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.
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4 comments:
Hi, I am student from US. I find your blog amazingly excellent in explaining the CIGS technology. I wonder if you mind that I use some pictures and your insights on your blog to illustrate this techonolgy in a academic presentation?
Hi Felix,
I thought I post my answer to a question you sent by e-mail here:
You haven't stated what kind of deposition process you're looking at, but I'm guessing you're talking evaporation - it's notoriously difficult to control. This is my area of expertise - my job initially at Ångström was to produce CIGS in a continuous process with set goals for efficiency, yield and throughput (all by myself, I might add!). I succeeded and Solibro was founded, which later became a part of Q cells in Germany.
Evaporation has yielded the highest efficiencies, but sputtering is also a method of choice because it doesn't have the deposition control issues. Nanosolar uses an ink-jet method, but has been strangely silent when prompted to show working modules.
For evaporation, the in-line deposition monitoring of choice is a thickness/film growth monitor (a QCM, quartz crytal microbalance) and an XRF, X-ray fluorescence, system which provides information on compositional make-up within a few minutes. Both methods have the possibility of being part of a PID feedback loop that would control the power to the four elemental sources for indium, copper, gallium and selenium. PID, if you're unfamiliar, is a feedback loop such as used by a home heating sysem in which the thermostat reads temperature and will adjust the home heating system to maintain a set value, but also makes sure that the there's no over- or under-shooting the mark. I have begged for an XRF unit to be attached to my vacuum system, but didn't get it. I now deposit a calibration substrate that I have to take out of the machine, run two floors up to the XRF unit, then run back down with the measurements and "tweak" the sources in a way that my experience tells me will give good results.
CIGS evaporation is so tremendously touchy not just because 4 elements are involved - that would be bad enough. It's because the deposition of the individual elements is not determined solely by their individual vapor fluxes, but there is also an interplay between the Se and the metals in the vapor phase that affects their "sticking co-efficients". For example, if I increase the vapor pressure of selenium, I will automatically get more indium into the growing CIGS film.
In addition to the these difficulties, I also have trouble guessing the performance of the individual sources because this is related to how depleted of material they are and corrosion of their heating and temperature sensing elements. In industry, a feedback system would be crucial, I think.
Best regards,
Marta
Hi Martha,
I will be interested in knowing more about your thickness optimization experiments. I worked myself on reducing thickness of Sulfides (CIGS2). What problems did you plan to solve while reducing thickness?
Parag.
Hi Parag,
I haven't decided whether it's good or bad that the CIGS term doesn't distinguish between sulphur and selenium - at least it's not as bad as as having the "C" referred to as carbon instead of copper, which happened in the local paper.
The thin CIGS was just a quick test to see what I could get and not a project at all. But I think I'll pursue IT after Feb when I hope to get a new copper source installed. There wasn't much science in the thinking, though. We have as an objective to produce a lab sized 13% module with 1 micron thick CIGS.
My result was 13.8% for 850 nm of CIGSe on a .5 sq cm cell, no AR. Then the copper source broke before I tried to make a module.
Marta
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