JP2007080744A - Hole injection layer and organic el (electroluminescent) element using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hole injection layer in which the film thickness can be controlled in molecule level and an organic electroluminescent element using the same. <P>SOLUTION: This is the hole injection layer in which a single layer polythiophene derivative is made into a super-thin film in molecule level using a mixed Langmuir-Blodgett's technique and an organic electroluminescent element using the same is provided. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a hole injection layer and an organic electroluminescence (EL) device using the same.

  An organic electroluminescent element is an element that injects holes and electrons from an anode and a cathode, recombines them to form an excited state of an organic substance, and emits energy as light when returning to a ground state. However, organic substances are inherently insulators and require a large overvoltage in order to inject holes and electrons, resulting in problems such as poor energy efficiency of light emission. In order to improve this problem, it is necessary to newly add a hole injection material to the device in order to lower the energy barrier of hole injection. Conventionally, the hole injection layer of the electroluminescent device is produced by a vacuum heating vapor deposition method in the case of a low molecular weight organic compound, and by a spin coating method or an ink jet printing method in the case of a polymer.

  However, it is impossible to produce an ultra-thin film that is uniform at the molecular level by these conventional film formation methods, and the film thickness cannot be controlled at the molecular level. There was a problem that the effect of.

  The present invention has been made in view of the above-described conventional situation, and it is an object to be solved to provide a hole injection layer capable of controlling a film thickness at a molecular level and an organic electroluminescence device using the same. .

  The hole injection layer of the first invention is characterized in that a single-layer polythiophene derivative is formed into an ultrathin film at a molecular level by using a mixed Langmuir Blodgett method. The hole injection layer of the second invention is characterized in that the polythiophene derivative of the first invention is polyalkylthiophene such as poly (3-hexylthiophene) (hereinafter referred to as “PHT”).

  The organic electroluminescence device of the third invention is characterized by using the hole injection layer of either the first invention or the second invention.

  A first embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the structure of the organic EL device fabricated and the energy level of the compound used. As the function of the hole injection layer, PHT (-5.2 eV) was selected because it is required to have a small work function difference from the ITO electrode (-4.8 eV). Also, since only the vicinity of the interface between the ITO electrode and the hole injection layer is important for hole injection, an ultrathin film is suitable. For this purpose, our previous studies show that PHT is mixed with N-dodecylacrylamide polymer (pDDA), which has good LB film-forming ability, and can be used to produce a PHT molecular-level ultrathin film. An injection layer was produced. As a control experiment, an organic EL device using a pDDA LB film instead of the hole injection layer was also fabricated. Except for the hole injection layer, the structure was the same as a typical organic EL device. ITO electrode as anode, triphenyldiamine derivative (TPD) as hole transport layer, aluminum quinoline complex (Alq ₃ ) as light emitting layer and electron transport layer, Ca as cathode and Al as protective film, hole injection layer All the layers except those were formed by vacuum evaporation, organic EL devices were fabricated, and characteristics of the organic EL devices were evaluated by introducing a PHT ultrathin film as a hole injection layer.
Current / luminance data for applied voltage obtained from ITO / PHT (0-5 layers) / TPD (100 nm) / Alq ₃ (100 nm) / Ca (100 nm) / Al organic EL devices is the degree of vacuum, deposition rate, etc. Since the data varies depending on the film forming conditions, the data was determined by averaging. FIG. 2 shows the relationship between the applied voltage and the luminance of the organic EL element in which the number of layers of the PHT mixed LB film is changed. When 0.1 cd / m ² is defined as the light emission starting voltage, the device without PHT was about 11V, but the device with one layer of PHT showed a decrease in the starting voltage of about 8V and 3V, and five layers were introduced. The device was about 7V. In the device in which the pDDA LB film is introduced instead of the PHT mixed LB film of the hole injection layer, the starting voltage rises by about 0.5 V, so that the PHT in the PHT mixed LB film functions as the hole injection layer. Became clear. FIG. 3 shows the change in luminance depending on the presence or absence of the PHT1 layer in the same electrode. As can be seen from FIG. 3, it was found that the emission luminance at the same voltage (14V) was clearly different by introducing the PHT1 layer, and that the hole injection barrier could be reduced by introducing the PHT1 layer. In addition, it was found that the luminous efficiency (luminance with respect to the injection current) is maximum in the device with one layer of the PHT mixed LB film, and on the contrary, the device with five layers is worse than the device without PHT. This is because the hole injection efficiency has increased due to the increase in the PHT mixed LB film layer, but the balance with the electron injection efficiency has deteriorated and the filter effect due to the PHT having an absorption peak at 560 nm is considered.
As described above, it is clear that the PHT mixed LB film is effective as the hole injection layer of the organic EL device, and it is clear that the luminous efficiency is improved in the PHT 1 layer device.

FIG. 3 is a diagram showing the structure of the organic EL device produced in Example 1 and the energy level of the compound used. It is a figure which shows the relationship between the applied voltage and luminance of the organic EL element which changed the number of layers of the PHT mixed LB film. It is a figure which shows the change of the brightness | luminance by the presence or absence of the PHT1 layer in the same electrode.

Claims (3)

A hole injection layer characterized in that a single-layer polythiophene derivative is formed into an ultrathin film at a molecular level by using a mixed Langmuir Blodgett method. The hole injection layer according to claim 1, wherein the polythiophene derivative according to claim 1 is polyalkylthiophene such as poly (3-hexylthiophene). An organic electroluminescent device using the hole injection layer according to claim 1.