OLED efficiency and life and device structure is closely related to the widely used structure belongs to the "sandwich" structure, that is, the light-emitting layer by the cathode and anode like a sandwich in the middle (one side of the transparent electrode in order to obtain the effect of surface light) structure.

Due to the low temperature of the OLED film, so generally use more indium tin oxide glass electrode (Indium Tin Oxide, ITO) as the anode. Single or multi-layer organic semiconductor films are prepared on the ITO electrode by vacuum vapor deposition or spin coating, and finally the metal cathode is prepared on top of the organic film.

According to the functions of organic semiconductor films, device structures can be broadly classified into the following categories:

1.1 Single-layer device structure

In the device between the ITO anode and metal cathode, the preparation of an organic semiconductor film as a light-emitting layer, which is the simplest single-layer OLED, the device structure as shown in Figure 1, it is only composed of anode, light-emitting layer and cathode, the structure is very simple and easy to prepare. This structure is more commonly used in polymer organic electroluminescent devices.

1.2 Double-layer device structure

As most organic electroluminescent devices are unipolar materials, while having the same hole and electron transport characteristics of bipolar (Bipolar) organic semiconductor materials are rare, so only a single transmission of electrons or holes in one. If the use of this unipolar organic material as a single-layer device light-emitting material, there will be an imbalance between electron and hole injection and transport, and easy to make the light-emitting region near the smaller mobility of the carrier injection side of the electrode, if the metal electrode, it is easy to lead to light-emitting burst, and this burst will reduce the exciton utilization rate, resulting in a reduction in device light-emitting efficiency.

Due to the existence of single-layer structure is more difficult to overcome the shortcomings of the current OLED devices are mostly multilayer structure. This landmark work was first proposed by Kodak in 1987, the structure can effectively achieve the purpose of adjusting the compound region of electrons and holes away from the electrode and balance the carrier injection rate, to a large extent, to improve the luminous efficiency of the device, so that the development of OLED into a new stage. The main feature of this structure is the light-emitting layer material with electron (hole) transport, the need to add a layer of hole (electron) transport material to regulate the rate and number of holes and electrons injected into the light-emitting layer, this layer of hole (electron) transport material also plays a role in blocking the electron (hole) layer, so that the injection of electrons and holes in the composite occurs near the light-emitting layer.

1.3 Three-layer and multilayer device structure

By the electron transport layer (Electron Transport Layer, ETL), hole transport layer (Hole Transport Layer, HTL) and light-emitting layer composed of a three-layer OLED device, as shown in Figure 3. The structure was first proposed by the Adachi group in Japan. The advantage of this device structure is that the three functional layers have their own functions, for the selection of functional materials and optimization of device structure performance are very convenient, is now often used in the OLED device structure.

In the actual OLED device structure design, in order to optimize the performance of OLED devices, and give full play to the role of each functional layer, to further improve the luminous brightness and luminous efficiency of OLEDs, people in the three-layer structure based on the use of multilayer device structure, the excess carriers to limit, deployment. This is currently the most commonly used device structure for OLEDs. This device structure not only ensures good adhesion between the functional layer of the organic electroluminescent device and the substrate (substrate), but also makes it easier to inject carriers from the anode and metal cathode into the organic semiconductor functional film.

To improve the performance of the device, a variety of more complex device structures continue to appear. However, because most organic materials have the property of insulation, only at very high electric field strength (about 10 V/cm) can carrier transport from one molecule to another, so the total thickness of organic semiconductor films can not exceed the hundred nanometer level, otherwise the driving voltage of the device will be higher.

1.4 Stacked string device structure

Based on the need for full color display, Forrest et al. proposed to stack three primary color devices vertically along the thickness direction and ensure that each device is controlled by its own electrode, which constitutes a color display device, as shown in Figure 4. Display devices made in this way can obtain a resolution better than that of conventional technology, and people use this idea to stack multiple light-emitting units vertically and add an electrode connecting layer in the middle, while using only the two end electrodes for driving, i.e., a stacked string structure device (Tandem OLED). This structure can extremely effectively improve the current efficiency of the device, so that the device can achieve very high brightness at a small current, which provides a convenient way to achieve high-efficiency, long-life organic electroluminescent devices.