OLED stands for Organic Light Emitting Diode. Its principle involves sandwiching an organic light-emitting layer between two electrodes. When electrons from the positive and negative electrodes meet in this organic material, light is emitted. Its component structure is simpler than the currently popular TFT LCD, and its production cost is only about 30-40% of that of TFT LCD.
Besides its lower production cost, OLED has many other advantages, such as its self-emissive nature. Currently, LCDs require backlight modules (lamp tubes added behind the liquid crystal), but OLEDs emit light themselves when powered, eliminating the weight, size, and power consumption of lamps (lamp tubes consume almost half the power of the entire LCD screen). This allows for products with a thickness of only about two centimeters, a lower operating voltage of 2 to 10 volts, and superior response time (less than 10ms) and color reproduction compared to TFT LCDs. Furthermore, its flexibility makes its applications extremely wide-ranging.
OLED Structure and Light Emitting Principle
The basic structure of an OLED consists of a layer of organic light-emitting material, tens of nanometers thick, fabricated on indium tin oxide (ITO) glass. Above this light-emitting layer is a low work function metal electrode, forming a sandwich-like structure.
The basic structure of an OLED mainly includes:
Substrate (transparent plastic, glass, metal foil) – This base layer supports the entire OLED.
Anode (transparent) – The anode eliminates electrons (increases electron "holes") when current flows through the device.
Hole transport layer – This layer is composed of organic material molecules that transport "holes" from the anode.
Emitting layer – This layer is composed of organic material molecules (different from the conductive layer), and the light-emitting process takes place in this layer.
Electron transport layer – This layer is composed of organic material molecules that transport "electrons" from the cathode.
Cathode (can be transparent or opaque, depending on the OLED type) – When current flows through the device, the cathode injects electrons into the circuitry.
OLEDs are dual-injection light-emitting devices. Driven by an external voltage, electrons and holes injected by the electrodes recombine in the emitting layer to form electron-hole pairs, i.e., excitons, which radiate and de-excite, emitting photons and producing visible light. To enhance electron and hole injection and transport capabilities, a hole transport layer is typically added between the ITO and the emissive layer, and an electron transport layer is added between the emissive layer and the metal electrode, thereby improving luminescence performance. Holes are injected from the anode, and electrons are injected from the cathode. Holes transport by hopping on the highest occupied molecular orbitals (HOMO) of the organic material, while electrons transport by hopping on the lowest unoccupied molecular orbitals (LUMO).
The luminescence process of OLEDs typically involves the following five basic stages:
**Carrier Injection:** Under the influence of an applied electric field, electrons and holes are injected from the cathode and anode, respectively, into the organic functional layer sandwiched between the electrodes.
**Carrier Transport:** Injected electrons and holes migrate from the electron transport layer and hole transport layer, respectively, to the emissive layer.
**Carrier Recombination:** After being injected into the emissive layer, electrons and holes are bound together by Coulomb forces to form electron-hole pairs, i.e., excitons.
Exciton migration: Due to the imbalance between electron and hole transport, the main exciton formation region usually does not cover the entire emissive layer, thus diffusion migration occurs due to the concentration gradient.
Exciton radiation de-excitation and photon emission: Excitons undergo radiative transitions, emitting photons and releasing energy.
The color of OLED light emission depends on the type of organic molecules in the emissive layer. Placing several organic thin films on the same OLED panel constitutes a color display. The brightness or intensity of light depends on the performance of the emissive material and the magnitude of the applied current. For the same OLED, the greater the current, the higher the brightness of the light.
OLED Manufacturing Principle
OLED components are composed of n-type organic materials, p-type organic materials, cathode metal, and anode metal. Electrons (holes) are injected from the cathode (anode), conducted through the n-type (p-type) organic material to the emissive layer (generally n-type material), and emit light through recombination. Generally, in the fabrication of OLED devices, ITO is first sputtered onto the glass substrate as the anode, and then p-type and n-type organic materials, and a low work function metal cathode, are sequentially deposited using vacuum thermal evaporation. Organic materials readily react with moisture or oxygen, creating dark spots that prevent the device from emitting light. Therefore, after vacuum coating, this device must undergo encapsulation in a moisture- and oxygen-free environment.
As for the electron transport layer, it is an n-type organic material with high electron mobility. When electrons travel from the electron transport layer to the hole transport layer interface, the lowest unoccupied molecular orbital (LUMO) of the electron transport layer is significantly higher than that of the hole transport layer, making it difficult for electrons to cross this energy barrier and enter the hole transport layer. Holes then travel from the hole transport layer to the vicinity of the interface and recombine with electrons to generate excitons. These excitons release energy in both light-emitting and non-light-emitting forms. In typical fluorescent material systems, selectivity calculations show that only 25% of electron-hole pairs recombine in a light-emitting manner, while the remaining 75% of the energy is dissipated as heat. In recent years, phosphorescent materials have been actively developed as next-generation OLED materials. These materials can overcome selectivity limitations, thereby increasing internal quantum efficiency to near 100%.
In two-layer devices, the n-type organic material—the electron transport layer—is also used as the emissive layer, and its emission wavelength is determined by the energy difference between HOMO and LUMO. However, a good electron transport layer—a material with high electron mobility—is not necessarily a material with good light emission efficiency. Therefore, the current common practice is to dope high-fluorescence organic pigments into the portion of the electron transport layer near the hole transport layer, also known as the emissive layer, with a volume fraction of approximately 1% to 3%. Doping technology development is a key technology for enhancing the fluorescence quantum absorption rate of raw materials, and dyes with high fluorescence quantum absorption rates are generally selected.
Traditionally, the cathode metal material uses low work function metals (or alloys), such as magnesium alloys, to facilitate electron injection from the cathode to the electron transport layer. Another common practice is to introduce an electron injection layer, which is an extremely thin low work function metal halide or oxide, such as LiF or Li₂O. This significantly reduces the energy barrier between the cathode and the electron transport layer, lowering the driving voltage. Since the HOMO value of the hole transport layer material differs from that of ITO, and ITO anodes may release oxygen after prolonged operation, damaging the organic layer and causing dark spots, a hole injection layer is inserted between ITO and the hole transport layer. Its HOMO value falls between that of ITO and the hole transport layer, which is beneficial for hole injection into OLED devices. Furthermore, the thin film's properties can block oxygen from the ITO from entering the OLED device, extending device lifespan.