Organic electroluminescence devices (referred to hereinbelow as organic EL devices) have been actively researched in recent years with the object of putting them to practical use. Organic EL devices can realize a high current density at a low voltage and are therefore expected to realize high emission luminance and emission efficiency. The organic EL device is provided with a first electrode and a second electrode that sandwich an organic EL layer, and the electrode on the light take-out side is required to have high transmittance transparent conductive oxide film (TCO) materials (for example, indium tin oxide (ITO), indium zinc oxide (IZO), and indium tungsten oxide (IWO)) are usually used as the electrode materials, and because these materials have a comparatively high work function of about 5 eV, they are used as hole injection electrodes (positive electrodes) for injecting holes into the organic material.
FIG. 1 shows a cross-sectional view illustrating an example of the conventional organic EL device using the conventional hole injection layer, and FIG. 2 illustrates the hole injection process. Emission in the organic EL device is obtained due to light emission occurring during excitation energy relaxation of excitons generated by holes injected from a positive electrode 202 provided on the substrate 201 into a highest occupied molecular orbital (HOMO) of the material of the light-emitting layer 205 and electrons injected from a negative electrode 207 into the lowest unoccupied molecular orbital (LUMO). The organic EL layer includes at least the light-emitting layer 205. In order to inject holes and electrons efficiently into the light-emitting layer, the organic EL layer typically has a stratified structure including a carrier transport layer in addition to the light-emitting layer. The carrier transport layer may also include any of a hole injection layer 203, hole transport layer 204, electron transport layer 206, and electron injection layer or all of these layers. In this case, as shown in FIG. 2, the injection is conducted from a Fermi level EF of the positive electrode 202 into the HOMO of the light-emitting layer 205 via the HOMO of the hole injection layer 203 and the HOMO of the hole transport layer 204.
A technique for increasing the effective mobility of carriers, reducing a barrier for carrier injection from the electrodes, and thus decreasing the drive voltage of the device by doping a carrier transport layer with a dopant in the organic EL device having the organic EL layer of the above-described stratified structure with the object of further reducing power consumption of the organic EL device has recently been disclosed in Japanese Patent Application Laid-open No. H4-297076, Japanese Patent Application Laid-open No. H11-251067, Japanese Translation of PCT Application No. 2004-514257, Organic Electronics, Vol. 4, Issues 2-3 (September 2003), page 89, and Applied Physics Letters, Vol. 73, Issue 20 (November 1998), page 2866 (see Patent Documents 1-3 and Non-Patent Documents 1 and 2).
With this technique, which is similar to p-type doping and n-type doping of inorganic semiconductors, in a case of hole injection layer or hole transport layer, by admixing a material with high electron accepting ability (acceptor) to a hole transport material constituting this layer, it is possible to reduce a barrier to hole injection from the electrode (reduce the difference between the work function of the positive electrode and the HOMO level of the adjacent hole transport material) and increase the effective mobility of holes, and in a case of electron injection layer or electron transport layer, by admixing a material with high electron donating ability (donor), it is possible to reduce a barrier to electron injection from the electrode (reduce the difference between the work function of the negative electrode and the LUMO level of the adjacent electron transport material) and increase the effective mobility of electrons.
A technique has also been suggested by which the driving voltage of the organic EL device is reduced by using an n-type semiconductor with a deep LUMO level or conduction band level in the hole injection layer and increasing the injection capacity of holes from the positive electrode. For example, Japanese Patent Application Laid-open No. 2000-223276, Applied Physics Letters, Vol. 87, Issue 19 (Nov. 7, 2005), Articles 193508, and Journal of Applied Physics, Vol. 101, Issue 2 (Jan. 15, 2007), Articles 026105 suggest techniques using an n-type inorganic semiconductor (see Patent Document 4 and Non-Patent Documents 3 and 4). Further, Japanese Translation of PCT Application No. 2003-519432 suggests a technique using an n-type organic semiconductor (see Patent Document 5; in Japanese Translation of PCT Application No. 2003-519432, the disclosed organic semiconductor material functions as a p-type semiconductor, but is actually an n-type semiconductor). FIG. 3 is a cross-sectional view illustrating an example of the conventional organic EL device using an n-type semiconductor hole injection layer, and FIG. 4 shows a hole injection process therein. The organic EL device shown in FIG. 3 has a positive electrode 302, an n-type semiconductor hole injection layer 303, a hole transport layer 304, a light-emitting layer 305, an electron transport layer 306, and a negative electrode 307 on a substrate 301. The injection of electrons from the negative electrode 307 into the LUMO of the light-emitting layer 305 via the electron transport layer 306 is conducted by a process similar to that in the conventional organic EL device (for example, the device shown in FIG. 1). The holes are formed because the electrons of HOMO of the hole transport layer 304 move to the LUMO to the n-type semiconductor hole injection layer 303 and leave holes in the HOMO of the hole injection layer 304. In this case, the electrons that have moved to the LUMO of the n-type semiconductor hole injection layer 303 are moved to the Fermi level EF of the positive electrode 302, and the holes of the HOMO of the hole transport layer 304 are moved to the HOMO of the light-emitting layer 305 by the electric field applied by the positive electrode 302 and negative electrode 307. As a total, with such a structure, hole injection is conducted from the positive electrode 302 to the light-emitting layer 305. In FIG. 4, the movement of holes is shown by a broken arrow and the movement of electrons is shown by a solid arrow (same hereinbelow).
In such a technique, the Fermi energy is matched among the positive electrode—hole injection material—hole transport material by introducing the aforementioned hole injection layer composed of the n-type semiconductor between the positive electrode and hole transport layer. In this process, the hole injection ability is increased by reducing a barrier for the charge movement at the layer interface. As a result, the drive voltage of the organic EL device decreases.
As for the movement of holes occurring when a voltage is applied to the organic EL device using such a technique, the electrons located on the HOMO (or valence band) of the hole transport layer move to the LUMO (or conduction band) of the hole injection layer and holes are generated in the hole transport layer. The electrons that have entered the hole injection layer are transported via the LUMO (or conduction band) level toward the positive electrode, eventually enter the Fermi level of the positive electrode, and are detected as an electric current. Meanwhile, the holes generated in the hole transport layer move inside the hole transport layer and are injected into the light-emitting layer and used to generate excitons. The technique for reducing the drive voltage of the organic EL device that uses the n-type semiconductor in the hole injection layer is especially effective when a gap between the Fermi level of the positive electrode material and the HOMO (or valence band) level of the hole transport layer is large.    Patent Document 1: Japanese Patent Application Laid-open No. H4-297076    Patent Document 2: Japanese Patent Application Laid-Open No. H11-251067    Patent Document 3: Japanese Translation of PCT Application No. 2004-514257    Patent Document 4: Japanese Patent Application Laid-Open No. 2000-223276    Patent Document 5: Japanese Translation of PCT Application No. 2003-519432    Non-Patent Document 1: Organic Electronics, Vol. 4, Issues 2-3 (September 2003), page 89    Non-Patent Document 2: Applied Physics Letters, Vol. 73, Issue 20 (November 1998), page 2866    Non-Patent Document 3: Applied Physics Letters, Vol. 87, Issue 19 (Nov. 7, 2005), Articles 193508    Non-Patent Document 4: Journal of Applied Physics, Vol. 101, Issue 2 (Jan. 15, 2007), Articles 026105
A means for reducing the thickness of the entire organic EL layer can be considered as a simple method for decreasing the drive voltage of the organic EL device. However, when the thickness of the entire layer is reduced, it can easily induce device defects by a short circuit between the positive electrode and negative electrode caused by particles that have adhered to the substrate. In particular, a large number of pixel defects and/or line defects appear in a flat panel display and the production yield of display panel decreases significantly. Therefore, from the standpoint of increasing the production yield, it is preferred that the thickness of the entire organic EL device be large, but this results in increased drive voltage. Thus, there is a tradeoff between a problem of reducing the drive voltage and a problem of increasing the yield.
When the effective mobility of carriers is increased and the driving voltage is decreased by doping a carrier transport layer with a dopant, as with the conventional technique, the following problem is encountered. Alkali metals such as Li and Cs or alkali metal salts such as LiF, CsF, LiO2, and Cs2CO3 are typically used as dopants in the electron transport layer. However, these materials have low stability in the air and require attention in handling. In addition, in most cases, an expensive vapor deposition source is required. Furthermore, because of their small atomic radii (subnanometer order), alkali metals easily migrate in the organic EL layer and can contaminate other layers constituting the organic EL layer. The possibility of using organic materials with a larger size (nanometer order) and having a comparatively shallow HOMO instead of the aforementioned materials has been studied, but such organic materials are instable and the effect obtained therewith is small.
A problem associated with the configuration using the n-type semiconductor as the hole injection layer is that the electron mobility of the n-type semiconductor is not necessarily high. Therefore, when the thickness of the hole injection layer is increased, an electric resistance rises. As a result, the drive voltage increases.
Further, a reflection component at the interface occurring when light is emitted from the device is especially large in an organic EL device using a low-molecular material. This is because the organic EL layer material and the transparent conductive film used for the electrode have a high refractive index (about n=1.8 to 2.1) and a difference in refractive index between them and a medium (for example glass having a refractive index of about 1.5 and air having a refractive index of about 1) that is in contact with the organic EL device is large. In addition, the emission spectrum and emission luminance of the organic EL device are strongly affected by multiple interference of light in the device. Therefore, from the optical standpoint, the thickness of the entire organic EL layer has to be designed so as to obtain the desired emission spectrum and sufficient emission luminance. However, it is somewhat difficult to design the thickness of each layer with consideration for the effect produced on the drive voltage and the balance of holes and electrons inside the light-emitting layer. In particular, in some cases, when the thickness is designed to obtain a sufficient yield, as described above, the drive voltage rises significantly.