In general, an organic light emitting device (OLED) includes two electrodes (an anode and a cathode) and at least one organic layer interposed between the two electrodes. In the organic light emitting device having this structure, when a voltage is applied between the two electrodes, holes from the anode and electrons from the cathode are injected into the organic layer. The electrons and the holes are recombined in the organic layer to thereby form excitons. When the excitons drop to a ground state again, photons corresponding to the energy difference are emitted. According to this principle, the organic light emitting device emits visible rays. Using this, it is possible to manufacture information display devices or illumination devices.
The organic light emitting device is widely applied to a small display that has a size of 5 inches or less, for example, an outside window on a cellular phone. An attempt to apply the organic light emitting device to a large display has been made. However, a liquid crystal display (LCD) is widely used in this field of application. In terms of price and performance, the organic light emitting device is not more excellent than the LCD. Therefore, it is difficult for the organic light emitting device to enter markets. Researchers have done large amounts of research on organic light emitting devices that differentiate from the LCD, one of which is a flexible organic light emitting device. The organic light emitting device is a self-emitting type and has a thickness of 0.2 μm or less in general. Therefore, a thin flexible display device can be manufactured with appropriate selection of the material and thickness of a substrate.
Flexible organic light emitting devices according to the related art are manufactured using three main methods.
First, an organic light emitting device is directly manufactured on a substrate that can be bent, such as plastic. Here, there is a disadvantage when it is difficult to use a large-area substrate because the substrate may be bent during the process.
Second, in order to complement the first method, glass that is conventionally used to manufacture the organic light emitting device is used as a mother substrate. A substrate that can be bent, such as thin plastic, is bonded to the mother substrate, and an organic light emitting device is manufactured thereon. Then, the plastic substrate is separated from the glass substrate. In this case, it is possible to use a large-scale substrate due to the rigid characteristic of the glass substrate.
However, since the above-described methods manufacture the plastic substrates through injection or a laminating process, the surface of the plastic substrates is rough. In addition, defects, such as dust formed when the plastic substrate is bonded to the mother substrate, have a fatal effect on life span of the organic light emitting device that is subsequently manufactured using vacuum evaporation.
The third method is a roll-to-roll process, which is the same as the first method in that a substrate, which can be bent, such as plastic or the like, is used. However, this method is different from the first method in that the process is performed while rolling a long substrate. The advantage of this method is that mass production is allowed. However, this method does not reach a technical level sufficient to be applied to manufacture organic light emitting devices.
Because of the above-described reasons, even though research on the flexible organic light emitting devices has actively been made, significant results of commercialization thereof have not been reported.