BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:
FIG. 1 is a circuit diagram showing a passive matrix type display device according to a first embodiment;
FIG. 2 is a circuit diagram explaining another operational state in the passive matrix type display device according to the first embodiment;
FIG. 3A is a cross sectional view showing a light emitting element, and FIG. 3B is a table showing materials in the light emitting element;
FIG. 4 is a perspective view showing the light emitting element;
FIG. 5A is a cross sectional view showing a light adjustment element according to a first example, and 5B is a table showing materials in the light adjustment element;
FIG. 6A is a cross sectional view showing a light adjustment element according to a second example, and 6B is a table showing materials in the light adjustment element;
FIG. 7A is a cross sectional view showing a light adjustment element according to a third example, and 7B is a table showing materials in the light adjustment element;
FIG. 8A is a cross sectional view showing a light adjustment element according to a fourth example, and 8B is a table showing materials in the light adjustment element;
FIG. 9A is a cross sectional view showing a light adjustment element according to a fifth example, and 9B is a table showing materials in the light adjustment element;
FIG. 10 is a circuit diagram showing a passive matrix type display device according to a second embodiment;
FIG. 11A is a circuit diagram showing a passive matrix type display device according to a third embodiment, and FIG. 11B is a circuit diagram showing a passive matrix type display device according to a fourth embodiment;
FIG. 12 is a circuit diagram showing a passive matrix type display device according to a fifth embodiment;
FIG. 13 is a diagram showing energy level in the passive matrix type display device according to the fourth embodiment;
FIG. 14 is a timing chart showing energization patterns in the light adjustment element;
FIG. 15 is a schematic view showing a chemical compound No. 1;
FIG. 16 is a schematic view showing a chemical compound No. 2;
FIG. 17 is a schematic view showing a chemical compound No. 3;
FIG. 18 is a schematic view showing a chemical compound No. 4;
FIG. 19 is a schematic view showing a chemical compound of rubren; and
FIG. 20 is a table showing electrical properties of various compounds.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
Now, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a circuit diagram showing one embodiment of a passive matrix type display device 1 according to the first embodiment. The passive matrix type display device 1 is configured including the following constituents:
scanning electrodes B (indicated as B1, B2, . . . , and Bn in the figure, in order to distinguish a plurality of ones on an array), wherein a plurality of scanning electrodes are arrayed at preset intervals in a first direction CD within a display area 120, and each of them is disposed so that it can be changed-over between a light-emission connection state capable of conducting a drive current I and a non-light-emission connection state incapable of conducting the drive current I;
data electrodes A (indicated as A1, A2, . . . , and Am with suffixes in the figure, in order to distinguish a plurality of ones on an array), wherein a plurality of data electrodes are arrayed at preset intervals in a second direction RD intersecting the first direction CD, within the display area 120;
drive current source, wherein the individual data electrodes A are connected to the drive current source so that each of them can be changed-over between a light-emission connection state capable of conducting a total current It and a non-light-emission connection state incapable of conducting the total current It, wherein the drive current source feeds currents to the data electrodes A while controlling conduction current quantities to predetermined values, and wherein this drive current source includes a stabilized power source Vc (or a battery +B), and constant-current circuits 7 which are connected to the stabilized power source Vc so as to individually correspond to the data electrodes A1, A2, . . . , and Am;
light emitting elements E (indicated as E1,1, E2,1, . . . , En,1, . . . , etc. with two-dimensional array suffixes in the figure, in order to distinguish a plurality of ones on a two-dimensional array), wherein the light emitting elements are formed at the intersection positions between the scanning electrodes B1, B2, . . . , and Bn and the data electrodes A1, A2, . . . , and Am within the display area 120, and they define display pixels, wherein, in this embodiment, the light emitting elements are configured as organic EL elements, and wherein the detailed structure of each light emitting element will be explained later;
scanning drive circuit 10, wherein the scanning drive circuit 10 scans and drives the plurality of scanning electrodes B1, B2, . . . , and Bn every predetermined scanning cycle so that only selected ones of the scanning electrodes B1, B2, . . . , and Bn may fall into the light-emission connection states, and that the scanning electrodes Bk to be selected may be successively changed-over on the array thereof, wherein the scanning switch circuit 10 is configured of a group of SPDT switches Y1, Y2, . . . , and Yn which connect the distal ends of the respectively corresponding scanning electrodes B1, B2, . . . , and Bn either to ground (corresponding to the light-emission connection states) or to a reverse bias supply voltage (corresponding to the non-light-emission connection states), wherein, as shown in FIG. 14, the SPDT switches Y1, Y2, . . . , and Yn are successively changed-over into the light-emission connection states (current conduction states) at predetermined time intervals within the scanning cycle Ts of one frame (one field in case of an interlaced scheme) by receiving a scanning signal SS from a control circuit 200, wherein, incidentally, a predetermined non-display period (a period during which all the scanning electrodes B1, B2, . . . , and Bn fall into cutoff states) is set between adjacent frames;
data drive circuit 9, wherein the data drive circuit 9 connects a specified one of the data electrodes A as is determined in accordance with any of the light emitting elements E to emit light, selectively to the drive current source every scanning cycle, wherein this data drive circuit 9 is configured of a group of SPDT switches X1, X2, . . . , and Xn which connect the end parts of the respectively corresponding data electrodes A1, A2, . . . , and Am on the power source connection sides thereof, selectively either to the drive current source sides thereof (corresponding to lit-up states) or to the ground sides thereof (corresponding to put-out states), wherein, in operation, the data drive circuit 9 sets the switches X which correspond to the light emitting elements to be lit up in the respective selection periods of the scanning electrodes B1, B2, . . . , and Bn, selectively at lit-up state positions by receiving a data signal DS from the control circuit 200, wherein, concretely, the data drive circuit 9 detects a horizontal sync signal corresponding to the selected scanning electrode B, it counts pixel transfer clocks with reference to the horizontal sync signal, thereby to specify the data electrode A corresponding to each display pixel, and it gives the command of the changeover of the SPDT switch X corresponding to the data electrode A, on the basis of the binary pulse level of display data expressive of the lit-up state of the pixel (light emitting element E) corresponding to the pertinent data electrode A;
light adjustment elements E′ (indicated as E′1,1, E′2,1, . . . , etc. with two-dimensional array suffixes in the figure, in order to distinguish a plurality of ones on a two-dimensional array), wherein the light adjustment elements E′ are disposed outside the display area 120, and they are connected in parallel with the light emitting elements E in each of the data electrodes A, whereby part of the total current It fed from the drive current source 7 through the data electrode A can be distributively conducted as a light adjustment current Id, wherein the data electrode A is connected to the constant-current circuit 7, and the total current It on the data electrode A is held constant, wherein in a case where the light adjustment current Id flows through the data electrode Ai, a remaining current obtained by subtracting the light adjustment current Id from the total current It is the drive current I (=It−Id), which is conducted to the light emitting elements Ei,j corresponding to the selected scanning electrode Bj, wherein the details of the structure of each light adjustment element E′ will be explained later; and
light adjustment control means 11, wherein the light adjustment control means 11 alters the distributive conduction quantity of the light adjustment current Id to the light adjustment elements E′, thereby to adjust the conduction quantity of the drive current I to the light emitting elements E on the corresponding data electrode A and to adjust the lights of the respective light emitting elements E, wherein the plurality of light adjustment elements E′1,1, E′1,2, . . . corresponding to the data electrodes A1, A2, . . . , and Am are connected in parallel with each other, at the intersection positions between light adjusting electrodes B′1 and B′2 and the respective data electrodes A1, A2, . . . , and Am by the light adjusting electrodes B′1 and B′2 which are arranged in adjacency at the distal end of the array of the scanning electrodes B1, B2, . . . , and Bn, wherein the light adjustment control means 11 alters the distributive conduction quantity of the light adjustment current Id through the light adjusting electrodes B′.
Each of the light adjusting electrodes B′ can be changed-over between a first connection state capable of conducting the light adjustment current Id and a second connection state incapable of conducting the light adjustment current Id, and the light adjustment control means 11 functions as light adjusting changeover control means for changing-over the light adjusting electrodes B′ between the first connection states and the second connection states. Concretely, the light adjustment control means 11 being the light adjustment switching circuit is configured of a group of SPDT switches Y′1 and Y′2 which connect the distal ends of the respective light adjusting electrodes B′1 and B′2 selectively either to the ground (corresponding to the current conduction states) or to the reverse bias supply voltage (corresponding to the cutoff states). The light adjustment control means 11 receives a light adjustment signal LS from the control circuit 200, and it subjects the group of SPDT switches Y′1 and Y′2 to the changeover control so that the light adjustment current Id corresponding to the content of the light adjustment signal LS may flow. Here, in FIG. 1, reference numeral 2 represents a display unit, reference numeral 3 represents a vertical unit, reference numeral 4 represents a horizontal unit, and reference numeral 6 represents a wire.
As shown in FIG. 4, the plurality of organic EL elements E are made unitary by an organic stacked body 150 which consists of a plurality of layers that are respectively formed continuously in the in-plane direction of the display area. The organic stacked body 150 which is shared by the individual light emitting elements E is formed by a vapor deposition method such as evaporation or high-frequency sputtering (effective in case of using low-molecular materials) or a solution coating method (effective in case of using molecular materials). The group of scanning electrodes B are arranged on one principal surface of the organic stacked body 150, while the group of data electrodes A are arranged on the other principal surface of the organic stacked body 150.
FIGS. 3A and 3B schematically show the sectional structure of one of the organic EL elements E. The organic EL element E is formed on a glass substrate 10 which forms a base material. The data electrode A which is connected to the drive current source is an anode 20, while the scanning electrode B and light adjusting electrode B′ which are grounded is a cathode. The anode 20 is made of a material of large work function φ so that the injection of holes into the organic stacked body 150 may proceed easily. In this embodiment, the anode 20 is made of ITO (Indium-Tin Oxide), but it may well be another oxide layer of zinc oxide, indium-zinc oxide, or the like. Besides, the cathode 80 needs to be small in work function φ so that the injection of electrons into the organic stacked body 150 may proceed easily. In this embodiment, the cathode 80 is made of Al (aluminum), but it is also possible to use an alloy (for example, Al—Li) in which the Al is doped with a metal smaller in work function than the Al, or an alloy such as Mg—In or Mg—Ag.
The organic stacked body 150 has a well-known structure in which an electron transportable material layer 160, a light emitting layer 50 and a hole transportable material layer 140 are stacked in this order from the side of the cathode 80. FIG. 13 schematically shows the energy structure of the organic stacked body 150. The electron transportable material layer 160 is arranged in contact with the cathode 80 and an electron transport layer 60 which is arranged in touch with the light emitting layer 50, and it has an electron injection layer 70 as to which the difference Δε1≡φc−Ac1 between the electron affinity Ac1 of its own and the work function φc of the cathode 80 is smaller than the difference Δε2≡φc−Ac2 between the electron affinity Ac2 of the electron transport layer 60 and the work function φc of the cathode 80. Thus, the levels of energy barriers pertinent to electron injections as are formed between the individual layers in the section between the cathode 80 and the light emitting layer 50 are decreased. Besides, in order to make difficult the occurrence of hole injection from the light emitting layer 50 into the electron transport layer 60, this electron transport layer 60 is selected so that the difference δE2 (≡Ec2−Ec0) between the ionization potential Ec2 thereof and the ionization potential Ec0 of the light emitting layer 50 may become larger than the difference δE4 (≡Ec0−Ec4) between the ionization potential Ec4 of a hole transport layer 40 to be stated later and the ionization potential Ec0 of the light emitting layer 50. Thus, the effect of confining holes in the light emitting layer 50 is heightened, and it contributes to enhancing the light-emission recombination probability of electrons—holes in the light emitting layer 50.
Well-known materials can be adopted as the constituent materials of the electron transport layer 60 and the electron injection layer 70. For the electron transport layer 60, it is possible to adopt an organic material composed of, for example, an aluminum-quinolinol complex (a concrete example of which is tris(8-quinolato) aluminum (so-called “Alq3”)) or an anthracene derivative. Besides, the electron injection layer 70 can be made of an alkali metal (such as Li, Na, K or Cs), alkaline earth metal (such as Be, Mg, Ca, Sr or Ba), or any of the inorganic compounds (for example, oxide (Li2O or the like) or halide (LiF or the like) of such metals.
Next, the hole transportable material layer 140 is arranged in contact with the anode 20 and the hole transport layer 40 which is arranged in touch with the light emitting layer 50, and it can be configured having a hole injection layer 30 as to which the difference ΔE1−Ec3−φa between the ionization potential Ec3 of its own and the work function φa of the anode 20 is smaller than the difference ΔE2≡Ec4−φa between the ionization potential Ec4 of the hole transport layer 40 and the work function φa of the anode 20. Thus, the levels of energy barriers pertinent to hole injections as are formed between the individual layers in the section between the anode 20 and the light emitting layer 50 are decreased, and this contributes to lowering the drive voltage of the element. Besides, in order to make difficult the occurrence of electron injection from the light emitting layer 50 into the hole transport layer 40, this hole transport layer 40 is selected so that the difference δE4 (≡Ac0−Ac4) between the electron affinity Ac0 of the light emitting layer 50 and the electron affinity Ac4 of this hole transport layer 40 may become larger than the difference δE2 (≡Ac2−Ac0: in FIG. 13, this value is a minus value, and an ohmic contact is established concerning electron transport) between the electron affinity Ac2 of the electron transport layer 60 and the electron affinity Ac0 of the light emitting layer 50. Thus, the effect of confining electrons in the light emitting layer 50 is heightened, and it contributes to enhancing the light-emission recombination probability of electrons—holes in the light emitting layer 50. Here, Δε0≡φcφAc0, Δε1<Δε2, ΔE0≡Ec0−φa, and ΔE1<ΔE2.
Well-known materials can be adopted as the constituent materials of the hole transport layer 40 and the hole injection layer 30. The hole injection layer 30 can be made of, for example, copper phthalocyanine, or a compound I whose structure is represented by the chemical formula No. 1 shown in FIG. 15.
Besides, the hole transport layer 40 can be made of a triphenylamine compound, for example, a compound II whose structure is represented by the chemical formula No. 2 shown in FIG. 16, or a compound III whose structure is represented by the chemical formula No. 3 shown in FIG. 17.
The light emitting layer 50 selects as its host material, a material in which an electron mobility is higher than a hole mobility (that is, an electron transportable material), whereby the recombination of electrons and holes occurs effectively near the interface of this light emitting layer 50 with the hole transport layer, and a light emission efficiency can be heightened. Any of various materials including the aluminum-quinolinol complex (for example, Alq3) mentioned above, a compound IV which is represented by the chemical formula No. 4 shown in FIG. 18, etc. can be adopted as such an electron transportable material constituting the light emitting layer 50:
Besides, the light emitting layer 50 can be formed as one whose host material is doped with a dopant (guest material) enhancing a fluorescent quantum yield. Thus, the light emission efficiency of the light emitting element E is heightened, and this contributes to the enhancement of an element lifetime. A well-known material can be adopted as such a dopant, and it is possible to adopt, for example, rubren having a structure represented by the chemical formula No. 5 shown in FIG. 19, or a coumarin derivative, DCM or quinacridone:
In FIG. 1, each light adjustment element E′ is formed by utilizing at least one layer of the organic stacked body 150 in FIG. 4 or FIGS. 7A and 7B. As stated before, the individual organic material layers constituting the organic stacked body 150 (in FIGS. 3A and 3B, the hole injection layer 30, hole transport layer 40, light emitting layer 50 and electron transport layer 60) are formed by the vapor deposition method such as evaporation or high-frequency sputtering (effective in case of using low-molecular materials) or the solution coating method (effective in case of using molecular materials). Such a process has the advantage that the layer or layers to be utilized for the light adjustment elements E′ can be collectively formed at the formation of the light emitting elements E.
From this viewpoint, a part of layers of the organic stacked body 150 is omitted or is replaced with a layer made of another material, whereby the light adjustment element E′ can be formed in various aspects as an element which exhibits a light emission intensity lower than that of the light emitting element E when both the elements are driven by an identical voltage, or as an element which does not emit light. In the case where the organic stacked body 150 forming the light emitting element E is configured as shown in FIGS. 3A and 3B, the light adjustment element E′ can effectively suppress its light emission by omitting at least the light emitting layer 50 or replacing it with the layer made of the other material.
FIGS. 5A, 5B, 6A, and 6B show examples in each of which the light adjustment element E′ is configured by omitting the light emitting layer 50 from the organic stacked body 150 in FIGS. 3A and 3B and at least the electron transport layer 60 from the electron transportable material layer 160, and leaving the hole transportable material layer 140 behind. Although the hole transportable material layer 140 is low in the light emission recombination probability, it is favorable in an electric conductivity itself originating from the hole transport. Accordingly, the light adjustment element E′ which is of non-light emission type and whose current conduction capacity is comparatively large can be easily configured by the omissions of the light emitting layer 50 and electron transport layer 60. The light adjustment element E′ in FIGS. 5A and 5B has a structure in which the electron injection layer 70 is further omitted. In this case, the electron injection energy barrier between the cathode 80 and the organic layer is rather increased by the omission of the electron injection layer 70, but holes are injected into the hole transportable material layer 140 more dominantly owing to the increased electron injection energy barrier. Therefore, unnecessary light-emission recombination becomes difficult to occur, and this is more convenient as the light adjustment element E′.
In either of the configurations in FIGS. 5A. 5B, 6A and 6B, the hole transportable material layer 140 included in the light adjustment element E′ can be configured of, at least, either of the hole transport layer 40 and the hole injection layer 30. That is, although the light adjustment element E′ succeeds to the partial structure of the light emitting element E, it no longer has any requirement concerning the enhancement of a light emitting function, and hence, it need not always be optimized so as to compare favorably with the light emitting element E in point of an energy barrier profile pertinent to the hole injection between the hole transportable material layer 140 and the anode 20. By way of example, the light adjustment element E′ which utilizes only the hole injection layer 30 in the hole transport layer 40 and hole injection layer 30 of the light emitting element E has the advantage that an energy barrier for the hole injection from the anode 20 into the hole injection layer 30 can be made small. However, the light adjustment element E′ can adopt a configuration in which only the hole transport layer 40 is utilized by omitting the hole injection layer 30, or it may well succeed to the stacked structure consisting of the hole injection layer 30 and the hole transport layer 40.
In a case where the anode 20 and the cathode 80 are respectively made of ITO and Al, the ionization potentials (Ec) and electron affinities (Ac) of the compounds I, II and III mentioned before, the work functions of the ITO and Al (denoted by φ below), the values Ec/Ac of the respective compounds, and the differences of the electron affinities from the work functions φ of the ITO or Al are collectively listed as indicated in Table 1 shown in FIG. 20. A minus value in the table signifies that any energy barrier is not existent, and that an ohmic contact is established between the compound and the electrode.
In the case of the organic EL element, holes are injected from the anode into the organic layer, and electrons are injected from the cathode into the organic layer. The injected holes and electrons are recombined in the organic layer, whereby light is emitted. In view of Table 1, the difference between the ionization potential Ec of each compound and the work function φ of the anode is smaller than the work function φ of the cathode and the electron affinity Ac of each compound, so that the electron injection energy barrier becomes relatively larger. Therefore, in the structure (FIGS. 5A and 5B) in which the hole transportable material layer 140 made of the compounds is sandwiched in between the anode and the cathode, the holes are injected more easily than the electrons, with the result that the structure functions as a hole current device, and the holes and the electrons are recombined in the organic layer at almost no probability. Accordingly, the structure becomes the device which emits no light in principle.
Incidentally, when the electron injection layer 70 is inserted between the cathode 80 and the hole transportable material layer 140 as shown in FIGS. 6A and 6B, an electron injection energy barrier at the interface of the electron injection layer 70 with the cathode 80 lowers. Therefore, electrons are easily injected into the hole transportable material layer 140, and the light-emission recombination which is undesirable as the light adjustment element E′ is sometimes liable to occur. Accordingly, it is more desirable to omit the electron injection layer 70 as shown in FIGS. 5A and 5B. Since the triphenylamine compound which is employed as the hole transport material is comparatively small in the ionization potential Ec and the electron affinity Ac, the hole injection energy barrier thereof with respect to the ITO forming the anode 20 is small, and the electron injection energy barrier thereof with respect to the Al forming the cathode 80 can be set large. Therefore, with the structure of FIGS. 5A and 5B in which such a hole transport material is sandwiched in between the electrodes, the hole current device can be easily obtained.
Next, the light adjustment element E′ in FIGS. 7A and 7B is an example of a configuration in which the light emitting layer 50 in the organic stacked body 150 is replaced with a substitute organic layer 50′ having a dopant added thereto in a quantity smaller than in the light emitting layer 50. Thus, unnecessary light emission in the light adjustment element E′ can be effectively suppressed. In FIGS. 7A and 7B, the substitute organic layer 50′ is not doped with rubren which the light emitting layer 50 in FIGS. 3A and 3B contains as the dopant, and the light emission thereof is suppressed.
Besides, a light adjustment element E′ in FIGS. 8A and 8B is such that the light emitting layer 50 of the organic stacked body 150 is replaced with a substitute organic layer 50″ which is made of a mixture consisting of an electron transportable organic material and a hole transportable organic material. In the case where the light emitting layer 50 of the light emitting element E is replaced with the substitute organic layer 50″ which is made of the mixture consisting of the electron transportable organic material and the hole transportable organic material, electrons and holes injected into the substitute organic layer 50″ migrate in a manner to be respectively localized in the regions of an electron transportable organic material phase and the regions of a hole transportable organic material phase as exist in mixed and dispersed fashion within the substitute organic layer 50″. As a result, a probability at which the carriers undergo light-emission recombination with each other within the layer 50″ is lowered, but the mobilities themselves of the corresponding carriers in the respective phase regions are high. Therefore, unnecessary light emission in the light adjustment element E′ can be effectively suppressed with a favorable conductivity ensured.
Concretely, the organic stacked body 150 of the light emitting element E in FIGS. 3A and 3B is considered as a structure to be referred to, and the substitute organic layer 50″ of the light adjustment element E′ in FIGS. 8A and 8B is configured in such a way that the hole transportable organic material (compound II or III) forming the hole transportable material layer 140 is mixed into the electron transportable organic material (compound IV) forming the light emitting layer 50. The constituent materials of the substitute organic layer 50″ can be shared with the electron transportable organic material forming the light emitting layer 50 and the hole transportable organic material forming the hole transportable material layer 140. Especially in case of configuring the substitute organic layer 50″ by the vapor deposition method, any material source dedicated to the substitute organic layer 50″ need not be assembled into a deposition equipment, and a deposition process and the deposition equipment can be simplified.
Concretely, the light adjustment element E′ is provided with a hole transport layer 40 in contact with the anode 20 side of the substitute organic layer 50″, while it is provided with a sub electron transport layer 61 made solely of the electron transportable organic material (compound IV) forming the light emitting layer 50, in contact with the cathode 80 side of the substitute organic layer 50″, and it is further provided with the same electron transport layer 60 and electron injection layer 70 as those of the light emitting element E, in contact with the cathode 80 side of the sub electron transport layer 61. As compared with the light emitting layer 50 of the light emitting element E, the substitute organic layer 50″ of the light adjustment element E′ becomes larger in the difference of an electron conduction level relative to the electron transport layer 60, in correspondence with the component of the hole transportable organic material mixed in the electron transportable organic material constituting this substitute organic layer 50″. Accordingly, when the substitute organic layer 50″ is brought into direct contact with the electron transport layer 60, it has the difficulty that an energy barrier level becomes somewhat high. In this regard, however, the sub electron transport layer 61 made solely of the electron transportable organic material forming the light emitting layer 50 is interposed on the cathode 80 side of the substitute organic layer 50″ as stated above, so that the increase of the energy barrier level can be effectively suppressed.
Incidentally, although the substitute organic layer 50″ of the light adjustment element E′ in FIGS. 8A and 8B is doped with the dopant (rubren) likewise to the light emitting layer 50 of the light emitting element E in FIGS. 3A and 3B, FIGS. 9A and 9B shows an example of a light adjustment element E′ in which a substitute organic layer 50″ is not doped with the dopant.
There will now be described an actual example of a light adjustment method in the passive matrix type display device 1 shown in FIG. 1. In FIG. 1, each of the light adjusting electrodes B′ can be independently changed-over between the current conduction state (first connection state) and cutoff state (second connection state) by the corresponding switch Y′. Any of the groups of light adjustment elements E′m connected in parallel by the corresponding light adjusting electrodes B′ has a current-conducting sectional area identical to that of the light emitting element E. The reason therefor is that the light adjusting electrode B′ is configured as the cathode 80 (refer to FIG. 4) identical in width to the scanning electrode B, so the intersection area between the anode 20 forming the data electrode A and the light adjusting electrode B′ becomes equal to the intersection area between the former and the scanning electrode B.
Assuming that all the light adjusting electrodes B′ are brought into the cutoff states, the light adjustment current Id which is distributed to the light adjusting electrodes B′ becomes zero, and the drive current I becomes It (first light-adjustment-element setting pattern). This drive current is the maximum current which flows to the light emitting element E. In addition, when only the light adjusting electrode B′1 is brought into the current conduction state as shown in FIG. 1, the drive current I which flows to the selected scanning electrode B becomes equal to the light adjustment current Id which is distributed to the light adjusting electrode B′1. Here, since the total current It flowing through the data electrode A is constant, the drive current I becomes It/2 (second light-adjustment-element setting pattern). That is, the light emission quantity of the light emitting element E can be decreased to ½ of the maximum value by bringing only the light adjusting electrode B′1 into the current conduction state. Besides, when the two light adjusting electrodes B′1 and B′2 are both brought into the current conduction states as shown in FIG. 2, the drive current I becomes It/3 considering that the total current It is equally distributed. Thus, the light emission quantity of the light emitting element E can be decreased to ⅓ of the maximum value (third light-adjustment-element setting pattern).
In this manner, the light adjustment elements E′m connected in parallel by the light adjusting electrodes B′ are disposed in the plurality of groups, and the combination of the groups of light adjustment elements E′m to be connected to the data electrodes A is altered at will, whereby the light adjustment current Id can be easily adjusted to any of the various levels corresponding to the respective combinations. Besides, the first to third light-adjustment-element setting patterns differ from one another in the number of the groups of light adjustment elements E′m which conduct currents. That is, the number of the light adjusting electrodes B′, in turn, the number of the groups of light adjustment elements E′m, to be connected to the data electrodes A is altered, whereby subtle light adjustments are permitted in accordance with the numbers of the groups of light adjustment elements E′m to be connected. In addition, the individual light adjustment elements E′ are formed as having voltage-current characteristics equal to one another. Therefore, as the number of the light adjusting electrodes B′ (the groups of light adjustment elements E′m) which are brought into the current conduction states is larger, the light adjustment current Id can be caused to flow more, and a larger light decrease level can be achieved.
Meanwhile, it is possible to adopt an aspect as shown in FIG. 10, in which a plurality of light adjustment elements E′ connected to one light adjusting electrode B′ in each group of light adjustment elements E′m are endowed with voltage-current characteristics equal to one another, and in which at least one of a plurality of light adjustment elements E′m is configured of light adjustment elements E′2 that are larger in a current-conducting sectional area than those of the remaining groups of light adjustment elements E′m. In this case, the levels of light adjustment currents Id can be collectively increased by selecting the group of light adjustment elements E′m configured of the light adjustment elements E′2 of larger current-conducting sectional area, and the sorts of settable light adjustment levels and the fluctuation width of the light adjustment levels can be expanded.
In the aspect shown in FIG. 10, each light adjustment element E′2 leading to the light adjusting electrode B′2 has a current-conducting sectional area which is three times as large as that of each light adjustment element E′1 leading to the light adjusting electrode B′1 or each light emitting element E leading to a scanning electrode B. In a case where only the light adjusting electrode B′2 is brought into a current conduction state, a light adjustment current flows three times more than in a case where only the light adjusting electrode B′1 is brought into a current conduction state, and the drive current I of the light emitting element E can be decreased to ¼ of the maximum value at one stroke (that is, light decrease to ¼ is possible). Such a light adjustment element E′2 can be easily fabricated in such a way that the width of the light adjusting electrode B′2 is made larger (three times larger) than the width of the light adjusting electrode B′1 or the scanning electrode B.
FIG. 11A shows an example provided with a current control circuit 107 by which the quantity of a conduction current to flow on a light adjusting electrode B′ is variably controlled in accordance with the number of data electrodes A connected to drive current sources 7. According to this configuration, the quantity of a light adjustment current Id on the light adjusting electrode B′ can be adjusted by the current control circuit 107 in accordance with the number of the data electrodes A connected to the drive current sources 7. Therefore, the light adjustment current Id which is conducted to each light adjustment element E′ can be stabilized irrespective of the number of the data electrodes A connected to the light adjusting electrode B′, and in turn, a stable light adjustment state can be realized.
In the example of FIG. 11A, the current control circuit 107 is disposed at a position at which currents from the individual data electrodes A join on the light adjusting electrode B′. In addition, a total current which flows from the data electrodes A into the current control circuit 107 in the selection period of each scanning electrode B increases in proportion to the number of the data electrodes A connected to the drive current sources, that is, the number of light emitting elements E brought into lit-up states. Accordingly, a control circuit 200 counts the number of levels corresponding to “light-up” among the binary pulse levels of the display data of individual pixels as are successively transferred on the basis of pixel transfer clocks, and it determines a control current level value with reference to the resulting count value, so as to give a command to the current control circuit 107 (control signal CS: here, it is an analog signal indicating an instructive current level). The current control circuit 107 causes the light adjustment current proportional to the number of the light emitting elements E which are brought into the lit-up states, to flow to the light adjusting electrode B′ in accordance with the instructive value. Thus, the constant light adjustment currents can be caused to flow to the light adjustment elements E′ corresponding to the light emitting elements E to-be-lit-up, irrespective of the number of the light emitting elements E to-be-lit-up on the scanning electrode B.
In this case, a reference light adjustment current value per light adjustment element E′ is determined beforehand, and the instructive current level for the current control circuit 107 is adjusted so that the ratio of the light adjustment current to flow through each light adjustment element E′, relative to the reference light adjustment current value may change, whereby the light emission level of the light emitting element E can be altered in accordance with the instructive current level. That is, the current control circuit 107 functions as means by which the quantity of the current to flow to the light adjustment element E′ through the data electrode A is variably controlled in accordance with a required light adjustment level. Incidentally, when the light adjusting electrode B′ is brought into a cutoff state, the light emitting elements E are lit up at the maximum intensity.
On the other hand, a configuration in FIG. 11B is an example of a circuit arrangement in which constant light adjustment currents can be conducted to corresponding light adjustment elements E′ in accordance with an instructive current level, without counting the number of light emitting elements E of lit-up states leading to a selected scanning electrode B. More specifically, in the example, a current control circuit 207 is configured of a current mirror circuit including an input side transistor T0 which receives a control voltage input CS for instructing a light adjustment level and which causes a control current corresponding to the control voltage to flow, and output side transistors T1 to Tm which are disposed in one-to-one correspondence with the individual light adjustment elements E′ on respective data electrodes A and are connected to the input side transistor T0 by sharing bases and which causes light adjustment currents Id whose value corresponds uniquely to the control current, to individually flow to the light adjustment elements E′.
The control voltage input CS has a voltage level reflecting the instructive current level and is converted into a current signal through a voltage/current conversion circuit 201, whereby the currents of identical level flow to the respective output side transistors T1 to Tm owing to the current mirror circuit. When the control voltage input CS is altered in accordance with the light adjustment level, the currents to flow through the input side transistor T0 and the respective output side transistors T1 to Tm are varied, and the light adjustment currents of desired level can be fed to the individual light adjustment elements E′. When the control voltage input CS is continuously changed, the light adjustment current level, in turn, the lit-up intensity of the light emitting elements E can be continuously changed. Incidentally, when the control voltage input CS is made zero, the light adjustment current can be made substantially zero. In this case, therefore, a light adjusting switch Y′ (switch circuit 11) can be omitted.
Incidentally, as shown in FIG. 12, each light adjustment element E′ may well be configured of an element which emits light by the conduction of a light adjustment current Id. Especially, in a case where the light adjustment element E′ is configured as the same element as a light emitting element E (that is, an element to which a stacked structure is common), the patterning etc. of the layers of parts corresponding to the light adjustment elements E′ are not required at all, and a manufacturing process can be sharply simplified. Moreover, since the current conduction characteristics of the light adjustment elements E′ can be brought into agreement with those of the light emitting elements E, light adjustment control specifications can be simplified. In this case, however, the light adjustment element E′ emits light at the same intensity as that of the light emitting element E, and it is therefore necessary to dispose a shield portion 15 which hinders a light emission flux from the light adjustment element E′ from leaking out in the viewing direction of a display area 120. The shield portion 15 can be formed in any of various aspects, for example, it is formed as part of a light-intercepting housing, or it is configured of a light-intercepting film or coating film.
Next, in case of adjusting light, a light adjustment electrode B′ through which a light adjustment current is caused to flow may well be brought into a current conduction state continuously over a plurality of frames, or it can be brought into a cutoff state in a non-display period (also in this case, it is brought into the current conduction state continuously within individual frame periods). On the other hand, the light adjustment electrode B′ can also be brought into the current conduction states intermittently in synchronism with the selection periods of individual scanning electrodes B, and this operating aspect is sometimes effective for, for example, the enhancement of the lifetime of the light adjustment elements E′. Besides, as shown at (1) to (5) in FIG. 14, the light adjustment electrode B′ can be brought into the current conduction state with at least one scanning line jumped. That is, only in the selection periods of at least one scanning electrode B within one frame, the light adjustment electrode B′ is brought into the current conduction state in synchronism with the selection period. Thus, the scanning lines composed of pixel strings can be decreased, for example, every predetermined number of lines, and the mean brightness of one frame can be adjusted in accordance with the number of the scanning lines to-be-decreased. On the other hand, as shown at (6) in FIG. 14, it is possible to perform a PWM control which reduces the current conduction period of the scanning electrode B (that is, the light emission period of the corresponding light emitting element E) at a predetermined duty ratio η, whereby the individual scanning lines can be uniformly decreased.
While the invention has been described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the preferred embodiments and constructions. The invention is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, which are preferred, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.
Patent Application
20080048952
