The present invention relates to a pixel circuit which drives light emitting elements using a driving transistor, a display device and an inspection method.
With a display device that uses current drive type light emitting elements, such as an organic EL element (OLED), a driving transistor is normally arranged in a pixel circuit. A display is made by driving the driving transistor based on display signals. However, because OLED is a current driving element, variable output current of the driving transistor is directly connected to a deterioration of visual quality. Therefore, a wide variety of proposals have been made to control variable driving current for example as in patent reference 1.
[Patent reference 1] Japanese unexamined patent application No. 2003-271095
[Patent reference 2] Japanese unexamined patent application No. 2004-191603
A switching transistor is used in the patent reference 1 to control the variation in the driving current, and the source electrode of this switching transistor and the cathode electrode of the light emitting elements are in common. Thus, the source electrode of the switching transistor becomes an open status before the light emitting elements are formed and it is difficult to conduct an inspection in such condition.
To conduct an inspection of pixels before the light emitting elements are formed has been proposed, for example, in patent reference 2. However, this patent reference 2 does not include a method of controlling variations in the driving current, and it is impossible to prevent deterioration of display quality as is.
A pixel circuit according to the present invention comprises a sampling transistor which is connected to a signal line by one end and is turned on and off by the first scanning line; a driving transistor with a gate being connected to the other end of the sampling transistor and with a drain being connected to the first power supply; light emitting elements which are connected in between a source of the driving transistor and the second power supply and is driven by the current applied to the said driving transistor; a retentive capacitance connected in between the gate and source of the said driving transistor; and a switching transistor which is arranged in between the source of the said driving transistor and a reference potential line and turned on and off by the second scanning line. The said sampling transistor and the said switching transistor are electrically connected during the period when a reference signal voltage is set to the said signal line, the difference in voltage between a reference signal voltage and a reference potential is charged to the said retentive capacitance under the condition of the voltage between the gate and source of the said driving transistor being equal to or greater than the threshold voltage of the said driving transistor, and the source voltage of the said driving transistor is set to the reference potential in order to make the voltage applied to the said light emitting elements equal to or lower than its threshold voltage. Subsequently, while a reference signal voltage is set to the said signal line, the said sampling transistor and the said switching transistor are electrically connected and by turning off the said switching transistor, the voltage equivalent to the threshold voltage of the said driving transistor is retained by the said retentive capacitance while maintaining the voltage applied to the said light emitting elements below its threshold voltage, and the said sampling transistor is electrically connected to sample the said signal voltage during the period when the display signal voltage is set to the said signal line to superimpose the said signal voltage on the threshold voltage retained by the said retentive capacitance.
Also the present invention is a display device having a plurality of pixels arranged in a matrix, comprising a plurality of signal lines; a signal line driving circuit for driving the plurality of signal lines; a plurality of the first scanning lines; the first scanning line driving circuit for driving this first scanning lines; a plurality of the second scanning lines; the second scanning line driving circuit for driving this first scanning lines; and a reference potential line for supplying reference potential, and each pixel with one end being connected to the signal line comprises a sampling transistor, switched between being on and off by a first scanning line; a driving transistor with a gate connected to the other end of the sampling transistor and a drain connected to the first power supply; a light emitting element which is connected in between the source of the driving transistor and a second power supply and driven by the current applied to the said driving transistor; a retentive capacitance connected to between the gate and source of the said driving transistor; and a switching transistor which is arranged between the source of the said driving transistor and the reference potential line and being switched between being on and off by a second scanning line. The said sampling transistor and the said switching transistor are electrically connected during the period when a reference signal voltage is set to the said signal line, the difference in voltage between a reference signal voltage and a reference potential is charged to the said retentive capacitance under the condition of the voltage between the gate and source of the said driving transistor being equal to or greater than the threshold voltage of the said driving transistor, and the source voltage of the said driving transistor is set to the reference potential in order to make the voltage applied to the said light emitting elements equal to or lower than its threshold voltage. Subsequently, while a reference signal voltage is set to the said signal line, the said sampling transistor and the said switching transistor are electrically connected and by turning off the said switching transistor, the voltage equivalent to the threshold voltage of the said driving transistor is retained by the said retentive capacitance while maintaining the voltage applied to the said light emitting elements equal to or lower than its threshold voltage, and the said sampling transistor is electrically connected to sample the said signal voltage during the period when the display signal voltage is set to the said signal line to superimpose the said signal voltage on the threshold voltage retained by the said retentive capacitance.
Also, it is preferred that the said reference potential line is common for two rows of pixels and arranged in the row direction per two rows of pixels.
Also, it is preferred that the said reference potential line is common for two columns of pixels and arranged in the column direction per two rows of pixels.
It is preferred that the said reference potential lines are connected in a group outside of the display area where the said pixels are arranged.
It is preferred that a probe point which is connected to the said reference potential line is a probe point which can be pointed by probe from outside at least before the said light emitting elements are formed.
Also, it is preferred that the said second scanning line is common for two rows of pixels and arranged in the column direction per 2 rows of pixels.
Also, it is preferred that the current-voltage characteristic of the driving transistor is measured before the said light emitting elements are formed, by connecting a probe to the reference potential line to control the said sampling transistor and the on and off of the switching transistor to detect current which flows out from the reference potential line
According to the present invention, threshold voltage at which current starts to flow to the driving transistor is corrected in a pixel circuit to make variations in the driving current small. Also, the cost reduction can be realized by not sending defective products to the next step, because pixels can be inspected before the said light emitting elements are formed.
An embodiment of the present invention will be explained based on the figures below.
A block diagram of the entire display device according to the embodiment is indicated in
Also, a signal line driving circuit DR for controlling the column direction signal lines, a first scanning line DSR in the row direction and a first scanning line driving circuit SRI for controlling the same, and a second scanning line drive circuit SR2 for controlling a second scanning line RSR in the row direction are arranged at the periphery of the display section in which pixels 10 are arranged. The second scanning line RSR and the reference potential line Vref_r are commonly connected to the pixels of upper and lower two rows.
Also, the reference potential line Vref_r may be in a line direction. In this case, the reference potential line Vref_r is common for every two lines and connected to the pixels in left and right two lines. The constitution of such is indicated in
The actual structure of a pixel circuit contained in the display device shown in
In
Also
Consequently, although the driving transistor 10 is turned on, current is not applied to the light emitting elements 10E. In the retentive capacitance 10B, Vgs-10C is retained.
This is expressed in the equations below:
Vgs
—10C=Vref−Vref—r>Vth—10C 1
Vgs
—11C=Vref−Vref—r>Vth—11C 2
VEE+Vth
—10E>Vref—r 3
VEE+Vth
—11E>Vref—r 4
The second scanning line here is common per two lines, the pixel having address (2n, m) requires a threshold detection period longer by 1H than the pixel having address (n+1, m). Also in
The steps of
Vs
—10C=Vref−Vth—10C 5
Vs
—11C=Vref−Vth—11C 6
Therefore, Vth_10C, Vth_11C are retained in the retentive capacitances 10B, 11B respectively.
Also at this time, the voltage applied to the light emitting elements 10E, 11E must be less than the threshold voltage Vth_10E, Vth_11E. That is, it must satisfy the following equations:
VEE+Vth
—10E>VS—10C 7
VEE+Vth
—11E>Vs—11C 8
In regards to 2nth column, Vref must satisfy equation 9 which is obtained from equations 5 and 7, and Vref_r must satisfy equation 1.
VEE+Vth
—10E+Vth—10C>Vref 9
At this time, the source electrode of the driving transistor 10C becomes:
Vs
—10C=Vref−Vth—10C+(Vsig0−Vref)×Cap—10E/ (Cap—10B+Cap—10E)+VEE×Cap—10B/(Cap—10B+Cap—10E) ={Cap—10BX(VEE+Vref+Cap—10ExVsig0}/(Cap10B+Cap—10E) −Vth—10C
The voltage between the gate electrode and source electrode becomes:
Vgs
—10C=Cap—10B/(Cap—10B+Cap—10E)(Vsig0−VEE−Vref) +Vth—10C
In
At this time, the source electrode of the driving transistor 11C becomes:
Vs
—11C=Vref−Vth—11C+(Vsig0−Vref)×Cap—11E/(Cap—11B+Cap—11E) +VEE×Cap—11B/(CAp—11B+Cap—11E)
The voltage between the gate electrode and source electrode becomes:
Vgs
—11C=Cap—11B/(Cap—11B+Cap—11E) ×(Vsig0−VEE−Vref)+Vth—11E
A characteristic formula for Ids of driving transistors is expressed by Ids=β/2(Vgs−Vth)2. If Vgs_10C and Vgs_11C are respectively input, it becomes:
Ids0=β/2{Cap—10B/(Cap—10B+Cap—10E)×(Vsig0−VEE−Vref)2
Ids1=β/2{Cap—11B/(Cap—11B+Cap—11E)×(Vsig1−VEE−Vref)2
The term Vth is corrected, and variations in drive current can be suppressed.
The sampling transistor 10A and the switching transistor 10D are made conductive and the signal potential is given to the gate electrode of the driving transistor 10C from the signal line DTCm. At this time, the current which flows between the drain electrode and source electrode of the driving transistor 10C is measured at the probe point connected to Vref r to check failures. That is, the second scanning line RSR is made H level and the first scanning line DSR is sequentially made H level. By doing so, the sampling transistors 10A of corresponding pixel are turned on, the potential of the signal line DTC is brought into a pixel, a current corresponding to the current is applied, and the current flowing from the probe point to an external ground is measured using a measuring device to confirm operation of pixel circuit.
In particular, IV characteristics including threshold voltage of the driving transistor 10C in one pixel circuit can be detected.
Also, by turning on the signal line DTC one by one, inspections of pixel is conducted one by one, but failures in elements can be detected when inspecting a group of pixels.
Although a n-channel transistor is used in the embodiment above, p-channel transistor may be used. When a p-channel transistor is used in the driving transistor 10C, the source electrode is arranged on the power supply VCC side and the light emitting elements 10# and the retentive capacitance 10B are also arranged on the power supply VCC side.
According to the embodiment of the present invention, threshold voltage at which current starts to flow to the driving transistor is corrected in each pixel circuit to make variations in the driving current small. Also, when inspection of pixel before light emitting elements are formed, that is, failures in sampling transistors, driving transistors, and switching transistors can be checked before the light emitting elements are formed. Consequently, by not sending failure products to the next step, cost reduction is realized.
10, 11 pixels, 10A, 11A sampling transistors, 10B, 11B retentive capacitances, 10C, 11C driving transistors, 10D, 11D switching transistors, 10E, 11E light emitting elements.
Number | Date | Country | Kind |
---|---|---|---|
2009-257527 | Nov 2009 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/US10/55368 | 11/4/2010 | WO | 00 | 10/5/2012 |