Chances are your flat-screen TV, your computer monitor, and your smartphone use OLED technology for their display. Have you ever wondered how these displays work, and how are they differ from LCDs? Because of their popularity in consumer devices there is a certain amount of marketing jargon floating around that makes understanding OLEDs more confusing than it needs to be.
We can start simplifying things by making the difference between OLED and LCD technologies clear. LCD (liquid crystal display) use the light-modulating properties of liquid crystals together with polarising films to display information by modifying the transmission or reflection of a light source. OLEDs (organic light-emitting diode) displays on the other hand, are an emissive electroluminescent technology where the display elements themselves emit light in response to an electrical signal.
This difference is illustrated in Figure 1 where a simplified cross section of both display types is shown. In the case of a transmissive LCD, a light source at the back of the display is polarised (horizontally in the diagram) then passed through the liquid crystal material, which is sandwiched between two pieces of glass which have transparent electrodes printed on them.
The uncharged liquid crystal rotates the polarisation of light 90˚ so the horizontally polarized light entering the liquid crystal emerges with vertical polarisation and passes through the vertically polarised filter on the other side. If the transparent electrodes are charged, the liquid crystal does not rotate the polarised light, and it therefore is attenuated by the final filter.
In the case of the OLED, the organic semiconductor material itself emits light if excited via the electrodes. The “organic” term simply means that the materials employed are principally made up of hydrogen and carbon atoms. Organic semiconductors are a class of molecular crystals or thin films that are normally insulating, but which can become conductive when charges are injected by doping or via electrodes.
To activate the OLED, electrons are injected into the semiconductor material at the cathode and holes are injected at the anode (electrons flow from cathode to anode). Electrostatic attraction causes these charged molecules to move towards each other and when they combine to neutralise each other’s charge, they release energy in the form of visible light.
You can see from Figure 1 that OLEDs are simpler in construction than LCDs, therefore they can be cheaper and easier to manufacture. They also have significantly better contrast since the light source is truly off when displaying “black”. In an LCD the degree of “blackness” depends on how effectively the crossed polarisation filters block the backlight. OLEDs are also thinner and generally have a better viewing angle than LCDs of a similar price and size.
This diagram contrasts the operation of an LCD and an OLED display. The LCD modulates an external light source using polarisation, while the OLED emits light directly.
You will see the term AMOLED used to describe OLED displays from some manufacturers. This is not a different technology, just a description of how the pixels are driven. AMOLED stands for Active Matrix OLED and this just means that there is a thin film transistor and a capacitor associated with each pixel. These components maintain the state of the pixel between refreshes – necessary for large displays with lots of pixels to address. This is exactly the same technology used in larger TFT LCD displays. In fact, most OLED and LCD displays use active-matrix technology – it is nothing special!
Just to add to the confusion, you will sometimes see LCD displays described as LED displays, particularly in consumer products. The LED in this context refers to the source of the backlight, not the display itself which operates on the usual LCD principle. This term is a little bit of sneaky marketing misdirection in my opinion. Of course, OLED displays aren’t perfect. They can be less energy-efficient than their LCD counterparts and can also have lower overall brightness, although this is improving rapidly. They are certainly an option to consider for your next display project.
Bibliography
“FAQ: What Is the Difference Between a TFT, LCD, and OLED?” Crystalfontz LCD Blog, November 1, 2016. https://www.crystalfontz.com/blog/faq-difference-tft-lcd-oled/.
“OLED.” In Wikipedia, December 6, 2024. https://en.wikipedia.org/w/index.php?title=OLED&oldid=1261454364.
“The Complete Guide to OLED Displays | RS.” Accessed December 6, 2024. https://au.rs-online.com/web/content/discovery/ideas-and-advice/oled-displays-guide?srsltid=AfmBOopwpE2JrZQS_WdRPpVgQp5fjH2WermRZw7_bq6aj6UNXgk5e8sI.
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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.
