6/19/2007

photoelectrochemical cells, quick & dirty

Alright, already. I wasn't planning on it, but there were enough curious comments on the last post that I'm actually going to tell you guys what that little device was.[1] However, I'm not in the lab now (else, I would be working and not writing, duh) so I'm short on notes and lit references (which are currently lying on my desk, in the lab...where I am not). Soooo...this is the quickie version from off the top of my head. Sadly, I have no pictures for you here. Feel free to correct me. (Especially YOU, Dave. Or YOU, Lakshmi. Or anyone else, really.)
Remember all those posts from a while ago on solar cells?[2] Well, I've been meaning to do a post on Graetzel (dye-sensitized) cells as well. Not that I've ever worked on them, but they're pretty cool. There's a back electrode (read: not transparent), and usually there's a lot of nanocrystalline TiO2 in them. This functions as an n-type semiconductor. Attached to the nanocrystalline titania is a dye. The "nanocrystalline" bit here is important--it provides a higher surface area for dye adsorption. Since titania's bandgap is too high to have much to do with absorbing sunlight, the dye is rather useful. After all, if it's a dye, it's not white like TiO2, so it's absorbing something, somewhere in the visible spectrum. Often, the dyes used for this purpose are ruthenium complexes, although price concerns may drive the development of alternatives. Usually there is some carboxylic acid functionality present, which enables the dye to adhere to the titania. Other (frequently oxide-based) semiconductors can be used in place of titania, which has the associated problem (advantage, maybe, elsewhere?) of nibbling on organic things.
So the dye absorbs light, and transfers an electron to the titania. And then...yeah, pretty much nothing, until I mention the electrolyte that is essential to the functioning device. What's generally used is I-/I3-. This, along with the front electrode, provides a cute little redox shuttle to restore the dye to its original state.
As I said, no pictures, since I have never made one of these and am not in the mood to Photoshop a nice little figure for other people to steal.[3] However, if you're interested, there are instructions for DIY dye-sensitized solar cells on the cheap here. Things like this are especially nice for outreach (HINT HINT).
Back to the original purpose of this post (which has thus far been neglected), the nanopopsicle I showed you a few days ago is not, in fact, a Graetzel cell. It is similar, though--the major difference is that the semiconductor used in my little device has a sufficiently low bandgap to absorb light in the visible spectrum.[4] That, and all the materials are completely different. The only appearance carbon makes in the nanopopsicle is in the insulating coat of epoxy.
Speaking of which, epoxy is not such a terrific thing for XPS, so I would be thrilled if someone had other quick & easy ideas for an alternative.

[1] In the early days of CBC, no one said much of anything to a device post. I know many of you are just bored synthetic chemists looking to see neat little structures, but...when there are no comments, we get lonely. And we stop doing device posts. Have the audience members experienced a change of heart?
[2] I'm not terribly satisfied with it, so a follow-up may be in the works. You have been warned.
[3] Not that you would want to, but anything of mine that appears on CBC is free for the taking as long as I am credited (and, yeah, it would be nice if you let me know you were going to use it).
[4] Never mind, y'know, all that infrared light from the sun. Bandgap's not THAT low.

12 comments:

Mitch said...

No pictures makes for a sad blog post.

Mitch

Matt Jenks said...

No pictures = sadness.

At least you get comments when there are no pictures.

sujit said...

This is a cool post, thanks; I was losing sleep trying to figure out what that thing is.

Uncle Al said...

There are lots of flash UV cure systems (e.g., dental composite, optical cement, plastic eyeglass lens hardcoat). Shield your goodie from the UV light. Too expensive, not viscous enough (add some fumed silica and sonicate).

Go for catalyzed peroxide cure filled styrene polyester resins - BONDO! Mix and work quickly. Ain't nothing better than Bondo! If you want to upscale, acrylic bone cement for fixating hip replacements.

http://www.bondo-online.com/
http://en.wikipedia.org/wiki/Bondo_(putty)
composition

Third possibility is plumbing epoxy putty. Gives you lots of working time and it sets to rock. Kinda pricey.

Ψ*Ψ said...

Huh. I didn't think of Bondo. We more or less need it to do two things: stand up to a few solutions and survive well for XPS (manage to pump down without screwing anything like the x-ray filament up).
Any idea how it does in vacuum?

milkshake said...

There is a teflon solution spray, in some organic solvent

David Eaton said...

Your explanation makes sense. I am glad to find out what the thing is, too.

Titania is used specifically to chew up organics on 'self-cleaning' glass.

I think the teflon spray is in some fluorinated solvent. If it is related to the stuff I've been spin coating it is hideously expensive.

Ψ*Ψ said...

Hmmmm...I think "hideously expensive" is a bit out of my price range.

kiwi said...

i've got a sprayable teflon suspension in dimethyl ether - great for fixing locks etc, but not sure about robustly coating anything with it. its pretty cheap (NZ$20/~200mL, US$0.70=NZ$1), but probably not very good for what you need it for

Hap said...

Dimethyl ether? Is it under pressure?

Jeremy said...

Not sure what your requirements are, but Epoxy Technologies (http://www.epotek.com/) makes a range of vacuum-compatible adhesives. I used their 377 product to template-strip gold from an Si wafer, and also one of their conductive epoxies to affix a conducting AFM tip to a metal base for use in a UHV AFM. No significant outgassing occurred with that one, though I can't remember the product number (I think it started w/ an H). Here's a reference where they performed XPS measurements on substrates that contained the 377 product: Langmuir 2006, 22, 2578-2587.

Ψ*Ψ said...

Yeah, what we were using was vacuum epoxy, and we did get the XPS to work. Still, the vacuum levels weren't quite as high as the (awesome) XPS guy wanted them.