JP2006100796A - Light emitting device using light emitting element, driving method of light emitting element, and lighting apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light emitting device having a light emitting element with a small luminance degradation by contriving a driving means, and to provide a driving method for reducing the luminance degradation of the light emitting element. <P>SOLUTION: A current density J of a current flowing through the light emitting element is increased with time in accordance with the following formula (1); J = J<SB>0</SB>exp[(k×t)<SP>β</SP>]where J<SB>0</SB>is an initialization of the current density in the light emitting element, t is an emitting time, and k and β denote positive parameters which are determined by characteristics of the light emitting element, respectively. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a light-emitting device having a light-emitting element that emits light when an electric current flows. In particular, the present invention relates to a light-emitting device having a light-emitting element with low luminance deterioration. The present invention also relates to a driving method of a light emitting element for reducing luminance deterioration.

2. Description of the Related Art In recent years, light-emitting elements that can obtain high luminance by flowing current, such as light-emitting diodes (LEDs) and light-emitting elements (organic light-emitting diodes; OLEDs) using a light-emitting organic compound, have attracted attention.

A basic structure of a light-emitting element using a light-emitting organic compound is obtained by sandwiching a layer containing a light-emitting organic compound between a pair of electrodes. By applying a voltage to this element, electrons and holes are respectively transported from the pair of electrodes to the layer containing a light-emitting organic compound, and current flows. Then, these carriers (electrons and holes) recombine, whereby the light-emitting organic compound forms an excited state, and emits light when the excited state returns to the ground state.

Note that the excited states formed by the organic compound can be singlet excited state or triplet excited state. Light emission from the singlet excited state is called fluorescence, and light emission from the triplet excited state is called phosphorescence. ing.

Since such a light emitting element is usually formed with a thin film of about submicron to several microns, it is a great advantage that it can be manufactured thin and light. In addition, since the time from the injection of the carrier to the light emission is about microseconds or less, one of the features is that the response speed is very fast. Further, since sufficient light emission can be obtained with a DC voltage of several volts to several tens of volts, power consumption is relatively small. Because of these advantages, the above-described light-emitting element has attracted attention as a next-generation flat panel display element.

In addition, since such a light-emitting element forms a pair of electrodes and a light-emitting layer in a film shape, planar light emission can be easily obtained by forming a large-area element. This is a feature that is difficult to obtain with a point light source typified by an incandescent bulb or LED, or a line light source typified by a fluorescent lamp, and therefore has high utility value as a surface light source applicable to illumination or the like.

By the way, the output (luminance) of a light-emitting element that emits light by passing a current as described above is determined by the amount of current that flows. Therefore, when the light emitting element emits light, the luminance suitable for the application can be set by setting the current amount to an appropriate value. At this time, the ratio of the luminance to the current density is called current efficiency.

If this current efficiency does not change, a certain luminance can be obtained by passing a certain current, but this is not practical. In a normal light emitting element, the current efficiency is gradually lowered by passing a current (or light is emitted), so that the luminance is gradually lowered even when a certain current is passed. In particular, in a light-emitting element using a light-emitting organic compound, this luminance deterioration is noticeable, which greatly hinders the development of the light-emitting element.

For this reason, in research and development in this field, many improvements have been made to materials and device structures in order to suppress as much as possible the deterioration in luminance when a constant current is passed (that is, the reduction in current efficiency during constant current driving). . As a result, at present, light-emitting elements having a luminance half-life of several tens of thousands of hours have been developed, and there are many viewpoints of practical use. Note that the constant current driving means that a current is continuously supplied at a certain current density to the electrodes of the light emitting element. Note that the current density represents the intensity of current per unit area of the electrode of the light emitting element.

However, since the difference in luminance (or luminance unevenness) is recognized even if it is about several percent, it is required to have low luminance deterioration as in the display application or lighting application used for personal computers and televisions. For application fields, it is still not reliable enough. In particular, high brightness is required for illumination and the like. However, considering the current state of light-emitting elements that brightness degradation is accelerated as the set brightness is increased, it has not yet reached a practical level.

Although there has not yet been a sufficient discussion regarding the mechanism of this luminance degradation, for example, fitting a luminance degradation curve (time-luminance curve) when a constant current continues to flow with a function called an extended exponential function results in luminance degradation. (See Non-Patent Document 1). A number of factors are complicatedly mixed in the luminance degradation, and unfortunately, it has not led to a fundamental clarification, but this function itself can fit the luminance degradation curve with high accuracy.

In any case, luminance degradation as described above, that is, a decrease in current efficiency, is considered to be largely due to the low durability of organic materials and the weakness of organic thin films. It can be said that improvement alone is not enough. Therefore, attempts have been made to suppress luminance deterioration from the viewpoint of the driving method (see, for example, Patent Document 1). In Patent Document 1, although the luminance half-life is improved to about twice by applying a reverse bias, the occurrence of luminance degradation itself has not been greatly suppressed.
Masahiko Ishii, 1 other person, Applied Physics Letters, vol. 80 (18), 3430-3432 (2002) JP 2003-323988 A

Accordingly, an object of the present invention is to provide a light-emitting device having a light-emitting element with small luminance deterioration by devising a driving unit. It is another object of the present invention to provide a driving method for reducing luminance deterioration of a light emitting element.

As described above, a light-emitting element exhibits a decrease in current efficiency when a current flows (or emits light), which is an unavoidable phenomenon due to the material and element structure. Therefore, in order to reduce the luminance deterioration, it is only necessary to increase the amount of current over time by correcting the amount of decrease in current efficiency over time.

However, if the amount of current is increased too much, there is a risk that the luminance will increase rather than decrease the luminance degradation. However, if the increase in the amount of current is too small, luminance deterioration cannot be suppressed. In addition, the method of increasing an appropriate amount of current for suppressing luminance deterioration greatly varies depending on the constituent material and structure of the light emitting element.

As a result of intensive studies, the present inventor has found that these difficulties can be overcome by the following method. That is, the structure of the present invention is a light emitting device having a light emitting element having a light emitting layer between an anode and a cathode, and means for increasing the current density J of the current flowing through the light emitting element over time according to the following formula (1). Device.

_{J = J 0 · exp [(} k · t) β] ··· (1)
(J ₀ is the initial current density in the light emitting element, t is the light emission time, and k and β are positive parameters determined by the characteristics of the light emitting element.)

According to another aspect of the present invention, there is provided a light emitting device having a light emitting layer between an anode and a cathode, means for increasing a current density J of a current flowing in the light emitting device over time according to the following formula (2), A light emitting device having

J = J ₀ · exp [(k ′ · ∫Jdt) ^β ] (2)
(J ₀ represents the initial current density in the light emitting element, t represents the light emission time, k ′ and β represent positive parameters determined by the characteristics of the light emitting element, and ∫ represents the integral from 0 to t. .)

According to another aspect of the present invention, there is provided a light emitting element having a light emitting layer between an anode and a cathode, a means for controlling an increase rate γ of a current density of a current flowing through the light emitting element according to the following formula (3), A light emitting device having

γ = exp [{(γ + 1) · k · t / 2} ^β ] (3)
(T represents the light emission time, k and β represent positive parameters determined by the characteristics of the light emitting element. Also, assuming that the initial current density in the light emitting element is J ₀ and the current density flowing in the light emitting element is J. , is γ = J / J _0.)

Furthermore, the present inventor appropriately increases the amount of current over time by increasing the voltage in accordance with the equation represented by the following equation (4) with respect to the light emitting element driven at a duty ratio n of 0 <n <100. It was found that the luminance degradation can be suppressed by increasing the luminance. That is, another configuration of the present invention includes a light emitting element having a light emitting layer between an anode and a cathode, a first means for driving the light emitting element with a duty ratio n of 0 <n <100, and the light emitting element. And a second means for increasing the voltage V of the element according to the following formula (4). Note that the rate of increase in the amount of current flowing through the light emitting element at this time can be set to an appropriate rate by selecting an appropriate duty ratio.

V = {J ₀ / g (Q ₁₀₀ )} ^{1 / f (t ′)} (4)
(J ₀ represents the initial current density in the light emitting element. F (t ′) is a monotonically decreasing function with the storage time t ′ as a variable. Also, g (Q) represents the light emitting element. A monotonically decreasing function with the total charge amount Q per unit area flowing as a variable, and Q ₁₀₀ per unit area flowing when the light emitting element is driven at a constant current with a duty ratio of 100 and a current density J ₀ . (It is a total charge amount, and when the driving time of the light emitting element is t ″, it is expressed by Q ₁₀₀ = J ₀ · t ″.)

At this time, as a method of applying the voltage represented by the formula (4) to the light emitting element, a monitor element having the same structure as that of the light emitting element has a constant current with a duty ratio = 100 and a current density = J _0. A method of driving and applying the voltage of the monitor element to the light emitting element by an operational amplifier can be considered. Therefore, the second means is applied to the monitor element having the same structure as the light emitting element, a constant current source for supplying a constant current having a current density of J ₀ to the monitor element, and the monitor element. A light-emitting device having an operational amplifier for applying a voltage to the light-emitting element is also included in the present invention. Note that the monitor element does not have to have the same structure as the light-emitting element as long as the voltage V of the light-emitting element can be increased according to the formula (4) corresponding to the light-emitting element.

Note that a reduction in current efficiency is relatively significant in a light-emitting element using a light-emitting organic compound, and thus the structure of the present invention is particularly useful for a light-emitting element using a light-emitting organic compound. As the light-emitting organic compound, a phosphorescent material is preferable.

Furthermore, the light-emitting device of the present invention is suitable for applications such as lighting fixtures that require high brightness and long life.

According to the gist of the present invention described above, the present invention can also provide a driving method for reducing luminance deterioration of a light emitting element. That is, the present invention includes a method for driving a light emitting element that increases the current density of the current flowing through the light emitting element over time according to any one of the above formulas (1) to (3). Alternatively, a driving method of a light emitting element is also included in which the light emitting element is driven with a duty ratio n of 0 <n <100, and the voltage of the light emitting element is increased according to the above formula (4).

Note that a light-emitting device in this specification refers to a light-emitting body using a light-emitting element, an image display device, or the like. In addition, a connector in which a connector, for example, a flexible printed circuit (FPC), a TAB (Tape Automated Bonding) tape or a TCP (Tape Carrier Package), is attached to the light emitting element, and a printed wiring board is attached to the end of the TAB tape or TCP. It is assumed that the light-emitting device includes all the modules provided or ICs (integrated circuits) directly mounted on the light-emitting elements by a COG (Chip On Glass) method.

  By implementing the present invention, a light-emitting device having a light-emitting element with low luminance deterioration can be obtained.

Hereinafter, embodiments of the present invention will be described in detail with reference to an operation principle and a specific configuration example. First, the operation principle of the present invention will be described.

It is assumed that the light emitting element before being continuously lit emits light with a luminance L ₀ by flowing a current having a certain current density J ₀ (hereinafter, J _{0 is referred} to as an initially set current density and L ₀ is referred to as an initial luminance). When a current is continuously supplied to the light emitting element at a current density of J ₀ (that is, driven at a constant current), the luminance of the light emitting element emitting light with the initial luminance L ₀ gradually decreases. 1, it is known that the luminance decreases according to the “expanded exponential function” expressed by the following equation (5).

L (t) = exp [− (t / τ) ^β ] (5)
(T is the light emission time, and L (t) represents the relative luminance (ratio of the luminance L to the initial luminance L ₀ ), that is, L (t) = L / L ₀ and L (0) = 1. In addition, τ and β are positive parameters determined by the light emitting element, τ is called decay time (unit; time), and β is called dispersion factor (no unit).)

Here, when k = 1 / τ, the above equation (5) can be rewritten as the following equation (6).

L / L ₀ = exp [− (k · t) ^β ] (6)

When β = 1, equation (6) can be written as L = L ₀ · exp [− (k · t)]. Since this equation is very similar to the reaction rate law of the first-order reaction, k can be regarded as a rate constant when “a molecule that contributes to light emission does not contribute to light emission in a first-order reaction” from the similarity. (In fact, the unit of k is also [time- ¹ ]). Further, β is a parameter that deforms the shape of the exponential curve. Specifically, in the range of 0 <β ≦ 1, Equation (6) becomes a curve with a larger initial deterioration as β decreases.

Incidentally, the luminance L of the light emitting element is generally represented by the following formula (7), where η is the current efficiency and J is the current density. In the case of constant current driving, by setting J = J ₀ , Equation (7) can be written as Equation (8) (J ₀ is the initial current density and is constant) Value).

L = η · J (7)
L = η · J ₀ (8)

Therefore, in the case of constant current driving, the following equation (9) can be derived from the equations (6) and (8).

η = L ₀ / J ₀ · exp [− (k · t) ^β ] (9)

Here, L ₀ / J ₀ is “initial luminance / initial setting current density”, and thus indicates the current efficiency η _{0 in} the initial state before continuous lighting. Therefore, this equation (9) shows the phenomenon that “current efficiency η decreases with time from η ₀ (= L ₀ / J ₀ ) when current is kept flowing at a certain current density J ₀ ”. Show. That is, it can be seen that the luminance deterioration curve at the time of constant current driving represented by the equation (6) is synonymous with the current efficiency decrease curve represented by the equation (9).

As long as the current efficiency η is reduced according to the equation (9), the luminance is also lowered in the method of keeping a constant current flowing. Therefore, the present inventor has thought that luminance deterioration can be suppressed by increasing the amount of current at an appropriate ratio.

What is important here is that if the amount of current increase is too small, the effect of suppressing luminance deterioration will be small, whereas if the amount of current increase is too large, the luminance will increase significantly. Another defect will occur. In other words, it is important to establish a method for increasing the amount of current so that the luminance does not increase greatly and deterioration is suppressed to a small extent, which is the gist of the present invention.

Further, in the present invention, one of the main points is that the method of increasing the amount of current is expressed by a function (formula) having the light emission time as a variable. This is because it is possible to easily obtain a light-emitting element with small luminance deterioration simply by programming a light-emitting element to be driven with such a mathematical formula and additionally providing a memory circuit capable of storing the light-emission time.

[Embodiment 1]
First, the following formula (10) is obtained by substituting the formula (9) into the formula (7) that is a formula representing a general current efficiency.

L = L ₀ / J ₀ · exp [− (k · t) ^β ] · J (10)

Here, in Equation (10), it is assumed that the luminance L is maintained as the initial luminance L ₀ (that is, there is no deterioration with time of luminance). In that case, the following formula (1) is obtained by substituting L = L ₀ into formula (10) and transforming it.

_{J = J 0 · exp [(} k · t) β] ··· (1)
(J ₀ is the initial current density in the light emitting element, t is the light emission time, and k and β are positive parameters determined by the characteristics of the light emitting element.)

Note that the light emission time refers to a time during which the light emitting element emits light.

Equation (1) shows how the luminance L can be maintained at the initial luminance L ₀ by increasing the current density J from the initial setting current density J ₀ over time when equation (9) holds. Is a theoretical expression. Therefore, by increasing the current amount of the light emitting element over time according to the formula (1), a light emitting element with small luminance deterioration can be obtained. Further, a light emitting element, means for increasing the current density J of the current flowing through the light emitting element over time according to the formula (1) (specifically, a memory circuit capable of storing the light emission time, and the light emitting element according to the formula (1)) A light-emitting device having a small luminance deterioration can be obtained.

Note that the parameters k and β determined by the light emitting elements are the luminance degradation curves (results obtained by driving the light emitting elements at a constant current L ₀ in advance at a constant current L ₀ (that is, driving at an initial current density J ₀ (= constant)). (Time-luminance curve) can be obtained by fitting with equation (6).

Therefore,
Decide the brightness you want to put to practical use,
Measure the current density needed to get that brightness,
Measure the luminance degradation curve by driving at that current density,
Parameters k and β are obtained from the luminance degradation curve,
Substituting the obtained parameters k and β into equation (1),
Program the current density J to increase according to equation (1) with the parameters k and β substituted,
By driving the light emitting element in accordance with the program, a light emitting element with small luminance deterioration and a light emitting device using the light emitting element can be obtained.

However, the parameters k and β may be obtained from an acceleration test. That is, if been obtained acceleration coefficient with respect to the luminance of the parameter k and beta, even 1000 cd / m ² luminance to be put to practical use, and parameters k actually be driven with a constant current at an initial luminance 5000 cd / m ² β may be obtained and converted into parameters k and β when driven at a constant current with an initial luminance of 1000 cd / m ² .

As described above, the first embodiment has an advantage that a light-emitting element with small luminance deterioration can be easily obtained by applying a very simple mathematical expression such as Expression (1).

[Embodiment 2]
Expression (9) described above is an expression representing a decrease in current efficiency during constant current driving. Therefore, when the amount of current is increased over time instead of constant current driving, the decrease in current efficiency may actually be accelerated more than in equation (9) (that is, the decrease in current efficiency is (There is a possibility of deviation from (9)).

In view of this, the second embodiment provides a theoretical formula that represents how to increase the amount of current so that a constant luminance can be theoretically obtained by correcting a shift in the decrease in current efficiency, although it is somewhat complicated.

First, in equation (9), if k = k ′ · J ₀ , the following equation (11) is obtained.

η = L ₀ / J ₀ · exp [− (k ′ · J ₀ · t) ^β ] (11)

Here, on the right side in the equation (11), “J ₀ · t” represents the total charge amount Q per unit area that has flowed to the light emitting element at the time of the light emission time t. That is, it can be considered that the decrease in current efficiency η is actually expressed by the following formula (12) using the total charge amount Q per unit area flowing through the light emitting element, regardless of whether the current drive is constant current or not.

η = L ₀ / J ₀ · exp [− (k ′ · Q) ^β ] (12)

Further, the total charge amount Q per unit area is expressed by an integral formula of the following formula (13) using the current density J of the current flowing through the light emitting element.

Q = ∫ Jdt (13)
(∫ represents the integral from 0 to t.)

Therefore, the following equation (14) is obtained from the equations (12) and (13).

η = L ₀ / J ₀ · exp [− (k ′ · ∫Jdt) ^β ] (14)

This equation (14) is not limited to constant current driving, but to what extent the current efficiency decreases depending on the total amount of electric charge (that is, the integral term) that has flowed through the light emitting element, even when the amount of current changes. It is a formula that can express. That is, as shown in the time-current density curve of FIG. 1A, even when the current density is not constant over time, the total charge amount Q per unit area that has flowed (see FIG. 1A It is possible to know how much the current efficiency is reduced in a certain time.

Substituting this equation (14) into equation (7), which is an equation representing general current efficiency, yields the following equation (15).

L = L ₀ / J ₀ · exp [− (k ′ · ∫Jdt) ^β ] · J (15)

Here, in Equation (15), it is assumed that the luminance L is maintained as the initial luminance L ₀ (that is, there is no deterioration with time of luminance). In that case, the following formula (2) is obtained by substituting L = L ₀ into formula (15) and transforming it.

J = J ₀ · exp [(k ′ · ∫Jdt) ^β ] (2)
(J ₀ represents the initial current density in the light emitting element, t represents the light emission time, k ′ and β represent positive parameters determined by the characteristics of the light emitting element, and ∫ represents the integral from 0 to t. .)

Expression (2) is an expression that theoretically represents how the luminance L can be maintained at the initial luminance L ₀ by increasing the current density J from the initial setting current density J ₀ over time. Therefore, by increasing the current amount of the light emitting element over time according to the formula (2), a light emitting element with extremely small luminance deterioration can be obtained. In addition, the light emitting element and means for increasing the current density J of the current flowing through the light emitting element over time according to the formula (2) (specifically, an ammeter for monitoring the current density, the light emission time and the current density flowing) A light-emitting device with low luminance deterioration can be obtained by manufacturing a light-emitting device having a memory circuit capable of storing the above and a program for driving a light-emitting element according to Equation (2).

Note that the parameters k ′ and β determined by the light-emitting elements are the luminance degradation curves obtained as a result of driving the light-emitting elements at a constant current L ₀ in advance with an initial luminance L ₀ (that is, driving with an initial current density J ₀ (= constant)). (Time-luminance curve) can be obtained by fitting with equation (6) (k and β are obtained from the fitting, but k ′ can be obtained from the definition of k = k ′ · J _0. ).

Therefore,
Decide the brightness you want to put to practical use,
Measure the current density needed to get that brightness,
Measure the luminance degradation curve by driving at constant current at that current density,
Parameters k ′ and β are obtained from the luminance degradation curve,
Substituting the obtained parameters k ′ and β into equation (1),
Program the current density J to increase according to equation (1) with the parameters k ′ and β substituted,
By driving the light emitting element in accordance with the program, a light emitting element with small luminance deterioration and a light emitting device using the light emitting element can be obtained.

However, the parameters k ′ and β may be obtained from an acceleration test. That is, if been obtained acceleration coefficient for the luminance parameter k 'and beta, the luminance to be used in practice is a 1000 cd / m ^2, actually by constant current driving at an initial luminance 5000 cd / m ² parameter k 'And β may be obtained and converted into parameters k' and β when driven at a constant current with an initial luminance of 1000 cd / m ² .

As described above, the second embodiment has an advantage that a light emitting element having a constant luminance with little deterioration in luminance can be obtained in theory.

[Embodiment 3]
In the third embodiment, the integral term of the equation (2) described in the second embodiment is approximated to provide a simpler theoretical formula.

The total charge amount Q per unit area flowing in the light emitting element is represented by the integral of the equation (13) as described above. In order to accurately obtain the value of Q, it is necessary to always monitor the current density at a certain time and integrate it as shown in FIG. Therefore, the present inventor has devised applying the approximation described below.

In the present invention, the main purpose is to gradually increase the amount of current in order to reduce the luminance degradation of the light emitting element. Therefore, the current density flowing through the light emitting element is basically a function of monotonically increasing with respect to time. FIG. 1 (b) is a time-current density curve, and the solid line in FIG. 1 (b) schematically represents this.

Here, when this monotonically increasing function is approximated by a straight line, it becomes as shown by a broken line in FIG. When this broken line is regarded as a change in current density with time (a straight line approximates a change in current density with time), the total charge amount Q per unit area can be expressed by the following equation (16).

Q = (J ₀ + J) · t / 2 (16)

By substituting equation (16) into equation (12), the following equation (17) is obtained.

η = L ₀ / J ₀ · exp [− {(J ₀ + J) · k ′ · t / 2} ^β ] (17)

This equation (17) is an equation that can approximately represent how much the current efficiency decreases with time when driving is performed to monotonously increase the amount of current. Substituting this equation (17) into equation (7), which is an equation representing general current efficiency, yields the following equation (18).

_{L = L 0 / J 0 ·} exp [- {(J 0 + J) · k '· t / 2} β] · J ··· (18)

Here, in Equation (18), it is assumed that the luminance L is maintained as the initial luminance L ₀ (that is, there is no deterioration with time of luminance). In that case, the following formula (19) is obtained by substituting L = L ₀ into formula (18) and transforming it.

J = J ₀ · exp [{(J ₀ + J) · k ′ · t / 2} ^β ] (19)

The equation (19) can be transformed as the following equation (20).

J / J ₀ = exp [{(J / J ₀ +1) · J ₀ · k ′ · t / 2} ^β ] (20)

Here, J ₀ · k ′ = k from the definition described in the second embodiment. J / J ₀ is the ratio of the current density actually flowing to the initially set current density, so it represents the rate of increase of the current density. When the current density increase rate J / J ₀ is represented by γ, the following formula (3) is obtained.

γ = exp [{(γ + 1) · k · t / 2} ^β ] (3)
(T represents the light emission time, k and β represent positive parameters determined by the characteristics of the light emitting element. Also, assuming that the initial current density in the light emitting element is J ₀ and the current density flowing in the light emitting element is J. , is γ = J / J _0.)

Expression (3) is an expression that approximately represents how the luminance L can be maintained at the initial luminance L ₀ approximately when the current density increase rate γ changes with respect to the light emission time. Therefore, by increasing the current amount of the light emitting element over time according to the formula (3), a light emitting element with extremely small luminance deterioration can be obtained. Further, a light emitting element, a means for increasing the current density increase rate γ of the current flowing through the light emitting element over time according to the equation (3) (specifically, a memory circuit capable of storing the light emission time, and the equation (3) Thus, a light-emitting device with small luminance deterioration can be obtained.

Note that the parameters k and β determined by the light emitting elements are the luminance degradation curves (results obtained by driving the light emitting elements at a constant current L ₀ in advance at a constant current L ₀ (that is, driving at an initial current density J ₀ (= constant)). (Time-luminance curve) can be obtained by fitting with equation (6).

Therefore,
Decide the brightness you want to put to practical use,
Measure the current density needed to get that brightness,
Measure the luminance degradation curve by driving at constant current at that current density,
Parameters k and β are obtained from the luminance degradation curve,
Substituting the obtained parameters k and β into equation (3),
Program the current density J to increase according to equation (3) with the parameters k and β substituted,
By driving the light emitting element in accordance with the program, a light emitting element with small luminance deterioration and a light emitting device using the light emitting element can be obtained.

However, the parameters k and β may be obtained from an acceleration test. That is, if been obtained acceleration coefficient with respect to the luminance of the parameter k and beta, even 1000 cd / m ² luminance to be put to practical use, and parameters k actually be driven with a constant current at an initial luminance 5000 cd / m ² β may be obtained and converted into parameters k and β when driven at a constant current with an initial luminance of 1000 cd / m ² .

As described above, the third embodiment has an advantage that a light-emitting element with extremely small luminance deterioration can be obtained without monitoring the total amount of charge that has flowed.

[Embodiment 4]
In Embodiment 4, focusing on the voltage-current characteristics of a light-emitting element, a mode in which a light-emitting element with small luminance deterioration is obtained by controlling the voltage is disclosed.

A voltage-current characteristic of a light emitting element that emits light by passing a current generally shows a so-called diode characteristic. Therefore, the voltage-current characteristic at the time of forward bias is a steeper curve than Ohm's law (J∝V). At this time, assuming that the current density of the flowing current is J and the voltage is V, in a practical luminance region (specifically, 100 cd / m ^{2 to} 10000 cd / m ² ), the diode characteristic is expressed by the following formula (21). Can be approximated by

J = S · V ⁿ (21)
(S and n represent positive parameters determined by the characteristics of the light emitting element, respectively, and n> 1.)

If a current continues to flow through the light emitting element (that is, continues to emit light), the diode characteristic becomes difficult to flow with time as shown in FIG. The solid line shows the diode characteristics before driving, and the broken line shows the diode characteristics after driving. In this case, in the above-described equation (21), S is a parameter that decreases depending on the total amount of charge per unit area, and n is a parameter that decreases only in storage time regardless of whether or not current is supplied. The inventor found that there is. That is, as S and n decrease, the change shown in FIG. 2A occurs.

When the logarithm of the formula (21) is taken, the following formula (22) is obtained. Therefore, in the equation (22), the y-intercept (logS) decreases depending on the total charge amount per unit area that has flowed, and the slope (n) decreases depending on the storage time. In other words, the threshold value of the diode characteristic has a property of shifting to the high voltage side depending on the total charge amount, and the slope has a property of falling asleep depending on the storage time.

  logJ = n · logV + logS (22)

Here, S is a parameter that decreases depending on the total charge amount per unit area that has flowed, and n is a parameter that decreases only in storage time, so that the total charge amount Q per unit area, storage time t, respectively. It can be expressed by the function of '. That is, the following formula (23) is obtained.

J = g (Q) · V ^{f (t ′)} (23)
(F (t ′) and g (Q) are both monotonically decreasing functions)

Here, in the equation (23), g (Q) changes depending on the total charge amount Q per unit area. Therefore, for example, a light-emitting element with a duty ratio of 100 and a duty ratio of n (0 < Naturally, the method of changing g (Q) is different from that of the light emitting element of n <100.

This phenomenon is schematically shown in FIG. 2 (b). A is a diode characteristic before driving, B is a diode characteristic when driving for a time with a duty ratio of 100, and C is a diode characteristic when driving with a duty ratio n for the same time. Conceptually, as shown in FIG. 2 (b), current flows more easily when C has a smaller total charge amount per unit area. This is due to the difference in g (Q) in equation (23). to cause. That is, in B, the following equation (24) is established, and in C, the following equation (25) is established. J ₁₀₀ is a current density of a current flowing through an element having a duty ratio of 100, and J _n is a current density of a current flowing through an element having a duty ratio of n. Further, Q ₁₀₀ is the total charge amount per unit area that has flowed through the element with a duty ratio of 100, and Q _n is the total charge amount per unit area that has flowed through the element with the duty ratio n.

J ₁₀₀ = g (Q ₁₀₀ ) · V ^{f (t ′)} (24)
J _n = g (Q _n ) · V ^{f (t ′)} (25)

Here, if a light emitting element with a duty ratio of 100 is driven with a constant current, J becomes a constant current density J ₀ . Therefore, Expression (24) becomes Expression (26).

J ₀ = g (Q ₁₀₀ ) · V ^{f (t ′)} (26)

Since both f (t ′) and g (Q ₁₀₀ ) are functions of monotonic decrease, the voltage V in the equation (26) is changed from the initial voltage V ₀ to V _t (V as shown in FIG. 2B). ₀ <shifts to V _t).

Here, the voltage V _t according to the element of the duty ratio of 100, when applied through a buffer amplifier or the like to the element of the duty ratio n, the voltage V of the formula (25) and equation (26) becomes common. Therefore, the following equation (27) is obtained from the equations (25) and (26).

J _n = {g (Q _n ) / g (Q ₁₀₀ )} · J ₀ (27)

In equation (27), g (Q) is a monotonically decreasing function and Q ₁₀₀ > Q _n , so that g (Q _n )> g (Q ₁₀₀ ) always holds. Further, as the driving time becomes longer, the difference between g (Q _n ) and g (Q ₁₀₀ ) gradually increases. Therefore, from equation (27), J _n gradually increases as the drive time increases.

FIG. 2B schematically shows how J _n increases. Further, since the curves B and C are separated as the driving time becomes longer, J _n gradually increases.

Since this increase rate can be controlled by controlling the duty ratio n, that is, Q _n , it is possible to achieve the gist of the present invention of gradually increasing the amount of current to reduce luminance deterioration. Therefore, the following formula (4) (obtained by modifying the above formula (26)) representing the voltage applied to the light emitting element when driving at a constant current with a duty ratio of 100 is obtained as a duty ratio n where 0 <n <100. By applying the voltage to the light emitting element driven at, the amount of current can be gradually increased and luminance degradation can be reduced.

V = {J ₀ / g (Q ₁₀₀ )} ^{1 / f (t ′)} (4)
(J ₀ represents the initial current density in the light emitting element. F (t ′) is a monotonically decreasing function with the storage time t ′ as a variable. Also, g (Q) represents the light emitting element. A monotonically decreasing function with the total charge amount Q per unit area flowing as a variable, and Q ₁₀₀ per unit area flowing when the light emitting element is driven at a constant current with a duty ratio of 100 and a current density J ₀ . (It is a total charge amount, and when the driving time of the light emitting element is t ″, it is expressed by Q ₁₀₀ = J ₀ · t ″.)

Note that the storage time (also referred to as elapsed time) t ′ is the elapsed time from an arbitrary point, for example, the time that has elapsed since the light emitting element was driven. The drive time t ″ is a time represented by t ″ = t · n / 100, where t is the light emission time of a light emitting element having a duty ratio of 100.

At this time, as a method of applying the voltage represented by the formula (4) to the light emitting element, for example, a monitor element having the same structure as that of the light emitting element has a duty ratio of 100 and a current density of J ₀ . There is a method of driving at a constant current and applying the voltage of the monitor element to the light emitting element by an operational amplifier. However, the present invention is not limited to this method.

[Embodiment 5]
In Embodiment 5, one embodiment of a light-emitting element will be described. The present invention can be applied to any light-emitting element that emits light when an electric current is passed. In Embodiment 5, a light-emitting element using a light-emitting organic compound will be described.

Note that in the light emitting element, at least one of the electrodes may be transparent in order to extract emitted light. Therefore, not only the conventional device structure in which a transparent electrode is formed on the substrate and the light is extracted from the substrate side, but also the structure in which the light is extracted from the opposite side of the substrate and the structure in which the light is extracted from both sides of the electrode are applied. Is possible.

Hereinafter, materials and element structures that can be used will be described with respect to the structure of the light-emitting element of the present invention. FIG. 3 shows a structure of a typical light emitting element, in which an anode 301, a light emitting layer 302, and a cathode 303 are stacked on a substrate 300. The light-emitting layer 302 only needs to contain at least a light-emitting organic compound, and uses a low molecular compound, a high molecular compound (polymer), an intermediate molecular compound such as an oligomer or dendrimer, or an inorganic compound that is not similar to them. Can be formed. As for the light-emitting organic compound, low molecular compounds and high molecular compounds (polymers) as well as medium molecular compounds such as oligomers and dendrimers that are not similar to them can be used.

In FIG. 3 of the fifth embodiment, the light emitting layer 302 includes a hole injection layer 311, a hole transport layer 312, a layer 313 containing a light emitting organic compound, an electron transport layer 314, and an electron injection layer 315. It is not necessarily limited to this configuration. Note that the hole injection layer is a layer that has a function of receiving holes from the anode, and the hole transport layer is a layer that has a function of delivering holes to a layer containing a light-emitting organic compound. The electron injection layer is a layer that has a function of receiving electrons from the cathode, and the electron transport layer is a layer that has a function of transferring electrons to a layer containing a light-emitting organic compound.

First, materials that can be used for each of these layers are specifically exemplified. However, materials applicable to the present invention are not limited to these.

The hole injection material can be used for the hole injection layer, it is effective to a phthalocyanine-based compound, phthalocyanine (abbreviation: H ₂ -Pc), copper phthalocyanine (abbreviation: Cu-Pc), vanadyl phthalocyanine (abbreviation: VOPc) Etc. can be used. In addition, there is a material obtained by chemically doping a conductive polymer compound, and polyethylenedioxythiophene (abbreviation: PEDOT) or polyaniline (abbreviation: PAni) doped with polystyrene sulfonic acid (abbreviation: PSS) can also be used. . Also effective are thin films of inorganic semiconductors such as molybdenum oxide (MoO _x ), vanadium oxide (VO _x ), nickel oxide (NiO _x ), and ultra-thin films of inorganic insulators such as aluminum oxide (Al ₂ O ₃ ). . In addition, 4,4 ′, 4 ″ -tris (N, N-diphenyl-amino) -triphenylamine (abbreviation: TDATA), 4,4 ′, 4 ″ -tris [N- (3-methylphenyl) -N-phenyl-amino] -triphenylamine (abbreviation: MTDATA), N, N'-bis (3-methylphenyl) -N, N'-diphenyl-1,1'-biphenyl-4,4'-diamine (Abbreviation: TPD), 4,4′-bis [N- (1-naphthyl) -N-phenyl-amino] -biphenyl (abbreviation: α-NPD), 4,4′-bis [N- (4- ( Aromatic amine compounds such as N, N-di-m-tolyl) amino) phenyl-N-phenylamino] biphenyl (abbreviation: DNTPD) can also be used. Furthermore, a substance showing acceptability to these aromatic amine compounds may be added. Specifically, 2,3,5,6-tetrafluoro-7,7,8,8 which is an acceptor for VOPc. - tetracyanoquinodimethane (abbreviation: F ₄ -TCNQ) obtained by adding or may be used after addition of MoO _x is an acceptor in alpha-NPD.

As the hole transport material that can be used for the hole transport layer, an aromatic amine compound is suitable, and the above-described TDATA, MTDATA, TPD, α-NPD, DNTPD, and the like can be used.

As an electron-transport material that can be used for the electron-transport layer, tris (8-quinolinolato) aluminum (abbreviation: Alq ₃ ), tris (4-methyl-8-quinolinolato) aluminum (abbreviation: Almq ₃ ), bis (10- Hydroxybenzo [h] -quinolinato) beryllium (abbreviation: BeBq ₂ ), bis (2-methyl-8-quinolinolato) (4-phenylphenolato) aluminum (abbreviation: BAlq), bis [2- (2-hydroxyphenyl) And metal complexes such as benzoxazolate] zinc (abbreviation: Zn (BOX) ₂ ) and bis [2- (2-hydroxyphenyl) benzothiazolate] zinc (abbreviation: Zn (BTZ) ₂ ). In addition to metal complexes, 2- (4-biphenylyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis [5- Oxadiazole derivatives such as (p-tert-butylphenyl) -1,3,4-oxadiazol-2-yl] benzene (abbreviation: OXD-7), 3- (4-tert-butylphenyl) -4 -Phenyl-5- (4-biphenylyl) -1,2,4-triazole (abbreviation: TAZ), 3- (4-tert-butylphenyl) -4- (4-ethylphenyl) -5- (4-biphenylyl) ) -1,2,4-triazole (abbreviation: p-EtTAZ) derivative, 2,2 ′, 2 ″-(1,3,5-benzenetriyl) tris [1-phenyl-1H-benzimidazole ] An imidazole derivative such as (abbreviation: TPBI), a phenanthroline derivative such as bathophenanthroline (abbreviation: BPhen), and bathocuproin (abbreviation: BCP) can be used.

Examples of the electron injection material that can be used for the electron injection layer include Alq ₃ , Almq ₃ , BeBq ₂ , BAlq, Zn (BOX) ₂ , Zn (BTZ) ₂ , PBD, OXD-7, TAZ, and p-EtTAZ. , TPBI, BPhen, BCP, and other electron transport materials can be used. In addition, an ultra-thin film of an insulator such as an alkali metal halide such as LiF or CsF, an alkaline earth halide such as CaF ₂ , or an alkali metal oxide such as Li ₂ O is often used. Alkali metal complexes such as lithium acetylacetonate (abbreviation: Li (acac)) and 8-quinolinolato-lithium (abbreviation: Liq) are also effective. In addition, a substance exhibiting a donor property may be added to these electron injection materials, and as the donor, an alkali metal, an alkaline earth metal, a rare earth metal, or the like can be used. Specifically, a material obtained by adding lithium as a donor to BCP or a material obtained by adding lithium as a donor to Alq ₃ can be used.

Next, materials that can be used as the light-emitting organic compound are listed, but the present invention is not limited to these, and any light-emitting organic compound may be used.

For example, blue to blue-green light emission is appropriately obtained by using perylene, 2,5,8,11-tetra-t-butylperylene (abbreviation: TBP), 9,10-diphenylanthracene (abbreviation: DPA) as a guest material. It is obtained by dispersing in a suitable host material. In addition, styrylarylene derivatives such as 4,4′-bis (2,2-diphenylvinyl) biphenyl (abbreviation: DPVBi), 9,10-di-2-naphthylanthracene (abbreviation: DNA), 9,10-bis It can be obtained from an anthracene derivative such as (2-naphthyl) -2-t-butylanthracene (abbreviation: t-BuDNA). A polymer such as poly (9,9-dioctylfluorene) may also be used.

For example, blue-green to green light can be emitted by coumarin dyes such as coumarin 30 and coumarin 6, or bis [2- (4,6-difluorophenyl) pyridinato-N, C ^{2 ′} ] (picolinato) iridium (abbreviation: FIrpic). Bis (2-phenylpyridinato-N, C ^{2 ′} ) (acetylacetonato) iridium (Ir (ppy) ₂ (acac)) as a guest material and obtained by dispersing in a suitable host material . It can also be obtained by dispersing the above-described perylene or TBP in a suitable host material at a high concentration of 5 wt% or more. It can also be obtained from metal complexes such as BAlq, Zn (BTZ) ₂ , bis (2-methyl-8-quinolinolato) chlorogallium (Ga (mq) ₂ Cl). A polymer such as poly (p-phenylene vinylene) may also be used.

For example, yellow to orange light emission is obtained from rubrene, 4- (dicyanomethylene) -2- [p- (dimethylamino) styryl] -6-methyl-4H-pyran (abbreviation: DCM1), 4- (dicyanomethylene) -2. -Methyl-6- (9-julolidyl) ethynyl-4H-pyran (abbreviation: DCM2), bis [2- (2-thienyl) pyridinato] acetylacetonatoiridium (Ir (thp) ₂ (acac)), bis (2 -Phenylquinolinato) acetylacetonatoiridium (Ir (pq) ₂ (acac)) or the like is used as a guest material and dispersed in a suitable host material. It can also be obtained from a metal complex such as bis (8-quinolinolato) zinc (abbreviation: Znq ₂ ) or bis [2-cinnamoyl-8-quinolinolato] zinc (abbreviation: Znsq ₂ ). Further, a polymer such as poly (2,5-dialkoxy-1,4-phenylene vinylene) may be used.

For example, light emission of orange to red is caused by 4- (dicyanomethylene) -2,6-bis [p- (dimethylamino) styryl] -4H-pyran (abbreviation: BisDCM), 4- (dicyanomethylene) -2,6- Bis [2- (julolidin-9-yl) ethynyl] -4H-pyran (abbreviation: DCM1), 4- (dicyanomethylene) -2-methyl-6- (9-julolidyl) ethynyl-4H-pyran (abbreviation: DCM2) ), Bis [2- (2-thienyl) pyridinato] acetylacetonatoiridium (Ir (thp) ₂ (acac)), bis (2-phenylquinolinato) acetylacetonatoiridium (Ir (pq) ₂ (acac)) , bis [2- (2'-benzothienyl) pyridinato -N, C ³ '] of (acetylacetonato) iridium (Ir (btp) ₂ (acac)) Used as a guest material, obtained by dispersing in a suitable host material. It can also be obtained from a metal complex such as bis (8-quinolinolato) zinc (abbreviation: Znq ₂ ) or bis [2-cinnamoyl-8-quinolinolato] zinc (abbreviation: Znsq ₂ ). A polymer such as poly (3-alkylthiophene) may also be used.

In addition, among the above-described light-emitting organic compounds, FIrpic, Ir (ppy) ₂ (acac), Ir (thp) ₂ (acac), Ir (pq) ₂ (acac), Ir (btp) ₂ (acac) It is preferable to use a phosphor such as A light-emitting element to which the present invention is applied has a large increase in power consumption because the amount of current increases with time. However, if these phosphors are used, power consumption can generally be reduced.

Note that in the above structure, a suitable host material may be any material that emits light with a shorter wavelength or has a larger energy gap than the light-emitting organic compound. Specifically, it can be appropriately selected from hole transport materials and electron transport materials represented by the examples described above. In addition, 4,4′-bis (N-carbazolyl) biphenyl (abbreviation: CBP), 4,4 ′, 4 ″ -tris (N-carbazolyl) triphenylamine (abbreviation: TCTA), 1,3,5- Tris [4- (N-carbazolyl) phenyl] benzene (abbreviation: TCPB) or the like may be used.

On the other hand, it is preferable to use a conductive material having a high work function as a material constituting the anode 301 in the light-emitting element. When light is extracted from the anode 301 side, a transparent conductive material such as indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), or indium tin oxide added with silicon oxide is used. Use it. If the anode 301 is to have a light-shielding property, the anode 301 is a laminated film of titanium nitride and a film mainly composed of aluminum in addition to a single layer film such as TiN, ZrN, Ti, W, Ni, Pt, or Cr. A three-layer structure of a titanium nitride film, a film containing aluminum as a main component, and a titanium nitride film can be used. Or the method of laminating | stacking the transparent conductive material mentioned above on reflective electrodes, such as Ti and Al, may be used.

Moreover, it is preferable to use a conductive material having a low work function as a material constituting the cathode 303. Specifically, an alkali metal such as Li or Cs, and an alkaline earth metal such as Mg, Ca, or Sr, In addition to these and alloys containing them (Mg: Ag, Al: Li, etc.), they can also be formed using rare earth metals such as Yb and Er. Further, another conductive material (such as aluminum) may be stacked on these conductive materials. Further, LiF, CsF, in the case of using an electron injecting layer such as CaF _2, Li ₂ O, may be used usual conductive thin film such as aluminum. When the cathode 303 side is the light extraction direction, an ultrathin film containing an alkali metal such as Li or Cs and an alkaline earth metal such as Mg, Ca or Sr, and a transparent conductive film (ITO, IZO, ZnO) Etc.) may be used. Or it is good also as a structure which laminated | stacked the layer which added the substance (alkali metal or alkaline-earth metal etc.) which shows donor property with respect to the electron transport material mentioned above, and a transparent conductive film (ITO, IZO, ZnO, etc.). Specifically, ITO may be stacked on a layer in which lithium as a donor is added to BCP or a layer in which lithium as a donor is added to Alq ₃ .

Note that in manufacturing the light-emitting element of the present invention described above, the stacking method of each layer in the light-emitting element is not limited. As long as lamination is possible, any method such as a vacuum deposition method, a spin coating method, an ink jet method, or a dip coating method may be selected.

[Embodiment 6]

One mode of a display device including a light emitting element and a monitor element is described with reference to FIG.

This display device includes a scanning line driving circuit 108, a data line driving circuit 109, and a pixel portion 111. In the pixel portion 111, pixels 110 including a switching transistor 106, a driving transistor 104, a capacitor 107, and a light emitting element 105 are arranged.

The data line driver circuit 109 includes a pulse output circuit 112, a first latch circuit 113, and a second latch circuit 114. In the data line driving circuit 109, when data is input to the first latch circuit 113, the second latch circuit 114 can output.

The pixel unit 111 includes scanning lines G1 to Gn connected to the scanning line driving circuit 108 and data lines D1 to Dm connected to the data line driving circuit 109. A scanning line G 1 to which a signal is input from the scanning line driver circuit 108 sends a signal to the gate of the switching transistor 106 of the pixel 110. The switching transistor 106 selected by the scanning line G1 is turned on, and the data signal output to the data signal line D1 by the second latch circuit 114 is written into the capacitor 107. The driving transistor 104 operates in accordance with the data signal written in the capacitor 107 to control the light emitting state or the non-light emitting state of the light emitting element 105. In other words, when the potentials of the power supply lines V1 to Vm are supplied to the light emitting element 105 through the driving transistor 104 which is turned on, an operation for setting the light emitting state is performed.

The number of monitor elements 102 can be selected as appropriate. There may be one monitor element or a plurality of monitor elements. The display device shown in FIG. 9 includes n (n> 1) monitor elements 102, which are provided in a number equal to one pixel column. By arranging n monitor elements 102, it is possible to average the characteristic variation in each monitor element.

The n monitor elements 102 shown in FIG. 9 are connected in parallel with the current source 101. While the light emitting element 105 takes a light emitting state or a non-light emitting state according to a data signal, the n monitoring elements 102 are driven at a constant current and are always lit. The potentials of the electrodes connected to the current source 101 of these n monitor elements 102 are detected, and the potentials are set to the power supply lines V1 to Vm by the voltage generation circuit 103. The voltage generation circuit 103 is composed of a voltage follower circuit.

According to this configuration, when the temperature of the display device changes while the n monitor elements 102 are driven at a constant current, the resistance values of the n monitor elements 102 change. As the resistance value changes, the potential between both electrodes of the n monitor elements 102 changes, and the potential can be detected by the voltage generation circuit 103. Accordingly, the change in the temperature of the display device can be reflected in the driving condition of the light emitting element 105. Further, when the light emission characteristics of the n monitor elements 102 change with time, the resistance values of the n monitor elements 102 change, and thus can be reflected in the driving conditions of the light emitting elements 105 in the same manner.

The pixel portion 111 can be configured by combining a plurality of light-emitting elements that exhibit different emission colors. For example, the pixel portion may be configured by combining light emitting elements exhibiting red (R), green (G), blue (B), or light emission colors close thereto. At that time, the n monitor elements 102 may be formed of a kind of light emitting element exhibiting red (R), green (G), blue (B), or a light emitting color close thereto. Similarly, the monitor element is composed of each light emitting element that exhibits red (R), green (G), blue (B), or an emission color close thereto. Further, the light-emitting element 105 that emits white light can be used. At that time, the n monitor elements 102 are similarly formed of white light emitting elements.

FIG. 10 illustrates another example that can be applied to the pixel 110 of the display device in FIG. 9. In the pixel 110 in FIG. 10A, the switching transistor 106 and the driving transistor 104 are provided with an erasing transistor 115 and an erasing gate line Ry. One of the light emitting elements 105 is connected to the driving transistor 104 and the other is connected to the counter power source 116. Since the erasing transistor 115 can forcibly create a state in which no current flows to the light-emitting element 105, the light-emitting period is the same as or immediately after the start of the data signal writing period without waiting for signal writing to the pixel 110. Can be provided. As a result, the duty ratio can be improved and the light emission and non-light emission periods are forcibly controlled, which is particularly suitable for displaying moving images.

In FIG. 10B, a transistor 118 and a transistor 119 are connected in series to function as a driving transistor. Further, a configuration of the pixel 110 provided with a power supply line Vax (x is a natural number, 1 ≦ x ≦ l) connected to the gate of the transistor 118 is shown. The power supply line Vax is connected to the power supply 117. In the pixel 110, the gate potential of the transistor 118 is fixed to a potential that operates in a saturation region by connecting the gate of the transistor 118 to the power supply line Vax having a constant potential. Since the transistor 119 operates in a linear region, a video signal including light emission or non-light emission information of the pixel 110 is input to its gate. Since the voltage between the source and the drain of the transistor 119 operating in the linear region is small, a slight variation in the voltage between the gate and the source of the transistor 119 does not affect the value of the current flowing through the light-emitting element 105. Accordingly, the value of the current flowing through the light emitting element 105 is determined by the transistor 119 operating in the saturation region. With the above structure, luminance unevenness of the light-emitting element 105 due to variation in characteristics of the transistor 119 can be improved and image quality can be improved.

As described above, this display device constitutes a compensation circuit for temperature and luminance deterioration by the current source, the monitor element, and the voltage generation circuit. That is, both the light emitting element and the monitor element equivalent to the light emitting element are operated under different driving conditions, and the ratio of the total amount of charge flowing through the light emitting element provided in the display unit and the monitor element has a certain relationship in consideration of luminance degradation. Can be controlled to meet.

[Embodiment 7]
One structural example of a display device using the light-emitting element described in Embodiment 6 will be described with reference to the drawings.

A pixel 110 illustrated in FIG. 11 illustrates a configuration including two transistors. In this pixel 110, a data line Dx (x is a natural number, 1 ≦ x ≦ m) and a scanning line Gy (y is a natural number, 1 ≦ y ≦ n) are provided so as to intersect with each other via an insulating layer. The pixel 110 includes a light emitting element 105, a capacitor 107, a switching transistor 106, and a driving transistor 104. The switching transistor 106 controls input of a video signal, and the driving transistor 104 controls light emission and non-light emission of the light emitting element 105. These transistors are field effect transistors, and for example, thin film transistors can be used.

The gate of the switching transistor 106 is connected to the scanning line Gy, one of the source and the drain is connected to the data line Dx, and further connected to the gate of the driving transistor 104. One of the source and the drain of the driving transistor 104 is connected to the second power supply line 121 through the power supply line Vx (x is a natural number, 1 ≦ x ≦ m), and the other is connected to the light emitting element 105. In the light emitting element 105, the other terminal not connected to the first power supply line 120 is connected to the second power supply line 121.

The capacitor 107 is provided between the gate and the source of the driving transistor 104. As the switching transistor 106 and the driving transistor 104, an n-channel type or a p-channel type can be selected. A pixel 110 illustrated in FIG. 11 illustrates a case where the switching transistor 106 is an n-channel type and the driving transistor 104 is a p-channel type. The potential of the first power supply line 120 and the potential of the second power supply line 121 are not particularly limited. Different potentials are set so that a forward voltage or a reverse voltage is applied to the light emitting element 105.

A plan view of such a pixel 110 is shown in FIG. A switching transistor 106, a driving transistor 104, and a capacitor 107 are provided. The first electrode 211 is one electrode of the light emitting element 105, and the light emitting element 105 connected to the driving transistor 104 is formed by stacking a light emitting layer thereon. In order to increase the aperture ratio, the capacitor 107 is provided so as to overlap with the power supply line Vx.

FIG. 13 shows a cross-sectional structure corresponding to the cutting line A-B-C shown in FIG. A switching transistor 106, a driving transistor 104, a light-emitting element 105, and a capacitor 107 are provided over a substrate 200 having an insulating surface such as glass or quartz. The switching transistor 106 is preferably a multi-gate in order to reduce off-state current. Various semiconductors can be used for forming the channel portion of the switching transistor 106 and the driving transistor 104. For example, an amorphous semiconductor containing silicon as a main component, a semi-amorphous semiconductor (also referred to as a microcrystalline semiconductor), or a polycrystalline semiconductor can be used. In addition, an organic semiconductor can also be used. The semi-amorphous semiconductor is formed using silane gas (SiH ₄ ) and fluorine gas (F ₂ ) or using silane gas and hydrogen gas. In addition, a polycrystalline semiconductor obtained by crystallizing an amorphous semiconductor formed by a physical film formation method such as a sputtering method or a chemical film formation method such as a vapor deposition method by irradiation with electromagnetic energy such as a laser beam may be used. it can. The gates of the switching transistor 106 and the driving transistor 104 are stacked in the order of tungsten nitride (WN) and tungsten (W) from the substrate side, molybdenum (Mo), aluminum (Al), molybdenum (Mo), or A structure in which molybdenum nitride (MoN) and molybdenum (Mo) are stacked in this order may be employed.

The wirings 204, 205, 206, and 207 connected to the source or drain of the switching transistor 106 and the driving transistor 104 are formed as a single layer or stacked layers using a conductive material. For example, a structure in which titanium (Ti), aluminum silicon (Al—Si), titanium (Ti), Mo, Al—Si, Mo, or MoN, Al—Si, and MoN are stacked in this order from the substrate side. These wirings 204, 205, 206, and 207 are formed on the first insulating layer 203.

The light emitting element 105 has a stacked structure of a first electrode 211 corresponding to a pixel electrode, a light emitting layer 212, and a second electrode 213 corresponding to a counter electrode. An end portion of the first electrode 211 is surrounded by a partition wall layer 210. The light emitting layer 212 and the second electrode 213 are stacked so as to overlap the first electrode 211 at the opening of the partition wall layer 210. This overlapping portion becomes the light emitting element 105. When both the first electrode 211 and the second electrode 213 have translucency, the light emitting element 105 emits light in a direction toward the first electrode 211 and in a direction toward the second electrode 213. That is, the light emitting element 105 is configured to emit light in both directions. In addition, when one of the first electrode 211 and the second electrode 213 has a light-transmitting property and the other has a light-shielding property, the light-emitting element 105 is directed toward the first electrode 211 or toward the second electrode 213. Emits light. That is, the light emitting element 105 performs top emission or bottom emission.

FIG. 13 illustrates a cross-sectional structure in the case where the light emitting element 105 performs bottom emission. The capacitor 107 is disposed between the gate and the source of the driving transistor 104 and holds a voltage between the gate and the source. The capacitor 107 includes a semiconductor layer 201 provided in the same layer as the semiconductor layer that forms the switching transistor 106 and the driving transistor 104, and a conductive layer provided in the same layer as the gates of the switching transistor 106 and the driving transistor 104. A capacitor is formed by the layers 202a and 202b (hereinafter collectively referred to as the conductive layer 202) and an insulating layer therebetween.

The capacitor 107 includes a conductive layer 202 provided in the same layer as the gates of the switching transistor 106 and the driving transistor 104, and wirings 204 and 205 connected to the sources and drains of the switching transistor 106 and the driving transistor 104. , 206, and 207, a capacitor is formed by the wiring 208 provided in the same layer and an insulating layer therebetween. As a result, the capacitor 107 can obtain a sufficient capacity to hold the gate-source voltage of the driving transistor 104. In addition, the capacitor 107 is formed so as to overlap with a conductive layer included in the power supply line, so that a reduction in aperture ratio due to the arrangement of the capacitor 107 is suppressed.

The thicknesses of the wirings 204, 205, 206, 207, and 208 connected to the sources or drains of the switching transistor 106 and the driving transistor 104 are 500 to 2000 nm, preferably 500 to 1300 nm. Since the wirings 204, 205, 206, 207, and 208 constitute the data line Dx and the power supply line Vx, as described above, the wirings 204, 205, 206, 207, and 208 are made thicker. , The influence of the voltage drop can be suppressed.

The first insulating layer 203 and the second insulating layer 209 are formed using an inorganic material such as silicon oxide or silicon nitride, an organic material such as polyimide or acrylic, or the like. The first insulating layer 203 and the second insulating layer 209 may be formed using the same material or different materials. As the organic material, a siloxane-based material may be used. For example, a skeleton structure is formed by a bond of silicon and oxygen, and an organic group (for example, an alkyl group or aromatic hydrocarbon) containing at least hydrogen as a substituent is used. It is done. 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.

[Embodiment 8]
A panel mounted with the pixel portion 111, the scan line driver circuit 108, and the data line driver circuit 109, which is one embodiment of the display device of Embodiment 611, will be described. Over the substrate 200, a pixel portion 111 including a plurality of pixels including the light-emitting element 105, a scan line driver circuit 108, a data line driver circuit 109, and a connection film 217 are provided (see FIG. 14A). The connection film 217 is connected to an external circuit.

14B is a cross-sectional view taken along a line AB of the panel, and shows a driving transistor 104, a light-emitting element 105, a capacitor 107, and a transistor provided in the data line driver circuit 109 provided in the pixel portion 111. . A sealant 214 is provided around the pixel portion 111, the scan line driver circuit 108, and the data line driver circuit 109, and the light-emitting element 105 is sealed with the sealant 214 and the counter substrate 216. This sealing process is a process for protecting the light-emitting element 105 from moisture. Here, a method of sealing with a cover material (glass, ceramics, plastic, metal, or the like) is used, but a thermosetting resin or ultraviolet light is used. A method of sealing with a curable resin or a method of sealing with a thin film having a high barrier ability such as a metal oxide or a nitride may be used. The element formed over the substrate 200 is preferably formed using a crystalline semiconductor (polysilicon) having favorable characteristics such as mobility as compared to an amorphous semiconductor. Realized. Since the number of external ICs to be connected is reduced, the panel having the above structure can be small, light, and thin.

14B, the first electrode 211 of the light-emitting element 105 has a light-transmitting property and the second electrode 213 has a light-blocking property. Accordingly, the light emitting element 105 emits light toward the substrate 200 side. As shown in FIG. 15A, as a structure different from the above, the first electrode 211 of the light-emitting element 105 can have a light-blocking property and the second electrode 213 can have a light-transmitting property. In this case, the light emitting element 105 performs top emission. Further, as shown in FIG. 15B, as a configuration different from the above, both the first electrode 211 and the second electrode 213 of the light-emitting element 105 are used as translucent electrodes to emit light from both sides. You can also In these embodiments, the monitor element may be provided with the same structure as the light emitting element.

Note that the pixel portion 111 may be formed using a transistor using an amorphous semiconductor (amorphous silicon) formed over an insulating surface as a channel portion, and the scan line driver circuit 108 and the data line driver circuit 109 may be formed using driver ICs. Good. The driver IC may be mounted on the substrate 200 by the COG method, or may be mounted on the connection film 217 connected to the substrate 200. An amorphous semiconductor can be easily formed on a large-area substrate by using the CVD method and does not require a crystallization step, so that an inexpensive panel can be provided. At this time, if a conductive layer is formed by a droplet discharge method typified by an ink jet method, a cheaper panel can be provided.

[Embodiment 9]
In this embodiment, various electric appliances completed using the light-emitting device of the present invention will be described with reference to FIGS.

As electric appliances manufactured using the light emitting device of the present invention, a television, a video camera, a digital camera, a goggle type display (head mounted display), a navigation system, a sound reproduction device (car audio, audio component, etc.), a personal computer A video game device, a portable information terminal (mobile computer, mobile phone, portable game machine, electronic book, etc.), an image playback device equipped with a recording medium (specifically, a recording medium such as a digital video disc (DVD)) , A device provided with a display device capable of displaying the image), a lighting fixture, and the like. Specific examples of these electric appliances are shown in FIGS.

FIG. 16A illustrates a display device, which includes a housing 9001, a support base 9002, a display portion 9003, speaker portions 9004, a video input terminal 9005, and the like. The display device 9003 is manufactured using a light-emitting device formed using the present invention. The display device includes all information display devices such as a computer, a TV broadcast receiver, and an advertisement display.

FIG. 16B illustrates a personal computer, which includes a main body 9101, a housing 9102, a display portion 9103, a keyboard 9104, an external connection port 9105, a pointing mouse 9106, and the like. It is manufactured by using a light-emitting device having the light-emitting element of the present invention for the display portion 9103.

FIG. 16C illustrates a video camera, which includes a main body 9201, a display portion 9202, a housing 9203, an external connection port 9204, a remote control reception portion 9205, an image receiving portion 9206, a battery 9207, an audio input portion 9208, operation keys 9209, and an eyepiece. Part 9210 and the like. It is manufactured by using a light emitting device having a light emitting element of the present invention for the display portion 9202.

FIG. 16D illustrates a table lamp, which includes a lighting portion 9301, an umbrella 9302, a variable arm 9303, a column 9304, a base 9305, and a power source 9306. A light-emitting device formed using the light-emitting element of the present invention is used for the lighting portion 9301. The lighting fixture includes a ceiling-fixed lighting fixture or a wall-mounted lighting fixture. Such a luminaire is required to have high luminance, and thus is a preferable form as an application example of the light emitting device of the present invention. In addition, unlike a display, a lighting fixture can be composed of at least one light-emitting element, so that the current density of the light-emitting element can be controlled by a program of mathematical formulas such as formulas (1) to (3), There is also an advantage that it is very easy to control the voltage of the light emitting element by the program of the mathematical formula as in 4).

Here, FIG. 16E illustrates a mobile phone, which includes a main body 9401, a housing 9402, a display portion 9403, an audio input portion 9404, an audio output portion 9405, operation keys 9406, an external connection port 9407, an antenna 9408, and the like. It is manufactured by using a light emitting device having a light emitting element of the present invention for the display portion 9403.

As described above, an electric appliance or a lighting fixture using the light emitting element of the present invention can be obtained. The applicable range of the light-emitting device having the light-emitting element of the present invention is so wide that the light-emitting device can be applied to electric appliances in various fields.

In this example, an example in which a light-emitting element with low luminance deterioration is manufactured by using the formula (1) described in Embodiment Mode 1 is specifically illustrated.

First, a light-emitting element using a light-emitting organic compound was manufactured. Since the element structure is as shown in FIG. 3, description will be made with reference to the reference numerals in FIG.

First, the anode 301 is formed over the substrate 300 having an insulating surface. ITO, which is a transparent conductive film, was used as a material, and a film thickness of 110 nm was formed by a sputtering method. The shape of the anode 301 was 2 mm × 2 mm.

After the substrate on which the anode 301 is thus formed is washed and dried, a light emitting layer 302 is formed on the anode 301. First, the substrate on which the anode 301 is formed is fixed to the substrate holder of the vacuum evaporation apparatus with the surface on which the anode 301 is formed facing downward, and the DNTPD is formed to a thickness of 50 nm by a vacuum evaporation method using a resistance heating method. A film was formed. This becomes the hole injection layer 311. Next, α-NPD, which is a hole transporting material, was formed to a thickness of 10 nm by the same method to form a hole transport layer 312.

Further, Alq ₃ and coumarin 6 were co-evaporated to form a layer 313 containing a light-emitting organic compound with a thickness of 37.5 nm. In the co-evaporation, the ratio of Alq ₃ and coumarin 6 was adjusted to be 1: 0.005 by mass ratio. Therefore, Alq ₃ acts as a host material and coumarin 6 acts as a light-emitting organic compound.

Next, Alq ₃ which is an electron transporting material was formed to 37.5 nm by a vacuum vapor deposition method to form an electron transport layer 314. Further, 1 nm of CaF ₂ was formed as the electron injection layer 315 by vacuum deposition. The above is the light emitting layer 302.

Finally, the cathode 303 is formed. In this example, aluminum (Al) was formed to 150 nm by a vacuum evaporation method using resistance heating to form the cathode 303.

In this example, first, the luminance for practical use was set to 1000 cd / m ² . In addition, when current was applied to the light-emitting element manufactured in this example to emit light, the current density required to emit light with a luminance of 1000 cd / m ² was 9.25 mA / cm ² .

Accordingly, first, a constant current driving test with an initial luminance of 1000 cd / m ² was performed by continuously supplying a constant current to the light emitting element at a current density of 9.25 mA / cm ² . The luminance degradation curve at that time is shown in FIG. The solid line in the figure is actual data, the horizontal axis represents time, and the vertical axis represents relative luminance (corresponding to L / L ₀ where the initial luminance is L ₀ and the luminance is L).

Next, the obtained data was fitted by the equation (6). The result is a dotted line in FIG. 4, and it can be seen that the fitting is very accurate. From this fitting, the values of parameters k and β in equation (6) were obtained. The results are shown in Table 1 below.

Therefore, from the formula (1), when the light emitting device of this example emits light with the initial luminance of 1000 cd / m ² , the luminance degradation is caused by increasing the current density J with respect to the light emission time t according to the following formula (1 ′). Can be obtained. In addition, when Formula (1 ') is graphed, it will become like FIG. 5 (a horizontal axis is light emission time t [h] and a vertical axis | shaft is current density J [mA / cm < ² >]).

J = 9.25 · exp [(0.00018414 · t) ^0.6531 ] (1 ′)
(J represents current density [mA / cm ² ], t represents light emission time [h])

  In this example, a specific example of manufacturing a light-emitting element with low luminance deterioration by using the formula (2) described in Embodiment Mode 2 will be specifically described.

As the light emitting element, the same one as in Example 1 was used. Therefore, the values of parameters k and β are the same as in Table 1. Since k = k ′ · J ₀ , the parameters k ′ and β in Equation (2) are as shown in Table 2 below.

Therefore, from the equation (2), when the light emitting device of this example emits light with an initial luminance of 1000 cd / m ² , luminance degradation is caused by increasing the current density J with respect to the emission time t according to the following equation (2 ′). Can be obtained.

J = 9.25 · exp [(0.0000199 · ∫Jdt) ^0.6531 ] (2 ′)
(J represents current density [mA / cm ² ], t represents light emission time [h])

In this example, a specific example of manufacturing a light-emitting element with low luminance deterioration by using the formula (3) described in Embodiment Mode 3 will be described.

As the light emitting element, the same one as in Examples 1 and 2 was used. Therefore, the values in Table 1 of Example 1 may be used for the parameters k and β.

Therefore, from the formula (3), when the light emitting device of this example emits light with an initial luminance of 1000 cd / m ² , the current density increase rate γ increases with respect to the emission time t according to the following formula (3 ′). Thus, a light-emitting element with low luminance deterioration can be obtained. In addition, when Expression (3 ′) is graphed, it is as shown in FIG. 6 (the horizontal axis is the light emission time t [h] and the vertical axis is the current density increase rate γ (= J / J ₀ ) [−]) .

γ = exp [{0.00009207 · (γ + 1) · t} ^0.6531 ] (3 ′)
(Γ represents a current density increase rate [−], and t represents a light emission time [h].)

In this example, an example in which a light-emitting element with low luminance deterioration is manufactured by using the formula (4) described in Embodiment Mode 4 is specifically illustrated. As a light emitting element, the same one as in Examples 1 to 3 was used.

First, in this light emitting element, A.I. B. Before turning on continuously. After storage without lighting for 1000 hours, C.I. The current was continuously supplied at a constant current density of 9.25 mA / cm ² to light for 1000 hours, and then the voltage-current characteristics were measured for each. The result is shown in FIG. The horizontal axis is voltage, and the vertical axis is current density. As shown in FIG. 7A, not only after lighting (C.) but also after storing without lighting (B.), it was difficult for current to flow.

Next, in the practical luminance region (100 to 10000 cd / m ² ; current density of 1 to 100 mA / cm ² ), the data of FIG. 7A was fitted using the equation (22). The result is shown in FIG. From FIG. 7B, it can be seen that the voltage-current density characteristics of the light-emitting elements are respectively fitted with straight lines with high accuracy according to the equation (22).

Further, from the fitting of FIG. , B. , C.I. For each, the values of S and n in equation (22) were obtained. The results are shown in Table 3 below. FIG. 8 is a graph of Table 3.

First, as can be seen from FIG. 8, n decreases even if it is stored without passing a current for 1000 hours, and the rate of decrease is almost the same as when the light is turned on with a current flowing for 1000 hours. That is, it is a parameter that decreases almost only with the passage of time regardless of whether or not current is passed. That is, n can be expressed as a function of the storage time t ′ (n = f (t ′)). Therefore, f (t ') can be obtained by conducting a similar experiment for storage times other than 1000 hours and plotting the value of n against the storage time t'.

On the other hand, as can be seen from FIG. 8, S is a parameter that hardly changes when stored for only 1000 hours, but decreases for the first time when a current is passed. Regardless of the time, depending on the current flow is expected to be a function of the total charge Q per unit area that has flowed (S = g (Q)). In this embodiment, since driving is performed at a constant current density of 9.25 mA / cm ² for 1000 hours, Q = 33300 [C / cm ² ]. Therefore, the same experiment is performed when the total charge amount per unit area other than 33300 [C / cm ² ] is flowed, and the value of S with respect to the total charge amount Q per unit area is plotted. Q) can be determined.

By substituting f (t ′) and g (Q) obtained as described above into Equation (4), a voltage is applied to the light emitting element driven with a duty ratio n of 0 <n <100 according to Equation (4). By applying the current density, the current density flowing in the light emitting element driven at a duty ratio n of 0 <n <100 gradually increases, and as a result, the luminance degradation can be reduced.

The figure which shows how to obtain | require the total electric charge amount per unit area. FIG. 11 shows a voltage-current density curve of a light-emitting element. The figure which shows the element structure of a light emitting element. FIG. 9 shows a luminance deterioration curve of a light-emitting element. FIG. 3 is a graph showing a change with time in current density in Example 1; The figure which shows the time-dependent change of the increase rate of the current density in Example 3. FIG. The figure which shows the voltage-current density curve in Example 4. FIG. The figure which shows the change of S and n in Example 4. FIG. FIG. 11 illustrates a structure of a display device including a monitor element and a light-emitting element according to the present invention. FIG. 9 illustrates a structure of a pixel circuit that can be used in a display device of the present invention. FIG. 14 illustrates an example of a circuit of a pixel in a display device of the present invention. FIG. 6 illustrates an example of a pixel in a display device of the present invention. The longitudinal cross-sectional view which shows the example of 1 structure of the display part in the display apparatus of this invention. FIG. 14 is a diagram showing a structural example of a display portion, a scan line driver circuit, and a data line driver circuit in the display device of the present invention. FIG. 14 is a diagram showing a structural example of a display portion, a scan line driver circuit, and a data line driver circuit in the display device of the present invention. An electric appliance to which the light emitting device of the present invention is applied.

Explanation of symbols

101 current source 102 monitor element 103 voltage generation circuit 104 driving transistor 105 light emitting element 106 switching transistor 107 capacitor element 108 scanning line driving circuit 109 data line driving circuit 110 pixel 111 pixel unit 112 pulse output circuit 113 first latch circuit 114 first 2 latch circuit 115 erasing transistor 116 opposing power supply 117 power supply 118 transistor 119 transistor 120 first power supply line 121 second power supply line

Claims (12)

A light-emitting device comprising: a light-emitting element having a light-emitting layer between an anode and a cathode; and means for increasing a current density J of a current flowing through the light-emitting element over time according to the following formula (1).
_{J = J 0 · exp [(} k · t) β] ··· (1)
(J ₀ is the initial current density in the light emitting element, t is the light emission time, and k and β are positive parameters determined by the characteristics of the light emitting element.) A light emitting device comprising: a light emitting element having a light emitting layer between an anode and a cathode; and means for increasing a current density J of a current flowing through the light emitting element over time according to the following formula (2).
J = J ₀ · exp [(k ′ · ∫Jdt) ^β ] (2)
(J ₀ represents the initial current density in the light emitting element, t represents the light emission time, k ′ and β represent positive parameters determined by the characteristics of the light emitting element, and ∫ represents the integral from 0 to t. .) A light-emitting device having a light-emitting element having a light-emitting layer between an anode and a cathode, and means for controlling an increase rate γ of a current density of a current flowing through the light-emitting element according to the following formula (3).
γ = exp [{(γ + 1) · k · t / 2} ^β ] (3)
(T represents the light emission time, k and β represent positive parameters determined by the characteristics of the light emitting element. Also, assuming that the initial current density in the light emitting element is J ₀ and the current density flowing in the light emitting element is J. , is γ = J / J _0.) A light emitting element having a light emitting layer between an anode and a cathode, a first means for driving the light emitting element with a duty ratio n of 0 <n <100, and a voltage V of the light emitting element is expressed by the following formula (4) And a second means for increasing in accordance with the light emitting device.
V = {J ₀ / g (Q ₁₀₀ )} ^{1 / f (t ′)} (4)
(J ₀ represents the initial current density in the light emitting element. F (t ′) is a monotonically decreasing function with the storage time t ′ as a variable. Also, g (Q) represents the light emitting element. A monotonically decreasing function with the total charge amount Q per unit area flowing as a variable, and Q ₁₀₀ per unit area flowing when the light emitting element is driven at a constant current with a duty ratio of 100 and a current density J ₀ . (It is a total charge amount, and when the driving time of the light emitting element is t ″, it is expressed by Q ₁₀₀ = J ₀ · t ″.) 5. The light emitting device according to claim 4, wherein the second means includes a monitor element, a constant current source for supplying a constant current having a current density of J ₀ to the monitor element, and a voltage applied to the monitor element. A light-emitting device having an operational amplifier for applying to the element.   The light-emitting device according to claim 1, wherein the light-emitting layer includes a light-emitting organic compound.   The light emitting device according to claim 6, wherein the light emitting organic compound is a phosphorescent material.   The lighting fixture using the light-emitting device as described in any one of Claims 1 thru | or 7. A driving method of a light emitting element, in which a current density J of a current flowing through the light emitting element is increased with time according to the following formula (1).
_{J = J 0 · exp [(} k · t) β] ··· (1)
(J ₀ is the initial current density in the light emitting element, t is the light emission time, and k and β are positive parameters determined by the characteristics of the light emitting element.) A driving method of a light emitting element, in which a current density J of a current flowing through the light emitting element is increased with time according to the following formula (2).
J = J ₀ · exp [(k ′ · ∫Jdt) ^β ] (2)
(J ₀ represents the initial current density in the light emitting element, t represents the light emission time, k ′ and β represent positive parameters determined by the characteristics of the light emitting element, and ∫ represents the integral from 0 to t. .) A driving method of a light emitting element that controls an increase rate γ of a current density of a current passed through the light emitting element according to the following formula (3).
γ = exp [{(γ + 1) · k · t / 2} ^β ] (3)
(T represents the light emission time, k and β represent positive parameters determined by the characteristics of the light emitting element. Also, assuming that the initial current density in the light emitting element is J ₀ and the current density flowing in the light emitting element is J. , is γ = J / J _0.) A driving method of a light emitting element, wherein the light emitting element is driven at a duty ratio n of 0 <n <100, and the voltage V of the light emitting element is increased according to the following formula (4).
V = {J ₀ / g (Q ₁₀₀ )} ^{1 / f (t ′)} (4)
(J ₀ represents the initial current density in the light emitting element. F (t ′) is a monotonically decreasing function with the storage time t ′ as a variable. Also, g (Q) represents the light emitting element. A monotonically decreasing function with the total charge amount Q per unit area flowing as a variable, and Q ₁₀₀ per unit area flowing when the light emitting element is driven at a constant current with a duty ratio of 100 and a current density J ₀ . (It is a total charge amount, and when the driving time of the light emitting element is t ″, it is expressed by Q ₁₀₀ = J ₀ · t ″.)

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