A CIGS solar cell

First, to make one thing perfectly clear: if I had been talking about cigarettes, I wouldn't have capitalized all the letters in CIGS. Nope, CIGS is an acronym for the ingredients in Copper-Indium-Gallium-Selenium solar cells. Actually, CIGS solar cells are made up of a number of micrometer thin layers of which CIGS is just one of those layers. But it's the most important one - it's the so-called absorber layer ie the layer that absorbs sunlight. In the illustration, it's the dark gray layer, gray being a fairly good color to be for soaking up sun rays.
Besides absorbing sunlight, the CIGS layer performs another very important trick. It separates charge. This is what distinguishes CIGS from other grey or black materials such as asfalt (have you ever heard of a solar cell made out of asfalt?). Electrical charges happen to come in two varieties known as positive and negative. If you've ever had to use a battery and were very observant, you would have noticed two things about your battery: it has two terminals and, with some luck, the terminals would have been labeled with a ' - ' and a ' + '. The battery has somehow managed to pile up positive charges at one terminal and negative at the other. A solar cell, aka solar-powered battery, must do the same. The CIGS absorber layer is responsible for the first step: the energy in sunlight is absorbed by electrons, which have a negative charge, giving them such an energetic 'kick' that they can free themselves from the positively charged atoms to which they are normally bound.
So far, so good, but there are a few more steps needed before a solar cell achieves 'battery' status. For one, it needs electrical terminals. Secondly, it needs to get the newly separated charges to their respective terminals. Unfortunately, one of the first things that happens to newly freed electrons is recapture. When this happens, the sun's energy is re-released, usually in the form of heat. This is entirely useless when trying to get electricity! However, a few ingenious people eg at http://www.arontis.se/ think that they can usefully recoup the heat and still produce electricity. But to get electricity, those freed electrons need to get out of the CIGS layer and onto some conducting terminal to eventually make their way into an external electrical circuit. To scientists, the discussion would also include 'holes' which describe the net positive charge created when an atom is deprived of its electron. Even holes can travel and be accumulated at terminals, thus contributing to solar cell function, but I don't intend to delve into that subject here.
Terminals, and how to get the charges there, is what the other layers in the solar cell are for. If you look at the illustration, you'll see the layers that act as terminals. One layer is made of a metal called molybdenum. It's a great electrical conductor just like copper, but it happens to be molybdenum, not copper. The choice reflects the need to satisfy other requirements such as chemical and mechanical compatibility. This layer forms the positive terminal of the CIGS solar cell where the 'holes' accumulate. Attach a wire to that layer and you're halfway there!
The negative teminal is the top layer labelled zinc oxide. By now, you would have noticed that the illustration is labelled in Swedish - that's thanks to Janne Sterner, a former PhD student at the Ångsröm Solar Center, who, besides being able to make really cute illustrations is also terribly Swedish! Anyway, the poor zinc oxide layer has three functions to perform. Firstly, it needs to be transparent to sunlight because it entirely covers the underlying CIGS layer which, as you remember, is the layer that absorbs the light. Secondly, this layer, in partnership with the CIGS layer, is responsible for creating the voltage difference that's so necessary to get the charges to travel to the terminals. And thirdly, it's the negative terminal where the electrons accumulate. Although not as conductive as a metal, zinc oxide can be mixed with a couple of percent aluminum to make it both transparent and conductive at the same time (being both transpartent and conductive is normally a conflict of interest). Attach the second wire to this layer and the solar cell is ready to be used! Oh, and don't forget to face it into the sun.
There's still another layer I haven't mentioned. If you look carefully at the illustration, you'll see a very thin layer labelled cadmium sulfide (in Swedish, of course). It's called a buffer and its purpose is to deal with compatibility and transition issues between the zinc oxide and CIGS. While it doesn't have much purpose in the explanation of how a CIGS solar cell works, it tends to get attention due to the fact that it contains a wee bit of cadmium, making it a possible environmental contaminant. And it's probably unhealthy to lick the solar cell, just in case the urge to do so should cross your mind. A large part of my own research has involved replacing this layer with a cadmium-free alternative, such as was the case for a solar module I wrote about in an earlier post.
I mentioned at the beginning how very thin all these layers are. The entire stack, in fact, is only 3 micrometers thick (there are 1000 micrometers in a millimeter). For comparison, a human hair is 50 to 100 micrometers. Without some supporting base, in this case a pane of window glass, it would be impossible to handle. But, instead of glass, wouldn't it be so much easier to handle if the supporting layer were a sheet of plastic? Being able to roll up solar cells and have them be light enough to carry around would open up so many possibilities! Well, with any luck, I'll get a chance to investigate this at Flisom (http://www.flisom.ch/). To be continued...
ps This picture at the bottom is a scanning electron micrograph photo of what the CIGS solar cell REALLY looks like. The thicker layer is the CIGS itself, characterized with more or less vertical crystals - where each "chunk" is a crystal - and the thinner layer on top is the zinc oxide, where the vertical structure of the crystals is a bit more obvious.