Gregory Mulholland

I guess since I am here to do research, all my family and friends should know a little about what I do here in terms of research. Plainly, I work with semiconducting oxides.

I know most of you know that I work with things that are trying to improve the efficiency of solar cells, that is, increase the amount of electrical energy output per unit light input of a solar cell. To do this, I am working with conductive and semiconductive oxides. These are typically metals mixed with oxygen. In my case, I am working with Titanium dioxide (TiO2) and Tin Oxide (SnO2).

Now, for some background: several years ago, a type of solar cell was devised that is called a hybrid organic-inorganic solar cell. These cells have a metallic or oxide part (inorganic) and a part made from a polymer (organic). These cells typically have a better efficiency than purely organic ones but do not do as well as purely inorganic ones. What is nice about them, though, is that they let us carefully examine how organic and inorganic layers connect.

Because these have been around for a bunch of years, a lot has been done with these cells. Different polymers have been tried, different oxides have been tried, lot of combinations have been made. Mine is a pretty new combination, but it is not exclusively the material that is the focus of my work.

Nanowires
Like I said before, I work with what happens at the interface between the two materials. What happens in the main part (officially the bulk) is well known. I am growing what are known as nanowires at the interface. If you want to be able to picture this (I will post real pictures at some point when I know what the rules are on publishing images), think flat hairbrush. There is a single flat surface with really fine, long tines sticking up from it. The reason we want nanowires is twofold. First, these wires are just that, wires. Charges (and thus, energy) can flow down them very efficiently in the long direction. Second, they greatly increase the surface area of the oxide. A flat surface has only one plane of contact. A comb-like surface has all those edges that can make contact with the surrounding material. Because the interface is so much larger now, its effect is maximized and I can compare the materials.

There are other challenges with nanowires: it is hard to get stuff between them because they are so small, they break easily, they are really hard to see (even with an electron microscope). All of these are in various stages of being overcome by me or one of the others working on my project.

Excitons are exciting!
Now into the nitty gritty. If you are not interested in reading about the hardcore science behind what I do, feel free to skip.

My ultimate goal is to hold on to, or keep alive, as many excitons as possible. What is an exciton? Well that is easy! It is a bound, electron-hole pair. If you are Saket, Ben, or Win, you probably understand this, otherwise, here is my quick sentence explanation:

Electrons sit on tracks. These tracks are at different energy levels. An electron can only travel on these tracks (officially known as bands, but tracks are a better common-sense descriptor for this example). So, if I have an electron on a track at 1 Volt, and I want to make it travel on a track at 2 Volts, I have to apply more energy to it to get it there. If if give 1/2 Volt to a 1 Volt electron, it can’t just jump to the 2 Volt track. Tracks are very well defined for all materials, the space between them is known as the band gap and varies by material.

I’m sure most people remember from high school chemistry that electrons (an just about everything else, including me) like to sit in the lowest energy state possible. If an electron can move to a lower track, it will. Thus, the low tracks get filled up fast. In fact, one can define a top level to the electrons. This (a simplification of Fermi level) means that the electrons are like sand grains on a beach. They all pack nicely into their layers with only slight variations on top. But, because they pack so tightly, there is little room for them to move. There is essentially an electron traffic jam on the lower tracks. The only way to allow and electron to move with any haste is to move it up to one of the unoccupied tracks.

Solar cells work by letting the sun provide the extra energy to move an electron from one track to the next one up. This allows the electron to zoom down the nearly empty track toward whatever you are trying to power with the solar cell. The electron leaves a space behind it where it formerly took residence on the lower track. Because the electrons are packed so tightly, they begin to, one by one, move to fill the void left by the zooming electron. This creates a propagating gap (known as a hole) down the lower track, just like when one car is able to get through in a traffic jam.

This electron-hole creation is called an exciton. My goal, like I said, is to make this last as long as possible. The longer I can make them last, the more electrons are able to escape into the next material and be used for energy. How do they die? This is simple, if the electron and hole do not get away from each other fast enough, they will be attracted to one another and the electron will fall back into the hole, and no energy will be extracted.

So, with all of that said, I am trying to grown Tin Oxide and Titanium Oxide nanowires and make them ideally suited for solar cells. As time goes on and I have more results (which are finally beginning to look rosy), I will post some pictures and descriptions for those of you who are interested.

If this all is totally unclear, let me know and I can try to explain it using a whole different set of metaphors

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