TECHNICAL FIELD

One embodiment of the present invention relates to a light-emitting element, and a light-emitting device, an electronic device, and a lighting device each including a light-emitting element.

BACKGROUND ART

A light-emitting element having a structure in which a light-emitting layer including an organic compound that is a light-emitting substance is provided between a pair of electrodes has attracted attention as a next-generation flat panel display element in terms of characteristics such as being thin and light in weight, high speed response, and direct current low voltage driving. Furthermore, a display device including this light-emitting element is superior in contrast, image quality, and has wide viewing angle.

A light-emitting element is said to have a light-emitting mechanism in which an electron and a hole are injected by application of voltage between a pair of electrodes and recombined to form an exciton, so that light is emitted. Electrons and holes each have a spin of ½. It is said that the recombination of electrons and holes allows generation of excitons including 25% spin-0 singlet excitons and 75% spin-1 triplet excitons on the basis of the spin statistics. Note that fluorescence is emitted from a singlet exciton; hence, the theoretical limit of internal quantum efficiency of fluorescence is 25%. The spin-1 triplet excitons constituting 75% of generated excitons relax to the ground state without emitting light in many cases, because they generally do not contribute to light emission. In some cases, triplet-triplet annihilation (TTA) due to a plurality of triplet excitons occurs. TTA is said to be a process in which, by collision between two triplet excitons, energy and a spin angular momentum are exchanged and delivered, generating a singlet exciton.

Development of a phosphorescent light-emitting element having internal quantum efficiency higher than a theoretical value is proposed as well as development of a phosphorescent compound (for example, see Patent Document 1) in order to efficiently use a triplet exciton generated in the light-emitting mechanism. Specifically, a compound with which triplet excitation energy can be converted to light emission (hereinafter referred to as phosphorescent compound) is used in addition to the above-described light-emitting substance, thereby providing phosphorescence due to transfer of energy between states with different multiplicities.

REFERENCE

Patent Document

[Patent Document 1] PCT International Publication No. WO 2005/105746

DISCLOSURE OF INVENTION

In one embodiment of the present invention, emission efficiency of a fluorescent light-emitting element is increased by using efficiently a triplet exciton generated in a light-emitting layer.

In one embodiment of the present invention, emission efficiency of a light-emitting element is improved in the following manner. By making triplet-triplet annihilation (TTA) occur efficiently in a light-emitting layer containing at least a host material and a guest material in the light-emitting element, a triplet exciton which does not contribute to light emission is changed to a singlet exciton; thus, the guest material (fluorescent dopant) is emitted by transfer of energy from the singlet exciton. Note that emission efficiency due to TTA is estimated to increase by approximately 15% from consideration based on the spin statistics. When the increase in emission efficiency is added to the internal quantum efficiency of 25% without TTA, an internal quantum efficiency of 40% can be achieved theoretically.

In order to increase the efficiency of occurrence of TTA due to the triplet exciton generated from the host material, it is important to increase the probability of collision of triplet excitons in the light-emitting layer.

To achieve this, a T1 level (lowest triplet exciton energy level) of the host material in the light-emitting layer is preferably lower than a T1 level of the guest material in the light-emitting layer. Generally, the proportion of a host material in a light-emitting layer is much higher than the proportion of a guest material in the light-emitting layer. When triplet excitons generated in the light-emitting layer are trapped by the guest material existing in only small amounts in the light-emitting layer, the triplet excitons are localized, leading to a reduction in the probability of collision of the triplet excitons. However, in the case of forming the light-emitting element so that the T1 level of the host material is lower than the T1 level of the guest material, a reduction in the probability of the collision of triplet excitons can be prevented, and accordingly, the probability of occurrence of TTA can be increased.

A hole-transport layer (HTL) formed on the anode side of the light-emitting element to be in contact with the light-emitting layer is preferably formed using a material having a T1 level higher than a T1 level of the host material of the light-emitting layer to prevent a triplet exciton generated in the light-emitting layer from exiting the light-emitting layer. With such a structure, a triplet exciton generated in the light-emitting layer is not likely to move from the light-emitting layer to the HTL.

Note that light emission provided when TTA occurs has an extremely longer lifetime (delayed fluorescence) than the fluorescence lifetime of a fluorescent material which is obtained in the case where TTA does not occur. The delayed fluorescence can be confirmed by observing the attenuation of light emission after steady injection of carriers is stopped at a certain point of time. Note that the delayed florescence refers to light which is continuously emitted for 1×10−6sec or longer after steady injection of carriers is stopped, with an intensity ratio of 0.01 or higher with respect to emission intensity during steadily injecting carriers. Note that in this case, a shape of a spectrum of delayed fluorescence coincides with a shape of an emission spectrum during steadily injecting carriers.

Thus, in a light-emitting element which is one embodiment of the present invention, an EL layer is provided between a pair of electrodes, the EL layer includes at least a light-emitting layer and a hole-transport layer, the light-emitting layer includes a guest material and a host material, a T1 level of the host material is lower than a T1 level of the guest material, the hole-transport layer includes a hole-transport material, and a T1 level of the hole-transport material is higher than the T1 level of the host material.

In a light-emitting element which is another embodiment of the present invention, an EL layer is provided between a pair of electrodes, the EL layer includes at least a light-emitting layer and a hole-transport layer, the light-emitting layer includes a guest material and a host material, a T1 level of the host material is lower than a T1 level of the guest material, the hole-transport layer includes a hole-transport material, and a T1 level of the hole-transport material is higher than the T1 level of the guest material and the T1 level of the host material.

In a light-emitting element which is another embodiment of the present invention, an EL layer is provided between a pair of electrodes, the EL layer includes at least a light-emitting layer and a hole-transport layer, the light-emitting layer includes a guest material and a host material, a T1 level of the host material is lower than a T1 level of the guest material, the hole-transport layer includes a hole-transport material, a T1 level of the hole-transport material is higher than the T1 level of the host material, a shape of a spectrum of delayed fluorescence due to TTA is the same as a shape of a fluorescent spectrum of the guest material, and a period where a ratio of an intensity of the delayed fluorescence with respect to an emission intensity during steadily injecting carriers is higher than or equal to 0.01 is 1×10−6sec or longer.

In a light-emitting element of another embodiment of the present invention, an EL layer is provided between a pair of electrodes, the EL layer includes at least a light-emitting layer and a hole-transport layer, the light-emitting layer includes a guest material and a host material, a T1 level of the host material is lower than a T1 level of the guest material, the hole-transport layer includes a hole-transport material, a T1 level of the hole-transport material is higher than the T1 level of the guest material and the T1 level of the host material, a shape of a spectrum of delayed fluorescence due to TTA is the same as a shape of a fluorescent spectrum of the guest material, and a period where a ratio of an intensity of the delayed fluorescence with respect to an emission intensity during steadily injecting carriers is higher than or equal to 0.01 is 1×10−6sec or longer.

Note that one embodiment of the present invention includes, in its scope, electronic devices and lighting devices including light-emitting devices, as well as light-emitting devices including light-emitting elements. The light-emitting device in this specification refers to an image display device and a light source (e.g., a lighting device). In addition, the light-emitting device includes all the following modules: a module in which a connector, such as a flexible printed circuit (FPC) or a tape carrier package (TCP), is attached to a light-emitting device; a module in which a printed wiring board is provided at the end of a TCP; and a module in which an integrated circuit (IC) is directly mounted on a light-emitting element by a chip-on-glass (COG) method.

In one embodiment of the present invention, a fluorescent light-emitting element with high efficiency can be provided. Furthermore, a light-emitting device, an electronic device, or a lighting device which consumes low power can be provided.

