So how do OLED's compare with respect to this liquid crystal based display?
And you'll see a lot of information out
there on the Internet companies, which make OLED's.
They will tout, you know, that OLED's have a much richer color gamut.
Black is really black (in an OLED display)
Companies, which make LCD's will say, you know, look how bright we are.
How how
nice the color is on our LCD display are.
So, now that we have some understanding of how these things work, you know, let's try
to do a fair technical analysis of OLED's where is says LCD's.
So let's start with, you know, areas in which
OLED fares better as compared to LCD.
So, one thing about OLEDs is
that you can make them very thin You can make these OLEDs
display for less than 0.3 microns, as in 300 nanometers
in thickness. Because essentially they only have these two layers.
And you can lay these layers on a flexible substrate.
So that's another advantage of these OLED- base displays that you can make them
on flexible substrate and, you know, There are always these sci-fi applications
like you can make newspapers out of
these OLED-based display, and so on. And another
thing, which I mentioned earlier is that, since you have a these
OLED based. displays they have these light source at
the very top and this light does not have to pass through many layers.
You can have a much larger viewing angle so
you can look at these displays from over here.
Or you can look at these display from over here and they'll still provide a large
contrast as compared to your LCD based display where your contrast falls
very rapidly as a function of your viewing angle.
So that's you know, three things in favor of my OLED based display.
Another thing that these OLED based display, they have a very high contrast.
So, OLED based display, they can easily achieve contrast of, you , 100,000 to one.
where it says LCD, which you only achieve
the contrast of maximum of around 1000 to 1.
And that comes from the fact that LCD, they have problem
Turning the light off and the black is never really black.
But over here the way you generate black
is essentially that you turn off these red,
green, and blue pixels.
So you have no light coming out and black is really black.
That is one of the the thing you'll see companies talking about.
That you know, the black is really black with the OLED based displays.
Another thing is that, you know, you since these are based
on light emitting diodes and light emitting diodes have a much faster
response time.
And you can switch any of these things up in 0.01 milliseconds.
That gives you a frequency, switching frequency of
up to 10^5 hertz.
Versus as compared to LCD-based displays, you, it takes, you know, at least a couple
of milliseconds to turn a, a liquid crystal.
Change the polarization of the liquid crystal, and that
limits your frequency to, you know, less than 500Hz.
And we'll see that some of the gaming application and especially when you are
do 3D (gaming) .You really see your switching
frequency really starts to approach this limit.
So OLED-based display have a much larger headroom to increase their, increase
their, switching frequency. Or they have a much faster response time.
Another that you will hear very often is that they have a much larger color gamut.
So a color gamut that you get from these OLED-based displays is often much larger
than the standard, which is out there. So the standard color
gamut that these displays are supposed to achieve is this sRBG gamut.
And OLED-based displays, many a times they even exceed or do much better than that.
On the LCD side, we saw that this color always comes at a compromise with
your brightness, because you need to use
need to use stronger color filters so there is always this
color vs. brightness trade-off. But since these light emitting diodes based
displayed they are generating, colors directly instead of using a color filter.
You tend to get larger color gamut with this display.
So alright we have talked about all of these pros, but
certainly you know, there must be some, some disadvantage of these display,
there must be something, which might be limiting these OLED
bases but otherwise, you know, we won't be having this conversation and everybody
would be using OLED based displays. So, one of the
things that is again, mostly a technological limitations,
these are not fundamental limitations to the OLEDs, but you
know, add the current status, that is, you know, speaking as of
November 2012. These OLEDs based display, they tend to
have less brightness, or lower brightness as compared to LCD based display.
You can At maximum. currently, you can buy
an OLED
based display.
it's brightness would be, you know, somewhere around 300
per meter square.
you can easily buy a LCD based display, which have a brightness of over 1,000
candela per meter square. And comes from the fact that, so
far, these light emitting diodes. these organic light emitting diodes.
Especially they efficiencies which are you know, for your
for your green LED (sorry let me write in the proper colors)
so for your red and your green the efficiencies
of these LEDs has been, you know, somewhere around 15 to 20%.
the blue has been really a weak point and it's efficiency has been between 4 to 6%.
And so the overall efficiencies of these organic light everything diodes.
They limit the brightness so far of these displays.
>> Another thing again that comes from the use of these
organic molecules for making light. Everything
So these displays they they
degrade over time. And, especially the blue LED, which is a
weak point it degrades much faster
so your brightness, overall brightness of your display degrades over time.
so far the resolution of these displays as been less as compared to OLED's as well.
The best as compared to LCD as well.
The best OLED display that you can buy currently in the market will have a
resolution of around 300 pixels per inch. You can easily
buy a LCD based display which have a resolution of over you
know 325 or. or even higher pixel per inch.
So the resolution is another sticking point.
and let me draw a line over here, since we are getting
crowded, and let me draw a dotted line which goes like this.
So, another, you know, one of the other
limitations of these OLED based displays is that so
far they have been available only in small size.
These are the, the most common application you see them in are
in these smart phones, especially all the smart phones that Samsung makes.
If you buy TVs or, you know, if you buy HDTVs made of these
OLED display they cost at least, you know, more than three or four grand and
as compared to LCD TVs you can easily buy the same TV for less than $500.
So they're available in small sizes and also they cost.
a lot more money.
So these are some of the limitations so far and to understand at least some
of these limitations, we can understand by looking at how these displays are made.
So let's look at this limitation first. So what limits the
resolution for OLED-based display to, you know, less than 300 pixels per inch.
So to understand that, let's look at how these displays are made.
So OLED-based display, it's a very simple manufacturing
process, because there are only a few layers
that you need to put down.
So the first thing you do is you start with a glass substrate.
Or you can start, with either a glass.
Or you can start with a flexible, substrate.
You, of course, clean it.
And, you know, pre-treat your, surface.
Then, what you do is you deposit these, you know, your cathode.
You deposit your whole transport layer.
And then the most important thing. And that's what limits
our resolution is you deposit these RGB layers.
So, you deposit this, you deposit this red layer.
You deposit these blue layers and these have to be placed right
next to each other because they, that's how you generate a individual pixel.
And that limits your resolution as we'll just see.
So, the next step you do is essentially you deposit these.
your cathode,
your electron transport layer, your hole transport layer.
And the final thing you do is you,
once you've deposited this layer, you encapsulate your display.
Specially these are organic based LEDs, they degrade, very fast if you
expose them to any moisture or atmosphere. So you seal
these displays off using different techniques and you encapsulate
it so that it's no longer exposed to atmosphere.
And the technique that is currently used to make these
RGB layers is, is a technique called fine metal mask.
Or called FMM, which stands for fine metal mask.
And it's a very fancy word for a very simple technique which is essentially very
similiar to if you have worked in a lab and use a shadow mask.
So it's the same technique as using a shadow mask.
So the way this works, is that you have, you know, one mask
for your your red layer, you have another mask for your green layer.
And you have yet another mask for your blue layer.
And, you you have these source of, so the source would be,
you know, containing your your organic layer, which regulates
the color. And, you evaporate this source and you
filter through this mask on. And, so you place this mask in this
red material only passes through the area,
which is, you know, open on this mask.
And then you deposit your red dye.
And then you, you know, use another mask, and you
expose the area right next to this red molecule.
And you deposit your green dye over there.
And then you do the same thing with your blue dye.
And you get these
three layer, which are, you know, placed adjacent to each other.
So, you know, that is a simple process but it wastes first
of all, it wastes it results in a lot of material waste.
So think about, you know, you are using this fine mask to deposit this material.
But you're depositing that material everywhere as on the mask too.
So it's wastes this materials and
these materials are not necessarily always cheap.
And the other thing is that since you're using this shadow mask or this metal
mask based process that Really limits your
resolution because you are depositing these different layers.
And many times these dyes are not even in a, in exactly a solid phase.
They might, you know, spread out. So,
and also there's a limit to how far, you know,
how close you can place these different layers because the,
since we are using a shadow mask process, is it's,
there's no good way to control this resolution very easily.
And that limits your overall resolution to
you know, somewhere between around 15 microns.
And that limits your overall pixel density
to less than 300 pixels per inch.
So it's it comes from the way that how we mask.
Are manufactured.
So people obviously have been, you know, there are smart people working in
this field have been looking at other ways you can deposit these materials.
So the most promising techniques so far which looks like to replace that
finite metal mask is this technique called LITI.
Or, it stands for Laser Induced Thermal Imaging.
So, in this case, you start with a dye, and you have this dye on a donor
wafer, and then you you essentially apply a laser and you
scan this laser over the regions where you want to print
that red pixel and shown here is how that layer is transferred.
So you have this donor film, which contains this,red layer
and then apply, I apply this laser, which is causes heat,
which, in turn, also causes a volume expansion and as a
result of that, that, this layer is now transferred from this donor
layer to this substrate.
And I'm left with this red layer on my
substrate, and this is one of the most you know
promising technique that you know, companies like Samsung have
been looking to, you know, replace this finite metal mask.
But this is not the only you know, the only technique out there.
People also have been they
realized, you know, that there are limitations to
these laser-based techniques, so they're also looking at.
Non laser based patterning techniques.
And again you see a lot of things that
you might know from used in other process technologies.
So you see things like lithography that of
course can give you a very nice resolution.
But it, it comes out the cost that that
you, you, it comes at increasing cost. And also
you have to figure out how to you know, pattern these different layers.
The other technique which is also being explored and can give you again, very
cheap, very low cost and very high throughput and also can be
applied to a flexible substrate is roll printing where you essentially
just use drum and you bring them on onto a flexible substrate
But again, the resolution might not be as good.
People have also been looking at things like ink jet printing similar to how you
print you know, color paper. Are using a InkJet printer, so you
just print where you want them. And, again, this can achieve reasonable
resolution but at the same time, it has low cycle time so it has low throughput.
But all these things are being explored.