**1.1. General descriptions of OLEDs**

In general, an organic light emitting diode (OLED) contains a system of thin organic layers sandwiched between two electrode layers (anode and cathode). When a voltage is applied, light is generated within the system of organic layers and emerges through one of the transparent electrodes. The OLED fabricated onto a glass substrate is considered as a surface light source less than 2 millimeters (mm) thick. In contrast to conventional light sources, OLEDs emit wide-area light with high-quality color rendering, which is very pleasant for the human eye, and also needs no reflectors to reduce glare. As a result, OLEDs are one of the hottest research areas being intensely developed in the photonic industry.

The thin organic layers of an OLED can be prepared either via the vacuum deposition or solution casting, with a total thickness of approx. 200-400 nanometers (nm). If the solution casting is employed to make organic or polymeric thin films, the under layer must be free from the destruction brought by the solvent of the upper layer, therefore, solution-processed OLEDs often have only one or two organic layers. However, if the vacuum deposition is adapted to make organic small-molecule thin films, the upper layer causes no harm to the under ones. Thus, vacuum-deposited OLEDs are always multi-layered. Compared to the solution casting, the vacuum deposition enables OLEDs to have much higher injection efficiency, luminance, and lifetime.
