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What is 'Organic' LED and how it works

What is 'Organic' LED and how it works

I'm going to assume most of you have heard of OLED, mostly known as that fancy screen technology used in phones and TVs. You might even know that OLED is short for 'Organic LED', but what is organic about it, what does that even mean? That's what this article is all about - How OLED works even at the molecular level, and how it compares to other screen technologies like LCD.


What is 'Organic' LED, how it actually works, why does OLED degrade, why use OLED at all, types of OLED technology, and how OLED is made?

In terms of functionality, OLED is advantageous because each individual pixel is its own light source, unlike LCD panels that have an LED backlight, then block the light of different colors using liquid crystals, only passing through colors you want.

What is 'Organic' LED and how it works

I also went into heavy detail about how the panel technology works as well. But the basic difference is that LCD selectively blocks light, where OLED selectively produces light.


Regular LEDs

But to really understand what's going on with OLED, first, we need to know about regular LEDs or light-emitting diodes. A diode is an electrical component that mostly just allows electrical current to pass through it in one direction, or in more scientific terms, it has low resistance in one direction, and high resistance in the other.

This is useful for converting alternating current to DC current for example because it effectively cuts off or blocks half the waveform when it reverses, but that's all beyond the point of this article.

What is 'Organic' LED and how it works

Anyway, a light-emitting diode is just a diode that emits light when electricity passes through it, no surprise there. With typical LEDs, the actual light-emitting part is made of a semiconductor die, which is a small cut-out piece of a semiconductor wafer, very similar to how CPUs are cut from large wafers. Then you hook up some wires to the semiconductor piece and it can make light.


What is 'Organic' LED and how it works



"Organic" LED

On the other hand, OLEDs are actually really different, both in what they're made of and their manufacturing process. You see, OLEDs are made of 'organic compounds' - ok but what does THAT mean. Well, an organic compound is simply ANY chemical compound or molecule that contains carbon-hydrogen bonds. Obviously, that's not pretty general, and there are literally millions of these compounds known, they're very common.

And organic compounds are called that because all known life is based on these types of molecules. So in the context of OLED, organic doesn't mean it uses materials harvested from plants or something, just that it uses similar molecules also found in life. But that doesn't mean every organic compound is necessarily found in life because there are synthetic organic compounds that are technically organic from the definition, but are purely man-made and don't exist in nature.


How do OLEDs work at the atomic level differently from LED? Why do OLEDs burn-in? Why do they use organic molecules anyway?


To explain how OLED works in the first place, I'm going to have to get into some physics. It's pretty complicated stuff, but I'll do my best to simplify it based on how I understand it myself. Admittedly, I'm not an expert on this stuff, so don't hate me if I make some small errors or leave something out, you can just let me know in the comments.

Alright, so, you might remember from school that atoms have layers of electrons surrounding them, called 'shells', and how each shell has different levels of energy depending on how close they are to the nucleus. You may also remember that electrons can actually be shared between atoms, for example with a covalent bond.


What is 'Organic' LED and how it works

however, electrons can be 'delocalized', which means they're not really closely associated or 'stuck' with any atom in particular.

What is 'Organic' LED and how it works

So if you have a crystal lattice of many molecules near each other, these molecules may sort of share electrons and pass them around. These are also called 'free electrons', and is often a big factor in how electrically conductive a material is.

What is 'Organic' LED and how it works

Alright so now we have some context, but what does this have to do with OLED? Well, everything! You see molecules used in OLED are special because they emit light when a current is passed through them. How this happens is when current passes through them, it adds energy which is absorbed by those molecules and they're electrons.

This kicks some electrons into a higher energy layer of the electron shell, known as the conduction layer. When this happens, this leaves an 'electron hole' in the lower energy level, or the valence shell, where the electron was - simple enough.

What is 'Organic' LED and how it works

There is a lot more physics on in this bit that I could possibly get into, but if you want, you can get more details if you look up the Wikipedia page for 'Excitons', which is the name for this combination of an electron and electron-hole.

What is 'Organic' LED and how it works

Anyway, eventually this excited state of the outer electron decays, and the electron falls back down from the higher energy to the lower energy level, and in doing so releases its extra energy in the form a photon, or light. This is called 'recombination', and the color of the photon depends on how much energy is released, which in turn depends on the molecule or material.

What is 'Organic' LED and how it works



Why does OLED degrade?

Now, over time OLED panels are known to degrade and potentially burn in. What causes that? Well in the recombination process we just mentioned, it doesn't always result in a photon. When it does release a photon like we want, it's called "Radiative Recombination". But there can also be 'Non-radiative recombination', where the energy from the electron falling back down is released in other ways.

For example, the energy may be transferred to another electron in the form of heat. Or if there is a defect in the crystalline structure in that part of the panel, there may be an extra energy band the electron can fall back into, so it doesn't release enough energy to make a photon.

What is 'Organic' LED and how it works

One form of non-radiative recombination is known as 'Auger' recombination, which is believed to be the main cause of the degradation of OLED. Basically, there is a very small chance for an electron to collide with another atoms' electrons, cause it to have such a high energy level that it actually breaks an atomic bond completely, therefore breaking the molecule apart.

This is called 'homolytic dissociation', and results in atoms or fragments of molecules being separated into 'radicals', which basically end up being defects in the structure of the panel at the molecular level. The more of these radicals, the more they interfere with the electrons around them, causing even more kinds of non-radiative recombination. Therefore this ends up decreasing the efficiency of the panels over time.


What is 'Organic' LED and how it works



Why use OLED at all?


So why use organic molecules at all then? Why not use regular inorganic LEDs? Well, we do, but only for huge displays, and it comes down to the difficulty of manufacturing LEDs at very small sizes. Typically LEDs are individually made components, each cut from a piece of semiconductor, so they have to be constructed and placed on the panel one by one.

Even for a 1080p screen, that's 2 million pixels, which will take a long time to make, and then you have all the RGB sub-pixels, so really it's 3 times that. You also have to have a way to wire up all these LEDs, and there's just a ton of room for error. Now like I said, for massive panels used on billboards or something, they are viewed very far away, so the LEDs don't have to be very small or close together, so it's easy to make them.

But the smaller the screen, the harder it is to pack individual LEDs together like pixels, so it's better to use other technologies like LCD at these high pixel densities. So what is it about how OLEDs are manufactured that make them able to overcome this is.


How OLED is made


Well, OLEDs are manufactured completely differently than regular LEDs. In fact, OLED pixels aren't made of a physical diode with wires and stuff at all. You see with LED, you have to cut it from a wafer, and wire it up to pass current through it. Organic molecules, from my understanding, are a lot easier to manipulate, so instead of using wires to pass current through them, you can just take one conductive sheet, spray OLED molecules onto it.

Then sandwich it between another conductive sheet, then the whole thing just puts electricity through it so it all lights up. In other words, OLED is deposited as a film between two layers which act as anode and cathode, when whatever parts of that film get power will light up.

What is 'Organic' LED and how it works

This means you can have squares of OLEDs for example that are not really an individual point of light, but rather the entire square can light up uniformly.

What is 'Organic' LED and how it works

Now to make individual pixels, you just have to find a way to divide up the anode and cathode layers so it can provide voltage to only certain tiny parts of the screen at once. So to make a pixel, you can create what is called a 'matrix' of anode and cathode layers, which is basically a big mesh that lets you choose and energize individual points, one for each pixel. Then you can deposit the OLED material onto the matrix in each spot.

What is 'Organic' LED and how it works

And you'll also probably need a way to control which types of OLED molecules you put where, because different molecules emit different colors of light. This can be done in a number of ways, including literal inkjet printing each pixel, so you can control exactly where each type of molecule is deposited.


What is 'Organic' LED and how it works


What is 'Organic' LED and how it works

Or, you can put the OLED material into a gas form then vacuum deposit it, so it's uniformly distributed. Of course remember, the way I'm explaining it is very over-simplified. Anyway, the main takeaway is that OLED is manufactured in a completely different way from LEDs, that is way easier than if you wanted to make tiny regular LEDs of the same size as OLED subpixels.


MicroLED

That being said though, you may have heard about MicroLED, which I've mentioned before on this channel, and is basically the holy grail of display technology. MicroLED refers to screens that do actually use traditional inorganic LEDs being used as individual pixel elements, just like with OLED.

If these elements could be shrunk down to what is necessary for say a TV or monitor, it would have the same benefits as OLED, such as not needing a backlight, and having perfect black levels as a result. But it would also have none of the degrading properties of OLED.

But MicroLED is still not here yet. Companies like Samsung and LG have made MicroLED TVs, but they have to be huge, like 100 inches plus because the LEDs are still relatively big, therefore the pixel density is still not nearly as good as LCD or OLED.

Also, because they are so tough to manufacture and it's a new technology, they are extremely expensive. However, these companies are obviously working on improved methods to shrink the LEDs and improve the speed of making them. I personally think eventually MicroLED will replace OLED, but it will take several years.


Types of OLED Technology


Now, you may be wondering more about these 'organic compounds' that are actually used in OLED, and there are several of them used, depending on the tech. The current two major types of OLED technologies in use and development are known as 'Small Molecule' and Polymers. And at least in 2019, basically, all OLEDs were the 'small molecule' type. In this context, a small molecule basically just means it's a carbon-containing molecule that is literally relatively small, specifically having a molecular weight under 1000 or so.

What is 'Organic' LED and how it works

On the other hand, polymers are very large molecules formed as long chains, but I'll get to them later. One example of a small molecule used in OLED is...

What is 'Organic' LED and how it works

well this name, abbreviated as AlQ3. This molecule contains aluminum and carbon, and is known as an 'organometallic chelate'. That's not really important but it sounds cool. Here, the aluminum is in the middle, surrounded by three branches of "8-hydroxyquinoline".

What is 'Organic' LED and how it works

According to a 2006 scientific paper, this organic compound is special because by modifying and substituting certain atoms, you can tune the frequency of light emitted, so it can be used to create various colors spanning the entire visible spectrum. In simpler terms, you can use it to make lots of different colored pixels.

What is 'Organic' LED and how it works

That's just one example of a molecule used in some OLED though, there are lots of other potential small molecules. The other type of technology we can talk about is called either PLED or P-OLED and uses light-emitting polymers or LEPs. A polymer is a general term for typically very large molecules with repeating subunits. So basically, a big molecule with a repeating pattern, forming a chain.

What is 'Organic' LED and how it works

For example, DNA is a polymer, along with nylon, silicone, rubber, and lots else you probably use every day.

What is 'Organic' LED and how it works

Anyway, P-OLED uses a type of polymer called 'electroluminescent conductive polymers', which is fancy talk for an organic polymer that lights up when charged with electricity. Apparently, by substituting different so-called 'side chains' on the molecule, you can determine what color it lights up as.

Here are some specific polymers used for different colors. There's Polyphenylene, which is a blue emitter. Poly(p-phenylene vinylene) is a green emitter. And Poly(alkyl thiophene) is a red emitter. You can see these show the base pattern, and the brackets indicate it repeats on either side.

What is 'Organic' LED and how it works

All that being said though like I mentioned, my understanding is Polymer-based OLED is still mostly in the development stage. Finally, there is a third possible type of OLED that is still being explored, called 'Phosphorescent' OLED, or PHOLED. These would work on the principle of phosphorescence, which I'm not going to get into because it would require even more physics I'm not prepared to explain.

But a major benefit of this type of OLED would be that it is way more efficient than other forms of OLED. Again though is still apparently pretty early in the research phase. So, by now you should be an OLED expert and probably learned way more than you'll ever need, but hopefully found it interesting. Let me know in the comments what you thought, and thanks for reading.

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