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How does OLED display technology work?

July 28, 2022


ABOUT THE AUTHOR

Gaurav Lokhande

Gaurav Lokhande, B.Tech in Electronics & Telecommunication Engineering. Currently, he is working at Lesics Engineers Pvt.Ltd as a Product Developer and Visual Educator. His areas of interest are Embedded Systems, IoT, and Printed Circuit Board (PCB) Designing. To know more about the author check this link


Everyone knows that an OLED display is better than LCD or LED. Let’s learn this technology in detail along with the advantages of it over LCD and LED. Before learning about OLED, first, we have to understand Pixel, this is the most important part of any display technology.

How does a Pixel create?

Interestingly, the fundamental image reproduction mechanism is the same in all the display technologies. The smallest display unit is an element called a pixel; You can see in the Fig:1A, three different colour filters inside a pixel. The most amazing thing is that we can achieve any colour just by illuminating these filters at different intensities. I have shown in the Fig below. Now, you might doubt that the filters are different pieces; how can these three different colours get mixed up and produce a new colour(Fig:2B)?

Fig:1A - Color filters inside pixel
Fig:1B -Combination of RGB colours produced new colour

I will explain this phenomenon with a simple example. We can see that the colours are distinctive! After a certain pixel size, the individual colours are not distinguishable. You will see the combined colour of all. This is due to the limited visual resolution of the human eye; it can’t differentiate between subpixels.

Practical image reproduction

Now, let’s convert these pixels into digital, so each pixel has its position and colour data. This data is stored in digital form for future reproduction of the image. Now, let’s see how image reproduction is done practically. I took a uniform white backlight source, and kept a colour filter containing multiple small red, blue, and green colours in front of it; again placed a glass screen in front of it. As soon as I turn on the backlight, all the filters will glow with equal intensity and what we get at the output is just white colour as you can see in Fig:2.

Fig:2-Image reproduction setup

LCD or LED Display

Now to get the other colours, I just need to get different brightness levels for the sub-pixels. To do this, I will use an LCD sheet and a small circuit. The polarisation of the LCD crystal can be adjusted, and we will easily get different brightness levels in the subpixels(Fig:3A). Now, it's time to convert the digital signal we stored to electrical signals. These electrical signals are fed into the circuit. When the signal is received, the crystal in the LCD rotates and polarises the light. In this way, we successfully produced our original image(Fig:3B).

Fig:3A - To get different colours, polarisation of the LCD crystal adjusted
Fig:3B - LCD’s crystal rotates by receiving the
electric signal and light has been polarised

There are several disadvantages of this display technology! The colour reproduction is not that accurate. For example, when we try to produce a perfect black colour using this technology, this is what we get. The energy consumption in those display types is quite high. What if we provide each pixel with its light source and control it?

This is a great idea. Instead of using a common light source, use minute and many light sources for every pixel. With this method, the LCD sheet can also be removed. However, the issue is that fabrication of such minute LEDs in the range of micrometres is not practical. Due to the issue of surface irregularities and their solid nature at room temperature, they cannot be miniaturised into micrometre ranges. This is why organic LED comes into the picture. They can be fabricated as small as 6.3 micrometres. Now I will explain it below in detail.

Fig:4 LED & OLED Manufacturing process

How does an OLED works?

Any LED technology works based on electron-hole pair recombination in semiconductor materials. Please note that only those materials with a suitable bandgap in their atoms can emit light in the visible range (fig).

Fig:5 - A suitable bandgap having materials that can emit light in the visible range.

In organic semiconductors, the energy levels of molecules are considered rather than atoms. The electrons in a stable state are located at the HOMO level, and those in an excitation state are located at the LUMO level(Fig:6). Let's connect this organic semiconductor to an external power supply using anode and cathode. Due to this, electrons move from the HOMO to the LUMO levels via a power supply and create holes. As soon as these electrons enter the LUMO layer, they recombine with the holes and emit light due to the natural tendency.

Fig:6 - Stable state electrons: HOMO level & Excitation state electrons: LUMO level

However, this process is not simple. You can see in Fig:7A the anode side first. When we connect the battery's positive terminal to the anode, it tries to extract electrons from the organic layer. However, there is an energy difference between the HOMO level of the organic layer and the anode, which will act as a barrier for electrons. The same is the case with the cathode side. So, the cathode won't be able to inject electrons easily and consumes more energy, as I have illustrated in Fig:7B below.

Fig:7A - Barrier between anode and semiconductor
Fig:7B - Recombination process

This problem is solved by adding two different layers between the electrodes and the organic semiconductor(Fig:8A). Due to the addition of those intermediate energy layers, the barrier will be reduced, and electrons can be easily injected or extracted from the organic layers. However, here, charges have very low mobility due to hopping between the molecules. For this reason, we add more intermediate layers to further reduce the energy barrier and reduce power consumption.

Fig:8A - Addition of transfer layers.
Fig:8B - Addition of injection layers

Let's place such three organic LEDs behind a filter to control each subpixel independently. Just by varying the external power supply, we can control the electron flow or recombination rate and reproduce any image(Fig:9). It is quite obvious that black colour reproduction can be perfectly achieved using this technology.

Fig:9 -Addition of injection layers

The current OLEDs produce only white light. A cool and promising feature of OLED technology is that we can even avoid the usage of colour filters with its help. What if we directly obtain RGB colour light emission from the OLED source itself? This is certainly a possibility. Currently, various OLED manufacturing companies are working on developing RGB colour emitting OLED devices by adding various doping materials in emission layers. Due to the addition of doping material, the bandgap of an emissive layer is changed accordingly, changing the colour of light emission(Fig:10).

Fig:10 -Different bands required for Producing RGB colours

That’s all about OLED Display Technology. I hope you now have a good understanding of OLED. Thank you for reading this article.


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