Devices based on organic semiconductors
Devices based on organic semiconductors
There are basically three product categories made using organic semiconductors: Organic light emitting diodes (OLED) used for display and lighting applications, organic solar cells, and organic electronics.
There are a number of applications for organic semiconductors. For instance, most of the copy machines and laser printers are already using organic photoconductors (Watch for the blue layer on the drum when you change the toner next time!). In the future, a number of new exciting devlopments are foreseeable. For instance, solar cells and low-cost organic electronics (smart labels) might become commercial products. Here, we briefly discuss some basic features of organic light-emitting diodes (OLED), which are already commercial.
Typical TV- or computer screens are still up 50cm thick, even though the only aim is to have a number of points in a single layer (the screen surface) emitting light. In contrast, organic light emitting diodes (OLEDs) only need an active layer of about 100nm (1/10 000mm), two metal contacts and a cheap substrate to reach the same brightness. As an alternative to LCD-TFT flat panel displays, they offer therefore an opportunity to replace the traditional TV-tube and many other display applications increasingly within the next decade.
First organic displays are already commercailly available (see below)). Many other products will soon follow because almost all key players in the electronics business actively work on organic display elements.
The production process for organic thin films is comparatively easy and suitable for low-cost devices. Organic LEDs can be prepared with any color - and with amazing brilliance that even does not considerably decrease if one looks onto the screen under a high angle. Their efficiency already exceeds standard light bulbs and approaches fluorescence lamps.
Working principles of OLED
The principle of organic LEDs is simple: An organic thin film is contacted by a noble metal on one side and a base metal on the other side. When a voltage is applied, positive charges (holes) are injected into the organic material from one contact (Me1) and negativ charges (electrons) from the other side. When two different charge carriers meet, they neutralize each other and set free their energy as a light particle (photon).
This simple setup involves the problem that exactly the same number of electrons and holes must be injected to make sure that no carrier passes through the sample without meeting a partner to recombine. A better two-layer setup has first been realized by Tang and coworkers (Kodak) in 1987: The electrons hit a barrier in the middle of the sample where they are blocked and wait until a positive charge carrier passes by.
Materials used for OLED
OLED have been realized both with small molecules (Tang and van Slyke 1987) and polymers (Friend et al. 1990). Currently, both classes of materials are actively investigated by a large number of groups worldwide. The first commercial devices are based on small molecules. However, there we be many polymer products on the market soon.
Both classes of materials have advantages and disadvantages: For small molecules, it is comparatively easy to prepare well-defined multi-layers systems with high purity. Also, the stability of small molecule materials tends to be better, in particular for blue emitters. Polymers, in contrast, can be processes by very low-cost techniques, which might offer many applications for which vacuum-prepared polymer devices are too expensive. It is also quite likely that in the future, polymer and small molecule layers will be combined.
Some examples for displays
The first commercially available product was a car radio from Pioneer, which had a green display. This radio was to our knowledge only marketed in Japan. Since 1998, a car radio with a three-color display based on blue small-molecule emitters with color converters was successfully sold wordwide (Image © Pioneer):
An example for the most recent technology is the 55" OLED television from Samsung:
Although the image quality of the OLED TV is superb, manufacturing issues cause so far rather high prices so that the market share of OLED TV is still extremely small. However, compared to the existing LCD Technology , OLED needs less resources so that efficient manufacturing should allow to reach low cost.