1. Field of the Invention
The present invention relates to a light emitting layer-forming solid material, an organic electroluminescent device (hereinafter may be referred to as “organic electroluminescence device” or “organic EL device”) and a method for producing the organic electroluminescent device.
2. Description of the Related Art
Conventionally, in formation of a white light emitting layer having a single layer structure, the dopant concentration must be controlled to be 1% by mass or less, and thus, a large-scale production of the white light emitting layer cannot be attained by a generally used co-evaporation method (see U.S. Pat. No. 5,683,823 and Japanese Patent Application Laid-Open (JP-A) No. 2004-228088).
Then, JP-A No. 2004-228088 discloses a method for producing a white light emitting layer by combining a plurality of light emitting layers formed from the corresponding light emitting layer-forming solid materials. As compared to the co-evaporation method, the layer formation is improved by this method. Nevertheless, this method requires separate solid materials corresponding to the light emitting layers, and also requires separate evaporation cells. As a result, the evaporation system becomes complicated and a time-consuming step is required in which the cells are filled with the materials.
In addition, an evaporation film formed from a pellet of several materials mixed may have a different composition from that being expected, since the materials have different sublimation temperatures. This problem has not yet been addressed by prior arts, and the compositions of solid materials have not yet been designed satisfactorily. Especially when powders of phosphorescent light-emitting materials (serving as light-emitting materials) are co-evaporated, device characteristics are not stable due to water or oxygen adsorbed on the surfaces of the powders, which is problematic.
Furthermore, JP-A No. 2003-249359 discloses that an organic light-emitting material and a thermally conductive material are pelletized into a solid. However, this literature has no description about the use of a phosphorescent light-emitting material as the organic light-emitting material. According to Examples thereof, a green-light emitting material Alq is used, and a white light-emitting layer having a single layer structure cannot be formed through evaporation of only one type of solid material.
Therefore, at present, keen demand has arisen for a light emitting layer-forming solid material, an organic electroluminescent device and a method for producing the organic electroluminescent device in which the heating temperature for the evaporation source (evaporation cell temperature) is changed while using only one type of light emitting layer-forming solid material, thereby controlling the composition of the resultant evaporation film, requiring no difficult-to-control co-evaporation, reducing variation in device performances and improving repetitive reproducibility.