A new display technology that is now starting to receive wide acceptance in the market is Organic Light-Emitting Diode (OLED) displays with sizes ranging from 0.66-in thumbnails to 40-in OLED TV screen now available from Samsung. There were even rumours floating around prior to the US CES 2009 show that Samsung would be launching a 50-in OLED TV screen there.
OLEDs can provide brighter, crisper displays on electronic devices and use less power than the conventional LCDs used today. OLEDs, like LEDs, emit light through a process called electrophosphorescence. The colour of the emitted light depends on the type of organic film used in the emissive layer. To make a colour OLED display, manufacturers use organic films with different chemical compositions to emit red, green and blue lights. The intensity or brightness of the light is proportional to the applied electrical current.
There are two major types of OLED displays available today: Passive Matrix (PMOLED) and Active Matrix (AMOLED).
PMOLEDs have anode strips that are laid perpendicular to the cathode strips with the intersection of the cathode and anode making up the pixels where the light is emitted. In between the cathode and anode layers are the emissive and conductive organic layers. External circuitry applies current to the selected strips of anode and cathode, with the pixel brightness dependent on the amount of applied current.
PMOLEDs are most efficient for text and icons and are best suited for small screens less than 2 inches diagonally, such as in mobile phones, PDAs and MP3 players. Even with the external circuitry, PMOLEDs consume less battery power than the LCDs still used in majority of these devices. PMOLEDs are also easier to make than AMOLEDs and are more economical to manufacture at screen sizes less than 2 inches diagonally. However although better than LCDs, PMOLEDs have greater power consumption than AMOLEDs.
AMOLEDs are built differently from PMOLEDs. Instead of strips, the cathode and anode layers are laid like whole sheets. In between the cathode and anode layers are the emissive and conductive organic layers. The anode layer overlays a thin film transistor (TFT) array matrix that determines which pixels are turned on to form an image. AMOLEDs consume less power than PMOLEDs due to the TFT array requiring less power than the PMOLED external circuitry. AMOLEDs also have ultra fast response time (less than 50 µsec) making them suitable for video applications. AMOLEDs are ideal for larger displays (greater than 2-in diagonally) due to their higher power efficiency and video capability.
OLED advantages
OLEDs offer many advantages over LCDs, even when compared with TFT LCDs that usually have the best performance within the LCD display range that includes CSTN, FSTN and STN displays.
OLED displays are much brighter, more vivid and have striking clarity due to higher luminance to power consumption ratio (nits per mW), higher contrast ratio (the luminance ratio between white and black), and higher colour gamut (complete subset of colours) compared to TFT LCDs. The higher contrast ratio and absence of backlighting makes OLEDs achieve much “blacker blacks” than LCDs. The ultra fast response time eliminates the motion-blurring problem commonly seen in cheaper LCD displays.
Initial reviews by industry experts on Sony’s 11-in OLED TV screen go as far as saying that OLED TV is better than Plasma TV in terms of deeper contrast, brighter highlights and better motion performance - not to mention its incredibly thin profile. OLED displays are so amazing compared to TFT LCDs that they never fail to stop visitor traffic at our stand during our various trade exhibitions.
OLED displays are also more sunlight readable and are practically viewable at any angle, compared to TFT LCDs (which have a limited viewing angle range due to additional colour filter and polariser layers).
OLED displays also consume less power than TFT LCDs since OLEDs are self-illuminating and consequently need no backlight. Furthermore, OLED power consumption is dependent on the displayed image unlike TFT LCDs which has the same power consumption level regardless of the image. Using the same image pattern at the same brightness level, AMOLEDs have been shown to consume about 22 to 60 percent less power than TFT LCDs. AMOLEDs can also be made to consume less power by using darker images such as a black background.
The self-illuminating nature of OLEDs also leads to simpler structure and construction with no colour filter, no backlight module and fewer polariser layers. This makes OLED displays much thinner and lighter than LCD displays.
Finally, the OLEDs’ substrate (the supporting back layer) can be made of very flexible metallic foils or plastics - rather than the rigid glass used in LCDs - allowing it to become flexible and even foldable. It is still very early days for this flexible OLED technology but this new frontier can open up many exciting possibilities such as “smart” clothing, digital newspapers, ultra-lightweight laptops and rollable large screen TVs.
OLEDs for portable products
Using the latest state-of-the-art passive and active matrix OLED displays in the market, 4D Systems, has developed OLED modules ranging in size from 0.96 to 2.83-in diagonally. These high quality OLED display modules are powered by 4D Systems’ graphics processors (PICASO or GOLDELOX), making it easier and quicker for design engineers to develop handheld and portable products that require high quality displays. (See picture.)
These 4D Systems OLED modules can be controlled in one of two modes: Serial Command Platform, and 4DGL Platform.
The Serial Command Platform allows the OLED module to be used as a slave device connected to an external host. The host can be any controller such as a PIC, AVR, ARM, Cyan, etc. or even a PC where all screen-related functions are sent using a simple protocol via the serial interface. Serial commands may comprise of a single byte or multiple bytes of data depending on the command type. The serial platform allows users to develop their application using their favourite microcontroller and software development tools. For USB operation simply add a µUSB interface controller.
4DGL (4D Graphics Language) is a high level graphics oriented language that permits applications to be developed and run directly on the embedded PICASO and GOLDELOX processors on the 4D range of OLED modules, saving on BOM cost since the external host controller is no longer included. An extensive library of graphics, text and file system functions and the ease of use of a language that combines the best elements of languages such as C, Basic, Pascal, etc., permits rapid development of graphics oriented applications that do not require a separate host controller. 4D Systems also provide software development tools such as its 4DGL Workshop IDE software and Graphics Composer which are freely downloadable from 4D Systems’ website.
Further information: Alan Suyko works for Glyn High Tech Distribution. The company distributes 4D Systems OLED display modules in Australia. Contact (02) 8850 0320 (Australia), (09) 441 9050 (New Zealand) or www.glyn.com.au.