JP4942971B2 - Driving method of light emitting device - Google Patents

Description

The present invention relates to a structure for performing accurate gradation display in a display device (light emitting device) having a light emitting element, and a driving method thereof.

In the conventional light emitting device, as shown in FIG. 9, a switching element 810 that is controlled to be turned on or off by a video signal input from a signal line 814, a transistor 811 for driving the light emitting element 813, and a gate of the transistor 811 A pixel configuration including a capacitor 812 provided between a power supply line 815 and a gate electrode of a transistor 811 in order to maintain a source-to-source voltage has been proposed (see Patent Document 1).
JP 2001-343933 A

An equivalent circuit of the light-emitting element described in Patent Document 1 is shown in FIG. As shown in FIG. 9, the light emitting element can be represented by a parallel circuit of a diode 816 and a capacitor (C _EL ).

Here, an operation when the current value of the current supplied to the light emitting element 813 varies will be described. First, it is assumed that a current having a current value I ₀ constantly flows through the light emitting element 813. Next, when the current value of the current flowing through the light emitting element 813 increases from I ₀ to I ₁ , the current value of the current flowing through the diode 816 immediately after that does not become I ₁ . This is because the increase in the current value is the sum of the increase in the current value of the current flowing through the diode 816 and the current value of the current flowing into the capacitor (C _EL ). Therefore, the current value of the current flowing through the diode 816 becomes equal to I ₁ when the charging of the capacitor (C _EL ) is completed.

Further, considering that the current of the current value I ₀ constantly flows through the light emitting element 813 and the current value decreases from I ₀ to I ₂ , the current value and the capacity ( The total value of the current discharged from C _EL ) is I ₂ , and the current value of the current flowing through the diode 816 when discharge from the capacitor (C _EL ) ends is equal to I ₂ . In such a case, the time until the current value of the steady current flowing through the diode 816 changes is equal to the time until the change in potential between the anode electrode and the cathode electrode of the light emitting element 813 ends, and The larger the capacitance (C _EL ), the longer it takes, and the longer the current value, the longer it takes.

Further, in the pixel circuit shown in FIG. 9, in addition to the capacitance (C _EL ) between both electrodes of the light emitting element 813, the overlap capacitance (C _gd ) between the gate electrode and the drain electrode of the driving transistor 811, and further the layout Therefore, there is a parasitic capacitance (Cp) caused by the overlap between the gate electrode and the anode electrode.

At this time, the switching element 810 is turned on, a current corresponding to the gradation signal input to the gate of the transistor 811 is supplied to the light emitting element 813, and the anode potential varies. However, when the capacitance (C _EL ) of the light-emitting element 813 is large and the current value of the current supplied to the light-emitting element 813 varies greatly, charging / discharging to the capacitor (C _EL ) ends, and the variation in the potential of the anode electrode occurs. It takes a long time to finish. Therefore, the fluctuation of the potential of the anode electrode may not end while the switching element 810 is on.

Then, if the potential of the anode electrode of the light emitting element 813 fluctuates (fluctuation value ΔV _A ) after the switching element 810 is turned off in FIG. 9, parasitic capacitance (Cp), overlap capacitance (C _gd ) , And the capacitive coupling by the storage capacitor (C _S ) 812, the potential of the gate electrode of the transistor 811 varies. The fluctuation value ΔV _B is expressed as ΔV _B = (C _p + C _gd ) / (C _p + C _gd + C _s ) × ΔV _A.

As described above, when the potential of the gate electrode of the transistor 811 fluctuates after the gradation signal is input to each pixel, the current value of the current supplied to the light emitting element 813 fluctuates. There is a problem that correct gradation display cannot be obtained. In particular, when black display is performed, a current may flow through the light emitting element, which makes it difficult to perform clean black display.

In view of the above, an object of the present invention is to provide a light-emitting device capable of performing accurate gradation display and a driving method thereof.

In view of the above problems, the present invention is characterized in that a signal for display is written a plurality of times within a predetermined timing period, or the writing operation period is lengthened. As a result, since the gate voltage is determined after the potential of the anode of the light-emitting element is stabilized, accurate gradation display can be performed.

A specific form of the present invention divides one frame period into a plurality of subframe periods SF1, SF2,... SFn (n is a natural number), and each subframe period SFn has a write operation period Ta, A light-emitting device having a timing chart in which a period Te for inputting an erasing signal is provided a plurality of times in at least one subframe period, and a driving method of the light-emitting device to which the timing chart is applied.

In another embodiment of the present invention, one frame period is divided into a plurality of subframe periods SF1, SF2,... SFn (n is a natural number), and each of the subframe periods SFn has a write operation period Ta, and at least 1 A light-emitting device having a timing chart in which a writing operation period Ta is provided a plurality of times in one subframe period, and a driving method of the light-emitting device to which the timing chart is applied.

Another embodiment of the present invention is a method for driving a light-emitting device that performs gradation display by inputting a video signal and an erasure signal, which are digital signals, in one frame period. A light-emitting device having a timing chart provided longer than an input period, and a driving method of the light-emitting device to which the timing chart is applied.

In a specific driving method, in a subframe period obtained by dividing one frame period into n (n is a natural number), a video signal is input to the pixel portion in the n subframe periods. In this method, an erase signal is input a plurality of times.

In another driving method, in a subframe period obtained by dividing one frame period into n (n is a natural number), a video signal is input to a pixel portion in a period out of the n subframe periods, and then the video signal This is a method of inputting the erase signal over a period longer than the period for inputting.

The timing chart of the present invention is written in the memory element of the light emitting device.

A pixel configuration of such a light emitting device includes a switching transistor in which a source electrode or a drain electrode is connected to a signal line, a gate electrode is connected to a scanning line, and a driving transistor in which a gate electrode is connected to the switching transistor. And a light emitting element connected to a source electrode or a drain electrode of the driving transistor.

In addition, the pixel configuration may further include an erasing transistor for discharging a charge corresponding to a voltage between the gate and the source of the driving transistor (hereinafter referred to as a gate-source voltage).

Further, the pixel configuration may further include a transistor connected in series to the driving transistor and having a fixed gate potential.

With the driving method of the present invention, a light-emitting device capable of performing accurate gradation display can be provided.

Hereinafter, embodiments and examples of the present invention will be described with reference to the drawings. However, the present invention can be implemented in many different modes, and those skilled in the art can easily understand that the modes and details can be variously changed without departing from the spirit and scope of the present invention. Is done. Therefore, the present invention is not construed as being limited to the description of this embodiment mode. Note that the same portions or portions having similar functions are denoted by the same reference symbols throughout the drawings for describing the embodiments and examples, and the repetitive description thereof is omitted.

(Embodiment 1)
In this embodiment, a driving method in the case where a predetermined signal is written a plurality of times will be described.

FIG. 1A shows an operation when an erase signal is input twice in digital gradation display (digital gradation method). First, a digital signal (video signal) to be written for display is input at a predetermined timing, and a first erase signal is input after a predetermined time has elapsed. At this time, if there is a capacitance (C _EL ) or a parasitic capacitance (Cp), the gate electrode potential (gate potential) of the driving transistor does not become relatively zero only by the first erase signal. Can not be. If it remains as it is, it is difficult to perform accurate gradation display, and gradation deviation occurs. Therefore, in the present invention, after a predetermined time elapses, the erase signal is input again, that is, the second erase signal is input. Then, the gate potential can be made relatively zero again, and the off operation can be performed. As a result, gradation shift is reduced and accurate gradation display can be performed.

Although FIG. 1A shows the case where the erase signal is input twice, it may be input three or more times. Further, the same video signal may be input twice or more instead of the erase signal.

In FIG. 1B, in analog gradation display (analog gradation method), when an analog signal (gradation signal) written for display is written twice, the gate potential of the driving transistor flows to the light emitting element. Indicates current. A dotted line indicates a state in which a gradation signal is written once as in the past.

First, the first gradation signal (SW) is input. Then, the gate potential of the driving transistor becomes a predetermined value. At this time, if the capacitance (C _EL ) exists, the gate potential gradually decreases. As a result, and due to the influence of the parasitic capacitance (Cp), the current flowing through the light emitting element is not maintained at a predetermined value and gradually increases. In this state, as indicated by the dotted line, the current flowing through the light emitting element remains high, resulting in a gradation shift. Therefore, in the present invention, after a predetermined time has elapsed, the gradation signal is input again, that is, the second gradation signal is input. Then, the gate potential of the driving transistor returns to a predetermined value, and the current flowing through the light emitting element also becomes a predetermined value.

Note that when the second gradation signal is input, the potential of the anode of the light-emitting element is stable to some extent, so that the gate potential does not fluctuate thereafter and the current flowing to the light-emitting element does not increase.

Although FIG. 1B illustrates the case where the gradation signal is input twice, the present invention is not limited to this, and the gradation signal may be input twice or more.

As described above, a light-emitting device that performs accurate gradation display can be provided by a driving method in which a predetermined signal such as an erasure signal or a gradation signal is input a plurality of times.

(Embodiment 2)
In this embodiment mode, a timing chart in the case of having a period for inputting a video signal and a period for inputting a plurality of erase signals in digital gray scale display will be described.

Although one frame period can be divided into n subframe periods SF1, SF2,... SFn (n is a natural number), FIG. 2 (A) shows that one frame period is divided into three subframe periods (SF1 , SF2, SF3), and is a timing chart in the case of displaying 6 gradations, and shows an example in which the erase signal is input twice in the subframe period SF3. FIG. 2B shows a timing chart when focusing on the i-th scanning line.

Each sub-frame period (SF1, SF2, SF3) has a write operation period (Ta1, Ta2, Ta3) (also referred to as a write signal input period) in which a video signal is input, and the written video signal It has a light emission period (Ts1, Ts2, Ts3) in which the light emission based on is performed. The length of each light emission period is set to satisfy Ts1: Ts2: Ts3 = 2 ² : 2 ¹ : 2 ⁰ .

The shortest subframe period SF3 is provided with periods Te3 (1) and Te3 (2) for inputting two erase signals. By providing the periods Te3 (1) and Te3 (2) for inputting the erase signal twice, the gate potential of the driving transistor can be accurately determined even if there is a capacitor (C _EL ). As a result, clear gradation display can be performed.

Such timing charts include a DRAM (Dynamic Random Access Memory), a SRAM (Static Random Access Memory), a FRAM (Ferroelectric Random Access Memory), and a PROM (Probable Memory Memory) provided in the light emitting device. (Electrically Programmable Read Only Memory), EEPROM (Electrically Erasable Read Only Memory), flash memory, and the like.

Note that when the erase signal is input in the subframe period SF3, the write signal in the subframe period SF1 of the next frame can be input immediately, and the duty ratio can be increased.

By using a pixel circuit having an erasing transistor for discharging a charge corresponding to the gate-source voltage of the driving transistor, the driving method of this embodiment can be realized. For example, the pixel circuit in FIG. 5B described below can be used.

Note that although a case where a period for inputting two erase signals is provided in the subframe period SF3 is described in this embodiment mode, the present invention is not limited to this. For example, a period for inputting the erasure signal three times or more may be provided, or may be provided other than the subframe period SF3. A plurality of write operation periods may be provided in order to input the same write signal multiple times. That is, according to the present invention, when it is difficult to perform accurate gradation display due to the presence of the capacitor (C _EL ), a plurality of input periods are provided in order to input a predetermined signal a plurality of times in order to eliminate this. And

(Embodiment 3)
In this embodiment, a timing chart in the case where a writing operation period (also referred to as a writing signal input period) is provided a plurality of times in digital gray scale display will be described.

Although one frame period can be divided into a plurality of subframe periods SF1, SF2,... SFn (n is a natural number), FIG. 3 (A) shows that one frame period is divided into three subframe periods (SF1, SF1,. FIG. 6 is a timing chart in the case of dividing into SF2 and SF3) and displaying 6 gradations, and additionally shows an example having a period in which a reverse voltage is applied. FIG. 3B shows a timing chart when focusing on the i-th scanning line.

Each subframe period (SF1, SF2, SF3) has a write operation period (Ta1 (W), Ta2 (W), Ta3 (W)), and a light emission period in which light emission based on the written signal is performed. (Ts1, Ts2, Ts3). The length of each light emission period is set to satisfy Ts1: Ts2: Ts3 = 2 ² : 2 ¹ : 2 ⁰ . In addition, the shortest subframe period SF3 is provided with an erase signal input period (Ta3 (E)). In the erase signal input period, the written signal is erased.

For example, the write operation period Ta1 is provided twice in the longest subframe period SF1 (represented as Ta1 (1) and Ta1 (2), respectively). Ta1 (1) and Ta1 (2) are provided with periods Ta1 (W) (1) and Ta1 (W) (2) for inputting video signals, respectively. In the first writing operation period Ta1 (1), a video signal is written (corresponding to Ta1 (W) (1)), and in the second writing operation period Ta1 (2), a video signal is written (Ta1 (W) (2 )). In this way, the video signal can be written a plurality of times. As a result, the gate potential of the driving transistor can be accurately controlled even in the presence of the capacitor (C _EL ).

Such timing charts include a DRAM (Dynamic Random Access Memory), a SRAM (Static Random Access Memory), a FRAM (Ferroelectric Random Access Memory), and a PROM (Probable Memory Memory) provided in the light emitting device. (Electrically Programmable Read Only Memory), EEPROM (Electrically Erasable Read Only Memory), flash memory, and the like.

Even if there is no erasing transistor for discharging the charge corresponding to the gate-source voltage of the driving transistor, the driving method of this embodiment can be realized. Since the erasing transistor is unnecessary, the aperture ratio of the pixel portion can be improved. For example, the pixel circuit in FIG. 5A described below can be used for the pixel portion. However, a driving circuit for providing the writing operation period Ta (W) and the period Ta (E) for inputting the erase signal is required.

During the period of applying the reverse voltage (FRB), the reverse voltage is applied to the light emitting element (RB). Further, an erasing signal input period (Ta (E)) is provided before the reverse voltage application period. In the erase signal input period, an operation of sequentially erasing data written in SF3 in this embodiment, a subframe period immediately before the erase signal input period is performed. This is because reverse voltage is applied to the light emitting elements all at once, and if data remains, there is a possibility that some elements emit light when the reverse voltage is applied. By applying such a reverse voltage to the light emitting element, the defective state of the light emitting element can be improved and the reliability can be improved. In particular, a light emitting element may have an initial failure in which the anode and the cathode are short-circuited due to adhesion of foreign matters, pinholes due to fine protrusions on the anode or cathode, and non-uniformity of the electroluminescent layer. When such an initial failure occurs, light emission and non-light emission corresponding to the signal are not performed, and most of the current flows in a shorted portion. As a result, there arises a problem that the image is not displayed favorably. In addition, this defect may occur in any pixel.

Therefore, as in this embodiment, when a reverse voltage is applied to the light emitting element, a local current flows through the shorted portion, and the shorted portion generates heat and can be oxidized or carbonized. As a result, the short-circuited portion can be insulated, and a current flows in a region other than that portion, so that the light emitting element can be operated normally. By applying the reverse voltage in this way, even if an initial failure occurs, the failure can be eliminated. Such insulation of the short-circuit portion is preferably performed before shipment.

In addition to the initial failure, a short circuit between the anode and the cathode may occur as time passes. Such a defect is also called a progressive defect. Therefore, by periodically applying a reverse voltage to the light emitting element as in this embodiment, even if a progressive failure occurs, the failure can be eliminated and normal operation can be performed. .

In addition, image burn-in can be prevented by applying a reverse voltage. Image burn-in occurs due to the deterioration state of the light-emitting element, but the deterioration state can be reduced by applying a reverse voltage. As a result, image burn-in can be prevented.

Although the deterioration of the light emitting element greatly progresses in the initial stage, the progress of the deterioration decreases with time. In other words, once the light emitting element has deteriorated, further deterioration with time is unlikely to occur. As a result, deterioration in the initial stage and deterioration with time are mixed, and the deterioration state of the light emitting element varies. Therefore, the deterioration state can be averaged by causing all the light emitting elements to emit light before shipping or when no image is displayed and causing the light emitting elements that have not undergone initial deterioration to deteriorate. Such a structure for causing all the light emitting elements to emit light may be further provided in the light emitting device.

Note that the period for applying the reverse voltage is not limited to that shown in FIG. For example, it may be provided at the beginning of one frame period. Further, the period during which the reverse voltage is applied is not necessarily provided for each frame period.

The period for applying the reverse voltage as described above can be provided also when the timing chart of FIG. 2 is used.

Note that although the case where two write operation periods are provided in the subframe period SF1 has been described in this embodiment mode, the present invention is not limited to this. For example, two or more write operation periods may be provided. Further, a plurality of write operation periods may be provided in other subframe periods. That is, the present invention is characterized in that a plurality of periods for inputting a predetermined signal are provided in order to eliminate the case where it is difficult to perform accurate gradation display due to the presence of the capacitor (C _EL ).

FIGS. 13A and 13B illustrate timing charts in the case where a plurality of, for example, two periods Ta (E) for inputting the erase signal are provided. FIG. 13A is a timing chart in the case where one frame period is divided into three subframe periods (SF1, SF2, and SF3) and six gradations are displayed, and in addition, a reverse voltage is applied. An example having a period is shown. FIG. 13B shows a timing chart when focusing on the i-th scanning line.

Each subframe period (SF1, SF2, SF3) has a write operation period (Ta1 (W), Ta2 (W), Ta3 (W)), and a light emission period in which light emission based on the written signal is performed. (Ts1, Ts2, Ts3). The length of each light emission period is set to satisfy Ts1: Ts2: Ts3 = 2 ² : 2 ¹ : 2 ⁰ .

A video signal is input in the write operation period Ta3 (W) of the shortest subframe period SF3, and an erase signal is input in the periods Ta3 (E) (2) and Ta3 (E) (3) in which two erase signals are input. Is done. As a result, the gate potential of the driving transistor can be accurately controlled even in the presence of the capacitor (C _EL ).

Such timing charts include a DRAM (Dynamic Random Access Memory), a SRAM (Static Random Access Memory), a FRAM (Ferroelectric Random Access Memory), and a PROM (EPROM), which are memory elements provided in the light emitting device. (Electrically Programmable Read Only Memory), EEPROM (Electrically Erasable Read Only Memory), flash memory, and the like.

In FIGS. 13A and 13B, a period in which a reverse voltage is applied to the light-emitting element is provided as in FIGS. 3A and 3B, so that the deterioration state of the element is reduced as described above. Can do.

(Embodiment 4)
In this embodiment, a driving method in the case where the time for inputting a predetermined signal is extended will be described.

FIG. 4A shows an operation in the case where the period for inputting the erase signal is made longer than the period for inputting the video signal in the digital gradation display. By extending the period during which the erase signal is input, fluctuations in the gate potential after the video signal is input can be suppressed, and the light emission of the light emitting element after the erase can be reduced. As a result, accurate black display can be performed. Of course, it is possible to provide an accurate video by making the period for inputting the video signal longer than the period for inputting the erasure signal.

FIG. 4B shows a case where a period for inputting a gradation signal is extended in analog gradation display. In particular, in low gradation display, since the current flowing to the light emitting element is small, the presence of a capacitor (C _EL ) becomes remarkable. Therefore, it is preferable to extend the period for inputting the gradation signal in the low gradation display rather than the high gradation display.

Note that the period for inputting the gradation signal is determined by the frame frequency, the number of pixels, and the number of columns in which signals are simultaneously written (hereinafter referred to as the number of parallel writing). The frame frequency and the number of pixels are related to display performance. The larger the numerical value, the shorter the period for inputting the gradation signal.

The write parallel number is a part related to the hardware configuration, and the smaller the parallel number, the shorter the period for inputting the gradation signal. In the line sequential writing method, the number of parallel writing is equal to the number of horizontal pixels.

As described above, with the increase in the number of pixels due to high image quality, the period for inputting a gradation signal is shortened.

On the other hand, since the current flowing through the light emitting element is reduced by increasing the efficiency of the light emitting element, it is desired to extend the period for inputting the grayscale signal.

Therefore, the period for inputting the gradation signal is lengthened particularly in the low gradation display. This can be achieved by lowering the frame frequency. As a result, gradation shift is reduced and accurate gradation display can be performed.

As described above, a light-emitting device that performs accurate grayscale display can be provided by a driving method that extends a time for inputting a predetermined signal.

(Embodiment 5)
In this embodiment mode, a timing chart in the case where the time for inputting a predetermined signal is extended in digital gradation display will be described.

Even in the case where the time for inputting a predetermined signal is lengthened, as shown in FIG. 2, a timing chart in which one frame period is divided into a plurality of subframe periods, or reverse application as shown in FIG. A timing chart or the like in the case of applying a voltage can be used.

For example, as shown in the timing chart of FIG. 11, the period Te3 (1) for inputting the erase signal in the subframe period SF3 may be set once and the period may be lengthened. For example, as shown in the timing chart of FIG. 12, the write operation period Ta3 and the erase signal input period Ta3 (E) in the subframe period SF3 may be lengthened.

Such timing charts include a DRAM (Dynamic Random Access Memory), a SRAM (Static Random Access Memory), a FRAM (Ferroelectric Random Access Memory), and a PROM (Probable Memory Memory) provided in the light emitting device. (Electrically Programmable Read Only Memory), EEPROM (Electrically Erasable Read Only Memory), flash memory, and the like.

Note that the other components of the timing chart of this embodiment can be the same as those in FIGS.

(Embodiment 6)
In this embodiment mode, an equivalent circuit diagram of a pixel included in the light-emitting device of the present invention will be described with reference to FIGS.

FIG. 5A illustrates an example of an equivalent circuit diagram of a pixel. A signal line 6114, a power supply line 6115, a scanning line 6116, a light-emitting element 6113, a switching transistor 6110, and a driver for controlling the light-emitting element are illustrated. A transistor 6111 and a capacitor 6112 are included. A video signal is input to the signal line 6114 by a signal line driver circuit, and the switching transistor 6110 is turned on or off by the video signal. The switching transistor 6110 can control supply of the potential of the video signal to the gate of the driving transistor 6111 in accordance with a selection signal input to the scan line 6116. The driving transistor 6111 can control supply of current to the light-emitting element 6113 in accordance with the potential of the video signal. The capacitor 6112 can hold the voltage between the gate and the source of the driving transistor 6111. Note that although the capacitor 6112 is illustrated in FIG. 5A, the capacitor 6112 is not necessarily provided when it can be covered by the gate capacitance of the transistor 6111 or another parasitic capacitance.

FIG. 5B is an equivalent circuit diagram of a pixel in which an erasing transistor 6118 and a scanning line 6119 are newly provided in the pixel shown in FIG. Since the gate and the source of the driving transistor 6111 have the same potential by the erasing transistor 6118 and a state in which no current flows through the light-emitting element 6113 can be forcibly created, the period from when video signals are input to all pixels In addition, the length of the subframe period can be shortened. As a result, the duty ratio can be increased.

FIG. 5C is an equivalent circuit diagram of a pixel in which a transistor 6125 and a wiring 6126 are newly provided in the pixel illustrated in FIG. The potential of the gate of the transistor 6125 is fixed by the wiring 6126. The driving transistor 6111 and the transistor 6125 are connected in series between the power supply line 6115 and the light emitting element 6113. Therefore, in FIG. 5C, the value of the current supplied to the light-emitting element 6113 is controlled by the transistor 6125, and whether or not the current is supplied to the light-emitting element 6113 can be controlled by the driving transistor 6111.

Note that the pixel circuit included in the light-emitting device of the present invention is not limited to the structure shown in this embodiment mode. This embodiment can be freely combined with the above embodiment.

(Embodiment 7)
In this embodiment, a cross-sectional structure of a pixel in the case where a driving transistor is a p-type thin film transistor (TFT) is described with reference to FIGS. Note that in the present invention, of the two electrodes of the anode and the cathode included in the light-emitting element, one electrode whose potential can be controlled by a transistor is a first electrode and the other electrode is a second electrode. 6 illustrates the case where the first electrode is an anode and the second electrode is a cathode, the first electrode may be a cathode and the second electrode may be an anode.

FIG. 6A is a cross-sectional view of a pixel in the case where the TFT 6001 is p-type and light emitted from the light-emitting element 6003 is extracted from the first electrode 6004 side. In FIG. 6A, the first electrode 6004 of the light-emitting element 6003 and the TFT 6001 are electrically connected.

The TFT 6001 is covered with an interlayer insulating film 6007, and a partition wall 6008 having an opening is formed over the interlayer insulating film 6007. A part of the first electrode 6004 is exposed in the opening of the partition wall 6008, and the first electrode 6004, the electroluminescent layer 6005, and the second electrode 6006 are sequentially stacked in the opening.

The interlayer insulating film 6007 is formed using an insulating film containing an Si—O—Si bond (hereinafter referred to as a siloxane insulating film) formed using an organic resin film, an inorganic insulating film, or a siloxane-based material as a starting material. can do. Note that siloxane corresponds to a resin including a Si—O—Si bond. Siloxane has a skeleton structure formed of a bond of silicon (Si) and oxygen (O). As a substituent, an organic group containing at least hydrogen (for example, an alkyl group or an aromatic hydrocarbon) is used. A fluoro group may be used as a substituent. Alternatively, an organic group containing at least hydrogen and a fluoro group may be used as a substituent. A material called a low dielectric constant material (low-k material) may be used for the interlayer insulating film 6007.

The partition wall 6008 can be formed using an organic resin film, an inorganic insulating film, or a siloxane-based insulating film. For example, acrylic resin, polyimide, polyamide, or the like can be used for the organic resin film, and silicon oxide, silicon nitride oxide, or the like can be used for the inorganic insulating film. In particular, a photosensitive organic resin film is used for the partition wall 6008, an opening is formed on the first electrode 6004, and the side wall of the opening is formed to have an inclined surface formed with a continuous curvature. Thus, the connection between the first electrode 6004 and the second electrode 6006 can be prevented.

The first electrode 6004 is formed with a material that transmits light or a thickness that transmits light, and a material that is suitable for use as an anode. For example, a light-transmitting oxide conductive material such as indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), or zinc oxide to which gallium is added (GZO) is used for the first electrode 6004. Is possible. Alternatively, indium tin oxide containing silicon oxide (hereinafter referred to as ITSO) or indium oxide containing silicon oxide mixed with 2 to 20% zinc oxide (ZnO) may be used for the first electrode 6004. . In addition to the light-transmitting oxide conductive material, for example, a single layer film made of one or more of TiN, ZrN, Ti, W, Ni, Pt, Cr, Ag, Al, etc., titanium nitride film and aluminum Alternatively, the first electrode 6004 can be formed using a stack of a film mainly containing silicon, a three-layer structure of a titanium nitride film, a film mainly containing aluminum, and a titanium nitride film. Note that in the case where a material other than the light-transmitting oxide conductive material is used, the first electrode 6004 is formed with a thickness enough to transmit light (preferably, about 5 nm to 30 nm).

The second electrode 6006 is formed of a material that reflects or blocks light, or a film thickness that reflects or blocks light, and is made of a metal, an alloy, an electrically conductive compound, a mixture thereof, or the like with a low work function. Can be formed. Specifically, alkali metals such as Li and Cs, alkaline earth metals such as Mg, Ca, and Sr, alloys containing these (Mg: Ag, Al: Li, Mg: In, etc.), and compounds thereof ( In addition to calcium fluoride such as CaF ₂ and calcium nitride such as Ca ₃ N ₂ , rare earth metals such as Yb and Er can be used. When an electron injection layer is provided, a conductive layer such as Al can be used.

The electroluminescent layer 6005 is composed of one or a plurality of layers. When composed of a plurality of layers, these layers can be classified into a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and the like from the viewpoint of carrier transport characteristics. In the case where the electroluminescent layer 6005 includes any of a hole injection layer, a hole transport layer, an electron transport layer, and an electron injection layer in addition to the light emitting layer, the first electrode 6004 to the positive hole injection layer, A hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer are laminated in this order. Note that the boundaries between the layers are not necessarily clear, and there are cases where the materials constituting the layers are partially mixed and the interface is unclear. For each layer, an organic material or an inorganic material can be used. As the organic material, any of a high molecular weight material, a medium molecular weight material, and a low molecular weight material can be used. The medium molecular weight material corresponds to a low polymer having a number of repeating structural units (degree of polymerization) of about 2 to 20. The distinction between a hole injection layer and a hole transport layer is not necessarily strict, and these are the same in the sense that hole transportability (hole mobility) is a particularly important characteristic. For convenience, the hole injection layer is a layer in contact with the anode, and the layer in contact with the hole injection layer is referred to as a hole transport layer to be distinguished. The same applies to the electron transport layer and the electron injection layer. The layer in contact with the cathode is called an electron injection layer, and the layer in contact with the electron injection layer is called an electron transport layer. The light emitting layer may also serve as an electron transport layer, and is also referred to as a light emitting electron transport layer.

In the case of the pixel shown in FIG. 6A, light emitted from the light-emitting element 6003 can be extracted from the first electrode 6004 side as shown by a hollow arrow.

Next, FIG. 6B is a cross-sectional view of a pixel in the case where the TFT 6011 is a p-type and light emitted from the light-emitting element 6013 is extracted from the second electrode 6016 side. In FIG. 6B, the first electrode 6014 of the light-emitting element 6013 and the TFT 6011 are electrically connected. In addition, an electroluminescent layer 6015 and a second electrode 6016 are sequentially stacked over the first electrode 6014.

The first electrode 6014 is formed using a material that reflects or shields light or a film thickness, and is formed using a material that is suitable for use as an anode. For example, in addition to a single layer film made of one or more of TiN, ZrN, Ti, W, Ni, Pt, Cr, Ag, Al, etc., a laminate of titanium nitride and a film containing aluminum as a main component, a titanium nitride film A three-layer structure of a film mainly containing aluminum and aluminum and a titanium nitride film can be used for the first electrode 6014.

The second electrode 6016 can be formed using a light-transmitting material or a light-transmitting film thickness, and can be formed using a metal, an alloy, an electrically conductive compound, a mixture thereof, or the like with a low work function. it can. Specifically, alkali metals such as Li and Cs, alkaline earth metals such as Mg, Ca, and Sr, alloys containing these (Mg: Ag, Al: Li, Mg: In, etc.), and compounds thereof ( In addition to calcium fluoride such as CaF ₂ and calcium nitride such as Ca ₃ N ₂ , rare earth metals such as Yb and Er can be used. When an electron injection layer is provided, a conductive layer such as Al can be used. Then, the second electrode 6016 is formed with a thickness enough to transmit light (preferably, about 5 nm to 30 nm). Note that a light-transmitting oxide conductive material such as indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), or zinc oxide added with gallium (GZO) can also be used. Alternatively, indium tin oxide containing silicon oxide (ITSO) or indium oxide containing silicon oxide mixed with 2 to 20% zinc oxide (ZnO) may be used. In the case of using a light-transmitting oxide conductive material, it is preferable to provide an electron injection layer in the electroluminescent layer 6015.

The electroluminescent layer 6015 can be formed in a manner similar to that of the electroluminescent layer 6005 in FIG.

In the case of the pixel shown in FIG. 6B, light emitted from the light-emitting element 6013 can be extracted from the second electrode 6016 side as shown by a hollow arrow.

Next, FIG. 6C is a cross-sectional view of a pixel in the case where the TFT 6021 is p-type and light emitted from the light-emitting element 6023 is extracted from the first electrode 6024 side and the second electrode 6026 side. In FIG. 6C, the first electrode 6024 of the light-emitting element 6023 and the TFT 6021 are electrically connected. Further, an electroluminescent layer 6025 and a second electrode 6026 are sequentially stacked over the first electrode 6024.

The first electrode 6024 can be formed in a manner similar to that of the first electrode 6004 in FIG. The second electrode 6026 can be formed in a manner similar to that of the second electrode 6016 in FIG. The electroluminescent layer 6025 can be formed in a manner similar to that of the electroluminescent layer 6005 in FIG.

In the case of the pixel shown in FIG. 6C, light emitted from the light-emitting element 6023 can be extracted from the first electrode 6024 side and the second electrode 6026 side as indicated by white arrows.

This embodiment mode can be freely combined with the above embodiment modes.

(Embodiment 8)
In this embodiment, a cross-sectional structure of a pixel in the case where an n-type TFT is used as a transistor that controls supply of current to a light-emitting element will be described with reference to FIGS. Note that although FIG. 7 illustrates the case where the first electrode is a cathode and the second electrode is an anode, the first electrode may be an anode and the second electrode may be a cathode.

FIG. 7A is a cross-sectional view of a pixel in the case where the TFT 6031 is an n-type and light emitted from the light-emitting element 6033 is extracted from the first electrode 6034 side. In FIG. 7A, the first electrode 6034 of the light-emitting element 6033 and the TFT 6031 are electrically connected. Further, an electroluminescent layer 6035 and a second electrode 6036 are sequentially stacked over the first electrode 6034.

The first electrode 6034 can be formed using a light-transmitting material or a light-transmitting film thickness, and can be formed using a metal, an alloy, an electrically conductive compound, a mixture thereof, or the like with a low work function. . Specifically, alkali metals such as Li and Cs, alkaline earth metals such as Mg, Ca, and Sr, alloys containing these (Mg: Ag, Al: Li, Mg: In, etc.), and compounds thereof ( In addition to calcium fluoride such as CaF ₂ and calcium nitride such as Ca ₃ N ₂ , rare earth metals such as Yb and Er can be used. When an electron injection layer is provided, a conductive layer such as Al can be used. Then, the first electrode 6034 is formed with a thickness enough to transmit light (preferably, about 5 nm to 30 nm). Further, a light-transmitting conductive layer is formed using a light-transmitting oxide conductive material so as to be in contact with or under the conductive layer having a thickness enough to transmit light, and the first electrode 6034 is formed. The sheet resistance may be suppressed. Note that only the first conductive layer using a light-transmitting oxide conductive material such as indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), or zinc oxide to which gallium is added (GZO) is used. It can also be used for the electrode 6034. Alternatively, indium tin oxide containing silicon oxide (ITSO) or indium oxide containing silicon oxide mixed with 2 to 20% zinc oxide (ZnO) may be used. In the case of using a light-transmitting oxide conductive material, it is preferable to provide an electron injection layer in the electroluminescent layer 6035.

The second electrode 6036 is formed of a material that reflects or shields light or a thickness that reflects or shields light, and is formed using a material that is suitable for use as an anode. For example, in addition to a single layer film made of one or more of TiN, ZrN, Ti, W, Ni, Pt, Cr, Ag, Al, etc., a laminate of a titanium nitride film and a film containing aluminum as a main component, titanium nitride A three-layer structure of a film, a film containing aluminum as its main component, and a titanium nitride film can be used for the second electrode 6036.

The electroluminescent layer 6035 can be formed in a manner similar to that of the electroluminescent layer 6005 in FIG. However, in the case where the electroluminescent layer 6035 includes any one of a hole injection layer, a hole transport layer, an electron transport layer, and an electron injection layer in addition to the light emitting layer, the first electrode 6034 to the electron injection layer The electron transport layer, the light emitting layer, the hole transport layer, and the hole injection layer are laminated in this order.

In the case of the pixel shown in FIG. 7A, light emitted from the light-emitting element 6033 can be extracted from the first electrode 6034 side as shown by a hollow arrow.

Next, FIG. 7B is a cross-sectional view of a pixel in the case where the TFT 6041 is an n-type and light emitted from the light-emitting element 6043 is extracted from the second electrode 6046 side. In FIG. 7B, the first electrode 6044 of the light-emitting element 6043 and the TFT 6041 are electrically connected. Further, an electroluminescent layer 6045 and a second electrode 6046 are sequentially stacked over the first electrode 6044.

The first electrode 6044 is formed of a material that reflects or shields light or a film thickness that reflects or shields light, and is formed of a metal, an alloy, an electrically conductive compound, a mixture thereof, or the like with a low work function. can do. Specifically, alkali metals such as Li and Cs, alkaline earth metals such as Mg, Ca, and Sr, alloys containing these (Mg: Ag, Al: Li, Mg: In, etc.), and compounds thereof ( In addition to calcium fluoride such as CaF ₂ and calcium nitride such as Ca ₃ N ₂ , rare earth metals such as Yb and Er can be used. When an electron injection layer is provided, a conductive layer such as Al can be used.

The second electrode 6046 is formed using a light-transmitting material or film thickness, and is formed using a material suitable for use as an anode. For example, a light-transmitting oxide conductive material such as indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), or zinc oxide added with gallium (GZO) is used for the second electrode 6046. Is possible. Alternatively, indium tin oxide containing silicon oxide (ITSO) or indium oxide containing silicon oxide mixed with 2 to 20% zinc oxide (ZnO) may be used for the second electrode 6046. In addition to the light-transmitting oxide conductive material, for example, a single layer film made of one or more of TiN, ZrN, Ti, W, Ni, Pt, Cr, Ag, Al, etc., titanium nitride film and aluminum Alternatively, the second electrode 6046 can be formed using a stack of a film mainly containing silicon, a three-layer structure of a titanium nitride film, a film mainly containing aluminum, and a titanium nitride film. Note that in the case where a material other than the light-transmitting oxide conductive material is used, the second electrode 6046 is formed with a thickness enough to transmit light (preferably, about 5 nm to 30 nm).

The electroluminescent layer 6045 can be formed in a manner similar to that of the electroluminescent layer 6035 in FIG.

In the case of the pixel shown in FIG. 7B, light emitted from the light-emitting element 6043 can be extracted from the second electrode 6046 side as shown by a hollow arrow.

Next, FIG. 7C is a cross-sectional view of a pixel in the case where the TFT 6051 is an n-type and light emitted from the light-emitting element 6053 is extracted from the first electrode 6054 side and the second electrode 6056 side. In FIG. 7C, the first electrode 6054 of the light-emitting element 6053 and the TFT 6051 are electrically connected. Further, an electroluminescent layer 6055 and a second electrode 6056 are stacked over the first electrode 6054 in this order.

The first electrode 6054 can be formed in a manner similar to that of the first electrode 6034 in FIG. The second electrode 6056 can be formed in a manner similar to that of the second electrode 6046 in FIG. The electroluminescent layer 6055 can be formed in a manner similar to that of the electroluminescent layer 6035 in FIG.

In the case of the pixel shown in FIG. 7C, light emitted from the light-emitting element 6053 can be extracted from the first electrode 6054 side and the second electrode 6056 side as indicated by white arrows.

This embodiment mode can be freely combined with the above embodiment modes.

(Embodiment 9)
As electronic equipment using the light emitting device of the present invention, a television device (television, television receiver), a camera such as a digital camera or a digital video camera, a mobile phone device (mobile phone), a portable information terminal such as a PDA, a mobile phone Type game machines, monitors, computers, sound reproduction apparatuses such as car audio, and image reproduction apparatuses equipped with recording media such as home game machines. A specific example will be described with reference to FIG.

A portable information terminal using the light-emitting device of the present invention illustrated in FIG. 8A includes a main body 9201, a display portion 9202, and the like. According to the present invention, accurate gradation display can be performed.

A digital video camera using the light-emitting device of the present invention illustrated in FIG. 8B includes display portions 9701 and 9702 and the like. According to the present invention, accurate gradation display can be performed.

A portable terminal using the light-emitting device of the present invention illustrated in FIG. 8C includes a main body 9101, a display portion 9102, and the like. According to the present invention, accurate gradation display can be performed.

A portable television device using the light-emitting device of the present invention illustrated in FIG. 8D includes a main body 9301, a display portion 9302, and the like. According to the present invention, accurate gradation display can be performed.

A portable computer using the light-emitting device of the present invention illustrated in FIG. 8E includes a main body 9401, a display portion 9402, and the like. According to the present invention, accurate gradation display can be performed.

A television device using the light-emitting device of the present invention illustrated in FIG. 8F includes a main body 9501, a display portion 9502, and the like. According to the present invention, accurate gradation display can be performed.

As described above, the light-emitting device of the present invention can be applied to any electronic device.

(Example)
In this example, the cathode current (cathode current) of the light emitting element was measured when the anode voltage (anode voltage) of the light emitting element was about 8 V and the number of light emitting rows was changed with 1 bit all white. The period for writing to the pixels for 320 rows (write operation period) is 1 μs, 500 ns, and 250 ns, and the erase signal is input only once (dotted line) at each time, and twice (the second time is 20 rows). Cathode current was compared with input (solid line).

FIG. 10 shows the results with the number of light emitting rows (for 320 rows) as the X axis and the cathode current as the Y axis. Ideally, the number of light-emitting rows and the cathode current should be in a proportional relationship, but when the erase signal is input only once, as shown by the dotted line, the number of light-emitting rows decreases as the write operation period becomes shorter. The cathode current increases. This indicates that low gradation display is not performed accurately. On the other hand, when the erase signal is input twice, as shown by the solid line, it can be seen that it is close to an ideal proportional relationship.

From the above, when the input of the erase signal is small, that is, when the period for inputting the erase signal is short, the capacitance holding the voltage between the gate and the source of the driving transistor is changed due to the presence of the capacitance (C _EL ). Therefore, it is considered that the influence becomes remarkable especially when the write operation period is short. In addition, it is considered that the influence becomes remarkable even when the number of light emitting rows is small. Therefore, a driving method in which an erase signal is input twice or more is desired as the write operation period becomes shorter.

It is the figure which showed the drive method of this invention It is the figure which showed the timing chart of this invention It is the figure which showed the timing chart of this invention It is the figure which showed the drive method of this invention It is the figure which showed the pixel circuit of this invention It is sectional drawing which showed the pixel structure of this invention It is sectional drawing which showed the pixel structure of this invention It is the figure which showed the electronic device of this invention It is the figure which showed the pixel structure of the light-emitting device. It is the graph which showed the experimental result of this invention It is the figure which showed the timing chart of this invention It is the figure which showed the timing chart of this invention It is the figure which showed the timing chart of this invention

Claims (2)

A first transistor having one of a source and a drain electrically connected to the signal line and a gate electrically connected to the scan line;
A second transistor having a gate electrically connected to the other of the source and the drain of the first transistor;
A third source in which one of a source and a drain is electrically connected to the gate of the second transistor, and the other of the source and the drain is electrically connected to one of the source and the drain of the second transistor; A transistor,
A light emitting device having a light emitting element electrically connected to the other of the source and the drain of the second transistor,
Subframe periods SF1, SF2,..., SFn divided into n (n is a natural number) one frame period,
The subframe period SF1 has a writing period Ta1 in which a video signal is input and a light emission period Ts1 in order.
The sub-frame period SF2 includes a writing period Ta2 in which a video signal is input and a light emission period Ts2 in order.
The subframe period SFn includes a writing period Tan in which a video signal is input and a light emission period Tsn in order.
The light emission periods Ts1, Ts2,..., Tsn become shorter in order.
The subframe period SFn has an erasing period Te for inputting an erasing signal after the light emitting period Tsn.
The erasing period Te has a first erasing period Te (1) and a second erasing period Te (2),
The method for driving a light-emitting device, wherein the third transistor is turned on in the first erasing period Te (1) and the second erasing period Te (2). A first transistor having one of a source and a drain electrically connected to the signal line and a gate electrically connected to the scan line;
A second transistor having a gate electrically connected to the other of the source and the drain of the first transistor;
A third source in which one of a source and a drain is electrically connected to the gate of the second transistor, and the other of the source and the drain is electrically connected to one of the source and the drain of the second transistor; A transistor,
A light emitting device having a light emitting element electrically connected to the other of the source and the drain of the second transistor,
Subframe periods SF1, SF2,..., SFn divided into n (n is a natural number) one frame period,
The subframe period SF1 includes a writing period Ta1 in which a digital video signal is input and a light emission period Ts1 in order.
The sub-frame period SF2 includes a writing period Ta2 in which a digital video signal is input and a light emission period Ts2 in order.
The sub-frame period SFn has a writing period Tan for inputting a digital video signal and a light emitting period Tsn in order.
The light emission periods Ts1, Ts2,..., Tsn become shorter in order.
The subframe period SFn has an erasing period Te for inputting an erasing signal after the light emitting period Tsn.
The erasing period Te has a first erasing period Te (1) and a second erasing period Te (2),
The method for driving a light-emitting device, wherein the third transistor is turned on in the first erasing period Te (1) and the second erasing period Te (2).

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