Recently, white organic electroluminescent (EL) devices are being actively developed because they can be used for a mono-color display device, a lighting application such as a back light, and a full-color display with color filters. In the case where white organic EL devices are used for lighting applications, they are required to have a high luminous efficiency, for example, which is equivalent to or more than that of fluorescent lamps.
Many methods of producing white light emission by an organic EL device have been disclosed. Few of the methods produce white light with only one kind of emitting material and a single organic EL device generally uses two or three kinds of emitting materials that emit light simultaneously. In the case of using two kinds of emitting materials, a blue emitting material and a yellow-to-red emitting material, yellow-to-red being the complementary color to blue, are selected. However, the yellow-to-red light emission becomes dominant in many cases, thereby yielding a reddish white color.
Patent document 1 proposes a white device in the type where an emitting layer is divided into two layers, the emission zone of which tends to be localized to the anode side. The tendency for red to be strong in color of emitted light is negated by using a blue emitting layer as an emitting layer on the anode side, and whose color change is suppressed. The level of the luminous efficiency was, however, not necessarily enough.
Patent document 2 discloses an organic EL device in which a red emitting layer, a blue emitting layer, and a green emitting layer are stacked in that order from the anode side. The patent document 2 also discloses technology of reducing a change in color due to an increase in driving current by doping the blue emitting layer with a red dopant used for the red emitting layer. However, the luminous efficiency of this organic EL device is not necessarily satisfactory.
As technology of obtaining white light in a well-balanced manner, technologies of providing a carrier barrier layer between emitting layers have been disclosed.
For example, patent document 3 discloses an organic EL device which emits white light and in which an anode, a hole transporting blue emitting layer, an electron transporting carrier recombination region control layer, an electron transporting red emitting layer, and a cathode are stacked in that order. However, since the affinity level of the carrier recombination region control layer is larger than the affinity level of the hole transporting blue emitting layer, the organic EL device requires a high driving voltage. Moreover, since electrons are injected into the hole transporting blue emitting layer to a smaller extent as the driving time increases, the emission intensity of the hole transporting red emitting layer decreases, whereby the emission color tends to be biased to the red light from the electron transporting emitting layer.
Patent document 4 discloses a white organic EL device in which two electron transporting emitting layers are disposed through a carrier barrier layer. However, since holes injected from the anode are almost completely consumed by the first emitting layer, only a small number of holes are supplied to the second electron transporting emitting layer through the carrier barrier layer. As a result, white luminous efficiency is decreased.
Patent document 5 discloses a white organic EL device in which an anode, first emitting layer, carrier barrier layer, second emitting layer, and cathode are stacked in that order, wherein the ionization potential of the carrier barrier layer is greater than the ionization potential of the first emitting layer in an amount of 0.1 eV or more, and the affinity level of the carrier barrier layer is smaller than the affinity level of the second emitting layer in an amount of 0.1 eV or more. However, since the carrier barrier layer has functions of both an electron barrier and a hole barrier, the driving voltage is increased.
Patent document 6 discloses an organic EL device in which a red emitting layer, a green emitting layer, and a blue emitting layer are stacked in that order from the anode side, and a hole transporting and electron blocking intermediate layer is provided at least between the green emitting layer and the blue emitting layer. However, this organic EL device exhibits an insufficient luminous efficiency.
Patent document 7 discloses an organic EL device using a naphthacene derivative and a periflanthene derivative. However, this organic EL device exhibits an insufficient luminous efficiency.    [Patent document 1] JP-A-2003-272857    [Patent document 2] JP-A-2004-235168    [Patent document 3] JP-A-8-78163    [Patent document 4] WO2005/099313    [Patent document 5] WO2005/112518    [Patent document 6] JP-A-2005-100921    [Patent document 7] US-A-2006/0088729
In view of the above-described problems, an object of the invention is to provide an organic EL device which exhibits color rendition suitable for displays and lighting applications, exhibits high luminous efficiency, and shows only a small change in chromaticity.
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