WO2008136583A1

WO2008136583A1 - Organic light emitting device

Info

Publication number: WO2008136583A1
Application number: PCT/KR2008/001829
Authority: WO
Inventors: Dae-Gyu Moon; Mi-Young Ha; Chang-Kyo Kim
Original assignee: Soonchunhyang University Industry Academy Cooperation Foundation
Priority date: 2007-05-02
Filing date: 2008-04-01
Publication date: 2008-11-13

Abstract

Provided is an organic light emitting device (OLED) capable of stably securing low- voltage performance by using Ph3PO for an organic material layer serving as an electron-transporting layer or a hole-blocking layer in the OLED. The OLED includes an anode, a cathode, and a plurality of organic thin films disposed between the anode and the cathode. One or both of the organic thin films serving as a hole-blocking layer and an electron-transporting layer are formed Of Ph3PO.

Description

ORGANIC LIGHT EMITTING DEVICE

Technical Field

[1] The present disclosure relates to an organic light emitting device (OLED), and more particularly, to an OLED capable of stably ensuring low- voltage performance by using Ph₃PO for an organic material layer serving as an electron-transporting layer or a hole- blocking layer in the OLED. Background Art

[2] Recently, demands for flat panel display devices with a small occupation area are increasing according to slimness of display devices. Examples of flat panel display devices include liquid crystal displays (LCDs), field emission displays (FEDs), plasma display panels (PDPs) and organic light emitting devices (OLEDs).

[3] The OLED includes an organic material layer, an anode and a cathode. The organic material layer includes a hole-injection layer, a hole-transporting layer, a light-emitting layer, a hole-blocking layer and an electron-transporting layer. The anode and the cathode are attached to both sides of the organic material layer. The OLED does not need an additional light source, and thus it is possible to reduce volume and weight of the device. Further, the OLED has advantageous characteristics such as wide viewing angle, high aperture ratio, high brightness, low-power consumption, high response rate and low weight. In addition, since a fabrication process of the OLED is simple, fabrication cost can be significantly reduced compared to a conventional LCD.

[4] When a positive voltage is applied to the anode and a negative voltage is applied to the cathode in the OLED, holes are injected from the anode and then supplied to the light-emitting layer via the hole-injection layer and the hole-transporting layer, whereas electrons are supplied from the cathode to the light-emitting layer via the electron-transporting layer. Subsequently, the holes and the electrons are coupled to form excitons, and light is thus generated and emitted to the outside while the excitons are being transited to a ground state. The OLED can be driven at a low voltage in a range of approximately 5 V to 20 V, and therefore various studies are being concentrated on the OLED.

[5] To improve the performance of the OLED, various structures have been proposed.

The OLED may have a simple structure of anode/hole-transporting layer/light-emitting layer/cathode (see FIG. 1), wherein the light-emitting layer also serves as an electron- transporting layer. Alternatively, the OLED may have a structure of anode/ light- emitting layer/electron-transporting layer/cathode (see FIG. 2), wherein the light- emitting layer also serves as a hole-transporting layer. Alternatively, by dividing the organic material layer into several sub-organic layers, the OLED may have a structure of anode/hole-injection layer/hole-transporting layer/light-emitting layer/ electron-transporting layer/electron-injection layer/cathode (see FIG. 3) or a structure of anode/hole-injection layer/hole-transporting layer/light-emitting layer/hole -blocking layer/electron-transporting layer/electron-injection layer/cathode (see FIG. 4).

[6] In order to realize low- voltage performance of the OLED, the anode and the cathode must have excellent thin film properties, and further a boundary between the electrode and the organic material layer, and each of the organic layers must have excellent characteristics.

Disclosure of Invention Technical Problem

[7] The present disclosure provides an organic light emitting device (OLED) capable of stably ensuring low-voltage performance by using Ph₃PO for an organic material layer which serves as an electron-transporting layer or a hole-blocking layer in the OLED. Technical Solution

[8] According to an exemplary embodiment, an organic light emitting device (OLED) includes an anode, a cathode, and a plurality of organic thin films disposed between the anode and the cathode, wherein at least one of the organic thin films serving as a hole- blocking layer and an electron-transporting layer are formed of Ph₃PO having the following chemical formula

[10] According to exemplary embodiments, at least one of the organic thin films serving as an electron-transporting layer and a hole-blocking layer are formed of Ph₃PO in a configuration of OLED including the anode, the cathode and the organic thin films provided therebetween, whereby a driving voltage can be significantly reduced.

Advantageous Effects

[11] According to exemplary embodiments, at least one of organic thin films serving as an electron-transporting layer and a hole-blocking layer are formed of Ph₃PO in configuration of OLED including an anode, a cathode and the organic thin films disposed between the anode and cathode, whereby a driving voltage can be significantly reduced. [12] Furthermore, by changing a structure of a light-emitting layer, the OLED of the exemplary embodiments of the present invention can emit white light even though fluorescent dopant is heavily doped.

Brief Description of the Drawings

[13] Exemplary embodiments can be understood in more detail from the following description taken in conjunction with the accompanying drawings, in which: [14] FIGS. 1 through 4 are sectional views illustrating various structures of organic light emitting devices (OLEDs);

[15] FIG. 5 is a sectional view of an OLED according to a first exemplary embodiment;

[16] FIG. 6 is a sectional view of an OLED according to a second exemplary embodiment;

[17] FIG. 7 is a sectional view of an OLED according to a third exemplary embodiment;

[18] FIG. 8 is a sectional view of an OLED according to a fourth exemplary embodiment;

[19] FIG. 9 is a sectional view of an OLED according to a fifth exemplary embodiment;

[20] FIG. 10 is a graph illustrating current density versus an applied voltage in the OLED of FIG. 5 according to the first exemplary embodiment; [21] FIG. 11 is a graph illustrating brightness versus an applied voltage in the OLED of

FIG. 5 according to the first exemplary embodiment; [22] FIG. 12 is a graph illustrating current density versus an applied voltage in a related art OLED of a first comparison example; [23] FIG. 13 is a graph illustrating brightness versus an applied voltage in the related art

OLED of the first comparison example; [24] FIG. 14 is a graph illustrating current density versus an applied voltage in the OLED of FIG. 6 according to a second exemplary embodiment; [25] FIG. 15 is a graph illustrating current density versus an applied voltage in a related art OLED of a second comparison example; [26] FIG. 16 is a graph illustrating current density versus an applied voltage in the OLED of FIG. 7 according to a third exemplary embodiment; [27] FIG. 17 is a graph illustrating brightness versus an applied voltage in the OLED of

FIG. 7 according to the third exemplary embodiment; [28] FIG. 18 is a graph illustrating a spectrum of the OLED of FIG. 7 according to the third exemplary embodiment; [29] FIG. 19 is a graph illustrating current density versus an applied voltage in the OLED of FIG. 8 according to a fourth exemplary embodiment; [30] FIG. 20 is a graph illustrating brightness versus an applied voltage in the OLED of

FIG. 8 according to the fourth exemplary embodiment; [31] FIG. 21 is a graph illustrating a spectrum of the OLED of FIG. 8 according to the fourth exemplary embodiment;

[32] FIG. 22 is a graph illustrating current density versus an applied voltage in the OLED of FIG. 9 according to a fifth exemplary embodiment;

[33] FIG. 23 is a graph illustrating brightness versus an applied voltage in the OLED of

FIG. 9 according to the fifth exemplary embodiment;

[34] FIG. 24 is a graph illustrating a spectrum of the OLED of FIG. 9 according to the fifth exemplary embodiment;

[35] FIG. 25 is a graph illustrating spectrum variations according to an applied voltage in the OLED of FIG. 9 according to the fifth exemplary embodiment; and

[36] FIG. 26 shows commission Internationale de I'Eclairage (CIE) Chromaticity coordinates of the OLED of FIG. 9 according to the fifth exemplary embodiment. Best Mode for Carrying Out the Invention

[37] Hereinafter, specific embodiments will be described in detail with reference to the accompanying drawings.

[38] Terms used in the present disclosure will now be defined. An OLED includes an anode, a cathode, and an organic material layer therebetween, wherein the organic material layer includes at least one organic thin films, necessarily including a light- emitting layer.

[39] At least one of the organic thin films serving as an electron-transporting layer and a hole-blocking layer are formed of Ph₃PO having the following chemical formula 1 in configuration of the organic material layer of the OLED according to exemplary embodiments.

[40] [Chemical Formula 1]

[42] As shown in FIGS. 1 through 4, the OLED may be modified and embodied as various structures. In a structure of anode/hole-transporting layer/light-emitting layer/ cathode of FIG. 1, the light-emitting layer serves as an electron-transporting layer, and the light-emitting layer of FIG. 1 can be formed of Ph₃PO to implement an OLED according to an exemplary embodiment. Ph₃PO is not a light-emitting material. Therefore, when the organic thin film corresponding to the light-emitting layer is formed of Ph₃PO, the hole-transporting layer serves as both the hole-transporting layer and the light-emitting layer. [43] FIGS. 2 and 3 respectively show a structure of anode/light-emitting layer/ electron-transporting layer/cathode, and a structure of anode/hole-injection layer/ hole-transporting layer/light-emitting layer/electron-transporting layer/ electron-injection layer/cathode. Since the electron-transporting layer is clearly provided in both structures, the electron-transporting layer can be formed of Ph₃PO to implement an OLED according to an exemplary embodiment.

[44] FIG. 4 shows a structure of anode/hole-injection layer/hole-transporting layer/ light-emitting layer/hole-blocking layer/electron-transporting layer/electron-injection layer/cathode. In the structure of FIG. 4, both the electron-transporting layer and the hole-blocking layer are provided. Thus, Ph₃PO can be used for at least one of the electron-transporting layer and the hole-blocking layer to implement an OLED according to an exemplary embodiment.

[45] Of course, Ph₃PO can be used in various OLEDs including organic thin films serving as an electron-transporting layer or a hole-blocking layer besides the OLEDs shown in FIGS. 1 through 4.

[46] Hereinafter, an OLED according to a first exemplary embodiment will be described in detail. FIG. 5 is a sectional view of an OLED according to the first exemplary embodiment, and FIG. 6 is a sectional view of an OLED according to a second exemplary embodiment. FIG. 7 is a sectional view of an OLED according to a third exemplary embodiment, FIG. 8 is a sectional view of an OLED according to a fourth exemplary embodiment, and FIG. 9 is a sectional view of an OLED according to a fifth exemplary embodiment.

[47] In a structure of the OLED of FIG. 5 according to the first embodiment, Ph₃PO is used for a hole-blocking layer. Referring to FIG. 5, an anode and a cathode are provided on a transparent substrate, and a hole-injection layer, a hole-transporting layer, a light-emitting layer, a hole blocking layer, an electron-transporting layer, and an electron-injection layer are sequentially stacked between the anode and the cathode.

[48] In detail, a glass substrate is used as the transparent substrate, and indium tin oxide

(ITO) is used for the anode, and (4,4',4"-tris)-N-(2-naphthyl)-N-phenyl-amino- tri- phenylamine (2-TNATA) of the following chemical formula 2 is stacked on the ITO to have a thickness of approximately 15 nm as the hole-injection layer. The hole-injection layer, i.e., 2-TNATA, is deposited at a rate of approximately 1.0 A/sec to 1.5 A/sec under pressure of approximately 2x10⁶ torr. [49] [Chemical Formula 2]

[51] N,N'-di(naphthanlene-l-yl)-N,N'-diphenylbenzidine (NPB or α-NPD) of the following chemical formula 3 is stacked on the hole-injection layer to have a thickness of approximately 35 nm as the hole-transporting layer. As in the deposition of 2-TNATA, the hole-transporting layer, i.e., NPB or α-NPD is also deposited at a rate of approximately 1.0 A/sec to 1.5 A/sec under pressure of approximately 2xlO^~6 torr.

[52] [Chemical Formula 3]

[54] 4,4'-bis(2,2'-diphenyl-ethene-l-yl)-biphenyl (DPVBi) of the following chemical formula 4 is stacked on the hole-transporting layer to have a thickness of approximately 40 nm as the light-emitting layer. Ph₃PO is then stacked on the light- emitting layer to have a thickness of approximately 5 nm as the hole-blocking layer. Also, tris(8-hydroxyquinolinato)aluminum (AIq₃) of the following chemical formula 5 is stacked on the hole-blocking layer to have a thickness of approximately 5 nm as the electron-transporting layer. Then, LiF is stacked thereon to have a thickness of approximately 0.5 nm as the electron-injection layer. Finally, aluminum (Al) is stacked on the electron-injection layer to have a thickness of approximately 100 nm as the cathode.

[55] [Chemical Formula 4]

[57] [Chemical Formula 5]

[59] Electrical characteristics of the OLED of FIG. 5 according to the first exemplary embodiment will now be described. FIG. 10 is a graph illustrating current density versus an applied voltage in the OLED of FIG. 5, and FIG. 11 is a graph illustrating brightness versus an applied voltage in the OLED of FIG. 5.

[60] As shown in FIG. 10, when a driving voltage of approximately 4 V is applied, a current of approximately 1 mA/cm² flows. Referring to FIG. 11, when a driving voltage of approximately 9 V is applied, brightness of 6000 cd/m² is obtained.

[61] Electrical characteristics of the OLED of FIG. 5 according to the first exemplary embodiment will be now described in comparison with a typical OLED. For comparison, a related art OLED is fabricated, and electrical characteristics thereof are measured.

[62] For comparison of the electrical characteristics of the OLED of FIG. 5 according to the first exemplary embodiment, a related art OLED of a first comparison example is fabricated. The OLED of the first comparison example has a structure of transparent substrate/anode/hole-injection layer/hole-transporting layer/light-emitting layer/ hole-blocking layer/electron transporting layer/electron-injection layer/cathode, like the OLED of the first exemplary embodiment. Further, the related art OLED of the first comparison example uses bathocuproine (BCP) of the following chemical formula 6 for the hole-blocking layer, whereas the OLED of the first exemplary embodiment uses Ph₃PO for the hole-blocking layer. Except for the hole-blocking layers, the OLED of the first exemplary embodiment and the related art OLED of the first comparison example are identical in materials, stack thicknesses, and stack methods of other elements.

[63] [Chemical Formula 6]

[64]

[65] The electrical characteristics of the related art OLED of the first comparison example will now be described. FIG. 12 is a graph illustrating current density versus an applied voltage in the related art OLED of the first comparison example, and FIG. 13 is a graph illustrating brightness versus an applied voltage in the related art OLED of the first comparison example.

[66] Referring to FIG. 12, a driving voltage of approximately 5.5 V is applied to obtain a current density of approximately 1 mA/cm². Referring to FIG. 13, when a driving voltage of approximately 9 V is applied, brightness is approximately 1000 cd/m². In comparison, the OLED according to the first exemplary embodiment requires a driving voltage of just approximately 4 V to obtain a current density of approximately lmA/cm², and achieves brightness of approximately 6000 cd/m² with a driving voltage of approximately 9 V. Accordingly, it can be seen that the OLED of FIG. 5 can realize a high current density and a high brightness characteristic with a relatively small driving voltage as compared to the related art OLED of the first comparison example.

[67] The OLED according to the first exemplary embodiment using Ph₃PO for the hole- blocking layer, and the electrical characteristics thereof have been described so far. Hereinafter, an OLED of FIG. 6 according to a second exemplary embodiment using Ph₃PO for an organic thin film serving as an electron-transporting layer, and electrical characteristics thereof will now be described.

[68] Referring to FIG. 6, in a structure of the OLED according to the second exemplary embodiment, an anode and a cathode are provided on a transparent substrate, and organic thin films are provided between the anode and the cathode, wherein the organic thin films serve as a hole-injection layer, a hole-transporting layer, a light- emitting layer, an electron-transporting layer, and an electron-injection layer.

[69] In detail, a glass substrate is used as the transparent substrate, indium tin oxide (ITO) is used for the anode, and (4,4',4"-tris)-N-(2-naphthyl)-N-phenyl-amino- tri- phenylamine (2-TNATA) of the chemical formula 2 is stacked on the ITO to have a thickness of approximately 15 nm as the hole-injection layer. The hole-injection layer, i.e., 2-TNATA is deposited at a rate of approximately 1.0 A/sec to 1.5 A/sec under pressure of approximately 2x10⁶ torr.

[70] N,N'-di(naphthanlene- 1 -yl)-N,N'-diphenylbenzidine (NPB or α-NPD) of the chemical formula 3 is stacked on the hole-injection layer to have a thickness of approximately 40 nm to serve as both the hole-transporting layer and the light-emitting layer. As in the deposition of 2-TNATA, the organic thin film serving as both the hole- transporting layer and the light-emitting layer i.e., NPB or α-NPD is deposited at a rate of approximately 1.0 A/sec to 1.5 A/sec under pressure of approximately 2xlO ⁶ torr.

[71] Ph₃PO is stacked on NPB or α-NPD to have a thickness of approximately 60 nm to serve as the electron-transporting layer. Then, LiF is stacked on the electron- transporting layer to have a thickness of approximately 0.5 nm as the electron-injection layer. Aluminum (Al) is stacked on the electron-injection layer to have a thickness of approximately 100 nm as the cathode.

[72] To describe electrical characteristics of the OLED according to the second exemplary embodiment, comparison of electrical characteristics between the OLED according to the second exemplary embodiment and a related art OLED of a second comparison example will be made as in the previous exemplary embodiment. For the comparison, the related art OLED of the second comparison example is fabricated, and the electrical characteristics of the OLED of the second exemplary embodiment and the related art OLED of the second comparison are measured, respectively.

[73] The related art OLED of the second comparison example includes organic thin films between an anode and a cathode, wherein the organic thin films serve as a hole- injection layer, a hole-transporting layer, a light-emitting layer, an electron- transporting layer, and an electron-injection layer like the OLED of the second exemplary embodiment. However, the organic thin films of the second comparison example has a structure of 2TNATA/NPD/Alq₃/LiF, whereas the organic thin films of the second exemplary embodiment has a structure of 2TNATA/NPD/Ph₃PO/LIF. That is, AIq₃ which serves as the light-emitting layer in the second comparison example is substituted with Ph₃PO in the OLED of the second exemplary embodiment. The OLED according to the second exemplary embodiment and the OLED of the second comparison example are identical in materials, stack thicknesses, and stack methods of other elements. There are functional differences between the OLED of the second exemplary embodiment and the OLED of the second comparison example. For example, in the OLED of the second exemplary embodiment , NPD serves as the hole- transporting layer and the light-emitting layer and Ph₃PO serves as the electron- transporting layer, whereas in the second comparison example, NPD serves as the hole-transporting layer, and AIq₃ serves as the light-emitting layer and the electron- transporting layer. However, even if there are such functional differences therebetween, operations of the entire OLEDs are performed in the same manner. Accordingly, the fact that AIq₃ is substituted with Ph₃PO does not affect measurement and comparison of the electrical characteristics thereof.

[74] The electrical characteristics of the OLED of the second exemplary embodiment is compared with the related art OLED of the second comparison example. FIG. 14 is a graph illustrating current density versus an applied voltage in the OLED of the second exemplary embodiment, and FIG. 15 is a graph illustrating current density versus an applied voltage in the related art OLED of the second comparison example.

[75] Referring to FIG. 14, a current of approximately lmA/cm² flows when a driving voltage of approximately 1 V is applied in the OLED of the second exemplary embodiment. However, as shown in FIG. 15, a driving voltage of approximately 6 V must be applied to obtain a current density of approximately 1 mA/cm² in the related art OLED of the second comparison example. Accordingly, it can be seen that the OLED of the second exemplary embodiment can realize a high current density with a relatively small driving voltage as compared to the OLED of the second comparison example.

[76] The electrical characteristics of the OLED of the second exemplary embodiment using Ph₃PO for the electron-transporting layer have been described so far. Hereinafter, there will be described an OLED of FIG. 7 according to a third exemplary embodiment which includes an electron-transporting layer formed of Ph₃PO and a light-emitting layer doped with rubrene of the following chemical formula 7, and electrical and optical characteristics thereof.

[77] [Chemical Formula 7]

[79] In the OLED shown in FIG. 7, an anode and a cathode are provided on a transparent substrate, and organic thin films are provided between the anode and the cathode, wherein the organic thin films serve as a hole-injection layer, a hole-transporting layer, a light-emitting layer, an electron-transporting layer, and an electron-injection layer.

[80] In detail, a glass substrate is used as the transparent substrate, and indium tin oxide

(ITO) is used for the anode, and (4,4',4"-tris)-N-(2-naphthyl)-N-phenyl-amino- tri- phenylamine (2-TNATA) of the chemical formula 2 is stacked on the ITO to have a thickness of approximately 15 nm to serve as the hole-injection layer. The hole- injection layer, i.e., 2-TNATA, is deposited at a rate of approximately 1.0 A/sec to 1.5 A/sec under pressure of approximately 2x10⁶ torr. [81] To form a layer serving as the light-emitting layer

N,N'-di(naphthanlene-l-yl)-N,N'-diphenylbenzidine (NPB or α-NPD) of the chemical formula 3 doped with 1% -rubrene of the chemical formula 7 is stacked on the hole- injection layer to have a thickness of approximately 10 nm. α-NPD or NPB is deposited at a rate of approximately 2.5 A/sec under pressure of approximately 2xlO⁶ torr, and the dopant is deposited at a rate of approximately 0.025 A/sec. Thereafter, α- NPD or NPB is further stacked to have a thickness of approximately 30 nm on the α- NPD or NPB doped with 1%-rubrene.

[82] Ph₃PO is stacked on the α-NPD to have a thickness of approximately 60 nm as the electron-transporting layer. Then, LiF is stacked thereon to have a thickness of approximately 0.5 nm as the electron-injection layer. Finally, aluminum (Al) is stacked on the electron-injection layer to have a thickness of approximately 100 nm as the cathode.

[83] Electrical characteristics of the OLED of FIG. 7 according to the third exemplary embodiment will now be described. FIG. 16 is a graph illustrating current density versus an applied voltage in the OLED of the third exemplary embodiment. When a voltage of approximately 2.5 V is applied, a current density of approximately 1 mA/cm 2 is obtained, and when a voltage of approximately 5.5 V is applied, a current density of approximately 140 mA/cm² is obtained. Since the current density greatly increases with respect to a small voltage increase by 3 V, it can be seen that the OLED of the third exemplary embodiment can realize a high current density with a low driving voltage. Optical characteristics of the OLED of the third exemplary embodiment will now be described. FIG. 17 is a graph illustrating brightness versus an applied voltage in the OLED of the third exemplary embodiment. When a voltage of approximately 3.0 V is applied, brightness of 6 cd/m² is obtained, and when a voltage of approximately 4.1 V is applied, brightness of 1,160 cd/m² is obtained. Since the brightness significantly increases with respect to a small voltage increase by 1 V, it can be seen that the OLED of the third exemplary embodiment can realize a highly efficient device having a high current density and high brightness.

[84] In the OLED of the third exemplary embodiment, light emission occurs at an area where α-NPD or NPB is doped with rubrene. FIG. 18 shows an emission spectrum of the OLED of the third exemplary embodiment.

[85] In FIG. 18, wavelengths of the highest two peaks are approximately 466 nm and approximately 557 nm. The peak shown at approximately 466 nm is for α-NPD or NPB, and the peak shown at approximately 557 nm is for rubrene. Emitted light of the OLED of the third exemplary embodiment is yellow.

[86] The aforementioned results are obtained by doping the light-emitting layer of the

OLED of the second exemplary embodiment with 1%-rubrene. Even if the OLED of the third exemplary embodiment has a lower current density than the OLED of the second exemplary embodiment, the brightness of the OLED of the third exemplary embodiment is more excellent than that of the OLED of the third exemplary embodiment. [87] The electrical and optical characteristics of the OLED of the third exemplary embodiment have been described so far, wherein the electron-transporting layer is formed of Ph₃PO and the light-emitting layer is formed of α-NPD or NPB doped with rabrene. Hereinafter, an OLED of FIG. 8 according to a fourth exemplary embodiment, and electrical and optical characteristics thereof will now be described. The OLED of the fourth exemplary embodiment is fabricated by using

4,4'-bis(2,2'-diphenyl-ethene-l-yl)-biphenyl (DPVBi) of the chemical formula 4 as a blue host material of a light-emitting layer instead of α-NPD or NPB according to the third exemplary embodiment FIG. 7.

[88] As shown in FIG. 8, in a structure of the OLED, an anode and a cathode are provided on a transparent substrate, and organic thin films are provided between the anode and the cathode, wherein the organic thin films serve as a hole-injection layer, a hole- transporting layer, a light-emitting layer, an electron-transporting layer, and an electron-injection layer like the OLED of the third exemplary embodiment.

[89] In detail, a glass substrate is used as the transparent substrate, and indium tin oxide

(ITO) is used for the anode, and 2-TNATA of the chemical formula 2 is stacked on the ITO to have a thickness of approximately 15 nm to serve as the hole-injection layer. The hole-injection layer, i.e., 2-TNATA, is deposited at a rate of approximately 1.0 A/ sec to 1.5 A/sec under pressure of approximately 2xlO^~6 torr.

[90] DPVBi of the chemical formula 4 doped with 1%-rubrene is stacked on the hole- injection layer to have a thickness of approximately 10 nm as a host material of the light-emitting layer. DPVBi is deposited at a rate of approximately 2.5 A/sec under pressure of approximately 2xlO ⁶ torr, and the dopant, i.e., rubrene is deposited at a rate of 0.025 A/sec. Thereafter, DPVBi is stacked further to have a thickness of approximately 30 nm on a structure prepared by doping DPVBi with 1%- rubrene.

[91] Ph₃PO is stacked on DPVBi to have a thickness of approximately 60 nm as the electron-transporting layer. Then, LiF is stacked to have a thickness of approximately 0.5 nm on the electron-transporting layer as the electron-injection layer. Finally, aluminum (Al) is stacked to have a thickness of approximately 100 nm on the electron- injection layer as the cathode.

[92] Electric characteristics of the OLED of the fourth exemplary embodiment will now be described. FIG. 19 is a graph illustrating current density versus an applied voltage in the OLED of the fourth exemplary embodiment. Referring to FIG. 19, in the OLED of the fourth exemplary embodiment, when a driving voltage of approximately 2 V is applied, a current density of approximately 1.3 mA/cm² is obtained. Also, when a driving voltage of approximately 5.5 V is applied, a current density of approximately 326 mA/cm² is obtained. The current density significantly increases with respect to a small voltage increase. This result is obtained by using Ph₃PO for the electron- transporting layer, which represents that Ph₃PO has an excellent electron-flow characteristic. FIG. 20 is a graph illustrating brightness versus an applied voltage in the OLED of the fourth exemplary embodiment. Referring to FIG. 20, in the OLED of the fourth exemplary embodiment, when a driving voltage of approximately 2.1 V is applied, brightness of 0.74 cd/m² is obtained, and when a voltage of approximately 3.0 V is applied, brightness of 1,017 cd/m² is obtained. The OLED of the fourth exemplary embodiment shows excellent brightness characteristics of 1,017 cd/m² with respect to a small voltage increase just by 0.9 V from 2.1 V.

[93] In the OLED of the fourth exemplary embodiment, light emission occurs at an area in which DPVBi is doped with rubrene, and an emission spectrum of the OLED of FIG. 8 is shown in FIG. 21.

[94] A peak wavelength of the light emission is shown at approximately 555 nm which is a wavelength of rubrene. Emitted light of the OLED of the fourth exemplary embodiment is yellow similar to a color of the emitted light of the OLED of the third exemplary embodiment. However, the emission spectrums are measure to be different. In FIG. 21, a wavelength area of DPVBi according to the current exemplary embodiment is rarely observed, whereas in FIG. 18, the wavelength of α-NPD or NPB, which is a blue host material, is easily observed in the emission spectrum of the OLED of the third exemplary embodiment. This indicates that exciplex takes place between the organic thin films in the OLED of the fourth exemplary embodiment.

[95] The results of the fourth exemplary embodiment are obtained by substituting DPVBi for α-NPD or NPB, a blue host material of the light-emitting layer of the OLED of the third exemplary embodiment, and doping DPVBi with 1%-rubrene. It can be seen that the current density and brightness characteristics of the OLED of the fourth exemplary embodiment are more excellent than those of the OLED the fourth exemplary embodiment.

[96] The electrical and optical characteristics of the OLED of the fourth exemplary embodiment have been described so far, wherein the rubrene-doped DPVBi is used for a blue host material of the light-emitting layer instead of α-NPD or NPB, and Ph₃PO is used for the electron-transporting layer. According to a fifth exemplary embodiment, a white-light emission is achieved by changing a structure of the OLED in which Ph₃PO is used for an electron-transporting layer. Hereinafter, an OLED according to the fifth exemplary embodiment will now be described. Also, electrical and optical characteristics of the white-light emitting OLED will be described.

[97] Referring to FIG. 9, in the OLED according to the fifth exemplary embodiment, an anode and a cathode are provided on a transparent substrate, and organic thin films are provided between the anode and the cathode, wherein the organic thin films serve as a hole-injection layer, a hole-transporting layer, a light-emitting layer, an electron- transporting layer, and an electron-injection layer.

[98] In detail, a glass substrate is used as the transparent substrate, and indium tin oxide

(ITO) is used for the anode, and 2-TNATA of the chemical formula 2 is stacked on the ITO to have a thickness of approximately 15 nm to serve as the hole-injection layer.

2-TNATA is deposited at a rate of approximately 1.0 A/sec to 1.5 A/sec under pressure of approximately 2x10⁶ torr.

[99] α-NPD or NPB of the chemical formula 3 is stacked on the hole-injection layer to have a thickness of approximately 3 nm to serve as the hole-transporting layer. In addition, the α-NPD or NPB layer prevents the 2-TNATA organic thin film and a DPVBi organic thin film from being stacked successively. As shown in FIG. 21, a blue peak is decreased due to the exciplex between the 2-TNATA organic thin film and DPVBi organic thin film that are successively deposited according to the fourth exemplary embodiment. The decrease of the blue peak makes white-light emission difficult. The decrease of the blue peak is prevented by disposing α-NPD or NPB between the 2-TNATA and DPVBi organic thin films, whereby white-light emission can be achieved. Accordingly, a thin layer of α-NPD or NPB of the chemical formula 3 is inserted to have a thickness of approximately 3 nm to prevent the exciplex. The α- NPD or NPB is deposited at a rate of approximately 1.0 A/sec to 1.5 A/sec under pressure of approximately 2xlO^~6 torr as in the deposition of 2-TNATA.

[100] A thin film of DPVBi of the chemical formula 4 is deposited to have a thickness of approximately 3nm on the hole-transporting layer at the same rate and pressure as those of the aforementioned organic thin film. Thereafter, DPVBi doped with 2%-rubrene of the chemical formula 7 is stacked to have a thickness of approximately IOnm on the DPVBi thin layer. Thereafter, DPVBi is further stacked to have a thickness of approximately 25 nm on the rubrene-doped DPVBi organic thin film. As in the deposition of 2-TNATA, the DPVBi organic thin films and the DPVBi:rubrene(2%) organic thin film serving as the light-emitting layer are deposited under pressure of approximately 2xlO ⁶ torr. DPVBi is deposited at a rate of approximately 2.5 A/sec, and rubrene is deposited at a rate of approximately 0.025 A/sec.

[101] Ph₃PO is stacked with a thickness of approximately 60 nm on the light-emitting layer as the electron-transporting layer. LiF is stacked thereon to have a thickness of approximately 0.5 nm as the electron-injection layer. Finally, aluminum (Al) is stacked on the electron-injection layer with a thickness of approximately 100 nm as the cathode.

[102] Electrical and optical characteristics of the OLED of the fifth exemplary embodiment will now be described.

[103] FIG. 22 is a graph illustrating current density versus an applied voltage in the OLED of the fifth exemplary embodiment, and FIG. 23 is a graph illustrating brightness versus an applied voltage in the OLED of the fifth exemplary embodiment. As shown in FIG. 22, a current density of approximately 0.77 mA/cm² is obtained in the OLED of the fifth exemplary embodiment by a driving voltage of approximately 3 V. Since the OLED of the fifth exemplary embodiment includes more organic thin films compared to other embodiments, a relatively small amount of current density is achieved. As shown in FIG. 23, the OLED of the fifth exemplary embodiment shows brightness of approximately 1.5 cd/m² when a driving voltage of approximately 2.6 V is applied.

[104] Although the electrical and optical characteristics of the OLED of the fifth exemplary embodiment are not better than those of the OLED of the fourth exemplary embodiment, the OLED of the fifth exemplary embodiment can advantageously emit white light, and prevent the exciplex.

[105] Hereinafter, a white-light emission spectrum of the OLED of the fifth exemplary embodiment, and commission Internationale de I'Eclairage (CIE) chromaticity coordinates will be described.

[106] FIG. 24 is a graph illustrating a spectrum of the OLED of the fifth exemplary embodiment. Two peaks in the graph have wavelengths approximately 455 nm and 557 nm, respectively. The peak of 455 nm is for DPVBi and the peak of 557 nm is for rubrene. The α-NPD or NPB organic thin film is disposed between the 2-TNATA organic thin film and the DPVBi organic thin film, thereby white-light emission is achieved.

[107] The white-light emitting layer of the fifth exemplary embodiment is a multi-stacked type. Color of the emitted light varies slightly according to an applied voltage, which is shown in FIG. 25.

[108] Spectrum variations according to the applied voltage can be seen in FIG. 25 representing the OLED of the fifth exemplary embodiment. Particularly, variations of a blue wavelength of approximately 455 nm are not serious.

[109] FIG. 26 shows CIE chromaticity coordinates of the OLED of the fifth exemplary embodiment.

[110] The OLED of the fifth exemplary embodiment has chromaticity coordinates of (0.33, 0.35) at 3 V, (0.33, 0.35) at 4 V, (0.34, 0.36) at 5 V, and (0.35, 0.36) at 6 V.

[I l l] In general, CIE chromaticity coordinates of white are (0.33, 0.33) or (0.34, 0.34).

[112] In the OLED of the fifth exemplary embodiment, the color variation of emitted light caused by changes of current and voltage is small as compared to a white-light emitting OLED including a single light-emitting layer where the color of emitted light varies drastically with a current and voltage. Therefore, according to the exemplary embodiment, a white-light emitting OLED with excellent electrical and optical characteristics and small color variations of emitted light can be achieved. Although the organic light emitting device has been described with reference to the specific embodiments, it is not limited thereto. Therefore, it will be readily understood by those skilled in the art that various modifications and changes can be made thereto without departing from the spirit and scope of the present invention defined by the appended claims.

Claims

[1] An organic light emitting device, comprising: an anode; a cathode; and a plurality of organic thin films disposed between the anode and the cathode, wherein at least one of the organic thin films serving as a hole-blocking layer and an electron-transporting layer are formed of Ph₃PO having the following chemical formula

[2] The organic light emitting device of claim 1, wherein the plurality of organic thin films has a structure in which a hole-transporting layer and a light-emitting layer are sequentially stacked, the light-emitting layer being formed of Ph₃PO.

[3] The organic light emitting device of claim 1, wherein the plurality of organic thin films has a structure in which a light-emitting layer and an electron-transporting layer are sequentially stacked, the electron-transporting layer being formed of Ph

[4] The organic light emitting device of claim 1, wherein the plurality of organic thin films has a structure in which a hole-injection layer, a hole-transporting layer, a light-emitting layer, an electron-transporting layer, and an electron-injection layer are sequentially stacked, the electron-transporting layer being formed of Ph

[5] The organic light emitting device of claim 4, wherein the light-emitting layer is doped with rubrene having the following chemical formula

[6] The organic light emitting device of claim 5, wherein the light-emitting layer has a stacked structure of N,N'-di(naphthanlene-l-yl)-N,N'-diphenylbenzidine (NPB or α-NPD), and NPB or α-NPD doped with 1%- rubrene.

[7] The organic light emitting device of claim 5, wherein the light-emitting layer and the hole-transporting layer has a stacked structure of

4,4'-bis(2,2'-diphenyl-ethene-l-yl)-biphenyl (DPVBi), and DPVBi doped with

1%-rubrene.

[8] The organic light emitting device of claim 4, wherein the organic thin film serving as the light-emitting layer is doped with rubrene having the following chemical formula

[9] The organic light emitting device of claim 8, wherein the light-emitting layer has a stacked structure of 4,4'-bis(2,2'-diphenyl-ethene-l-yl)-biphenyl (DPVBi),

DPVBi doped with 2%-rubrene, and DPVBi. [10] The organic light emitting device of claim 1, wherein the plurality of organic thin films has a structure in which a hole-injection layer, a hole-transporting layer, a light-emitting layer, a hole-blocking layer, an electron-transporting layer, and an electron injection layer are sequentially stacked, and one or both of the hole-blocking layer and the electron-transporting layer are formed of Ph₃PO.