The making of a CIGS solar cell

Finally the nitty gritty of how to make thin film CIGS solar cells. Let's see, where to start?
The beginning comes to mind. "Substrate" in thin film lingo means the base upon which the solar cell will be formed. I'm using glass, ordinary window glass. Actually, I'm already cheating because the glass I use has a low iron content so it's not quite ordinary. Iron is a no-no for CIGS.

Step 1: Wash the glass. A hot ultrasonic soap bath and lots of rinses in purified water does it for me. Ends with a spin dry.

Step 2 is to coat the glass with a metal. Not just any metal. Molybdenum. In the pic you can see 4 pieces of the glass I use, 12.5 x 12.5 centimeters which has just been coated with molybdenum by a method called "sputtering". Since I'm going to focus on the CIGS layer, I won't describe what that means at this time (ok, briefly it means that highly energetic argon gas bombards a molybdenum "target" to knock off atomic chunks of the metal which conveniently land and stick to the glass substrate that's been placed nearby).

Step 3 is the fun part. CIGS, that is. Copper-indium-gallium-diselenide. As you may guess from the name, it's made up of four elements: three metals, copper, indium and gallium and one semi-metal, selenium. In a vacuum chamber where all the air has been pumped out, these metals get "evaporated". Literally. I have three pots for copper, indium and gallium. I heat these up way beyond melting. They get heated so much that they vaporize. Just for curiosity, the temperatures required are over 1000 degrees centigrade and about 1500C just for the copper. There's also a pyrex container with selenium that's vaporized, too, at a relatively measly 300C. These four vapors get mixed together in the vacuum. I then send the molybdenum-coated glass on a trip through this metal-selenide vapor and out it comes with a fine grey-black coating. If it happens that I get just the right amounts of all the elements and they coalesce at just the right temperature to form just the right crystal structure then I can get amazingly efficient solar cells, almost 20% efficient. But, really, it often seems like black magic to get it all just right like that. And therein lies the make-it or break-it future of CIGS. I, myself, have been able to make world record CIGS solar cells, but have been hardly reliable. Mostly I'm fixing corroded heating elements and trying to get this CIGS stuff to form in the right way. I think of it as making the world's best pizza with the tricky part being to make the world's best every single time I make a pizza. By the way, for those of you into character analysis, a lot of my analogies have to do with food.

Step 4 is the buffer layer. To refresh your memory on all the coatings involved I'll refer you to an earlier post about a CIGS solar cell. The standard buffer is a cadmium sulfide layer which involves a wet chemical dip in a mixture that turns intensely yellow and leaves a thin coating on the glass. I consider this process to be entirely disgusting and also a nuisance in that the used solutions contain cadmium and need to be treated as toxic waste. Good thing that alternatives to CdS exist. Even better that I use a non-toxic alternative myself, a zinc oxide - zinc sulfide mix which I deposit using another vacuum process. The process this time is called atomic layer deposition. It involves letting various gases flow over the substrate in turn, each leaving an atomic coating on the glass.

Step 5 is actually made up of two different zinc oxide layers. It's another sputtering process and is, in fact, carried out in the same vacuum chamber where the initial molybdenum was deposited. The first layer is pure zinc oxide. It's actually an insulator, but the layer is so extremely thin that charge carriers are able to cross it anyway by means of what physicists call "tunneling". The second layer is also zinc oxide, but it's mixed with 2% aluminum to make it conductive. It's a relatively thick layer because it needs to carry all the solar cell's current while minimizing resistive losses.

The last step is to deposit a metal grid on top. This layer is deposited by evaporating aluminum through a template in the pattern that you can see in the pic with the finished cells. It forms a convenient contact pad and the metal fingers help collect the generated current, thus providing some current carrying relief to the underlying ZnO:Al.

I typically carry out all of these steps on my own. From start to finish it could go as fast as a couple of days, but I'll usually spread it out over a week. Once the cells are made, they need to be characterized under sunlight and otherwise evaluated. Very often the CdS buffer will be replaced by some experimental layer. The CIGS evaporation system can make 24 substrates at a time, so it's quite time consuming to process and evaluate them all. I end up making a CIGS "run" only 2 or 3 times a month. Which is good, since it takes 12 or 13 hours to run the CIGS machine.

And in between, there are the endless repairs.

Spikning och disputation

Hardly anyone managed to get work done in our department yesterday. For one student, Olof Bengtsson, it was the day of his dissertation defense. His examiner - "opponent" as he is called - is an invited expert in the field. The defense involves short presentations by both the defendant and the opponent, after which the opponent and a judging board of three others may ask questions of the defendant based on the written thesis. This process takes 2 and half to three hours. In Olof's case, he started at 10am and was out a bit before 1pm. The judging board retreats for discussion while champagne and snacks are prepared for the expected outcome. If anyone ever fails the defense, I've never heard of it. Olof passed, of course, and we drank champagne and congratulated him in the meeting room. A formal dinner at one of the student "nations" was held that night with the usual toasts, brännvin and singing. I was seated with international students invited from the university in Gävle where Olof conducted much of his work. The students were from various countries - Lebanon, Pakistan, Peru, Sudan, India and I think a couple more. There were also a few Swedes. It made for a fun and interesting evening.

But there was more happening yesterday. Another student, Ulf Malm from our solar cell group will also be defending his thesis in three weeks. It's a requirement that the thesis be made public at least three weeks prior to the defense. Not unexpectedly, this event is also steeped in tradition (and alcohol). A copy of the thesis is quite literally nailed to a wall in a place for this purpose. Our department simply uses a board which can easily be taken down and moved. Ulf successfully nailed his thesis on the second attempt, having first bent his carefully selected spike from the 1930's to an unuseable shape. Afterwards, beer (spiköl) and chips were served while Ulf signed the copies with which we were personally presented. This event was conveniently scheduled at 4pm, suitably distant from both the 1pm champagne and the evening's brännvin.

I didn't even think of making a solar cell all day.

John Ellis and the Large Hadron Collider

One of the advantages of working in a place like Ångström is the access to the numerous interesting seminars given by distinguished scientists. Some are traditional annual events of which the most notable are the invited talks by the Nobel prize winners in physics. But throughout the year there will be invited speakers who are at the leading edge of scientific knowledge in their fields.

Todays' seminar was given by John Ellis, a key scientist at the forefront of supersymmetry theory at CERN. The subject of the talk was, of course, the Large Hadron Collider and the talk was entitled "What the Future May Hold". Of course, it's not always possible to know beforehand how many equations are likely to appear during the talk. I was somewhat apprehensive about having to sit through an hour of calculations designed to demonstrate why the proposed Higgs boson that's so sought afer should have precisely the mass that theory predicts. But this talk was clearly intended for a lesser informed audience and was very enjoyable for me.

Dr. Ellis' enthusiasm for the subject was infectious. Here we are at the brink of a deepening understanding of our world and universe. Not only are we not the center of our Copernican solar system, we may be, as Dr. Ellis phrased it, mere pond scum on the surface of a universe made up of matter that we can experience despite an overwhelming existence of something called antimatter. The large hadron collider will take us on a journey into realms where Newtonian physics is ludicrous and quantum mechanics merely fails, where 3 spatial dimensions are quaint and Einsteinian warping of spacetime old hat.

The collider will likely lead to insight into compelling questions about the origins of mass and where all the antimatter has gone. But I suspect the most likely result will be the generation of new questions that no one knew to ask.

A CIGS day

Last week I was finally able to make decent solar cells after some failed experiments and lots of equipment downtime. 16% conversion efficiency is what I expect out of M. Pilote, the name of my CIGS layer machine and that's what I got. Today I decided on a new experiment to see how much current the devices lose if I make the CIGS layer with only half the usual thickness - 1 micrometer instead of 2. The idea with CIGS is all about how to make solar cells good, fast and cheap. To make the devices thin, I decided for a first approach to run my carousel with the substrates on it at twice the usual speed cutting the deposition time in half. I was delighted that this also cut an hour off my work day, which ended up being 12 hours today anyway since I decided to continue with the rest of the layers that make up solar cells. It takes a long time for the warm-up, stabilization and cool-down phases of CIGS depostion. I realize that I have yet to properly describe this process - I'll have to do something about that! I should have finished devices by tomorrow, so I'll find out how today's run went.

It was already 9:30 pm when I left. I decided to take the carpool car home - too lazy to bicycle. As an excuse I used the car to pick up some gasoline for the lawn mower in the hopes of cutting the lawn one last time before winter. Since it's too late for a fire tonight, the house remains a cold 15C. I'm sitting here under blankets as I type.

Wish the house had insulation.

Cool weather

The temperature's dropped to 15C - inside the house, that is. Yesterday's nighttime low outside was 3C. Today I lit the living room fire for the first time after summer. I've already been using the small kitchen woodstove for the past several weeks. It certainly feels like it's an "in-between" season, part summer and part fall. I'm still picking apples from the tree and also the last blackberies went into my muesli and yogurt this morning. But I wished I had mittens when I bicycled home from work yesterday evening.

Dreams

It's been a year now that I've been contemplating finding a new job somewhere and moving. Even though I nearly have a job offer in Zurich, I don't actually have it yet. I suspect it's been the reason for the dreams I've been having where the recurring theme is trains and planes and me running to catch them but typically ending up taking them in the wrong direction while forgetting in the meantime what my destination was. On waking I'm usually tired and confused.
Last night's dream was fairly typical, but with an unusual twist. While running to catch the tram that would take me to the airport, the large travel bag I was carrying broke its zipper. Out popped multitudes of miniature penguins. The more I scooped them up and put them back, the more they popped out again and I was losing precious time. The dream finished as usual: I got to the airport but ultimately missed the flight because I couldn't remember which flight I was supposed to be on.

Cambridge: planning

Daniele will soon be off to Cambridge for his year as an exchange student from Uppsala University. After a busy summer involving lots of travel with the European Youth Parliament and a month of work at the Ångström lab, he's finally getting ready for his year abroad. Upon discovering the need to ship several boxes of books and other things, we've decided on an alternative method of transport for his things - I will go there myself and bring them. The finances for the decision were straightforward: 1600 crowns to ship a couple of boxes vs 570 crowns for me to fly roundtrip and bring them myself. Of course, the actual cost with public transport and lodging for 3 nights comes to a total of 1700 crowns - pretty darn good, especially considering that I view the trip as a mini vacation. This evening has been spent in all my train/plane/lodgings bookings. Exciting for both me and Daniele!

Apple and wood, part 2