Organic chemists can go ahead and run away screaming, because this post is about direct carrier multiplication and not dichloromethane.  No carbon this time—troublesome, eh?  But it’s been about…five months since I last put up a serious post on semiconductors.  Entirely too long.  To be fair, in that time I’ve been reading up on band theory and solid-state awesomeness, so I haven’t just been lazy.

Direct carrier multiplication, for the uninitiated (such as myself—physicists can have a field day correcting me here), occurs when an exciton—an electrostatically bound electron-hole pair—creates more excitons.  Naturally, the first exciton has to be pretty high-energy in order for this to happen…say, between 2 and 3 times the bandgap.  For those of you who are scratching your heads and asking why, and haven’t considered who’s writing the post and just how much she likes applied research…I’ll give you a hint.  It has something to do with solar cells.

In conventional (and sadly carbonless) solar cell materials, light is absorbed like mad above the bandgap, and pretty much not at all below it.  (“Like mad” is a scientific term—google a CdSe thin film or nanocrystal absorption spectrum if you don’t believe me!)  But what about WAY above the bandgap?  Hopefully we can agree that blue photons are higher in energy than red photons.[1]  What happens to the extra energy?  Usually, the answer is…nothing exciting.  It just gets turned into heat.

IF, instead, we could harvest the extra energy, we’d have more efficient solar cells.  Through DCM, you get extra charge carriers from that excess energy.  The question, then, is how?  First, light is absorbed (this is, of course, the starting point for all things solar)—in this case, really high-energy light.  So the electron that gets excited doesn’t go straight to the LUMO (or whatever you would call the equivalent for a crystal?), it goes to some higher energy level.  We call this a hot carrier.[2]

Normally, the hot carrier loses its energy through phonon-assisted decay—it emits phonons (lattice vibrations; raise your hand if you thought I misspelled “photon”?) and ends up back in the LUMO.  I say “normally” because this is typical of bulk semiconductors.  Nanocrystals are a little different: less energy levels to worry about.  With this decay process inhibited to some degree, what can happen to the hot carriers?

The answer: impact ionization (hereafter II, not to be confused with Roman numeral II).  Instead of dumping energy into the lattice and making it vibrate (as in, emitting phonons?) the super-excited electron can transfer its energy to other electrons, thereby producing more excitons.  Of course, there is an annoying reverse process to worry about: Auger recombination.[3]  This occurs when one of the excitons recombines, and the energy released goes toward exciting another exciton to a higher-than-LUMO energy level.

At this point, I’ll leave you with a very interesting and very recent paper on the subject by a few people out at NREL.  Unlike me, the authors definitely know what they’re talking about.  Organikers beware: there are some equations lurking about.[4]

[1] If you disagree, you need a math flogging.
[2] It’s hot cos it’s fly. You ain’t cos you not.
[3] You know you’re a nerd if you hear Auger when someone says OJ.
[4] If this frightens you, I suggest drawing a benzene ring around the curly S of death, as a protective sigil against the evils of integration.

This entry was posted on Tuesday, August 26th, 2008 at 9:52 pm and is filed under cool papers, solar cells, xtal porn. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.