JP2018037570A - Display unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a display unit capable of controlling a threshold voltage of an OLED provided on each pixel.SOLUTION: An electron transport layer 44 includes a positive hole blocking layer 43 formed between the electron transport layer 44 formed from a material storing a negative fixed charge on the positive hole blocking layer 43 side and a luminous layer 42. When a positive charge by a positive hole which the positive hole blocking layer 43 blocks and a negative fixed charge cancel each other, current supply into an OLED starts.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a display device.

  The organic EL display device has an organic light emitting diode (OLED) including an anode, a cathode, and a light emitting layer in each pixel. Some organic EL display devices have a light emitting layer formed separately on a plurality of pixels. In the following Patent Document 1, a blue light emitting layer, a green light emitting layer, and a red light emitting layer are formed in a blue pixel, a green pixel, and a red pixel, respectively. Moreover, in patent document 1, the green light emitting layer and the red light emitting layer are covered with the electron carrying layer and the blocking layer, and the blue light emitting layer is covered only with the electron carrying layer.

JP 2002-260858 A

  The threshold voltage of an OLED is determined by its built-in potential. Therefore, since the material of the OLED (for example, the material of the light emitting layer) differs depending on the pixel, the threshold voltage of the OLED may differ depending on the pixel. For example, in a display device in which a blue light emitting layer, a green light emitting layer, and a red light emitting layer are formed in three pixels, the threshold voltage of the OLED may be different in the three pixels. For this reason, for example, when the same voltage is applied to a blue pixel, a red pixel, and a green pixel in order to display white, the luminance of a pixel having a low threshold voltage (for example, a red pixel or a green pixel) is increased, and the threshold voltage is increased. The luminance of a pixel (for example, a blue pixel) becomes relatively low, and as a result, there arises a problem that pure white is not displayed.

  An object of the present invention is to provide a display device capable of controlling the threshold voltage of an OLED provided in each pixel.

The display device according to the present invention is formed between an anode, a cathode, a light emitting layer formed between the anode and the cathode, and between the cathode and the light emitting layer, and is supplied from the cathode. An electron transport layer that transports electrons to the light emitting layer, and a hole blocking layer that is formed between the electron transport layer and the light emitting layer and blocks holes from the anode toward the cathode. Yes. The electron transport layer is formed of a material that accumulates negative fixed charges on the hole blocking layer side of the electron transport layer. According to this display device, the threshold voltage of the OLED can be controlled.
Further, another display device according to the present invention includes a light emitting layer, a first electrode that supplies the first charge, which is one of electrons and holes, to the light emitting layer, and the light emitting layer interposed therebetween. A second electrode that is disposed on the opposite side of the first electrode and supplies the second charge, which is the other charge of electrons and holes, to the light emitting layer, and between the first electrode and the light emitting layer A first charge transport layer disposed to transport the first charge supplied from the first electrode to the light emitting layer, disposed between the first charge transport layer and the light emitting layer, and A second charge blocking layer that blocks the second charge from the two electrodes toward the first electrode. The material of the first charge transport layer is a material that accumulates the first charge on the second charge blocking layer side of the first charge transport layer. This display device can also control the threshold voltage of the OLED.

It is a top view which shows the organic electroluminescence display which concerns on embodiment of this invention. It is sectional drawing of the display apparatus shown in FIG. It is a figure which shows the pixel circuit currently formed in each pixel. It is a figure which shows typically the laminated structure of the organic light emitting diode in a blue pixel, a green pixel, and a red pixel. It is a figure which shows the laminated structure of an organic light emitting diode. In this figure, a state (initial state) before the start of electron supply from the cathode to the light emitting layer is schematically shown. It is a figure which shows the example of the energy level of the layer which comprises an organic light emitting diode. This figure shows a state (initial state) before the start of electron supply from the cathode to the light emitting layer. It is a figure which shows the laminated structure of an organic light emitting diode. In this figure, a state in which the supply of electrons from the cathode to the light emitting layer is started is schematically shown. It is a figure which shows the example of the energy level of the layer which comprises an organic light emitting diode. In this figure, a state in which the supply of electrons from the cathode to the light emitting layer is started is schematically shown. It is a figure which shows the relationship between the voltage applied to an organic light emitting diode, and the current density which flows through an organic light emitting diode. It is a figure which shows the relationship between the voltage applied to an organic light emitting diode, and the current density which flows into an organic light emitting diode.

  Hereinafter, an embodiment of the present invention will be described. FIG. 1 is a plan view of an organic ElectroLuminescence display device 1 which is an example of an embodiment of the present invention. FIG. 2 is a cross-sectional view of the organic EL display device 1. FIG. 3 is a diagram for explaining an example of a circuit formed in the circuit layer 30 of the TFT substrate 10. FIG. 4 is a diagram illustrating a stacked structure of organic light emitting diodes (OLED) 4 in three pixels having different emission colors.

  Hereinafter, an embodiment of the present invention will be described. It should be noted that the disclosure of this specification is merely an example, and any modifications that can be easily conceived by those skilled in the art while maintaining the gist of the present invention are included in the scope of the present invention.

  As shown in FIG. 1, a display area D is defined in the display device 1. In the display area D, a plurality of pixels that respectively emit light in a plurality of colors are defined. For example, the display device 1 includes a red pixel Pr, a green pixel Pg, and a blue pixel Pb as a plurality of pixels (see FIG. 2). An OLED 4 (see FIG. 3) is formed in each pixel. The color of the pixel and the number of colors are not limited to the example of the display device 1.

  As shown in FIG. 1, the display device 1 has a TFT substrate 10. The display device 1 may further include a counter substrate 9 that faces the TFT substrate 10. As shown in FIG. 2, the TFT substrate 10 has a base material 10a. The base material 10a is, for example, glass or plastic. The TFT substrate 10 has a circuit layer 30 formed on the base material 10a. The circuit layer 30 is formed with a pixel circuit that controls a current supplied to the OLED 4.

  As shown in FIG. 2, the anode 2 is formed on the upper side of the circuit layer 30. Specifically, a planarization film (not shown) is formed on the upper side of the circuit layer 30, and the anode 2 is formed on the upper side of the planarization film. The anode 2 is located in each pixel. The TFT substrate 10 has a cathode 3 facing the anode 2. The cathode 3 is formed over a plurality of pixels. That is, the cathode 3 is continuous in a plurality of pixels. As shown in FIG. 4, an organic layer 40 having a light emitting layer 42 and an electron transport layer 44 described later is formed between the anode 2 and the cathode 3. An OLED 4 is constituted by the organic layer 40, the anode 2, and the cathode 3.

  The circuit layer 30 has a pixel circuit formed in each pixel. The anode 2 of each pixel is connected to a pixel circuit formed in the circuit layer 30. As shown in FIG. 3, the pixel circuit includes a plurality of TFTs (Thin Film Transistors) 31a and 31b, a capacitor 31c, and the like. The circuit layer 30 includes a scanning signal line 31B extending in the X direction, a video signal line 31C extending in the Y direction, and a drive power supply line 31D extending in the Y direction. The anode 2 is connected to the drive power supply line 31D via the drive TFT 31b. The cathode 3 is connected to the ground potential. The scanning signal line 31B is provided in each of a plurality of pixel rows arranged in the Y direction. The video signal line 31C is provided in each of a plurality of pixel columns arranged in the X direction. The scanning signal lines 31B are sequentially selected by a scanning line driving circuit (not shown). A voltage for turning on the switching TFT 31a is applied to the selected scanning signal line 31B. A voltage corresponding to the video signal of the pixel connected to the selected scanning signal line 31B is applied to the video signal line 31C. This voltage is applied to the capacitor 31c via the switching TFT 31a. A current corresponding to the voltage applied to the capacitor 31c is supplied from the drive power supply line 31D to the OLED 4 through the drive TFT 31b. Thereby, the OLED 4 of the pixel corresponding to the selected scanning signal line 31B emits light. The pixel circuit is not limited to the example shown in FIG. 3, and various changes may be made.

  As shown in FIG. 2, a bank layer 21 is formed on the anode 2. The bank layer 21 has an opening for exposing the anode 2 to each pixel. The organic layer 40 is formed on the anode 2 and the bank layer 21, and is in contact with the anode 2 inside the opening of the bank layer 21. The cathode 3 is covered with a barrier layer 5 that prevents the penetration of moisture into the organic layer 40. The barrier layer 5 may have a laminated structure including a plurality of layers. In the example of the display device 1, the barrier layer 5 includes a first barrier layer 5a and a third barrier layer 5c formed of an inorganic material such as SiN or SiO, and between the first barrier layer 5a and the third barrier layer 5c. And a second barrier layer 5b formed of an organic material such as acrylic. The structure of the barrier layer 5 is not limited to the example of the display device 1. In the example of the display device 1, the counter substrate 9 is disposed on the upper side of the barrier layer 5, and the filler 6 is formed between the barrier layer 5 and the counter substrate 9.

  As described above, the OLED 4 is formed in each pixel. The OLED 4 has an anode 2, a cathode 3, and an organic layer 40. The display device 1 is a top emission type display device. The anode 2 is composed of a metal film having light reflectivity or a laminated film of a metal film having light reflectivity and a transparent conductive film. For example, as the material of the anode 2, for example, a laminated film of ITO and Ag, an AlNd alloy film, a laminated film of IZO and silver, or the like can be used. As a material of the cathode 3, for example, a transparent conductive material such as ITO, IZO, or ZnO can be used. A semi-transparent and semi-reflective conductive material may be used for the cathode 3. The cathode 3 may contain, for example, magnesium or silver, or an alloy of magnesium and silver may be used. The display device 1 may be a bottom emission type. In this case, the anode 2 is formed of a transparent conductive material or a translucent, semi-reflective conductive material, and the cathode 3 includes a material that reflects light. The materials of the anode 2 and the cathode 3 are not limited to those described above, and may be changed as appropriate.

  As shown in FIG. 4, the organic layer 40 has a light emitting layer 42 formed of an organic material between the anode 2 and the cathode 3. The light emitting layer 42 is separately formed on a plurality of pixels. That is, a light emitting layer that emits light in the color of the pixel is formed in each pixel. For example, the red pixel Pr is formed with a light emitting layer 42 that emits red light, the green pixel Pg is formed with a light emitting layer 42 that emits green light, and the blue pixel Pb is formed with a light emitting layer 42 that emits blue light. .

  As shown in FIG. 4, the organic layer 40 has a hole transport layer 41 formed between the light emitting layer 42 and the anode 2. In other words, the hole transport layer 41 is formed on the upper side of the anode 2, and the light emitting layer 42 is formed on the upper side of the hole transport layer 41. The hole transport layer 41 transports holes supplied from the anode 2 to the light emitting layer 42. The organic layer 40 may have a hole injection layer between the hole transport layer 41 and the anode 2. Further, the organic layer 40 has an electron transport layer 44 between the light emitting layer 42 and the cathode 3. The cathode 3 is formed on the upper side of the electron transport layer 44. The electron transport layer 44 transports electrons supplied from the cathode 3 to the light emitting layer 42. The organic layer 40 may have an electron injection layer between the electron transport layer 44 and the cathode 3. Further, the organic layer 40 has a hole blocking layer 43 between the electron transport layer 44 and the light emitting layer 42. The hole blocking layer 43 is formed on the upper side of the light emitting layer 42, and the electron transport layer 44 is formed on the upper side of the hole blocking layer 43. The hole blocking layer 43 blocks holes from the anode 2 toward the cathode 3.

  The electron transport layer 44 is formed of a material that accumulates negative fixed charges on the hole blocking layer 43 side. Here, the fixed charge is a charge that does not move to the hole blocking layer 43 side. Specifically, the electron transport layer 44 includes a material that causes orientation polarization, and accumulates negative fixed charges on the hole blocking layer 43 side by orientation polarization. The orientation polarization is polarization caused by the direction of the permanent dipoles of the material (molecules) of the electron transport layer 44 being aligned by the electric field generated between the anode 2 and the cathode 3. For example, in the state where there is a potential difference between the cathode 3 and the anode 2 but no electrons are supplied from the cathode 3 to the light emitting layer 42, the material (molecules) of the electron transport layer 44 is oriented and polarized in a predetermined direction. Then, negative fixed charges due to the orientation polarization of the electron transport layer 44 accumulate on the hole blocking layer 43 side of the electron transport layer 44. Thereafter, even if the potential difference between the cathode 3 and the anode 2 is eliminated, the accumulated fixed charge is maintained on the hole blocking layer 43 side of the electron transport layer 44. When the potential difference between the cathode 3 and the anode 2 is eliminated, the direction of the permanent dipole of the electron transport layer 44 may be random, and the accumulated fixed charge may be eliminated. As described above, the organic layer 40 has the hole blocking layer 43 between the electron transport layer 44 and the light emitting layer 42. According to this structure, the threshold voltage of the OLED 4 can be controlled by adjusting the thickness of the hole blocking layer 43.

  5A, 5B, 6A, and 6B are diagrams for explaining a mechanism of threshold voltage control by the electron transport layer 44 and the hole blocking layer 43. FIG. 5A and 6A are diagrams showing a laminated structure of the OLED 4. FIG. 5A schematically shows a state (initial state) before the start of electron supply from the cathode 3 to the light emitting layer 42. FIG. 6A schematically shows a state where supply of electrons from the cathode 3 to the light emitting layer 42 is started. FIG. 5B and FIG. 6B show examples of energy levels of the layers constituting the OLED 4. FIG. 5B shows a state (initial state) before the start of electron supply from the cathode 3 to the light emitting layer 42, as in FIG. 5A. FIG. 6B schematically shows a state where the supply of electrons from the cathode 3 to the light emitting layer 42 is started, as in FIG. 6A.

  As described above, the electron transport layer 44 is formed of a material that causes orientation polarization. Therefore, when a voltage is applied between the cathode 3 and the anode 2, as shown in FIG. 5A, in an initial state where electrons are not supplied from the cathode 3, that is, in a state where the voltage is lower than the threshold voltage of the OLED 4, Due to the electric field between the cathodes 3, the electron transport layer 44 undergoes orientational polarization. As a result, a negative charge appears at the interface of the electron transport layer 44 on the hole blocking layer 43 side. Due to the negative charge generated in the electron transport layer 44, a potential gradient occurs in the LUMO of the electron transport layer 44 as shown in FIG. 5B. Due to this potential gradient, the electrons of the cathode 3 do not pass through the electron transport layer 44. That is, no current flows through the OLED 4. In this state, as shown in FIG. 5A, the anode 2 is supplied with holes to the hole transport layer 41 and the light emitting layer 42. A hole blocking layer 43 is formed between the electron transport layer 44 and the light emitting layer 42. As shown in FIG. 5B, the hole blocking layer 43 has a HOMO that is sufficiently lower than the HOMO of the light emitting layer 42. Therefore, the holes that have entered the light emitting layer 42 cannot travel to the hole blocking layer 43 and remain at the interface of the light emitting layer 42 on the hole blocking layer 43 side.

  As the potential difference between the anode 2 and the cathode 3 increases, the amount of positive charge remaining at the interface on the hole blocking layer 43 side of the light emitting layer 42 increases. As shown in FIG. 6A, when the positive charge amount remaining at the interface of the light emitting layer 42 on the hole blocking layer 43 side becomes equal to the negative charge amount due to the orientation polarization of the electron transport layer 44, as shown in FIG. The LUMO potential gradient of layer 44 is eliminated. As a result, the electrons of the cathode 3 pass through the electron transport layer 44 and the hole blocking layer 43 and enter the light emitting layer 42. Then, recombination of holes and electrons occurs in the light emitting layer 42. That is, the light emission layer 42 starts to emit light.

As shown in FIGS. 6A and 6B, when the positive charge amount accumulated at the interface of the light emitting layer 42 at the time when the electron supply from the cathode 3 to the light emitting layer 42 is started is Qh, Qh is expressed by the following equation. (The positive charge amount Qh is the same as the negative charge amount generated by the orientation polarization of the electron transport layer 44).
Qh = C ・ V
In the above equation, C is the capacitance of the OLED 4 and V is the voltage applied to the OLED 4. The capacitance C is inversely proportional to the thickness of the hole blocking layer 43. Therefore, the voltage V increases as the thickness of the hole blocking layer 43 increases. That is, the voltage (ie, the threshold voltage of the OLED 4) required to cancel the negative charge generated by the orientation polarization of the electron transport layer 44 increases as the thickness of the hole blocking layer 43 increases.

  FIG. 7 is a diagram showing the relationship (current-voltage characteristics) between the voltage applied to the OLED 4 and the current density flowing through the OLED 4. In this figure, the horizontal axis is voltage, and the vertical axis is current density. In this figure, three types of marks indicated by circles, triangles, and squares represent current-voltage characteristics for three OLEDs 4 having different hole blocking layer thicknesses, respectively. The circle mark represents the characteristic of the OLED 4 having the thinnest hole blocking layer 43, and the square mark represents the characteristic of the OLED 4 having the thickest hole blocking layer 43. As shown in this figure, the threshold voltages Vth1, Vth2, and Vth3 at which current starts to flow through the OLED 4 increase as the thickness of the hole blocking layer 43 increases. That is, the threshold voltage of the OLED 4 can be controlled by adjusting the thickness of the hole blocking layer 43.

As a material included in the electron transport layer 44, that is, a material in which molecules having a permanent dipole are oriented, for example,
TPBi: 1,3,5-tris (1-phenyl-1H-benzimidazol-2-yl) benzene,
BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline,
Alq3: tris- (8-hydroxyquinolate) aluminum,
OXD-7: 1,42is [2- (4-tert-butylphenyl) -1,3,4-oxadiazo-5-yl] benzene or the like can be used.

As a material included in the hole blocking layer 43, for example,
PPT: 2,8-bis (diphenylphosphoryl) dibenzo [b, d] thiophene,
DPEPO: bis (2- (diphenylphosphino) phenyl) ether oxide,
TAZ: 3-pheny-4- (1′-naphthyl) -5-phenyl-1,2,4-triazole and the like can be used.

The material included in the hole blocking layer 43 is not limited to the above example. As a material of the hole blocking layer 43, a material having a high electron mobility and a low hole mobility may be used. For example, the material of the hole blocking layer 43 may be a material having an electron mobility of 10 ⁻⁵ cm ² / Vs or more and a hole mobility of 10 ⁻⁸ cm ² / Vs or less. Instead, as the material of the hole blocking layer 43, a material having high electron mobility and deep HOMO may be used. For example, the material of the hole blocking layer 43 is a material having an electron mobility of 10 ⁻⁵ cm ² / Vs or more and a HOMO greater than 6.5 eV (a HOMO deeper than 6.5 eV). May be.

  As shown in FIG. 4, the OLED 4 is formed in each pixel. The organic layer 40 includes a light emitting layer 42, a hole blocking layer 43, and an electron transport layer 44 in each pixel. That is, the organic layer 40 includes a light emitting layer 42, a hole blocking layer 43, and an electron transport layer 44 in a plurality of pixels having different colors (in the example of the display device 1, a red pixel Pr, a green pixel Pg, and a blue pixel Pb). have.

  The electron transport layer 44 is formed over a plurality of pixels having different colors (in the example of the display device 1, a red pixel Pr, a green pixel Pg, and a blue pixel Pb). That is, the electron transport layer 44 is common to a plurality of pixels. The hole transport layer 41 is also formed over a plurality of pixels. Unlike the electron transport layer 44 and the hole transport layer 41, the light emitting layer 42 is formed separately according to the color of the pixel. In the example of the display device 1, a red light emitting layer 42 that emits red light, a light emitting layer 42 that emits green light, and a light emitting layer 42 that emits blue light are formed in the red pixel Pr, the green pixel Pg, and the blue pixel Pb, respectively. . Further, the hole blocking layer 43 is also separately formed on a plurality of pixels having different colors. Specifically, the hole blocking layer 43 is formed separately for each of the red pixel Pr, the green pixel Pg, and the blue pixel Pb.

  Among the plurality of pixels having different colors, the hole blocking layer 43 of some pixels is thinner than the hole blocking layer 43 of some other pixels. By doing so, when there is a difference in the threshold voltage of the OLED 4 due to the material of the light emitting layer 42, the difference can be reduced. In the example of the display device 1, as shown in FIG. 4, the hole blocking layer 43 of the blue pixel Pb is thinner than the hole blocking layer 43 of the green pixel Pg and the red pixel Pr. In other words, in a pixel having a relatively short wavelength of light emitted from the light emitting layer 42 (blue pixel Pb in the example of the display device 1), a pixel having a relatively long wavelength of light emitted from the light emitting layer 42 (of the display device 1). In the example, the hole blocking layer 43 is thinner than the red pixel Pr and the green pixel Pg).

  Generally, the threshold voltage of an OLED that emits light having a short wavelength tends to be higher than the threshold voltage of an OLED that emits light having a long wavelength. For example, the threshold voltage of an OLED that emits blue light tends to be higher than the threshold voltage of an OLED that emits green or red light. In the example of the display device 1, the thickness of the hole blocking layer 43 in the blue pixel Pb is smaller than the thickness of the hole blocking layer 43 in other color pixels. Thereby, the difference between the threshold voltage of the OLED 4 of the blue pixel and the threshold voltage of the OLED 4 of another color can be reduced.

  When it is assumed that there is no hole blocking layer 43, the threshold voltage (in other words, the built-in potential) of the OLED 4 of the blue pixel Pb is higher than the threshold voltage of the OLED 4 of the other pixels Pr and Pg. In other words, in the example of the display device 1, the relatively thin hole blocking layer 43 is originally provided in the OLED 4 having a high threshold voltage, and the thick hole blocking layer 43 is originally provided in the OLED 4 having a low threshold voltage. By doing so, the hole voltage layer 43 can reduce the difference in threshold voltage originally between pixels. Note that the pixel on which the relatively thin hole blocking layer 43 is provided is not necessarily the blue pixel Pb. That is, when the threshold voltage of one of the two pixels is higher than the threshold voltage of the other pixel, a relatively thin hole blocking layer 43 is provided on the one pixel, and A thick hole blocking layer 43 may be provided.

  FIG. 8 is a diagram showing the relationship (current-voltage characteristics) between the voltage applied to the OLED 4 and the current density flowing through the OLED 4. In this figure, solid lines Lr2 and Lb2 indicate lines indicating current-voltage characteristics in the red pixel Pr and blue pixel Pb, respectively (hereinafter, these lines are referred to as current-voltage characteristic lines). Broken lines Lr1 and Lb1 indicate current-voltage characteristics obtained when it is assumed that the hole blocking layer 43 is not provided in the red pixel Pr and the blue pixel Pb. The current-voltage characteristic line for the green pixel Pg is omitted in FIG.

  In the display device 1, the organic layer 40 includes the electron transport layer 44 and the hole blocking layer 43 in both the red pixel Pr and the blue pixel Pb. Therefore, the current-voltage characteristic lines Lr1 and Lb1 of the two pixels are shifted to the high voltage side by providing the hole blocking layer 43. That is, the current-voltage characteristic lines Lr1 and Lb1 are changed to the current-voltage characteristic lines Lr2 and Lb2 by providing the hole blocking layer 43.

  As described above, in the example of the display device 1, the hole blocking layer 43 of the red pixel Pr is thicker than the hole blocking layer 43 of the blue pixel Pb. Therefore, the current-voltage characteristic line Lr1 for the red pixel Pr is shifted more than the current-voltage characteristic line Lb1 for the blue pixel Pb. The difference between the threshold voltages of the two pixels is reduced by providing the hole blocking layer 43 (in FIG. 8, Vr1 and Vb1 are the threshold voltages of the red pixel Pr when there is no hole blocking layer 43, respectively). The threshold voltages of the blue pixel Pb and Vr2 and Vb2 are the threshold voltage of the red pixel Pr and the threshold voltage of the blue pixel Pb, respectively, when the hole blocking layer 43 is formed.

  As shown in FIG. 4, in the example of the display device 1, the hole blocking layer 43 provided in the red pixel Pr and the hole blocking layer 43 provided in the green pixel Pg have substantially the same thickness. doing. That is, in the example of the display device 1, among the three pixels having different colors, the hole blocking layer 43 of the two pixels has the same thickness, and only one pixel has the relatively thin hole blocking layer 43. Have. Therefore, the hole blocking layer 43 may be formed across the red pixel Pr and the green pixel Pg. By doing so, the manufacturing process of the display device 1 can be reduced. Instead of the example of the display device 1, the hole blocking layer 43 of the green pixel Pg may be thinner than the hole blocking layer 43 of the red pixel Pr. That is, the thickness of the hole blocking layer 43 may be different among the three pixels having different colors.

  As described above, in the display device 1, the electron transport layer 44 is formed of a material that accumulates negative fixed charges on the hole blocking layer 43 side. A hole blocking layer 43 is formed between the electron transport layer 44 and the light emitting layer 42. According to this structure, the threshold voltage of the OLED 4 can be controlled by adjusting the thickness of the hole blocking layer 43.

The present invention is not limited to the embodiment described above, and various changes may be made.
The hole blocking layer 43 is not necessarily formed on all pixels. For example, the hole blocking layer 43 does not necessarily have to be formed in a pixel that originally has a high threshold voltage or a pixel that emits light having a relatively short wavelength (specifically, the blue pixel Pb). Then, the hole blocking layer 43 may be formed only on the pixels of other colors.

  In addition, an orientation polarization material is used for the hole transport layer, an electron blocking layer is disposed between the hole transport layer and the light emitting layer, and a positive fixed charge is accumulated on the hole transport layer electron blocking layer side. May be. Also in this structure, the threshold voltage can be controlled by adjusting the thickness of the electron blocking layer, as in the above embodiment.

  The OLED 4 may be a multi-photo emission element. That is, the organic layer 40 may have a plurality of light emitting layers 42 in each pixel. A charge generation layer may be formed between the two adjacent light emitting layers 42. Even in this case, the threshold voltage of each OLED 4 can be controlled by the hole blocking layer 43 and the electron transport layer 44.

  DESCRIPTION OF SYMBOLS 1 Display apparatus, 2 anode, 3 cathode, 4 organic light emitting diode (OLED), 5 barrier layer, 6 filler, 9 counter substrate, 10 TFT substrate, 10a base material, 21 bank layer, 30 circuit layer, 31B scanning signal line , 31C video signal line, 31D drive power supply line, 31a switching TFT, 31b drive TFT, 31c capacitor, 40 organic layer, 41 hole transport layer, 42 light emitting layer, 43 hole blocking layer, 44 electron transport layer, Pb blue pixel , Pg Green pixel, Pr Red pixel.

Claims (14)

An anode,
A cathode,
A light emitting layer formed between the anode and the cathode;
An electron transport layer that is formed between the cathode and the light emitting layer and transports electrons supplied from the cathode to the light emitting layer;
A hole blocking layer that is formed between the electron transport layer and the light emitting layer and blocks holes from the anode toward the cathode;
The display device characterized in that the material of the electron transport layer is a material that accumulates a negative fixed charge on the hole blocking layer side of the electron transport layer. 2. The display according to claim 1, wherein the electron transport layer includes a material that causes orientation polarization, and accumulates negative fixed charges on the hole blocking layer side of the electron transport layer by the orientation polarization. apparatus. A first pixel having the light emitting layer;
A second pixel having the light emitting layer,
The display device according to claim 1, wherein a light emission color of the light emitting layer of the first pixel and a light emission color of the light emitting layer of the second pixel are different from each other. Both the first pixel and the second pixel have the anode, the light emitting layer, the cathode, the electron transport layer, and the hole blocking layer,
The display device according to claim 3, wherein the hole blocking layer of the first pixel and the hole blocking layer of the second pixel are formed separately. The first pixel has the anode, the light emitting layer, the cathode, the electron transport layer, and the hole blocking layer,
The second pixel has the anode, the light emitting layer, the cathode, and the electron transport layer,
A hole blocking layer thinner than the hole blocking layer of the first pixel is formed in the second pixel, or a hole blocking layer is not formed in the second pixel. The display device according to claim 3 or 4. The display device according to claim 5, wherein the light emitting layer of the second pixel emits light having a shorter wavelength than light emitted from the light emitting layer of the first pixel. The emission color of the light emitting layer of the first pixel is green or red,
The display device according to claim 6, wherein an emission color of the light emitting layer of the second pixel is blue. A first pixel having the anode, the light emitting layer, the cathode, the electron transport layer, and the hole blocking layer;
A second pixel having the anode, the light emitting layer, the cathode, and the electron transport layer;
In the second pixel, a hole blocking layer thinner than the hole blocking layer of the first pixel is formed, or a hole blocking layer is not formed,
When it is assumed that there is no hole blocking layer in the first pixel and the second pixel, the threshold voltage between the anode and the cathode that emits light from the second pixel is that the first pixel emits light. The display device according to claim 1, wherein a threshold voltage between the anode and the cathode is larger. A first pixel having the light emitting layer;
A second pixel having the light emitting layer;
A third pixel having the light emitting layer,
The emission color of the emission layer of the first pixel, the emission color of the emission layer of the second pixel, and the emission color of the emission layer of the third pixel are different from each other,
The display device according to claim 1, wherein the hole blocking layer having the same thickness is formed in the first pixel and the third pixel. The display device according to any one of claims 1 to 9, wherein the electron transport layer includes at least one of TPBi, BCP, Alq3, and OXD-7. The display device according to any one of claims 1 to 10, wherein the hole blocking layer includes at least one of PPT, DPEPO, and TAZ. The hole blocking layer has an electron mobility of 10 ⁻⁵ cm ² / Vs or more and a hole mobility of 10 ⁻⁸ cm ² / Vs or less. The display device according to claim 11. A light emitting layer;
A first electrode for supplying a first charge, which is one of electrons and holes, to the light emitting layer;
A second electrode disposed on the opposite side of the first electrode across the light emitting layer and supplying a second charge that is the other charge of electrons and holes to the light emitting layer;
A first charge transporting layer disposed between the first electrode and the light emitting layer for transporting the first charge supplied from the first electrode to the light emitting layer;
A second charge blocking layer disposed between the first charge transport layer and the light emitting layer and blocking the second charge from the second electrode toward the first electrode;
The material of the first charge transport layer is a material that accumulates the first charge on the second charge blocking layer side of the first charge transport layer. The first charge transport layer includes a material that causes orientation polarization, and the first charge is accumulated on the second charge blocking layer side of the first charge transport layer by the orientation polarization. 13. The display device according to 13.

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