Methods of forming each constituent layer, such as an organic layer, in production of organic electronic devices including an organic EL element, are loosely divided into dry processes by way of a deposition method or the like, and wet processes by way of a coating method using a solution in which an organic material is dissolved in an organic solvent.
The dry process has an advantage that a uniform film can be formed to have a desired film thickness, substantially without contamination of moisture, oxygen or impurities, since an organic layer and a metal are formed as films usually under a high vacuum of 10−4 to 10−6 Pa. Further, since the organic layer, the metal oxide and the metal can be continuously formed as films, thus it is easy to attain a high efficiency of an element and optimization of an element structure by providing each layer with discrete function. On the other hand, the method has problems in that uniform film forming in a large area is difficult, a material usage efficiency is low, cost is high, and the like.
On the other hand, the wet process has a comparatively simple film forming process and allows flexible film forming of a large area at low cost, thus attracts attention in recent years, and is also used in research and development of organic electronic devices such as, not only an organic EL element, but also an organic transistor, an organic thin film solar cell, and the like.
Specific method includes coating methods such as a spin coating method, a casting method and a spray method, as well as immersing methods such as a dip method, a self-organization method and an LB method, and printing methods such as an ink-jet method, a screen printing method and a roll-to-roll method.
In the coating method by way of a spin coating method, film forming is performed to have a desired film thickness by dissolving an organic material in various solvents and controlling the dropping amount and concentration of the solution, the number of revolutions of a spin coater and the like, in the atmosphere or under an inert gas atmosphere in a glove box or the like.
The electron injection layer in the coating-type organic electronic device conventionally employs metals such as Ba and Ca that are water-soluble or alcohol-soluble and have a low work function, in combination with Al or the like, but these metals are very active, thus are susceptible to moisture or oxygen in the atmosphere and are unstable.
Further, it is often the case that an organic material used in a coating-type organic electronic device is essentially unipolar, i.e., it has a charge transport property which allows transport of either holes or electrons. Thus, there is an electric charge which passes to an electrode and does not contribute to electric charge recombination, and there is also a problem that such low carrier balance causes the organic electronic device to reduce its efficiency.
Therefore, in order to attain a high efficiency of the coating-type organic electronic device, an electron injection layer or an electron transport layer that can prevent the electric charge from passing through due to its laminated structure, and is stable and applicable in the atmosphere, is required.
In response to this, for example, Patent Literature 1 describes that a liquid material containing an aryl compound having a PO group and an alkali metal ion or alkaline-earth metal ion obtained by being dissolved in alcohol is applied to form an electron transport layer, whereby an electron injection property and an electron transport property can be improved.
On the other hand, Patent Literature 2 describes that, use of an organic-inorganic composite material in which zinc oxide (ZnO) particles and an aryl compound having a PO group are combined can improve the electron injection property and the electron transport property without using an alkali metal, an alkaline-earth metal, and a compound thereof.