The OLED mode of operation
OLEDs (Organic Light Emitting Diode) are millimeter-thin glass wafers with organic materials enclosed in them. These form layers which are around 400 nanometers thin, which the current can flow through. The organic layers are enclosed by an anode and a cathode layer, which function as the electrical contacts from both sides. The organic layer contains molecules which begin to glow when an electric current passes through them. The particular molecular structure determines the color of the light. The organic layers are coated to protect them from external effects.
An organic LED consists of several organic semiconducting layers between two electrodes, at least one of which is transparent. While manufacturing an OLED, organic layers are applied in onto a conductive substrate, followed by another conductive electrode. In general, two different classes of material are used in the manufacture of organic, light-emitting components: Polymer substances and what are known as small molecule materials, which have no orientation properties and thus form amorphous layers.
Organic molecules generally have a broad emission spectrum. Consequently all light wavelengths are present in the spectrum. This permits an especially natural illumination of objects. OLED emissions can be adjusted to practically any color, including white, with every possible color temperature. Most white OLEDs consist of a red, a green and a blue emission layer, which together produce high quality white light.
One technical challenge for the large-area OLED is the limited conductivity of the electrode material. For example, the conductivity of indium tin oxide (ITO) is about two orders of magnitude lower than that of aluminum. The result: A significant voltage drop in the transparent electrode and a reduction in the local operating voltage of the active layers. Consequently, the intensity of the radiation reduces from the edges to the center point. To minimize the effect, conductive auxiliary structures (conducting strips, also known as busbars) made of metal can be attached to the ITO anode. Thus a uniform distribution of the lighting density is achieved. A further possibility for making large-area OLEDs glow homogeneously is the stacking approach. Here, several OLED stacks are vapor deposited on top of one another, each contributing its share to the total luminance.
The mean service life of an incandescent lamp is around 1000 hours. After that, the filament burns out and the lamp must be replaced. With OLEDs on the other hand, a gradual drop in the luminous flux occurs. That is, the OLED does not burn out in the usual sense, but rather loses luminous flux and lighting power over the course of its service life. Typically the service life is defined as the period during which the light output falls to 70% of the original output (L70). Some manufacturers also specify the value for the drop in light output to 50% (L50).
Typical lifetimes of current OLEDs are around 10,000 hours (L70) at a brightness and luminance of 2000 candelas per square meter.