supply voltage or current can be adjusted by the anode or by the cathode.
For example purposes, the addressing circuit illustrated in
In alternative embodiments illustrated in
Thus, as illustrated in
Practically, the product of the instantaneous luminance L of the diode by its supply time (Lxton) corresponds to the mean luminance of the sub-pixel over period Δt. The global luminance of pixel 1 then depends on the mean luminance of the different selected sub-pixels.
Thus, the supply voltage or current of each of organic light-emitting diodes 3 being fixed according to the color of the corresponding sub-pixel (VR, VG, VB or IR, IB, IG), the color of pixel 1 and its luminance are determined by the control circuit by selecting the appropriate sub-pixels (command SL of
As illustrated in
The invention is not limited to the embodiments described above. In particular, the addressing circuit of
In another particular embodiment, the display system also called display device comprises a matrix of pixels 1 that is identical to the foregoing embodiments. The display device also comprises an addressing circuit specific to each sub-pixel 2 in order to select the required sub-pixel 2 and to control its supply time and supply conditions. This specific addressing circuit can be the one represented for example in
Each addressing circuit of sub-pixel 2 comprises a reset terminal which controls the supply time of sub-pixel 2, i.e. the supply time of light-emitting element 3. Each addressing circuit also comprises a selection terminal enabling it be defined whether the light-emitting element 3 of sub-pixel 2 has to be supplied with current or not. Each addressing circuit further comprises a power supply control terminal which enables the supply conditions of sub-pixel 2 to be modulated. As explained in the foregoing, the cathode can be common to sub-pixels of a determined color, and therefore to a group of sub-pixels 2, or the cathode can be specific to each sub-pixel 2.
As in the foregoing embodiments illustrated in
The control input of the supply conditions of sub-pixel 2 can be formed by an input terminal of first transistor T1, the output terminal whereof is connected to the control terminal of second transistor T2. In this way, depending on the value applied on the control line, also called data line DL, second transistor T2 modulates the quantity of current that can be applied on light emitter 3. Modulation of the current in light emitter 3 is only effective if first transistor T1 is in on state.
The reset input of sub-pixel 2 can be formed by the control input of third transistor T3 which is connected between the control input of second transistor T2 and ground or supply voltage Vdd. In this way, depending on the voltage applied on the control terminal of third transistor T3, second transistor T2 is in an off state or an on state.
In this embodiment which is particularly advantageous as it is compact, the different addressing circuits associated with a color sub-pixel 2, i.e. with a color of color filter 4, are connected to the same reset line RL. The control circuit is connected to all the sub-pixels 2 by means of their respective addressing circuits. The control circuit is connected independently to each sub-pixel 2 by a specific selection line SL and by a specific supply condition control line DL. The control circuit is also connected to the different sub-pixels 2 by reset lines RL which fix supply time ton of sub-pixel 2. However, these specific reset lines are dedicated to a color of sub-pixel 2. The control circuit therefore comprises as many reset lines RL as there are sub-pixels 2 of different colors in a pixel 1. The same is the case for the reset inputs in a pixel 1. On the contrary, the control circuit comprises as many selection lines SL and supply condition control lines DL as there are sub-pixels 2 in the matrix. In this way, the quantity of independent lines in the display device can be reduced, while at the same time ensuring independence of use of the different sub-pixels 2 and enhanced energy performances. Reset line RL can be physically common to all the sub-pixels of the same color. This can be the case for example when the reset line is connected to the anode or to the cathode of a diode.
In general manner according to the different addressing circuits illustrated in
As far as the addressing circuits illustrated in
For example purposes illustrated in
As reset line RL controls the supply time of sub-pixels 2 of the same color, all the red sub-pixels are supplied during a first predetermined time ton(R), all the green sub-pixels are supplied during a second predetermined time ton(G) and all the blue sub-pixels are supplied during a third predetermined time ton(B). The different sub-pixels 2 are supplied on the condition that first transistor T1 is in on state, i.e. that they have been selected to emit light. In this way, a sub-pixel 2 is supplied if the information on its selection line SL authorizes this and sub-pixel 2 is then only supplied during the time that is defined by reset line RL.
In this particular embodiment, modulation of the luminance of each sub-pixel 2 and therefore of pixel 1 is achieved by modulating the supply voltage at the terminals of each sub-pixel 2. Indeed, as specified in the foregoing, according to the supply conditions of each diode 3, modulation of the emitted color, but also of the instantaneous luminance, is performed. For a given supply condition, there are therefore a predefined color and a predefined instantaneous luminance. Modulation of the luminance for a given color is therefore performed by modulating the supply conditions for each of the sub-pixels 2. Each sub-pixel 2 naturally remains supplied in a range such that the emitted color is close to that of the associated color filter 4 so as to preserve an energy interest for this architecture. The color emitted by sub-pixel 2 is defined by the intersection between the transmission spectrum of the color filter and the emission spectrum of light-emitting element 3.
In a display device comprising this control circuit associated with a plurality of identical pixels 1 with variable chromatic coordinates with sub-pixels 2 and their associated addressing circuit, the operating conditions are fixed in the following manner.
In a pixel 1, each sub-pixel 2 (each light emitter 3) is supplied according to different conditions to determine the most energetically favorable supply conditions with the associated color filter 4 of sub-pixel 2. In this way, light emitter 3 of each sub-pixel 2 presents an emission spectrum which is as close as possible to the transmission spectrum of the associated color filter.
Depending on the supply conditions retained for each sub-pixel 2, the latter present different instantaneous luminances from one another. Each sub-pixel 2 is then supplied according to a specific predetermined time so that the corresponding pixel 1 emits a predetermined color and luminance. Typically, the supply times of each of the sub-pixels are chosen such that the pixel emits a white color under the most favorable supply conditions between each light emitter 3 and color filter 4 that is associated thereto.
Reset lines RL being associated with a sub-pixel color, all the sub-pixels of the same color normally have the same supply times. This means that all the pixels emit a white light when they are supplied under their most favorable supply condition with their color filter. In order for the different pixels to emit colors and luminances which are proper to them, each sub-pixel is supplied under different conditions (voltage and/or current). In this embodiment, modulation of the characteristics of the radiation emitted by the pixel is achieved by modulating the supply conditions of the sub-pixels which compose the latter.
In an alternative embodiment where the pixel electrode (the electrode commanded by transistor T2) represents the anode of the light-emitting diode, the cathode is common to all the sub-pixels of the same color, and control of the supply time can then be performed by means of the cathode. In this particular case, the reset line is formed by the cathode common to all the sub-pixels of the same color. This reset results in the appearance of a lower potential difference than a threshold voltage at the terminals of diode 3. It does in fact have to be considered that the anode control voltage can vary according to the displayed level, and a constant voltage at the terminals of the diode can therefore not be guaranteed. The use of a threshold voltage is then very advantageous. In this embodiment, it is possible to control the supply conditions of each of the diodes independently by means of second transistor T2. In the case where the pixel electrode represents the cathode of the light-emitting diode, it is also possible to perform the same modulation by means of the anode which is the common to all the sub-pixels of the same color. In these two particular embodiments, third transistor T3 can be eliminated.
Thus, if two pixels emit different colors and/or luminances, the only difference that exists between these two pixels is related to the supply conditions of each of the sub-pixels in each pixel.
| Number | Date | Country | Kind |
|---|---|---|---|
| 08/02584 | May 2008 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2009/000533 | 5/6/2009 | WO | 00 | 10/20/2010 |
