Patent Grant
8310469
This application claims priority to Korean Patent Application No. 10-2006-0134801 filed on Dec. 27, 2006, and all the benefits accruing therefrom under 35 U.S.C. §119, the contents of which in its entirety are herein incorporated by reference.
(a) Field of the Invention
The present invention relates to an organic light emitting device and a driving method thereof. More particularly, the present invention relates to an organic light emitting device having improved screen uniformity, and a driving method thereof.
(b) Description of the Related Art
In general, active flat panel displays respectively include a plurality of pixels arranged in a matrix, and control the light intensity of each pixel on the basis of predetermined luminance information to display images. Among the active flat panel displays, an organic light emitting device is a display in which fluorescent organic materials are electrically excited to display images. The organic light emitting device is self-emissive and has low power consumption, a large reference viewing angle, and a high pixel response speed. Accordingly, the organic light emitting device is suitable for displaying a motion picture at a high definition.
The organic light emitting device includes organic light emitting diodes (“OLEDs”) and thin film transistors (“TFTs”) for controlling the OLEDs. The TFTs are classified as polysilicon TFTs and amorphous silicon TFTs, depending on the type of active layer.
Since the amorphous silicon can be deposited at a low temperature to form a thin film, it can be applied to a display that has a glass substrate having a low melting point. However, amorphous semiconductor has low electron mobility, which hinders a display device from being enlarged. In addition, when the amorphous silicon TFT is continuously supplied with a direct voltage at its control terminal, a threshold voltage of the amorphous silicon TFT is changed which degrades the performance of the TFT and thus, a reduction of the life span of the organic light emitting device may result.
Therefore, it is required to apply a polysilicon TFT having high electron mobility, excellent high frequency operation characteristics, and a low leakage current. Particularly, a low temperature polysilicon (“LTPS”) backplane can remarkably solve the problem of the life span. However, laser shot marks that are made in a laser crystallization process cause deviation of the threshold voltages of driving transistors in a device, thereby causing deterioration in screen uniformity.
Exemplary embodiments of a display device according to the present invention include a light emitting element, a capacitor, a driving transistor, a first switching unit, a second switching unit, and a third switching unit. The light emitting element emits light having an intensity dependent on a magnitude of a driving current. The capacitor is connected between a first node and a second node. The driving transistor outputs the driving current and has an input terminal connected with a first voltage, an output terminal, and a control terminal connected with the second node. The first switching unit selects one of a data voltage and a second voltage, and connects a selected voltage to the first node. The second switching unit switches a connection between the second voltage and the second node. The third switching unit selects one of the second node and the light emitting element, and connects a selected one of the second node and the light emitting element to the output terminal of the driving transistor.
The third switching unit may connect the second node to the output terminal of the driving transistor while the first switching unit connects the data voltage to the first node. The third switching unit may connect the light emitting element to the output terminal of the driving transistor while the first switching unit connects the second voltage to the first node. The second switching unit may connect the second node to the second voltage and may then disconnect the second node from the second voltage while the first switching unit connects the data voltage to the first node.
The capacitor may store a threshold voltage of the driving transistor while the first switching unit connects the data voltage to the first node and the third switching unit connects the second node to the output terminal of the driving transistor.
The first switching unit may include a first switch and a second switch. The first switch may switch a connection between the data voltage and the first node. The second switch may switch a connection between the second voltage and the first node. The second switching unit may include a third switch. The third switching unit may include a fourth switch and a fifth switch.
The fourth switch may switch a connection between the second node and the output terminal of the driving transistor. The fifth switch may switch a connection between the light emitting element and the output terminal of the driving transistor.
The first, second, fourth, and fifth switches may be controlled by a first control signal.
The first switch and the fourth switch may be field effect transistors (“FETs”) of a first channel type, and the second switch and the fifth switch may be FETs of a second channel type, and the second channel type may be different from the first channel type.
The third switch may be controlled by a second control signal, and may be an FET of the first channel type.
The driving transistor may have the first channel type.
The first to fifth switches and the driving transistor may include polysilicon.
The driving current may not be dependent on a threshold voltage of the driving transistor, and may depend on the data voltage and the second voltage.
Other exemplary embodiments of a display device according to the present invention include a light emitting element, a first capacitor, a driving transistor, a first switching transistor, a second switching transistor, a third switching transistor, a fourth switching transistor, and a fifth switching transistor. The first capacitor is connected between a first node and a second node. The driving transistor has an input terminal connected with a first voltage, an output terminal, and a control terminal connected with the second node. The first switching transistor is controlled by a first control signal, and is connected between a second voltage and the first node. The third switching transistor is controlled by a second control signal, and is connected between the second node and the second voltage. The fourth switching transistor is controlled by the first control signal, and is connected between the second node and the output terminal of the driving transistor. The fifth switching transistor is controlled by the first control signal, and is connected between the second node and the output terminal of the driving transistor.
The first, third, and fourth switching transistors may have a channel type that is different from a channel type of the second and fifth switching transistors.
The driving transistor, the second switching transistor, and the fifth switching transistor may be p-channel field effect transistors.
The first to fifth switching transistors and the driving transistor may include polysilicon.
In sequentially consecutive first to third periods, the first, third, and fourth switching transistors may turn on and the second and fifth switching transistors may turn off during the first period, the first and fourth switching transistors may turn on and the second, third, and fifth switching transistors may turn off during the second period, and the second and fifth switching transistors may turn on and the first, third, and fourth switching transistors may turn off during the third period.
The first control signal may be a scanning signal from a scanning driver and the second control signal may be a light-emission signal from an emission driver.
Exemplary embodiments of a driving method according to the present invention drive a display device having a light emitting element, a capacitor connected between a first node and a second node, and a driving transistor having an input terminal, an output terminal, and a control terminal that is connected to the second node. The driving method includes connecting a data voltage to the first node and connecting the second node to the output terminal of the driving transistor, connecting a second voltage to the second node, disconnecting the second node from the second voltage, and connecting the second voltage to the first node and connecting the light emitting element to the output terminal of the driving transistor.
Connecting the second voltage to the second node and disconnecting the second node from the second voltage may be sequentially performed while the data voltage is connected to the first node, and while the second node is connected to the output terminal of the driving transistor. The second voltage may be connected to the first node and the light emitting element may be connected to the output terminal of the driving transistor while the second node and the second voltage are disconnected from each other.
The above and other aspects, features, and advantages of the present invention will become more apparent by describing in further detail exemplary embodiments thereof with respect to the accompanying drawings, in which:
The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout.
It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown.
An organic light emitting device according to an exemplary embodiment of the present invention will now be described in detail with reference to
Referring to
The display panel 300 includes a plurality of signal lines G1 to Gn, D1 to Dm, and S1 to Sn, a plurality of voltage lines (not shown), and a plurality of pixels PX that are connected to the signal lines G1 to Gn, D1 to Dm, and S1 to Sn, and are arranged approximately in a matrix shape.
The signal lines G1-Gn, D1-Dm, and S1-Sn include a plurality of scanning signal lines G1 to Gn, also known as gate lines, transmitting scanning signals, a plurality of data lines D1 to Dm transmitting data signals, and a plurality of light-emission signal lines S1 to Sn transmitting light-emission signals. The scanning signal lines G1 to Gn and the light-emission signal lines S1 to Sn substantially extend in a row direction, such as a first direction, in parallel with one another. The data lines D1 to Dm substantially extend in a column direction, such as a second direction, in parallel with one another. The first direction may be substantially perpendicular with the second direction.
The voltage lines of the signal lines include driving voltage lines (not shown) transmitting a driving voltage.
As shown in
Referring to
One terminal of the capacitor Cst is connected with the control terminal of the driving transistor Qd at the node N2, and the other terminal is connected with the switching transistor Qs1 at a node N1.
The switching transistor Qs1 is connected between a data voltage Vdat and the node N1. The switching transistor Qs2 is connected between a sustain voltage Vsus and the node N1, and the switching transistor Qs3 is connected between the sustain voltage Vsus and the node N2. The switching transistor Qs4 is connected between the node N2 and the output terminal of the driving transistor Qd, and the switching transistor Qs5 is connected between the output terminal of driving transistor Qd and the organic light emitting element LD.
The switching transistors Qs1, Qs2, Qs4, and Qs5 operate in response to the scanning signal Vgi, from the scanning signal lines G1 to Gn, and the switching transistor Qs3 operates in response to the light-emission signal Vsi, from the light-emission signal lines S1 to Sn. The switching transistors Qs1 and Qs2 select one of the data voltage Vdat and the sustain voltage Vsus and connect the selected one to the node N1. The switching transistor Qs3 switches a connection between the sustain voltage Vsus and the node N2, and the switching transistors Qs4 and Qs5 select one of the node N2 and the light emitting element LD and connect the selected one to the output terminal of the driving transistor Qd. The switching transistors Qs1 and Qs2 may constitute a first switching unit, the switching transistor Qs3 may constitute a second switching unit, and the switching transistors Qs4 and Qs5 may constitute a third switching unit.
The switching transistors Qs1, Qs3, and Qs4 are n-channel polysilicon field effect transistors (“FETs”), and the switching transistors Qs2 and Qs5 and the driving transistor Qd are p-channel polysilicon FETs. An example of the FET is a thin film transistor (“TFT”), and the TFT may contain amorphous silicon rather than polysilicon. The channel type of each of the switching transistors Qs1 to Qs5 and the driving transistor Qd may be reversed, and accordingly, signal waveforms for driving them may be reversed.
An anode and a cathode of the organic light emitting element LD are respectively connected to the switching transistor Qs5 and a common voltage Vss. The organic light emitting element LD emits light having an intensity according to the magnitude of the output current ILD of the driving transistor Qd that is supplied through the switching transistor Qs5 so as to display an image. The magnitude of the output current ILD depends on the voltage difference between the control terminal and the input terminal of the driving transistor Qd.
Referring to
The emission driver 700 is connected with emission signal lines S1 to Sn of the display panel 300, and applies emission signals Vsi to the emission signal lines S1 to Sn. Each of the emission signals Vsi is a combination of the high voltage Von and the low voltage Voff.
The high voltage Von can turn on the switching transistors Qs1, Qs3, and Qs4 and turn off the switching transistors Qs2 and Qs5, and the low voltage Voff can turn off the switching transistors Qs1, Qs3, and Qs4 and turn on the switching transistors Qs2 and Qs5. The sustain voltage Vsus is sufficiently low to turn off the switching transistors Qs1, Qs3, and Qs4 and turn on the switching transistors Qs2 and Qs5, as does the low voltage Voff.
The data driver 500 is connected with the data lines D1 to Dm of the display panel 300 and applies the data voltages Vdat to the data lines D1 to Dm.
The signal controller 600 controls the scanning driver 400, the data driver 500, and the emission driver 700.
The respective elements 400, 500, 600, and 700 may be directly mounted on the display panel 300 in the form of at least one integrated circuit (“IC”) chip, may be mounted on a flexible printed circuit (“FPC”) film (not shown) that is mounted on the display panel 300 in the form of a tape carrier package (“TCP”), or may be mounted on a separate printed circuit board (“PCB”) (not shown). Alternatively, the elements 400, 500, 600, and 700 may be integrated into the display panel 300 together with, for example, the signal lines G1 to Gn and D1 to Dm and the transistors Qs1 to Qs5 and Qd. In another exemplary embodiment, the elements 400, 500, 600, and 700 may be integrated into a single chip. In this case, at least one circuit of the elements 400, 500, 600 and 700 may be disposed outside the single chip.
The operation of the organic light emitting device will be described in detail with reference to
The signal controller 600 receives input image signals R, G, and B and input control signals from an external graphics controller (not shown) for controlling the display thereof. The input image signals R, G, and B contain luminance information of the pixels PX, and the luminance has a predetermined number of grays (e.g., 1024(=210), 256 (=28), or 64 (=26)). The input control signals, for example, include a vertical synchronization signal Vsync, a horizontal synchronizing signal Hsync, a main clock signal MCLK, and a data enable signal DE.
On the basis of the input image signals R, G, and B and the input control signals, the signal controller 600 processes the input image signals R, G, and B to be suitable for the operation conditions of the display panel 300, and generates scanning control signals CONT1, data control signals CONT2, and emission control signals CONT3. The signal controller 600 sends the scanning control signals CONT1 to the scanning driver 400, the emission control signals CONT3 to the emission driver 700, and the data control signals CONT2 and output image signals DAT to the data driver 500.
The scanning control signals CONT1 include a scanning start signal STV for instructing to start scanning of the high voltage Von into the scanning signal lines G1 to Gn and at least one clock signal for controlling an output period of the high voltage Von. The scanning control signals CONT1 may further include an output enable signal OE for defining the duration of the high voltage Von.
The data control signals CONT2 include a horizontal synchronization start signal STH for notifying the start of transmission of the digital output image signals DAT for a row of pixels PX, a load signal LOAD for instructing to apply analog data voltages to the data lines D1 to Dm, and a data clock signal HCLK.
The emission control signals CONT3 include a synchronization signal for instructing to start the scanning of the high voltage Von into the emission signal lines S1 to Sn and at least one clock signal for controlling output of the high voltage Von. The emission control signals CONT3 may further include a signal for defining the duration of the high voltage Von.
The following description will be focused on one exemplary pixel row, for example, on the i-th pixel row.
With reference to
Within period T1, the scanning driver 400 converts the scanning signal Vgi applied to the scanning signal line Gi into the high voltage Von according to the scanning control signals CONT1 from the signal controller 600, and the emission driver 700 converts the emission signal Vsi applied to the emission signal line Si into the high voltage Von, according to the emission control signals CONT3 from the signal controller 600.
Then, with reference to
As shown in
Subsequently, the emission driver 700 changes the emission signal Vsi into the low voltage Voff, as shown in
Then, with the switching transistor Qs3 turned off, the node N2 is separated from the sustain voltage Vsus as shown in
Therefore, a voltage VN2 at the node N2 converges to a voltage value given by Equation 1.
VN2=Vdd+Vth [Equation 1]
In this case, since the voltage VN1 at the node N1 stays at the data voltage Vdat, the voltage stored in the capacitor Cst can become:
VN1−VN2=Vdat−(Vdd+Vth). [Equation 2]
Then, the scanning driver 400 turns off the switching transistors Qs1 and Qs4 and turns on the switching transistors Qs2 and Qs5 by changing the scanning signal Vgi to the low voltage Voff so that an emission period T3, shown in
Then, the node N1 is separated from the data voltage Vdat and connected to the sustain voltage Vsus through the switching transistor Qs2, and the control terminal of the driving transistor Qd is floating, as shown in
Therefore, the voltage VN2 at the node N2 can be obtained as given in Equation 3.
VN2=Vdd+Vth−Vdat+Vsus [Equation 3]
When the switching transistor Qs5 is turned on, the output terminal of the driving transistor Qd is connected to the organic light emitting element LD, and the driving transistor Qd outputs an output current ILD with a magnitude that varies in accordance with the voltage difference between the control terminal and the input terminal of the driving transistor Qd, as given in Equation 4.
where K denotes a constant K (K=μ×Ci×W/L) determined by the characteristics of the driving transistor Qd, μ denotes a field effect mobility, Ci denotes a capacitance of a gate insulating layer of the driving transistor Qd, W denotes a channel width of the driving transistor Qd, and L denotes a channel length of the driving transistor Qd.
As given in Equation 4, the magnitude of the output current ILD in the emission period T3 is determined by the data voltage Vdat and the sustain voltage Vsus that is fixed. Therefore, the output current ILD is not affected by the threshold voltage Vth of the driving transistor Qd, and is thus not affected by any deviation in the threshold voltage Vth.
The output current ILD is supplied to the organic light emitting element LD, and the organic light emitting element LD emits light with an intensity that varies depending on the magnitude of the output current ILD to thereby display an image.
Therefore, a uniform image can be obtained even when a deviation occurs in the threshold voltages Vth of driving transistors Qd or when the threshold voltage Vth of a driving transistor Qd varies as a function of time.
The emission period T3 lasts until the initialization period T1 of the next frame for the i-th pixel row starts, and the operations in the respective periods T1 to T3 are repeated for the next row of pixels PX in the above-described manner. However, the initialization period T1 of the (i+1)-th pixel row is set to start after the compensation period T2 of the i-th pixel row is terminated. In the above-stated manner, the operations in the initialization period T1, the compensation period T2, and the emission period T3 are sequentially performed for all the scanning signal lines G1 to Gn and the emission signal lines S1 to Sn to thereby display the corresponding image on all the pixels PX.
The length of the respective periods T1 to T3 may be adjusted as necessary.
Simulation results in the presence of a deviation of the threshold voltages Vth of the driving transistors Qd of an organic light emitting device according to an exemplary embodiment of the present invention will be described with reference to
The waveforms of
The result of the simulation shows that the deviation of the threshold voltage Vth of the driving transistors Qd can be compensated by the organic light emitting device according to the exemplary embodiment of the present invention.
As described, the deviation of the threshold voltage of the driving transistors can be compensated by using a pixel circuit including only five switching transistors, one driving transistor, one capacitor, and one organic light emitting device, according to the exemplary embodiment of the present invention.
While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
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