Active Matrix Organic Light-Emitting Diode (AMOLED) displays, novel pixel circuits therefore, and methods of programming and driving the pixel circuit are disclosed. A pixel circuit includes five TFT transistors, a light-emitting device and a storage capacitor coupled to an external voltage supplied through a voltage line and is driven using a plurality of operation states effecting in-pixel compensation.

FIELD OF THE PRESENT DISCLOSURE

The present disclosure relates to active-matrix organic light-emitting diode (AMOLED) displays and particularly to pixel circuits thereof and methods of driving pixel circuits to emit light.

BRIEF SUMMARY

According to a first aspect there is provided a display system including: an array of pixel circuits arranged in rows and columns, a pixel circuit of the array of pixel circuits including: a drive transistor coupled between a first and a second supply voltage and including a source terminal coupleable to a data line of the display system; a storage capacitor coupled across a gate terminal of the drive transistor and a voltage line; and a light-emitting device coupled between the first supply voltage and the source terminal of the drive transistor, and a controller for driving the pixel circuit in a drive mode including a plurality of operation states for the pixel circuit including a programming and in-pixel compensation state at least for programming the storage capacitor of the pixel circuit with use of a data voltage provided over the data line.

In some embodiments, the voltage line is kept at a constant voltage level.

In some embodiments, the constant voltage level is a voltage level different from voltage levels of the first and the second supply voltages.

Some embodiments further provide for an initialization transistor coupled across a drain terminal and the gate terminal of the drive transistor.

In some embodiments, the initialization transistor is for coupling the gate and drain terminals of the drive transistor during an initialization state.

In some embodiments, the initialization transistor is for coupling the gate and drain terminals of the drive transistor during a programming and in-pixel compensation state, in which the drive transistor discharges a gate voltage of the gate terminal until the drive transistor turns off.

Some embodiments further provide for a first emission transistor coupled between the first supply voltage and the drain terminal of the drive transistor and a second emission transistor coupled between the source terminal of the drive transistor and the second supply voltage, the first and second emission transistors for allowing current to pass between the first and second supply voltages and though the light-emitting device during an emission state.

Some embodiments further provide for a write transistor coupled between the source terminal of the drive transistor and the data line, for said programming the storage capacitor with use of the data voltage during the programming and in-pixel compensation state.

In some embodiments, the pixel circuit includes transistors which are only N-type TFTs, and said light-emitting device is an organic light-emitting diode (OLED) device.

According to another aspect there is provided a method of driving a display system, the display system including an array of pixel circuits arranged in rows and columns, a pixel circuit of the array of pixel circuits including: a drive transistor coupled between a first and a second supply voltage and including a source terminal coupleable to a data line of the display system; a storage capacitor coupled across a gate terminal of the drive transistor and a voltage line; and a light-emitting device coupled between the first supply voltage and the source terminal of the drive transistor, the method comprising: driving the pixel circuit in a plurality of operation states for the pixel circuit including: during a programming and in-pixel compensation state, programming the storage capacitor of the pixel circuit with use of a data voltage provided over the data line.

In some embodiments, during the plurality of operation states the voltage line is kept at a constant voltage level.

In some embodiments the constant voltage level is kept at a voltage level different from voltage levels of the first and the second supply voltages.

In some embodiments, the display system includes an initialization transistor coupled across a drain terminal and the gate terminal of the drive transistor, and driving the pixel circuit in the plurality of operation states further includes: during an initialization state, coupling the gate and drain terminals of the drive transistor with the initialization transistor.

In some embodiments, driving the pixel circuit in the plurality of operation states further includes: during the programming and in-pixel compensation state, using the initialization transistor to couple the gate and drain terminals of the drive transistor allowing the drive transistor to discharge a gate voltage of the gate terminal until the drive transistor turns off.

In some embodiments, the display system includes a first emission transistor coupled between the first supply voltage and the drain terminal of the drive transistor and a second emission transistor coupled between the source terminal of the drive transistor and the second supply voltage, and driving the pixel circuit in the plurality of operation states further includes: during an emission state turning the first and second emission transistors on to allow current to pass between the first and second supply voltages and though the light-emitting device.

In some embodiments, the display system includes a write transistor coupled between the source terminal of the drive transistor and the data line, and driving the pixel circuit in the plurality of operation states further includes: during the programming and in-pixel compensation state, using the write transistor to program the storage capacitor with use of the data voltage.

The foregoing and additional aspects and embodiments of the present disclosure will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments and/or aspects, which is made with reference to the drawings, a brief description of which is provided next.

DETAILED DESCRIPTION

An Organic Light-Emitting Diode (OLED) device is a light-emitting device in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current. This layer of organic material is situated between two electrodes; typically, at least one of these electrodes is transparent. Compared to conventional Liquid Crystal Displays (LCDs), Active Matrix Organic Light Emitting Device (AMOLED) displays offer lower power consumption, manufacturing flexibility, faster response time, larger viewing angles, higher contrast, lighter weight, and amenability to flexible substrates. An AMOLED display works without a backlight because the organic material of the OLED within each pixel itself emits visible light and each pixel consists of different colored OLEDs emitting light independently. The OLED panel can display a deep black level and can be thinner than an LCD display. The OLEDs emit light according to currents passing through them supplied through drive transistors controlled by programming voltages. The power consumed in each pixel has a relation with the magnitude of the generated light in that pixel.

The quality of output in an OLED-based pixel depends on the properties of the drive transistor, which is typically fabricated from materials including but not limited to amorphous silicon, polysilicon, or metal oxide, as well as properties of the OLED itself. In particular, the critical drawbacks of OLED displays include luminance non-uniformity due to the electrical characteristic variations of the drive transistor such as threshold voltage and mobility as the pixel ages and image sticking due to the differential aging of OLED devices. In order to maintain high image quality, variation of these parameters are compensated for by adjusting the programming voltage. In some approaches, those parameters are extracted from the driver circuit. The measured information can then be used to inform subsequent programming of the pixel circuits so that adjustments may be made to the programming taking into account the measured degradation. In some approaches, in-pixel compensation which adjusts the programming voltage in-pixel taking into account the degradation of that pixel is utilized.

Aspects of the present disclosure include a novel pixel circuit in display panels and methods to drive the pixel in ways which take into account the parameters of the pixel which affect performance. The pixel circuit includes a light-emitting device, such as an Organic Light Emitting Diode (OLED), a storage capacitor and Thin Film Transistors (TFTs). Methods include supplying voltage or current to the pixel circuit from the source via the data line over a number of cycles or states such that in-pixel programming is compensated, at least in part, for degradation of the pixel.

FIG.1is a block diagram of an exemplary display system100according to an embodiment. The display system100includes a display panel108, a source driver110which may include a Readout Circuit (ROC)112, a gate driver104, a controller114, a memory storage116, a voltage source106, and a supply voltage102. The display panel108includes a plurality of pixels200arranged in “n” rows and “m” columns. Each pixel200has a pixel circuit including five Thin Film Transistors (TFTs), a storage capacitor and a light-emitting device as shown inFIG.2. Each pixel200is individually programmed to emit light with specific luminance values. The digital controller114receives digital video data indicative of information to be displayed on the display panel108. The controller114sends signals136comprising digital video data to the source driver110and signals134to the gate (address) driver104to drive the pixels200in the display panel108on a row-by-row basis to display the information indicated. The plurality of pixels200associated with the display panel108thus comprise a display array (“display screen”) adapted to dynamically display information according to the input digital data received by the controller114. The display screen108can display, for example, video information from a stream of video data (not shown) received by the controller114. The supply voltage102provides constant or adjustable supply voltages (e.g. ELVDD) for the display panel108which is controlled by the signals132from the controller114. The voltage source106provides constant voltage VINIfor the display panel108which is controlled by the signals140from the controller114.

FIG.1is illustrated with only two pixels200aand200bin the display panel108for sake of simplicity and illustration. The display system100can be implemented with a plurality of similar pixels, such as the pixel200and the display panel size is not restricted to a particular number of rows and columns of pixels. For example, the display system100can be implemented with a display panel with a number of rows and columns of pixels commonly available in displays for mobile devices, monitor-based devices, TVs, and projection devices.FIG.1is illustrated with only two pixels200aand200bin the display panel108.

As shown inFIG.1, the pixel200aillustrated as the top-left pixel in the display panel108represents a “ith” row and “jth” column pixel and is coupled to an emission signal line120ifor a first emission signal EM[i] and the emission signal line120i+1of the next row for the second emission signal EM[i+1] which is the first emission signal of the next row, coupled to a write signal line122ifor a write signal WR[i], a initialization signal line124ifor an initialization signal INIT[i], coupled to a supply line128jfor a supply voltage ELVDD[j], coupled to a data line130jfor a data voltage VDATA[j], and coupled to a voltage line126ifor a voltage VINI[i]. The pixel200billustrated as the bottom-right pixel200in the display panel108represents a “nth” row and “mth” column pixel and is coupled to an emission signal line120nfor a first emission signal EM[n] and to an emission signal line120n+1for a second emission signal EM[n+1] (delayed by one programming cycle from EM[n], e.g. seeFIG.3), coupled to a write signal line122nfor a write signal WR[n], coupled to an initialization signal line124nfor an initialization signal INIT[n], coupled to a supply line128mfor a supply voltage ELVDD[m], coupled to a data line130mfor a data voltage VDATA[m], and coupled to a voltage line126nfor a voltage VINI[n].

As shown inFIG.1, the gate driver104provides the EM, WR, and INIT signals for the emission signal lines120i,120n,120i+1,120n+1, the write signal lines122i,122n, and the initialization signal lines124i,124n. These signals are utilized to control the pixels200in the display panel108in order to program and drive the pixels200. The data line130conveys programming information such as a programming voltage VDATAto the pixel200from the source driver110to the pixel200in order to program the pixel200to emit a desired amount of luminance according to the digital data received by the controller114. The programming voltage can be applied to the pixel200during a programming operation of the pixel200so as to charge a storage device within the pixel200, such as a storage capacitor, thereby enabling the pixel200to emit light with the desired amount of luminance during an emission operation following the programming operation. For example, the storage device in the pixel200can be charged during a programming operation to keep the data voltage and then apply it to a gate terminal of the driving transistor during the emission operation, thereby causing the driving transistor to convey the driving current through the light-emitting device according to the voltage stored on the storage device. In some embodiments a programming operation is combined with in-pixel compensation.

Generally, in the pixel200, the driving current that is conveyed through the light-emitting device by the driving transistor during the emission operation of the pixel200is a current that is supplied by the supply line (e.g. the supply line128jand128m). The supply line128can provide a positive supply voltage (e.g., the voltage commonly referred to in circuit design as “ELVDD”). In some implementations, a zero (0V) or negative supply voltage ELVSS[j] can be provided over a second supply line to the pixel200. For example, as described in association withFIGS.2and3, each pixel can be coupled to a first supply line128coupled to ELVDD and a second supply line (not shown) coupled to ELVSS, and the pixel circuits200can be situated between the first and second supply lines to facilitate driving current between the two supply lines during emission or other states of the pixel circuit. Although ELVDD and ELVSS may be provided on a column-by-column basis, in some embodiments ELVDD and ELVSS are each single common voltage values provided to all pixels of all columns.

According to an embodiment, an exemplary pixel circuit200of a display system ofFIG.1, is shown inFIG.2, the pixel circuit comprising five N-type TFTs (T1, T2, T3, T4 and T5)201202203204205, a light-emitting device (D1)210(such as an OLED), a storage capacitor (CS)212, and input with four control signals. A drive transistor T1201is coupled in series with the light-emitting device D1210, and the storage capacitor (CS)212is coupled across a gate214of the drive transistor T1201and a voltage line126providing the voltage VINI. Transistor T4204, controlled by the first emission signal EM[i], is coupled between the source of the drive transistor T1201and ELVSS. Transistor T3203, controlled by the write signal WR[i], is coupled between the source of the drive transistor T1201and the data line130, while transistor T2202, controlled by the initialization signal INIT[i], is coupled between the gate of the drive transistor T1201and the drain of the drive transistor T1201. Transistor T5205, controlled by the second emission signal EM[i+1] is coupled between the drain of the drive transistor T1201and the light-emitting device D1210.

Control signals EM[i], WR[i], and INIT[i] are control signals of a pixel circuit200of the ith row. The second emission signal EM[i+1] is the first emission signal for the (i+1)th row and is also coupled to the ith row. As will be seen inFIG.3, the EM[i+1] lags behind EM[i] by the duration of one operation cycle or state. All the control signals are provided by the gate driver104, as controlled by the controller114, as shown inFIG.1.

The constant voltage VINIis common for all pixels located in each row. These voltages VINI[i] . . . VINI[n] are provided over voltage lines126i. . .126nby the voltage source106. In some embodiments, a common voltage VINIis common to and provided for all pixels in all rows. The pixel circuit200includes a storage capacitor Cs212, for storing a voltage including a data voltage VDATAprovided by the source driver110over the data line130and for allowing the pixel circuit200to drive the light-emitting device D1210after being addressed. As such, the display panel108including a pixel circuit200, is an active-matrix display array. The present disclosure includes a novel pixel circuit in display panels which includes the N-type TFT transistors because the N-type TFT transistors have far less threshold voltage variation than their p-type TFT transistor counterparts. Therefore, time for the programming and In-Pixel Compensation (IPC) state (referred to below) can be reduced in order to reduce the total time for the driving mode described below. Although, the transistors utilized in the pixel circuit200are N-type Thin Film Transistors (TFTs), implementations of the present disclosure are not limited to pixel circuits having a particular polarity of transistor or only to pixel circuits having thin-film transistors.

In some embodiments, the display system100also includes a Readout Circuit (ROC)112which is integrated with the source driver110. The data line (130j,130m) connects the pixel200to the readout circuit112. The data line (130j,130m) allows the readout circuit112to measure an electrical signals (voltage or current) associated with the pixel200and thereby extract information indicative of a degradation of the pixel200. The Readout circuit112converts the associated current into a digital value which is sent to the digital control114for further processing or compensation.

Drive Mode

A timing diagram for the control signals of the pixel circuit200in the drive mode300is shown inFIG.3. The drive mode300ofFIG.3comprises four states which include, initialization301, programming and an In-Pixel Compensation (IPC) state302, an off state303, and an emission state304during which the pixel emits light.

During the initialization state301, the first emission signal EM[i] is pulled low and the write signal WR[i] is kept low, causing transistor T3203to stay off and transistor T4204to turn off, while the second emission signal EM[i+1] is kept high and the initialization signal INIT[i] is pulled high, causing transistor T5205to stay on and transistor T2202to turn on. Consequently, during the initialization state301, the storage capacitor Cs212is charged to ELVDD−VTHLED−VINI, where VTHLEDis the threshold voltage of the light emitting diode D1210(i.e. the voltage required to turn on, and hence for current to flow through, the light-emitting device D1210). Moreover, the voltage Vgat the gate of the drive transistor T1201is charged to ELVDD−VTHLED.

During the programming and In-Pixel Compensation (IPC) state302, the first emission signal EM[i] stays low and the second emission signal EM[i+1] is pulled low, causing transistor T4204to stay off and transistor T5205to turn off, while the initialization signal INIT[i] is kept high and the write signal WR[i] is pulled high, causing transistor T2202to stay on and transistor T3203to turn on. The appropriate VDATA[i] for the pixel circuit200is also provided on the data line130. Consequently, the voltage Vgat the gate214of the drive transistor T1201discharges to VDATA+VTHT1, where VTHT1is the threshold voltage of the drive transistor T1201, at which point the drive transistor T1201turns off, and the voltage stored in the capacitor Cswill have dropped to VDATA+VTHT1−VINI.

During the off state303, the first emission signal EM[i] is pulled high causing transistor T4204to turn on, while the second emission signal EM[i+1] stays low, the initialization signal INIT[i] and the write signal WR[i] are pulled low, keeping transistor T5205off, while, causing transistors T2202and T3203to turn off. Consequently, all the transistors except for T4204are off.

During the emission state304, the first emission signal EM[i] stays high and the second emission signal EM[i+1] is pulled high, causing transistor T4204to stay on and transistor T5205to turn on, while the initialization signal INIT[i] and the write signal WR[i] are kept low, keeping transistors T2202and T3203off. Consequently, the gate-source voltage at the drive transistor T1201is:

The drive transistor T1201drives the light-emitting device D1210with a pixel current Ipixelcorresponding to the gate-source voltage Vgs and the characteristics of the drive transistor T1201. The current passing through the drive transistor T1201(and also through the light-emitting diode D1210) is:

where μ is the charge carrier mobility, Coxis the oxide capacitance density, W/L is the width to length ratio of the drive transistor T1201. Hence, both the current passing through the pixel200and the luminance of the light-emitting device are independent of the threshold voltage VTHT1of the drive transistor T1201.

Although the embodiments have been described with functionality of the transistors resulting from the application of particular example voltage values such as “ELVDD” or “0” or “ELVSS”, it is to be understood that in different contexts, the application of “high” and “low” voltages of appropriate different voltage values may be used to effect the same functionality from transistors and do not represent a departure from the embodiments disclosed above.