6/25/2007

gadget fixation: OLET, part one

Since people actually seemed interested when I posted about photoelectrochemical cells a few days ago, I'm going to test the waters again with another device post. The inspiriation for this is a recent paper that caught my eye. I thought it was kinda cool, so I'm going to ramble about it.
In order to get excited about the subject of this post, a little background on displays is needed.[1] If you're viewing this from your laptop, you're staring at an LCD screen (sometimes I like to state the obvious). The most important concept behind an LCD is that two cross-polarizers will not allow light to pass through. Parallel polarizers, however, will transmit light.[2]

These aren't actually sunglasses, they're toys.

That said, nematic liquid crystalline materials will orient themselves when an electric field is applied, and the direction of their polarization changes. Applying an electric field in the right direction will turn off a pixel (you see colors because there's a color filter). Of course, a critical element of any LCD is a light source, and the backlight is the most power-hungry component of the display.
Ideally, the light would be OFF until something turned it ON, rather than the backasswards working of an LCD. This is why you're starting to hear about OLED (organic light-emitting diode)-driven displays lately (if you've been paying attention). These screens are more efficient than LCDs and they're pretty bright. Also, you don't see viewing angle problems like you do with an LCD (tilt your laptop screen up or down if you don't know what I'm talking about).

If you have a bit of an engineering or gadget-nerd background, you're probably familiar with the term active-matrix display (also seen as TFT-LCD). If you haven't, I suggest you check out sections 3 through 5 here (incentive: it's not that long and there are lots of pretty pictures). It focuses on TFT-LCDs, but active matrix OLED displays aren't that different (IIRC, they also stick capacitors in there). This is very nice, but it requires teeny little transistors in each pixel.[3] The more complex these things become, the more difficult it is to make them, and the more expensive they tend to be. Savvy? So...wouldn't it be so much better if it were possible to integrate a few components of an active-matrix OLED display into a single device?

This brings us, at long last, to OLETs (organic light-emitting transistors). The first LET of the non-carbon variety was developed by some people at UIUC around 2003ish and emitted infrared photons. In addition to the potential display applications (which I am clearly excited about), these little gadgets are also likely to become important in telecommunications, since they give both electrical and optical output. Finally, they will probably provide new ways for bored engineers to set up their circuits.




Of course, this post was entirely background.[4] I'm going to make you wait until part two for anything actually related to the paper or the carbon-based goodies people are using in these devices.


[1] Again, I'm a few thousand miles from home this summer and my notes are not with me. Besides which, I'm a chemist and not an engineer. Correct me if I'm wrong. I hate being wrong.
[2] I've been momentarily confused numerous times by looking at my cell phone display when the polarizer was at a right angle to my polarized sunglass lenses. ("What? The LED is flashing! The battery isn't dead...WTF?...Oh. Duh.")
[3] If you don't know what a pixel is, I might cry. And then lock you in a room with a graphics program until you figure it out.
[4] As far as I can tell (because they're the only regular commenters), synthetic chemists seem to comprise a majority of the CBC readership. I'm not entirely sure how much they know about the inner workings of their laptop screens, and I'm also not entirely sure how thoroughly I can write about them before I start putting people to sleep. Suggestions are nice for these things (HINT HINT).

20 comments:

kiwi said...

i have a sony mp3 player with a simple monochrome OLED disply, and its terrible in bright light (luckily thats not an issue for people who live their lives within a 5m radius of their fumehood). But i sure hope they have a fix for that on the way.

Anonymous said...

There's at least one computer geek reading. I await part two...

Matt Jenks said...

If I tell you I don't know what a pixel is, can you lock me in a room until just after I'm supposed to leave to go on my "vacation" at my in-laws' house?

Ashutosh said...

i am assuming "home" actually means "lab"

chemist said...

Same here... eagerly waiting for part two.

milkshake said...

This is what I know about the screens: in the old times when the desktop color monitors were cathode tube, you can generate impressive rainbow circles on the screen with a strong magnet. Electrons are charged, you know. (Once I overdid it with a magnet puck out of stirplate - and ruined the monitor in this way; the coils got out of tune so I ended up with a purplish splotch over half of my screen).

The magnet trick does not work on LCDs but you can make grey maltese crosses on it by poking it with a sharp object.

mevans said...

Bring on Part 2!

Liquidcarbon said...

Are those just sunglasses or with dioptres too? If they are, I totally want them. It's the first time I see something like this, for some reason (I mean from both sides).

Ψ*Ψ said...

They're just polarized sunglasses with prescription lenses. I don't think you want them, though, because my prescription is a little weird.

David Eaton said...

I am looking forward to reading what you have to say next. This is an area that is very near and dear to my heart.

OLEDs are where the money is right now. They have the lifetime and economics to make a big splash, and the market is about to explode, with Samsung and Sony both poised to start making big displays. OLETs might be really interesting, depending on how their i-V characteristics change with light emission, and whether the light emission will age in step with the FET characteristics. If you don't mind me getting long-winded, I'll explain. I think that this is an area where chemists can do a lot of good, but I am not sure most people know what the issues are.

OLEDs don't age all that well, but incorporation of a silicon phototransistor in the display backplane (on the glass where all the Si switching transistors go) allows monitoring of the light, and using this feedback, one can do adjustments of the current to keep the output consistent. The important point here is that one can use the changes in the light output as a signal to tell the drive electronics how to adapt to the aging of the OLED. It isn't that the OLED 'goes bad'- it just changes, and to make the light stay the same, supporting electronics are necessary.

Even with this added complexity, OLEDs can be more economical than LCDs, which is amazing, considering the maturity of LCD technology. Samsung and Sony seem to think it is worthwhile, though, and have demonstrated large-format displays recently, and are promising products in the near term, within a year.

Where I work the ratio is about 20 EEs to every chemist, so I have picked up a few things from them. The most important thing for us to solve with respect to organic electronics isn't performance- the performance of OFETs, OLEDs and O-whatevers is good enough for many applications today. Stability is the problem. Turn-on voltage, i-V characteristics, etc could all be better, no doubt. But the killer is the changes with these over time. Without complicated schemes for compensation for changes, it means the operating point of the circuits using them changes, and this is bad. All of the assumptions one makes to design electronics are based on knowing that within stated tolerances, the electrical characteristics of an electronic part will be the same over the lifetime of its use.

In the case of OLEDs, you can justify using conventional 'tronics to overcome this, because even with the added expense, there are benefits. But needing a silicon transistor to make your OFET behave is unlikely to be worth it, unless you are specifically asking the OFET to do something, like act as a sensor, that the Si-FET can't do as well.

So one reason the OLET might be interesting, maybe even outside display tech, is that you might be able to monitor and react to changes in its characteristics. If the light emission could be used to probe the electrical characteristics of the device, then the circuit might be able to adapt to changes. Since OFETs are likely to be much less expensive than Si, adding more electronics this could be huge.

Ψ*Ψ said...

This is why I love it when someone who knows much, much more than I do about these things has something to say!! I was aware of the lifetime issues for OLEDs, but didn't know supporting electronics could be used. That's...really kinda cool. Maybe you should do a post on these. I know you know more about them than I do.

The Lil'est Naturist said...

I know so little...so very very little.

What are the base and collector?

David Eaton said...

Emitter base and collector are the terminals of a transistor. There are historical reasons why they are called this, but the bottom line is this- to a first approximation the current that flows from the collector to the emitter is dependent on the current flowing into the base (this is really not precisely right, but it is how engineers sometimes describe it. Maybe I will do a post on transistors...)

The whole point of the device is that a bunch of current through the device is controlled by a little current into the base. Thus, the base current is amplified.

FETs work differently, in that the voltage at the gate determines the current that flows from the drain to the source. Gate, source and drain are analogs of base, emitter and collector.

David Eaton said...

If you guys don't know about it, one of the best electronics books for scientists is "The art of Electronics" by Horowitz and Hill. It is by far the most useful.

If you are mathophobic at all (I know pchem types aren't but some organickers are) it uses very little math, and if you follow it, you can actually learn to design electronic things. It's good to have around to help you understand how instruments work, or alternatively, why they don't.

The Lil'est Naturist said...

Thank you, Mr. Eaton! I am indeed an organicker (gosh...now that I've said that word out loud, it sounds kinda taboo) and though I am not terribly mathophobic, I pretty much slept through E&M in college. I decided I didn't want to learn it when there was so much O-chem and pharm-chem just waiting to be learned!

mevans said...

Seconded on Horowitz and Hill! The book is thick, and looks intimidating, but it's actually a moderately fun read and ridiculously informative.

Ψ*Ψ said...

mevans: do you have a copy? if so, can i check it out after i get home?

mevans said...

Yes I do...it's in Louisville right now; I'll bring it back next time I go home.

David Eaton said...

If you are a complete noob, there is a lab manual that goes with the book (I have a copy. No, I didn't need it for a class. Yes I'm a complete nerd). It's got a lot of interesting background in it that the book doesn't mention, as well as some reasonably cool experiments.

Sometimes actually building something is a good way to figure out whether you understand it. I find that electronics 'pings' the same happy spots in my brain that synthesis does, and I can get away with doing electronics stuff in my house without attracting unwanted attention from the police.

Ψ*Ψ said...

OMG. I think I need to get a copy of that lab manual and take over Dad's garage. Sounds fun!