JPH03245490A - Organic membrane luminous element - Google Patents

Abstract

PURPOSE:To obtain an organic membrane luminous element with memory function by giving electron pouring property to one side electrode holding organic membranes, and giving hole pouring property to the other side electrode. CONSTITUTION:Of the electrodes to hold organic membranes 4 and 3, one side electrode 5 is given an electron pouring property, and the other side electrode 2 is given a hole pouring property. As a result, when a bias current is applied, electrons and holes are poured to the organic membranes 4 and 3 respectively and simultaneously, and they are accumulated. And, in the condition to generate a carrier recombination, the charging condition of the organic membranes is neutralized, and thereby the carrier movable degree is increased. In other words, a conductivity modulation is generated in the organic membranes to reduce the resistance, and a large current flows. As a result, a hysteresis property is generated in a constant voltage driving, while a negative resistance property is generated in a constant current driving, in the electrical property, and a memory function is generated in the luminous property.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a light emitting device using an organic film containing an organic dye, and particularly to an organic film light emitting device exhibiting memory characteristics.

(Prior Art) In recent years, research and development of organic film light emitting devices used as display devices, lighting devices, etc. has been actively conducted. For example, Shogo Saito of Bonyo University reported in 1986 an organic two-layer structure device using a metal electrode/aromatic dye/polythiophene/transparent electrode (J, J, AI) pl, Phys.
, 25. L773.198B). Here, the thickness of the organic film is 1 μm or more, and the applied voltage is as large as 100V. On the other hand, Kodak's C, W, Tang, etc.
By using an organic two-layer structure of Mg-Ag/AIq3/diamine/ITO and reducing the thickness of the organic film to 1000 Å or less, an element that can be driven at an applied voltage of 10 V or less and exhibits sufficient brightness for practical use was obtained. is reported (A
P.L.

5], 913.1987). These light emitting elements are
The basic idea is to combine an electron-injecting dye and a hole-injecting dye to form an organic two-layer structure, make the organic film as thin as possible, and use a metal electrode with a small work function on the electron-injecting side. The main characteristics include selecting a material that does not cause electrical defects when forming an organic film by vacuum evaporation or sublimation. Furthermore, in 1988, Shogo Saito of Bonyo University proposed an organic three-layer structure element consisting of an electron injection layer, a light emitting layer, and a hole injection layer, and achieved high brightness by selecting a dye with high photoluminescence for the light emitting layer. It was shown that luminescence could be obtained (J, J
, Apl)1. Phys, , 27. L2B9.
1988).

In addition, we have discovered that light-emitting device structures are formed by combining various organic films, that even a single-layer organic film can emit light to some extent by mixing a luminescent agent and a hole-injecting agent, and that Studies on the deterioration of the characteristics of A iq3, which is a chemical compound, have been reported one after another, and many similar patent applications have been filed.

Incidentally, flat panel displays, which have recently been used in various electronic devices, have a large number of display pixels arranged in a matrix and are driven in a time-division manner. There are two types of methods: a simple matrix method and an active matrix method, but in either case, the driving voltage applied to one pixel is in the form of a pulse, and the voltage application time is very short. Therefore, in order to obtain a high-quality image, each pixel needs to have a memory function to some extent. For example, in a liquid crystal display, this memory function is provided by the capacity of the liquid crystal itself or by the capacity provided in parallel with the liquid crystal.

Organic film light-emitting devices also need to have a memory function when applied to such matrix-driven flat panel displays, etc., but so far there has been no information on such memory functions for organic film light-emitting devices. No report has been made.

(Problems to be Solved by the Invention) As described above, research on organic film light-emitting devices is being actively conducted in various places, but there is no mention at all of the memory function desired as a matrix-driven display device.

The present invention has been made in view of these points, and an object of the present invention is to provide an organic film light emitting device with a memory function.

[Structure of the invention]

(Means for Solving the Problems) The organic film light emitting device according to the present invention has an organic film containing at least one kind of organic dye between an electron injection first electrode and a hole injection second electrode. It has a sandwiched structure, and the first. Second
When a positive bias is applied to the second electrode between the electrodes, electrons injected from the first electrode and holes injected from the second electrode are accumulated in the organic film, and the memory It is characterized by emitting light.

(Function) When electron-injecting electrodes are provided on both surfaces of a luminescent organic film, only electrons are injected into the organic film by applying a bias. Organic materials used in light emitting devices have a small carrier mobility of at most 1O-3CII+2/v-5ec, and the injected electrons form a space charge, but the current that flows at this time is an ohmic current (depending on the voltage) in a low electric field. (proportional), and at high electric fields there is a space charge limited current (proportional to the square of the voltage) limited by the internal electric field.

There is no memory function in this case.

In contrast, in the present invention, one electrode sandwiching the organic film has an electron injection property, and the other electrode has a hole injection property. Then, when a bias is applied, electrons and holes are simultaneously injected into the organic film and accumulated. In a state where carrier recombination occurs, the charge state of the organic film becomes neutral, thereby increasing carrier mobility. That is, so-called conductivity modulation occurs within the organic film, resulting in low resistance and a large current flowing. As a result, in the electrical characteristics of the device, a hysteresis characteristic occurs when driven at a constant voltage, a negative resistance characteristic occurs when driven at a constant current, and as a result, a memory function occurs in the light emitting characteristics. Furthermore, this memory function appears as an afterimage phenomenon.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a cross-sectional structure of a light emitting device according to one embodiment.

This element has a first electrode (Ml) when viewed from above.
4) NJliilli (01) 4. 2nd (7)G
It is composed of a seven-layer membrane (02)3 and a second electrode ll2)2. In this embodiment, the second electrode 2 is a transparent electrode made of ITO or the like formed on the glass substrate 1, and light is extracted from the substrate 1 side. A compound semiconductor may be used as the transparent electrode. The manufacturing process for this element will be explained in detail later, but films are sequentially formed on a substrate by vacuum evaporation, vacuum sublimation, or the like.

FIG. 2 shows a band diagram in a state where each layer constituting this light emitting element is independent. The conduction band level of the first organic film 4 is E. 0. The Fermi level is E11, the valence band level is Ev, and the conduction band level of the second organic film 3 is E.

2. When the Fermi level is E2 and the valence band level is Ev2, as shown in the figure, ECI>EC21EVI>Ev
Two materials have been selected.

Further, the first electrode 5 has a work function EMI such that EMI<E,
The relationship is selected so that electrons can be easily injected into the first organic film 4. The second electrode 2 has a work function EM2 of Ev2>E2, and is selected so that holes can be easily injected into the second organic film 3.

FIG. 3 is a band diagram of a light emitting device in which these layers are bonded together in a thermal equilibrium state. In thermal equilibrium, the Fermi levels of the system coincide. Therefore, from the magnitude relationship between the work function of the electrode and each energy level of the organic film shown in FIG. 2, as shown in FIG.
A junction is formed between which electrons can be easily injected from the first electrode 5.

A junction is formed between the second electrode 2 and the second organic film 3 into which holes can be easily injected from the second electrode 2. First organic film 4
and the second organic film 3, there is a ΔE (=Ec+
A barrier called EC2 is formed, and ΔEv=E in the valence band.
A barrier of v+Ev2 is formed.

The operating principle of the light emitting device of this example will be explained with reference to FIG. FIG. 4(a) is a band diagram of the device when a positive bias voltage V is applied to the second electrode 2 with respect to the first electrode 5. From the first electrode 5, the first organic film 4
Electrons are injected into the second organic film 3 from the second electrode 2.
Holes are injected into the first . Second
is accumulated at the barrier junction interface of the organic film 3.4. This accumulated carrier will form an electric double layer.

Since the thickness of this electric double layer is equal to the intermolecular distance of the dye (approximately 10 people), a large electric field is generated here as a result. Then, as shown in FIG. 4(b), when the 1 Iass voltage exceeds a certain threshold value and reaches V2, carriers forming the electric double layer are tunnel-injected into the adjacent layer through the barrier junction. The holes injected from the second organic film 3 into the first organic film 4 recombine with electrons, which are majority carriers, within the first organic film 4 . When the first organic film 4 is a light emitting layer, light emission at the first wavelength λ1 is thereby obtained.

The electric charge r injected from the first organic film 4 to the second organic film 3 recombines with holes, which are majority carriers, within the second organic film 3. When the second organic film 3 is a light emitting layer, light emission at the second wavelength λ2 is thereby obtained.

Which of the first wavelength light emission and the second wavelength light emission is dominant depends on the first. Barrier height ΔEC for electrons and barrier height Δ for holes of the barrier junction of the second organic films 4 and 3
Determined by the relationship between Ev.

By selecting the material you want, ■ the first . A light-emitting element that can simultaneously emit light of a second wavelength; ■ A light-emitting element that emits only the first wavelength light at the first threshold and multiple light emission at the second threshold; With this value, it is possible to obtain a light-emitting element that emits light only at the second wavelength and can obtain multiple light emission at the second threshold value.

Figures 5(a) and 5(b) show that Eel EC2<EVI
FIG. 3 is a band diagram for explaining the operation of a light emitting element whose material is selected to satisfy Ev2. As is clear from the previous explanation of the principle, electrons and holes are injected by the bias voltage V to form an electric double layer, but when the bias voltage V exceeds the first threshold value v thi, as shown in FIG. (
As shown in a), electrons are tunnel-injected from the first organic film into the second organic film, and are radiatively recombined in the second organic film to generate light with a wavelength λ2. If the bias voltage ■ is further increased to exceed the second threshold value V th2, as shown in Fig. 5 (
As shown in b), tunnel injection of holes from the second organic film to the first organic film also begins, and the first organic film emits light and recombines, and the light emission of wavelength λ1 overlaps.

FIG. 6(a) and (b) show that E c+−E C2>E
FIG. 3 is a band diagram for explaining the operation of a light emitting element whose material is selected to satisfy vlE y2. In this case, the fifth
Contrary to the figure, light emission (wavelength λ1) occurs in the first organic film at the first threshold value v tht, and at the second threshold value V th2
The light emission (wavelength λ2) from the second organic film overlaps. Note that 1. in Figure 6. second threshold vtht, vth
2. Wavelength λ1. λ2 are generally not the same as those in FIG.

According to this embodiment, the first. As a result of injecting electrons and holes into the second organic film 43 and accumulating them in the barrier junction, a memory function appears in the light emission characteristics.

FIG. 7 shows the voltage-current characteristics when the device of this example is driven at a constant voltage, and hysteresis characteristics appear as a result of the simultaneous injection effect of electrons and holes and the memory function due to carrier accumulation. Similarly, FIG. 8 shows voltage-current characteristics when the device of this embodiment is driven at a constant current, and a negative resistance characteristic appears as a result of the memory function. The current on the vertical axis in these figures corresponds to the luminance.

It can be confirmed by the following experiment that the device of this example has the above-mentioned memory function.

First, when the DC electrical characteristics of the element are measured, rectification characteristics are obtained as shown in FIG. The polarity is negative on the first electrode 5 side or the forward direction. As mentioned above, the forward current flows by tunnel injection due to the formation of an electric double layer.

The reason why no current flows under reverse bias is because there is no carrier injection from the picture electrode.

Next, a triangular AC bias is applied to this element and the displacement current is measured. The measurement region is a minute current region near the origin of the voltage-current characteristics, as shown by the broken line in FIG. 10th
The figure shows the potential-current characteristics obtained. First, in the reverse bias region, that is, on the right side of point A in FIG. 10, carrier injection does not occur in the organic film as described above, so some charge is accumulated between the two electrodes. At this time, the space between the two electrodes is the same as a capacitor in which two layers of organic films as insulators are sandwiched, and therefore the capacitance is small. As a result, the displacement current is also small. Displacement current I is generally expressed as I-C-dV/d t with respect to capacitance C, and in this case, capacitance C is 1. second organic film 4,
This is because the series capacitance is 3.

Next, when the bias approaches zero bias from the reverse bias region, carriers begin to flow from the electrode to the organic film 4. In general, electron injection from the first electrode 5 to the first organic film 4 and second
Hole injection from the electrode 2 to the second organic film 3 does not occur at the same time, so in this case, if the electron injection occurs first, then at point A in FIG. First organic film 4
The injection of electrons into the cell begins. The injected electrons are the first. Second
is accumulated at the barrier junction interface between the organic films 4 and 3 to form a barrier junction. At this time, the capacitance becomes approximately twice the value determined by the second organic film 3, and therefore the displacement current also increases approximately twice.

When the bias increases and hole injection from the second electrode 2 to the second organic film 3 begins, an electric double layer is formed at the interface of the organic film as described above, and the capacitance becomes the capacitance of that double layer. becomes extremely large. Therefore, as shown in FIG. 10, the displacement current increases significantly at point B where hole injection begins. A large forward current flows due to tunnel injection, and light emission is observed.

By measuring the displacement current characteristics in this way, the state of carrier injection and accumulation in the device of this example can be determined.

Next, a more specific example will be described. In the device shown in FIG. 1, first electrode 5: ytterbium first organic film 4: bi(9-malononitrilefluorenyl) second organic film 3: bipyrenyl (-light emitting layer) second electrode 2
:ITO was used.

A 1,000-bibyrenyl thin film was formed on an ITO substrate by vacuum sublimation method (vacuum degree ~ 10 Torr), and then
10 ill of 9-malononitrilefluorenyl) thin film
0 people were formed, and finally 1000 ytterbium electrodes were formed by direct air deposition.

It was confirmed that the displacement current characteristics of the obtained element were as shown in FIG. Furthermore, when a positive forward bias was applied to this element on the ITO electrode side, the hysteresis characteristic shown in FIG. 11 was obtained. Furthermore, as a result of applying a pulsed bias that was positive on the ITO'4 pole side, an afterimage effect was observed in the emission intensity as shown in FIG.

The present invention is not limited to the above embodiments.

In the examples, a two-layer stacked structure of organic films was used, but a stacked structure of two or more layers can also be used. In short, it is sufficient that the six-layer membrane and the electrodes sandwiching it satisfy the condition that electrons and holes are simultaneously injected into the organic membrane and only these are accumulated. Electrodes for electron injection include ytterbium, lanthanum (La), and neodymium (Nd).
), gadolinium (Gd) and other rare earth elements are preferred.

[Effects of the Invention] As described above, according to the present invention, it is possible to obtain an organic film light emitting device whose light emitting characteristics have a memory function by utilizing both carrier injection and accumulation of electrons and holes.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing an organic film light emitting device according to an embodiment of the present invention, FIG. 2 is a band diagram showing the state of each layer before bonding, and FIG. 3 is a band diagram of the device after bonding. Figure 4 (a)
5(b) is a band diagram for explaining the light emitting operation of the device. FIGS. Figure 6 (a)
(b) is a band diagram for explaining the operation when the light emission of the first organic film takes priority, FIG. 7 is a diagram showing the hysteresis characteristics of the device of this example due to constant voltage eternity, and FIG. 8 is Similarly, Figure 9 is a diagram showing the negative resistance characteristics due to constant current drive, Figure 9 is a diagram showing the DC current-voltage characteristics of the element, Figure 10 is a diagram showing the displacement current characteristics of the element, and Figure 11 is a diagram showing the specific FIG. 12 is a diagram showing the hysteresis characteristics of the element of the example. FIG. 12 is also a diagram showing the afterimage characteristics during pulse driving. 1. Glass substrate, 2. Second electrode, 3. Second organic film, 4. First organic film, 5. First electrode.

Claims (2)

[Claims] (1) An organic film containing at least one type of organic dye is sandwiched between a first electron-injecting electrode and a hole-injecting second electrode, and the first and second electrodes During this period, when a positive bias is applied to the second electrode side, electrons injected from the first electrode and holes injected from the second electrode are accumulated in the organic film, and the memory An organic film light-emitting device characterized by: exhibiting light emission with specific characteristics. (2) The organic film light emitting device according to claim 1, wherein the organic film has a laminated structure of a first organic film and a second organic film forming a barrier junction for electrons and holes.