GB2364592A

GB2364592A - Pixel driver for an organic electroluminescent device

Info

Publication number: GB2364592A
Application number: GB0016815A
Authority: GB
Inventors: Simon Tam
Original assignee: Seiko Epson Corp
Current assignee: Seiko Epson Corp
Priority date: 2000-03-31
Filing date: 2000-07-07
Publication date: 2002-01-30
Also published as: KR20020032571A; KR100493555B1; WO2001075853A1; GB2360870A; GB0007879D0; GB0016816D0; GB2364593A; GB0016815D0

Abstract

A compensated pixel driver circuit comprises a p-channel transistor and an n-channel transistor connected as a complementary pair of transistors to provide analog control of the drive current for an organic electroluminescent device (OELD). The transistors, being of opposite channel, compensate for any variation in threshold voltage W V<SB>T</SB> and therefore provide a drive current to the OELD which is relatively independent of W V<SB>T</SB>. The complementary pair of transistors can be applied to either voltage driving or current driving pixel driver circuits.

Description

2364592 Organic ElectroLuminescent Device Compensated Pixel Driver Circuit

The present invention relates to an organic electroluminescent device and particularly to a compensated pixel driver circuit thereof.

An organic electroluminescent device (OELD) consists of a light emitting polymer (LEP) layer sandwiched between an anode layer and a cathode layer. Electrically, this device operates like a diode. Optically, it emits light when forward biased and the intensity of the emission increases with the forward bias current. It is possible to construct a display panel with a matrix of OELDs fabricated on a transparent substrate and with one of the electrode layers being transparent. It is also possible to integrate the driving circuit on the same panel by using low temperature polysilicon thin film transistor (TFT) technology.

In a basic analog driving scheme for an active matrix OELD display, a minimum of two transistors are required per pixel. Such a driving scheme is illustrated in Figure 1. Transistor T, is provided to address the pixel and transistor T 2 is provided to convert a data voltage signalVDam into current which drives the OELD at a designated brightness. The data signal is stored by a storage capacitor Cstorage when the pixel is not addressed. Although p-channel TFTs are shown in the figure, the same principle can also be applied for a circuit utilising n-channel TFTs.

There are problems associated with TFT analog circuits and OELDs do not act like perfect diodes. The LEP material does, however, have relatively uniform characteristics. Due to the nature of the TFT fabrication technique, spatial variation of the TFT characteristics exists over the extent of the display panel. One of the most important considerations in a TFT analog circuit is the variation of threshold voltage, AVT, from device to device. The effect of such variation in an OELD display, exacerbated by the non perfect diode behaviour, is the non-uniform pixel brightness over the display area of the panel, which seriously affects the image quality. Therefore, a built-in compensation circuit is required.

A simple threshold voltage variation compensation, current driven, circuit has been proposed. The current driven circuit, also known as the current programmed threshold voltage compensation circuit, is illustrated in figure 2. In this circuit, transistor T is provided for addressing the pixel. Transistor T 2 operates as an analog current control to provide the driving current to the OELD. Transistor T 3 connects between the drain and gate of transistor T 2 and toggles transistor T 2 to act either as a diode or in a saturation mode. Transistor T 4 acts as a switch in response to an applied waveformVGP. Either Transistor Ti or transistor T 4 can be ON at any one time. Initially, at time to shown in the timing diagram of Figure 2, transistors T, and T 3 are OFF, and transistor T 4 is ON. When transistor T 4 is OFF, transistors Ti and T 3 are ON, and a current 1DAT of known value is allowed to flow into the OELD, through transistor T 2. This is the programming stage because the threshold voltage of transistor T 2 is measured with transistor T3 turned ON which shorts the drain and gate of transistor T 2 Hence transistor T 2 operates as a diode while the programming current is allowed to flow through transistors T, and T 2 and into the OELD. The detected threshold voltage of transistor T 2 is stored by a capacitor C1 connected between the gate and source terminals of transistor T 2 when transistors T 3 and Ti are switched OFF. Transistor T 4 is then turned ON by driving waveformVGp and the current through the OELD is now provided by Supply VDD' If the slope of the output characteristics for transistor T 2 were flat, the reproduced current would be the same as the programmed current for any threshold voltage of T 2 detected and stored in capacitor C1. However, by turning ON transistor T 4, the drain-source voltage of transistor T 2 is pulled up, so a flat output characteristic will maintain the reproduced current at the same level as the programmed current. Note that AVT2 shown in figure 2 is imaginary, not real. It has been used solely to represent the threshold voltage of transistor T 2.

A constant current is provided, in theory, during a subsequent active programining stage, which is signified by the time interval t2 to t 5 in the timing diagram shown in figure 2. The reproduction stage starts at time t 6 - The circuit of Figure 2 does provide an improvement over the circuit shown in Figure 1 but variations in the threshold value of the control transistor are not fully compensated and variations in image brightness over the display area of the panel remain.

The present invention seeks to provide, therefore, an improved compensated pixel driver circuit in which variations in the threshold voltages of the pixel driver transistor can be further compensated, thereby providing a more uniform pixel brightness over the display area of the panel and, therefore, improved image quality.

According to a first aspect of the present invention there is provided a compensated pixel driver circuit for an electroluminescent device, the circuit comprising an n-channel transistor and a complementary p-channel transistor connected so as to operatively control, in combination, the current supplied to the electroluminescent device.

Preferably, the compensated pixel driver circuit also comprises respective storage capacitors for the n-channel and p-channel transistors and respective switching means connected so as to establish when operative respective paths to the n- channel and p-channel transistors for respective data voltage pulses.

Advantageously, the compensated pixel driver circuit may also comprise respective storage capacitors for storing a respective operating voltage of the n-channel and the pchannel transistors during a programming stage, a first switching means connected so as to establish when operative a first current path from a source of current data signals through the nchannel and p-channel transistors and the electroluminescent device during the programming stage, and a second switching means connected to establish when operative a second current path through the n-channel and p-channel transistors and the electroluminescent device during a reproduction stage.

In a further embodiment, the first switching means and the source of current data signals are connected so as to provide when operative a current source for the electroluminescent device In an alternative embodiment, the first switching means the source of current data signals are connected so as to provide when operative a current sink for the electroluminescent device.

According to a second aspect of the present invention there is also provided a method of compensating the supply current to an electroluminescent device comprising providing an n-channel transistor and a p-chann el transistor connected so as to operatively control, in combination, the supply current to the electroluminescent device.

Preferably, the method further comprises providing respective storage capacitors for the n-channel and p-channel transistors and respective switching means connected so as to establish when operative respective paths to the n-channel and p-channel transistors for respective data voltage pulses thereby to establish, when operative, a voltage driver circuit for the electroluminescent device.

Advantageously, the method may comprise providing a programming stage during which the n-channel and p-channel transistors are operated in a first mode and wherein a current path from a source of current data signals is established through the n-channel and the p-channel transistors and the electroluminescent device and wherein a respective operating voltage of the n-channel transistor and the p-channel transistor is stored in respective storage capacitors, and a reproduction stage wherein a second mode and a second current path is established through the n-channel transistor and the p-channel transistor and the electroluminescent device.

According to a third aspect of the present invention, there is also provided an organic electroluminescent display device comprising a compensated pixel driver circuit as claimed in any one of claims 1 to 11.

The present invention will now be described by way of further example only, with reference to the accompanying drawings in which: - Fig. I shows a conventional OELD pixel driver circuit using two transistors; Fig. 2 shows a known current programmed OELD driver circuit with threshold voltage compensation; Fig. 3 illustrates the concept of a compensated pixel driver circuit including a complementary pair of driver transistors for providing threshold voltage compensation in accordance with the present invention; Fig. 4 shows plots of characteristics for the complementary driver transistors illustrated in Fig. 3 for various levels of threshold voltages; Fig. 5 shows a compensated pixel driver circuit arranged to operate as a voltage driver circuit in accordance with a first embodiment of the present invention.

Fig. 6 shows a compensated pixel driver circuit arranged to operate as a current programmed driver circuit in accordance with a second embodiment of the present invention; Fig. 7 shows a compensated current programmed driver circuit in accordance with a third embodiment of the present invention, and Figs 8 to 11 show SPICE simulation results for the circuit illustrated in Fig. 6.

The concept of a compensated pixel driver circuit according to the present invention is illustrated in Fig. 3. An OELD device is coupled between two transistors T11 and T12 which operate, in combination, as an analog current control for the current flowing through the OELD. Transistor T11 is a p-channel transistor and transistor T15 is an n- channel transistor which act therefore, in combination, as a complementary pair for analog control of the current through the OELD.

As mentioned previously, one of the most important parameters in a TFT analog circuit design is the threshold voltageVT. Any variation, AVT within a circuit has a significant effect on the overall circuit performance. Variations in the threshold voltage can be viewed as a rigid horizontal shift of the source to drain current versus the gate to source voltage characteristic for the transistor concerned and are caused by the interface charge at the gate of the transistor.

It has been realised with the present invention that in an array of TFT devices, in view of the fabrication techniques employed, neighbouring or relatively close TFT's have a high probability of exhibiting the same or an almost similar value of threshold voltage AVT' Furthermore, it has been realised that as the effects of the sameAVT on p-channel and n- channel TFT's are complementary, compensation for variations in threshold voltage AVT can be achieved by employing a pair of TFT's, one p-channel TFT and one n-channel TFT, to provide analog control of the driving current flowing to the OELD. The driving current can, therefore, be provided independently of any variation of the threshold voltage. Such a concept is illustrated in figure 3.

Figure 4 illustrates the variation in drain current, that is the current flowing through the OELD shown in figure 3, for various levels of threshold voltage AVT, AVT1, AVT2 for the transistors T11 and T,,. Voltages V,, V2 and V,, are respectively the voltages appearing across transistor T11, T,2 and the OELD from a voltage source VDD. Assuming that the transistors T,j and T12 have the same threshold voltage and assuming that AVT = 0, then the current flowing through the OELD is given by crossover point A for the characteristics for the p-channel transistor T,, and the n-channel transistor T,2 shown in figure 4. This is shown by value I0.

Assuming now that the threshold voltage of the p-channel and n-channel transistors changes to AVT11 the OELD current I, is then determined by crossover point B. Likewise, for a variation in threshold voltage to AV21 the OELD current 12 is given by crossover point C. It can be seen from figure 4 that even with the variations in the threshold voltage there is minimal variation in the current flowing through the OELD.

Figure 5 shows a compensated pixel driver circuit configured as a voltage driver circuit. The circuit comprises p-channel transistor T,2 and nchannel transistor T15 acting as a complementary pair to provide, in combination, an analog current control for the OELD. The circuit includes respective storage capacitors C12 and C,, and respective switching transistors TA and T,3 coupled to the gates of transistors T,2 and T15. When transistors TA and T,, are switched ON data voltage signals V, and V2 are stored respectively in storage capacitors C,2 and C15 when the pixel is not addressed. The transistors TA and T13 function as pass gates under the selective control of addressing signals j and 2 applied to the gates of transistors TA and TB- Figure 6 shows a compensated driver circuit according to the present invention configured as a current programmed OELD driver circuit. As with the voltage driver circuit, p-channel transistor T12 and n-channel transistor T15 are coupled so as to function as an analog current control for the OELD. Respective storage capacitors C,, C2 and respective switching transistors T, and T6 are provided for transistors T12 and T15. The driving waveforms for the circuit are also shown in figure 6. Either transistors T,, T3 and T6, or transistor T4 can be ON at any one time. Transistors T, and T6 connect respectively between the drain and gate of transistors T12 and TO and switch in response to applied waveform VSEL to toggle transistors T12 and T,, to act either as diodes or as transistors in saturation mode. Transistor T3 is also connected to receive waveform VSEL. Transistors T, and T6 are both p-channel transistors to ensure that the signals fed through these transistors are at the same magnitude. This is to ensure that any spike currents through the OELD during transitions of the waveform VSEL are kept to a minimum.

The circuit shown in figure 6 operates in a similar manner to known current programmed pixel driver circuits in that a programming stage and a display stage are provided in each display period but with the added benefit that the drive current to the OELD is controlled by the complementary opposite channel transistors T12 and T15Referring to the driving waveforms shown in figure 6, a display period for the driver circuit extends from time to to time t6. Initially, transistor T4 is ON and transistors T,, T3 and T6 are OFF. Transistor T4 is turned OFF at time t, by the waveform VGp and transistors TI, T3 and T6 are turned ON at time t3 by the waveform VSEL. With transistors T, and T6 turned ON, the p-channel transistor T12 and the complementary n- channel transistor T1. act in a first mode as diodes. The driving waveform for the frame period concerned is available from the current source 1DAT at time t. and this is passed by the transistor T3 when it switches on at time t3. The detected threshold voltages of transistors T12 and TI, are stored in capacitors C, and C2. These are shown as imaginary voltage sources AVT12 and AVT15 in figure 6.

Transistors TI, T3 and T6 are then switched OFF at time t4 and transistor T4 is switched ON at time t. and the current through the OELD is then provided from the source VDD under the control of the p-channel and nchannel transistors T12 and TI, operating in a second mode, i.e. as transistors in saturation mode. It will be appreciated that as the current through the OELD is controlled by the complementary p-channel and n-channel transistors T12 and Tl,, any variation in threshold voltage in one of the transistors will be compensated by the other opposite channel transistor, as described previously with respect to figure 4.

In the current programmed driver circuit shown in figure 6, the switching transistor T3 is coupled to the p-channel transistor T,21 with the source of the driving waveform IDAT operating as a current source. However, the switching transistor T3 may as an alternative be coupled to the n-channel transistor T,5 as shown in figure 7, whereby 1DAT operates as a current sink. In all other respects the operation of the circuit shown in figure 7 is the same as for the circuit shown in figure 6.

Figures 8 to 11 show a SPICE simulation of an improved compensated pixel driver circuit according to the present invention.

Referring to figure 8, this shows the driving waveforms IDAT, VGP, VSEL and three values of threshold voltage, namely -lvolt, Ovolts and +lvolt used for the purposes of simulation to show the compensating effect provided by the combination of the p-channel and n-channel transistors for controlling the current through the OELD. From figure 8, it can be seen that, initially the threshold voltage AVTwas set at -1volt, increasing to Ovolts at 0.3 x 10-4 seconds and increasing again to + lvolt at 0.6 x 10-4 seconds. However, it can be seen from figure 9 that even with such variations in the threshold voltage the driving current through the OELD remains relatively unchanged.

The relative stability in the driving current through the OELD can be more clearly seen in figure 10, which shows a magnified version of the response plots shown in figure 9.

It can be seen from figure 10 that, using a value of 0 volts as a base for the threshold voltage AVT, that if the threshold voltage AVT changes to -1volts there is a change of approximately 1.2 % in the drive current through the OELD and if the threshold voltage AVT is changed to + 1 volt, there is a reduction in drive current of approximately 1. 7 % compared to the drive current when the threshold voltage AVT 'SO volts. The variation of drive current of 8.7% is shown for reference purposes only as such a variation can be compensated by gamma correction, which is well known in this art and will not therefore be described in relation to the present invention.

Figure 11 shows that for levels of IDAT ranging from 0.2LA to 1.0tA, the improved control of the OELD drive current is maintained by the use of the p-channel and opposite nchannel transistors in accordance with the present invention.

It will be appreciated from the above description that the use of a pchannel transistor and an opposite n-channel transistor to provide, in combination, analog control of

I I the drive current through an electroluminescent device provides improved compensation for the effects which would otherwise occur with variations in the threshold voltage of a single p-channel or n-channel transistor.

Preferably, the TFT n-channel and p-channel transistors are fabricated as neighbouring or adjacent transistors during the fabrication of an OELD display so as to maximise the probability of the complementary p-channel and n-channel transistors having the same value of threshold voltage AVT. The p-channel and n-channel transistors may be further matched by comparison of their output characteristics.

The improved compensated pixel driver circuit of the present invention may be used in display devices incorporated in many types of equipment such as mobile displays e.g. mobile phones, laptop personal computers, DVD players, cameras, field equipment; portable displays such as desktop computers, CCTV or photo albums; or industrial displays such as control room equipment displays.

The aforegoing description has been given by way of example only and it will be appreciated by a person skilled in the art that modifications can be made without departing from the scope of the present invention.

Claims

1. A compensated pixel driver circuit for an electroluminescent device, the circuit comprising an n-channel transistor and a complementary pchannel transistor connected so as to operatively control, in combination, the current supplied to the electroluminescent device.

2. A compensated pixel driver circuit as claimed in claim 1, wherein the complementary n-channel and p-channel transistors comprise polysilicon thin filin transistors.

3. A compensated pixel driver circuit as claimed in claim 2, wherein the complementary n-channel and p-channel transistors are spatially arranged in close proximity to each other for providing a complementary pair of nchannel and p-channel transistors having approximately equal threshold voltages.

4. A compensated pixel driver circuit as claimed in any one of claims I to 3 connected so as to establish when operative a voltage driver circuit comprising respective storage capacitors for the n-channel and p-channel transistors and respective switching means connected so as to establish when operative respective paths to the n-channel and p-channel transistors for respective data voltage pulses.

5. A compensated pixel driver circuit as claimed in any one of claims 1 to 3 comprising respective storage capacitors for storing a respective operating voltage of the n-channel and the p-channel transistors during a programming stage, a first switching means connected so as to establish when operative a first current path from a source of current data signals through the n-channel and p-channel transistors and the electroluminescent device during the programming stage, and a second switching means connected to establish when operative a second current path through the n-channel and p-channel transistors and the electroluminescent device during a reproduction stage.

6. A compensated pixel driver circuit as claimed in claim 5, wherein the first switching means and the source of current data signals are connected so as to provide when operative a current source for the electroluminescent device.

7. A compensated pixel driver circuit as claimed in claim 5, wherein the first switching means and the source of current data signals are connected so as to provide when operative a current sink for the electroluminescent device,

8. A compensated pixel driver circuit as claimed in any one of claims 5 to 7, further comprising respective further switching means respectively connected to bias the n-channel transistor and the p-channel transistor to act as diodes during the programming stage.

9. A compensated pixel driver circuit as claimed in claim 8, wherein the respective further switching means comprise p-channel transistors.

10. A compensated pixel driver circuit as claimed in any one of claims 5 to 9, wherein the circuit is implemented with polysilicon thin film transistors.

11. A compensated pixel driver circuit as claimed in claim 4, wherein the circuit is implemented using polysilicon thin film transistors.

12. A method of compensating the supply current to an electroluminescent device comprising providing an n-channel transistor and a p-channel transistor connected so as to operatively control, in combination, the supply current to the electroluminescent device.

13. A method as claimed in claim 11, comprising the ftirther step of providing the n channel transistor and the p-channel transistor as polysilicon thin film transistors.

14. A method as claimed in claim 12 comprising the further step of spatially arranging the n-channel and p-channel polysilicon thin film transistors in close proximity to each other.

15. A method as claimed in any one of claims 12 to 14 comprising providing respective storage capacitors for the n-channel and p-channel transistors and respective switching means connected so as to establish when operative respective paths to the n-channel and pchannel transistors for respective data voltage pulses thereby to establish, when operative, a voltage driver circuit for the electroluminescent device.

16. A method as claimed in any one of claims 12 to 14 comprising providing a progranuning stage during which the n-channel and p-channel transistors are operated in a first mode and wherein a current path from a source of current data signals is established through the n-channel and the p-channel transistors and the electroluminescent device and wherein a respective operating voltage of the n-channel transistor and the pchannel transistor is stored in respective storage capacitors, and a reproduction stage wherein a second mode and a second current path is established through the n-channel transistor and the p-channel transistor and the electroluminescent device.

17. A method as claimed in claim 15, wherein the first mode comprises operating the n channel and p-channel transistors as diodes.

18. An organic electroluminescent display device comprising a compensated pixel driver circuit as clairned in any one of claims I to 11.