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How stuff works: flexible solar cells

So today's installment in my ongoing series, tentatively titled "Occasional ramblings on science type things", is going to be more of a "How stuff works" segment. Partly because I couldn't find anything inspiring in my favourite journals this morning and partly because if I write my own stuff, I can make my own figures and don't have to fret about internet copyrights (except that top image, I'm fretting about that, source: NRC Canada. Link.). Also partly, I hope, because people might be interested. It has become pretty clear in my short time here that everyone is really smart, so I worry that this may be too basic, but then again not everyone knows everything about everything. Even smart people. For example, ask me what the endoplasmic reticulum does, go ahead, ask, I don't have a clue.

So today I'm going to write about solar cells. And since that is a huge topic, I'm going to write about organic solar cells only. And since that is still pretty broad, I'm going to write about bulk heterojunction organic solar cells.


I like to think of organic solar cells as the hipsters of the solar cell family . They're not going to be the ones in the big solar cell arrays down in Arizona, and they're not (yet) the one you see on people's rooves. They're the ones being looked at for niche applications. The solar cells you find on messenger bags so you can charge your phone on the go, or the ones you might spot on the top of a beach umbrella so you can surf the net instead of the waves. Kind of a novelty, kind of impractical, a little out of the mainstream: solar energy hipsters.

The reason they are so potentially interesting, if we can just get them to work right, is that they are flexible and (relative to silicon cells) they are dirt cheap. They can also be fabricated pretty easily on a large scale using something akin to a printing press. Because I'm interested in organic photovoltaic (OPV) research myself, I'm going to hide their disadvantages at the end of this very long article.

So let's take a look at their structure. Here (Fig. 1) is a very (over) simple bilayer OPV.


At the top, there's a transparent electrode, so the light gets through. Indium tin oxide is very popular for this, though there are others and that could be an article in itself. In the middle there are active layers. The active layers are made out of materials that absorb a photon and use that energy to excite an electron. The best active layers can do this at many different wavelengths so that as much of the sun's energy as possible is used. At the bottom, is a second electrode, which need not be transparent. Aluminum is pretty popular.

In this basic example, we have two active layers, one of which likes to accept electrons and one of which likes to donate electrons. Why is that? Let's zoom in (Fig. 2).


When a photon hits the active material, it generates an electron-hole pair, called an "exciton" and represented in my picture as a blue dot (electron) and a white dot (hole) in an orange circle. Excitons can do three things: they can recombine (a), they can migrate (b), and they can separate into free charges (c). Recombination is the enemy - the material is just back to the way it started and the photon energy used to generate the exciton has been wasted. What we're after is the charge separation. Then the free electron and hole can head to opposite electrodes and we've got an electric current. Charge separation tends to occur at a donor-acceptor (D-A) interface, because now there is some incentive for the electron to prefer the acceptor material and the hole to prefer the donor material.


So to recap, we want a situation such that the generated exciton will migrate to a D-A interface before it recombines. This gives us a little more criteria for our active layers as well, not only should they absorb light at a number of wavelengths and be strongly electron donating/withdrawing, they should also allow excitons to migrate over as long a distance as possible.

But say we've already optimized the active layer materials, and the efficiency of our OPV is still around 1%. 1% is not very good. Silicon cells can get like 25%, NASA's fancy cells are getting 45%. Organic cells may be cheap and flexy, but 1% still isn't going to cut it. Time for a new idea: what if we mix up our active layers, so that there isn't one D-A interface, but many (Fig. 3).


Now the exciton diffusion length can stay exactly the same, but the exciton is more likely to encounter an interface. This style is what's known as a bulk heterojunction OPV and these are getting efficiencies on the order of 10%. So still not on par with silicon cells, but getting pretty good!


I suppose now I have to break the bad news though... low efficiency isn't the only problem with organic cells. They also have durability issues. The most popular active materials in the cells degrade upon exposure to moisture, air and light. Yep, light. So, are you an enterprising young materials scientist with an idea for a brand new active layer that's stable to the elements? Leave it in the comments so I can unceremoniously steal it and present it to my boss as my own.

This article came entirely out of my own brain and could be riddled with errors. If you would prefer to read something fact-checked and peer-reviewed, I'd recommend these articles:


Efficiency of bulk-heterojunction organic solar cells

Charge carrier recombination in organic solar cells

Or this book:

Organic Photovoltaics: Materials, Device Physics, and Manufacturing Technologies

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