The three-layer structure olED device is composed of HTL, ELL, ETL 3 layers of organic materials, each layer of organic materials perform their duties, HTL is responsible for adjusting the injection speed and injection amount of holes, ETL is responsible for adjusting the injection speed and injection amount of electrons, injected electrons and holes interact in the ELL, combined in the binding state to form excitons, exciton decay radiates out photons. This structure is convenient for adjusting the electro-optical characteristics of OLED, and is currently the OLED structure commonly used.
(1) Three-layer A type (threelayer-A abbreviation TL-A) device structure is proposed by Professor Saito group of Kyushu University in Japan, the most important feature of which is to place a layer of light between HTL and ETL, and this layer of light is as thin as Langmuir-Blodgettfilm, so that excitons are limited to this layer to produce strong luminescence. The structure of the three-layer A type standard OLED device is from bottom to top: a thin and transparent indium tin oxide (ITO) with conductive properties on the glass substrate layer, a cathode metal composition, and the organic material layer is sandwiched like a sandwich, and the organic material layer includes a hole transport layer (HTL), a light emitting layer (ELL), and an electron transport layer (ETL). When a three-layer A standard OLED device is channeled with the appropriate current, the hole injected into the positive electrode is combined with the charge from the cathode in the light emitting layer, and the released energy stimulates the organic material to produce light, and different organic materials emit different colors of light.
(2) Three-layer B type (threelayer-B abbreviated as TL-B) The structure of the three-layer A standard OLED device was proposed by the Kimo group of Yamagata University in Japan, and its structure is similar to that of TL-A. But the main feature is the exciton limit layer (excitonconfinementlayer for short ECL) between HTL and ETL. The thickness of the exciton limit layer can be adjusted to adjust the position of the light emitted, one side of the control device can emit light or both sides emit light, if the ECL is adjusted appropriately, the exciter can be generated in HTL and ETL at the same time, so that HTL and ETL emit light at the same time, and the light emitting is mixed into white light. The structure of the three-layer B-type standard OLED device is bottom-up: glass substrate, ITO, HTL, ECL, ETL/metal cathode.
For OLED devices with a three-layer structure, the role of the injection layer is to make the work function of the anode and the LUMO quasi-position, and the work function of the cathode match well with the HOMO quasi-position, so that the electrons and holes can flow smoothly from the electrode to the transport layer. The hole injection layer material is mainly allylamine system or copper titanium cystine system, and is matched with anode material with high work function. Electron injection layers usually use aluminum as a cathode with metals or metal fluorides with lower functions such as lithium or calcium.
The role of the transport layer is so that the hole injected from the anode can be transported laminarly through the hole to the light emitting layer, and the electrons from the cathode are blocked so that they do not directly transmit the stream to the anode; and the electrons injected from the cathode can flow through the electron transport laminar layer to the light emitting layer, and block the holes from the anode so that they are not directly transmitted to the cathode. Therefore, the transport layer must use a material with a high carrier mobility and a potential barrier that can block the flow of electrons and holes between the transport layer and the luminescence layer, so that the electrons and the holes can be recombined in the luminous layer and emit light. Although the material of the transport layer is divided into holes and electron transport layers, it is mainly based on allylamine compounds (TPDs) containing nitrogen. At present, the development of the electronic transport layer lags behind the hole transport layer.
The role of the luminescent layer is to make the injected electrons and the holes produce a recombinant excitation effect and emit light, the light emitting layer material is usually a lower luminous capacity of the diameter formula 8-hydroxyquinoline aluminum (Alq3) or bismuth-fault compound (Bebq2) as the host material, and then a small amount of doped high luminescence capacity of the guest (guest) material. In addition to improving the efficiency of luminescence, doped objects can also be used to change the color of luminescence. The mechanism of luminescence can be activated by the main material and then transfer energy to the guest molecule, so that the guest molecule is excited and emits light; another way is that the electron and the hole are directly combined on the guest molecule and then emit light. The recombination mechanism can be divided into two types: fluorescence and phosphorescence. The luminous efficiency of phosphorous rotation is about 2 to 5 times higher than that of fluorescent rotation (about 2 times red light and about 5 times green light), so the use of phosphor optical light layer can reduce power consumption and improve life.
As for the electron transport layer, which is an n-type organic material, its characteristics are that it has a high electron mobility, when the electron is from the electron transport layer to the hole electron transport layer interface, because the LUMO of the electron transport layer is higher than the LUMO of the hole transport layer, the electron is not easy to cross this energy barrier into the hole transport layer, and is blocked at this interface. At this time, the hole is transmitted from the hole transport layer to the vicinity of the interface and recombines with the electrons to produce excitons, and the excitons will release energy in the form of emitting light and non-emitting. In the case of general fluorescent material systems, only 25% of the electron hole pairs are reconjugated in the form of luminescence due to the selection rate (Selectionrule), and the remaining 75% of the energy is dissipated in the form of exothermic heat. In recent years, phosphorescent materials are being actively developed to become a new generation of OLED materials that can break the limits of the selection rate to improve internal quantum efficiency to nearly 100%.
In the actual device design, in order to optimize and balance the performance of the device, a variety of function layers with different functions are introduced. For example, the electron injection layer and the hole injection layer often reduce the opening and operating voltage of the device; the electron barrier layer and the hole barrier layer often reduce the current flowing directly through the device without forming excitons, thereby improving the efficiency of the device. In a particular device, several layers may be included. However, because most organic materials are insulators, effective current input can only be achieved at higher electric field strength, so the thickness of the organic film cannot be too thick, otherwise the driving voltage of the device is too high, losing the practical application value of OLED. There are many practical applications of hole barrier layers. In general, in a two-layer or three-layer device, there are more holes than electrons, and a large part of the holes form a leakage current. Therefore, it is very necessary to introduce a hole barrier layer to limit the flow of holes, thereby improving the efficiency of the device.