The basic structure of an OLED consists of several thin organic layers sandwiched between two electrodes, typically a transparent anode and a metallic cathode, all deposited on a substrate (glass or plastic). The key layers include the hole injection layer (HIL), hole transport layer (HTL), emissive layer (EL), electron transport layer (ETL), and electron injection layer (EIL). The anode, often made of indium tin oxide (ITO) due to its transparency and conductivity, injects holes (positive charge carriers) into the HIL. The cathode, usually a metal like aluminum or magnesium, injects electrons into the EIL.
The light-emitting process begins when an electric current is applied across the electrodes. Holes from the anode move through the HIL and HTL to the emissive layer, while electrons from the cathode travel through the EIL and ETL to the same layer. When holes and electrons meet in the emissive layer, they recombine to form excitons (energetic electron-hole pairs). These excitons then relax to a lower energy state, releasing energy in the form of photons (light). The color of the emitted light depends on the organic materials in the emissive layer—for example, red, green, and blue OLEDs use different organic compounds, which can be combined to produce full-color displays.
In some OLED structures, especially active-matrix OLEDs (AMOLEDs), a thin-film transistor (TFT) layer is added to control the current for each pixel individually, enabling high-resolution displays. The simplicity of the structure (fewer layers than LCDs) allows for flexibility and thinness, while the direct recombination process ensures efficient light emission, contributing to the technology’s high contrast and fast response times.
