The photoelectric effect

Now that I'm home healing an injury, I might as well put the time to some use and write about something that interests me: how to explain the inner workings of a solar cell in plain language. Admittedly any discussion that involves the submacroscopic world that can't be experienced directly takes a leap of faith, a willingness to believe that theories, evidence, calculations and measurements can and do lead to successful predictions that we can actually experience in the macroscopic world. Such is the world of atoms, electrons and photons and of waves and particles. It is also a fundamental part of understanding electricity and solar cells.

It's been ages since I've written about solar cells. In some of those early posts I made an attempt at explaining solar cells in posts labelled CIGS. In this post I want to focus on the mechanism of how the stuff of sunlight, photons, interacts with the stuff of electricity, electrons. The simplest example is actually quite straightfoward: one photon gets absorbed by one electron. The photon then ceases to exist (see above regarding leap of faith) and the electron picks up all its energy. If this electron was originally trapped by an atom and if absorbing a photon gives it enough energy to break free of the atom then you have what is called the photoelectric effect.The photon itself can be of higher or lower energy, blue light being the more energetic variety and red light the less.

An electron absorbing a photon lies at the very heart of a solar cell. Remember, the end goal is for the solar cell to set electrons in motion around some external circuit and get them to do some work, like turn on a light bulb. Interesting, in a light bulb electrons can create a photons where none existed  (go back to the leap of faith bit) and send them out to be enjoyed by you and me. But now I'm digressing. The slow and tired electron can then make its way back to the solar cell to see if it can get another chance at capturing a new photon.

I've only mentioned the first step in the proper functioning of a solar cell, that is, the absorption of sunlight by bound electrons enabling them to become free.

Diagram from APlusPhysics

By no means does this guarantee that they'll do any useful work. There's still the bit about setting the electrons in motion to be explained and the story of their journey is filled with pifalls and traps, walls and barriers.

But freeing electrons is where it all starts.

Zurich...

...it is after all! 2 weeks to go before I move, again. The big world of solar cell production in Berlin proved to be a bit too distant from my sheltered research life to date. My whimsical and impulsive approach to experiments had been replaced with goal-driven planning and motivation. I learned how to write experiments in the smallest details and analyse and report the results without ever seeing a solar cell - a different world indeed. While my helter-skelter ways did need a bit of organizing, I'm afraid being 100% methodical is not a sustainable work style for me. I hope that I will be able to find a balance between the two worlds in Zurich. I'll be working with flexible CIGS solar cells at a tiny spin-off company from ETH (Swiss Federal Institute of Technology) with a tremendous amount of work needed before any solar cells roll off a production line. I think I'll like it!

How does a CIGS solar cell respond to sunlight?

Well, hopefully the solar cell will act like a battery when the sun shines on it. But, of course, I want to go into a bit more detail than that. I've already written a fair number of posts regarding the making and workings of CIGS solar cells. You'll find them by clicking on the "CIGS" label at the bottom of this post. If you're new to the CIGS world, I suggest you start there first.

By now, you must be tremendously curious about the wonderful graph my colleague Sebastian put together for me, my own computer skills being somewhat limited in this regard. First off, you probably can't read the labeling on the graph very well because it's too small. This is easily remedied by a single click on the graph itself to see it full size - a handy trick that's useful for websites in general. So go take a closer look now and I'll explain what you're seeing.

First off, I've actually got two graphs for the price of one. I'm "allowed" to put them together because they have a meeting ground in their shared x-axis, the one labelled wavelength. This refers to the wavelength of light. The units are nanometers, nm (1 nm = 0.0000001 centimeters - does that help?).

One graph is the solar irradiance aka solar spectrum. I'm pleased to display the visible part of the solar spectrum in the pretty colors. The y-axis on the left is for irradiance of the solar spectrum in units of watts per square meter per nanometer. The "square meter" part refers to sun power falling on a square meter of surface pointed directly at the sun. The "nanometer" part refers to the color or wavelength interval of incoming light so that you can see the relative power contribution by the various colors. To get the total solar power you'd have to integrate or add up all the individual contributions by wavelength interval over the entire spectrum including the wavelengths that lie outside the regions of my graph. Understandably a bit confusing, but I'm trying.

The graph of the solar spectrum is labelled very scientifically as "AM 1.5 global solar irradiance at sea level". This is a bit more information than I had intended to discuss, but, well, Sebastian is very much a scientist (I'm more of an engineer). Anyway, here goes. This spectrum is what's referred to as a "standard sun" for the purpose of comparing the performance of solar cells with each other under the same sunlight conditions. AM 1.5 stands for air mass 1.5 and refers to the sun when its zenith angle (the angle from vertical) is 48.2 degrees. This corresponds to sunlight that travels through an atmosphere that's "1.5 atmospheres thick". AM 1 would, of course, refer to a sun that's directly overhead with sunlight travelling through an atmosphere that's 1 atmosphere thick. The "global" part of the label refers to the fact that both direct and diffuse light are included. The diffuse light is the sunlight available to you when standing shaded from direct sunlight.

I think the spectrum must be what you might get in some fairly pristine environment and it's not the same as you would get where there are high pollutant levels. Humidity will also affect the spectrum. In fact, much of the "choppiness" in the spectrum is due to absorption by water molecules. In any case, the sun tends to be at some angle in the sky other than the zenith angle of 48.2 degrees. However, when you see a power rating of a solar module, it will have been calculated using this AM 1.5 spectrum. So a module rated at a certain power output will rarely produce that particular power. It's just a means of comparing solar module A to solar module B. Nevertheless, this particular spectrum has been chosen because it's a good indication of average solar power per square meter arriving at the earth's surface.

Now that I've gone through that lengthy discussion I might as well tell you how much sun power arrives at the earth's surface for the AM 1.5 spectrum. It's, conveniently, 1000 watts per square meter. Just out of curiosity, I went and read what the max sun power was according to our sensor (pyranometer) on the roof at the lab. Three days ago we had full sun and the power was impressively over 450 W for an hour or two. Being near the winter solstice, the sun rises only 6 degrees above the horizon.

Ok, onto the other part of the graph labelled quantum efficiency. The y-axis for this graph is on the right and its measure is percent. Quantum efficiency is also called spectral response, the former term being preferred by solar scientists. It's a measure of how well a solar cell can produce electric current from the incoming sunlight color by color, wavelength by wavelength. For scientists, the sunlight is "quantized" into photons, the minimum energy packets by which light can give its energy up. The quantum efficiency measures the percentage of incoming photons that will result in an electron coming out of the solar cell. For the units of incoming sun power, I could have replaced the "watts" with "photons per second".

You'll see that the top and relatively flat part of the quantum efficiency curve lies at about 90%. So if you shine pure red light, say, onto the solar cell 90% will be converted to electricity. If the quantum efficiency were 90% over all wavelengths, then you'd have a 90% efficient solar cell. But it isn't, and you don't. The reasons? Some light is reflected away and never even makes it into the cell. Some light produces overly energetic electrons that shrug off the extra energy as heat. That's not good when you'd rather just have electricity. And some light never frees up an electron at all, it just produces heat directly. And then, some freed electrons get trapped before making it out. They're forced to give up their energy and wait around for another photon to come by for another chance at freedom. The result is as you see in the graph. A solar cell has a fairly limited working range.

The particular solar cell in the graph is a pretty good one. It's a CIGS cell I made and measured and this one is my best ever at 18.5% efficient. In the graph you can see that where the sun is producing most of its power the cell is also at its most efficient, that is 90%. Going to the violet and ultraviolet there is less sun power available (thanks in part to ozone, but I digress) and what little there is is used poorly by the solar cell. For any CIGS savvy readers, you'll also notice that the characteristic absorption by the cadmium sulfide layer isn't there and you'd guess rightly that this cell is cadmium free. Going to the infrared part of the spectrum at longer wavlengths, again the sun power falls off, choppily and a bit more slowly. At a wavelength a bit over 1000 nm there's less than half the max sun power available while at the same time the quantum efficiency is down to about 50%. Half of a half is only a quarter as much power produced than at the max part of both curves (very approximately). Beyond 1200 nm, the solar cell is totally useless. Whatever sun power it picks up will only go as heat.

The information in the two graphs is sufficient to calculate the electrical current that a solar cell will (not just "can") produce, at least when the sun is looking like the AM 1.5 spectrum. Since the quantum efficiency curve is a real measurement of the current produced at each wavelength, this is considered a real measurement of maximum current output of the solar cell when the external circuit is in short circuit mode. Just take the incoming photons per second at each wavelength, multiply this by the quantum efficiency at each wavelength to get some reduced number, add up or integrate the contributions from all the wavelengths and there you have a measure of the short circuit current. This is considered a better measure of current than that measured under a solar simulator, because the simulator uses a light bulb whose spectral output differs from the sun's. The quantum efficiency method gives a wavelength by wavelength breakdown of the picture with soooo much more info.

Ok, so what's my point, anyway? Bit by bit, I'm hoping to explain why solar cells aren't better than they are. Why couldn't they be 80% efficient, or how about 60? 40% maybe? But not even 20?? That will be for another time. I'll try and make future posts much shorter than this one.

Hopefully my new blog contributors, Per-Oskar and Sebastian, will soon be spouting lots of CIGS wisdom of their own.

Hello World

You may have already noticed the appearence of two new names on the sidebar<-- And as one of the new contributors to this sporadically solar energy oriented blog, I thought I'd begin with a Hello and a brief introduction. Right now I am in the cleanroom, which is quite different from just any ordinary clean room. The cleanroom is a particle-controlled (and temperature/humidity they claim) environment common to most who work in microelectronics or thin film processing. Essentially, it's just very very clean. I am a phd student, and I do most of my work in this clean room. That takes up about half of my day; the other being spent sifting through articles or my own data for something of value. Like the originator of the blog, I am working with CIGS solar cells at the Ångstrom Solar Center, a research group within the division of Solid State Electronics, a part of the department of Engineering Sciences of the University of Uppsala. You see? It's all a big mess, at least when you try ordering anything by mail. 4 out of 5 packages end up at the division of Electricity research, or maybe Solid State physics or.. (wow, I am digressing rapidly..).
As a phd student I am working on any and most aspects regarding solar modules, or solar panels; i.e. the medium-to-large scale arrangement of solar cells for electricity production. By any and most I refer to such things as: interconnection of cells, patterning of thin films, isolation and edge deletion, encapsulation, long-term stability, climate testing and stability, etc. etc. At some point these things may be adressed and explained on this blog.
I also work part-time as a development engineer for a (ok, I'm gonna introduce another word here: Photovoltaic, which is the word I prefer to use for our type of electricity generating solar cells and modules, since the word means just this; electricity from light. The word is often abbreviated PV) PV startup company called Solibro. This puts me in touch with reality in a welcome and interesting way, as I get insight in what different challenges need to be adressed when you go from making a few sqare cm's of cells in a week, to making 1000 squar m's a day.
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The rest of my time is divided between The Girl, Photography, Tv-series, Music and Videogames. Roughly. I try to squeeze in things like eating and drinking as often as I can but you know how it is. I've been an occasional blogger for a while now, sometimes writing journal entries and sometimes just spitting out quotes that I like or uploading pictures. The ambition here is to write about PV and energy in a relaxed but interesting way, and hopefully in a way that is accessible to most people who are interested. If any readers have questions, I'd be happy to try my best at answering.
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So, Hello everybody!
/Per-Oskar

Fun with setbacks

Be forewarned - this is a boring post about my latest difficulties with CIGS.

The CIGS evaporator is down again, this time for a corroded thermocouple (a temperature measuring device) that's needed to regulate the copper evaporation source. I haven't been permitted to keep a spare heater on hand due to the 13000 euro price tag, which means that a replacement can't be bought until failure occurs. Delivery time for the new source: 3 months.

However, far from putting an early end to my CIGS activities at the Ångström Lab, the missing source gives me a wonderful opportunity to get really creative in making solar cells. The heaters for the three metals, copper, indium and gallium, are namely interchangeable (not selenium because it evaporates at a relatively low temperature). The crystal structure for CIGS requires about 50% selenium, 25% copper and 25% indium and/or gallium. Indium and gallium are, in fact, interchangeable in the crystal. It's thus possible to make CIS (copper indium diselenide) or CGS (copper gallium diselenide) or something in between known as CIGS with varying electronic properties that are still useable for solar cells. Copper, however, is a must and so is selenium. So, I can either ditch the gallium source and use its heater for copper to make CIS, or ditch the indium source and make CGS.

But wait, there are more possibilities. What if I keep both gallium and indium and figure out a new way of getting copper where it's needed? To refresh your memory on how CIGS is made I'll refer you to an earlier post entitled The making of a CIGS solar cell. What if I start with glass and its molybdenum coating and also precoat it with all the copper I think I'll need? Then when indium, gallium and selenium come along, these four materials will all mix together anyway and still come out with the right crystal structure. I think, anyway. Molybdenum doesn't mix in but keeps itself safely separate for its future function as metallic contact.

Problem is, I don't have any good method of precoating with copper. Sputtering would be my first choice - then I could sputter copper on right after sputtering molybdenum onto the glass. But, drat it, I don't have a copper "target" that would be needed. There is, though, a small evaporator that can handle copper. Too bad it can coat only one substrate at a time and at best it could do two in a day. Ideally I would like to have a couple dozen. But I can still have some fun doing some preliminary tests with just a few.

And who knows? Maybe it'll work so well that CIGS solar maufacturers will all end up using this method. A more likely scenario is that it won't work very well. But at least I get to have fun playing with this stuff.

Now if I could just get equally creative with setbacks in my personal life.

Patent application

It appears that an interest has been taken in a patent application that Uwe, Per-Oskar and I wrote up a couple of years ago about being able to streamline CIGS module fabrication using some new technology. But at the time neither the university nor Solibro, the solar spin-off that's now partially owned by Q-Cells in Germany, was interested in the costs involved in securing the patent. It was given back to us to apply for a patent on our own, if we wished. We didn't, being the cash-strapped researchers that we are. The idea we were trying to patent was going to take quite a bit of development. In the past 2 years, Per-Oskar has been working on it with Uwe's help and has made significant headway. As for me, I haven't done anything except throw the initial idea to Uwe and P-O to implement.

Too bad that we're not going to get rich off of it - we merely get a one-time insignificantly small sum to keep us happy. There'll probably be a celebratory cake at some point too, I expect.

Is it coincidence that Uwe and P-O also happen to be fellow carpoolers?

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.

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.

Nanosolar

I'm not always aware of which topics are hot at any given time since I don't own a television (hmm - a future blog topic?) although I do get the daily Dagens Nyheter. But I've been curious about a company called Nanosolar for a while and I've decided get some perspective about their technology and claims in fabricating CIGS solar cells, the same kind that I research. The suggestion to do so at this time came from Bengt Axmacher's comment to my post about building a solar charger. If I'm surmising correctly, he's wondering how a company such as Solibro that uses expensive vacuum deposition equipment in a relatively slow and materials inefficient manner to painstakingly produce solar modules one glass pane at a time could possibly compete with Nanosolar's cheap ink printing method using a metal foil roll-to-roll process with 100% materials utilization to produce solar cells that are - apparently - just as good.

Some fact gathering is needed here and I started at Nanosolar's website. Before getting into their technology, I first was curious about the end result: the efficiency of their solar panels (I'm one of those people who likes to find out how the story ends before reading the book). That's apparently a secret - I couldn't find it anywhere even in a broader web search. But they do advertise the efficiency of their absolute best cell as being 14.6% in a paper published March 2006. It was a half square centimeter in size and was made on a glass substrate. Incidentally, March 2006 was also when I fabricated my own record cell of 18.5% efficiency using a process identical to Solibro's. Unfortuneately, it can be very tricky to compare results between two different cells. In this case, the Nanosolar cell was actually measured to be only 13.95% efficient but was calculated to be 14.6 had it not had a top metal contacting grid on it. Similarly, my cell would have been 19.1% if I'd used the same trick of calculating away the contact area. Other things can also affect efficiencies such as antreflective coatings or whether the cells are encased for durability. When making large cells there will be more inherent losses and they are also more difficult to manufacture uniformly. Modules compound the losses due to the interconnects of the individual cells and problems in cell matching. Trying to calculate what the efficiency of a fully assembled solar panel might be based on the results of a weentsy little lab cell is not straightforward. If you take a look at my posting about a solar module you'll see it was 14.8% efficient, i.e. not even close to the 18.5% I got for the single cell. So here goes: I'm going to guess wildly and predict 8 to 9% efficiencies for Nanosolar's fully assembled panels.

On to some technology. The copper-indium-gallium-selenium ink that Nanosolar uses to print their solar cells is proprietary, but apparently it's made of nanoparticles of metal selenides which are printed onto a metal foil which forms one of the electrical contacts. It then goes through a heat treatment of 4 or 5 hundred degrees. Additionally, solar cells require a buffer layer and a top contact to form a structure that I described in a CIGS solar cell. I couldn't find out which method is used to deposit these layers, but Nanosolar emphasizes that vacuum deposition is not it. Hopefully, I'll get around to describing the vacuum deposition method someday since it's the way I make my solar devices.

I'm undeniably intrigued by Nanosolar's process. Vacuum deposition methods are indeed wasteful. Materials usage can only be minimized when huge glass panels are processed since the waste is mostly at the edges. Applied Materials makes thin film silicon on glass panes that are 5.4 square meters - I think it's the biggest in the industry. But vacuum equipment is terribly expensive, takes a tremendous amount of energy to run, corrosive selenium gets into everything and, in my experience, the heaters for the metals keep burning out - I don't just melt copper, I evaporate it at about 1500C. It's difficult to get uniform coating and control the respective quantities of copper, indium and gallium in the resultant films. And this is just for one of the layers needed to make cells. Modules also require patterning steps to form the interconnects between cells. Nanosolar doesn't require this in their process, but instead they have to assemble cells into modules and make interconnects. As far as esthetics go, I think Solibro has the upper hand. I suspect their sleek black panels are easier for an architect to place on buildings.

Back to the original question of whether Solibro can compete with Nanosolar: I conclude that it would be entirely premature to predict the demise of Solibro.

But neither company has yet made a product available to the public.

Building a solar charger

It's July and this has special meaning for Swedes. They disappear. Where once the town was vibrant with the comings and goings of students, it's now quiet with a preponderance of people who are, well, over 40. Many shops have shut and signs of "sommarstängt" abound. At work, the doors that are normally open now require a passcard and code to enter. Many co-workers left on Friday with wishings of "trevlig sommar". So what about the rest of us who chose to stay on and continue working in the silent halls of the lab? While I don't know what everyone else is up to, I can describe Uwe's and my just-for-fun project: building solar chargers.

Previously I've described a solar module. I've made many modules during my years at Ångström, but almost all have been used for some very destructive tests to see what they can tolerate. Last fall, I decided to make a batch of 20 modules that I would keep for fun. I ended up destroying 5 or so while trying to sandblast the edges to remove areas where the modules would short-circuit themselves and while trying to glue protective cover glass on them. Some modules were duds, but I think there are about 10 left for fun projects.

To get started, Uwe and I needed to buy a couple of things. I headed to a store called "Gallerix" where I bought a picture frame. This will house the module and it also has a stand for support. Uwe went off to "PC City" to buy a cell phone/ipod charger kit designed to be powered by the 12 volt outlet in cars aka cigarette lighter. The output of the charger needs to be a regulated 5 or so volts. Uwe guesses the charger to be about 85% effective - in other words, it will sap 15% of the incoming power. Our financial outlay: 50 crowns for the frame, 100 for the charger kit and one stolen solar module .

Next, Uwe gutted the charger to get at the electronics inside. We measured the module under the solar simulator to see what kind of ouput it has: 11.8% efficient, with 12.3 volts and 76 milliamps at maximum power output. Uwe said we needed a larger capacitor on the output side to prevent the charger from going into short-circuit mode. I included a picture of the I-V curve with the charger electronics and the new capacitor to be installed for the benefit of my geekier friends.

With all the parts in place, the rest was assembly. Uwe replaced the capacitor, soldered a couple of wires from the solar module to the charger and soldered the connection cable that came with the kit on the output side. To house the electronics, I cut the soft plastic packaging that came with the kit and we glued it with the electronics inside to the back of the picture frame. We also stuck on a wire holder so that the cable can be neatly coiled when not being used. A quick test under sunlight revealed that, yep, the thing works. The voltmeter reads 5.4 volts and the little led charger light is lit.

The next day, today that is, I was eager to see whether the module was worth the effort. A quick check of the connectors that came with the kit revealed that I didn't have the one needed to charge my digital camera which takes a USB connector. Nevermind, I did get the connector to charge my cell phone, a Sony Ericsson K220i. The cell phone indicates battery life in fourths. Great - only one fourth left! I headed to my covered veranda to point the solar module into the sun while keeping the phone itself out of the sun and, possibly, out of the rain. The immediate indication from the phone was good - when I plugged it in, the display showed "charging". I spent the afternoon checking progress and repositioning the module while writing this post. The weather varied with a couple of hours of sun and then clouding up. I finally brought it in a half hour ago. It's completely cloudy now and the phone no longer indicated it was charging even though the little led on the back of the module was still lit. Result: the battery's 3/4 charged now.

I'm pleased.

My solar installation

With the introduction I've now given to some nitty gritty solar cell details, I expect you're breathlessly waiting to see my own personal solar installation at home. So I've included a photo of my somewhat funky looking house. In the pic you'll notice the more or less red clay tiles on the roof and the gray-white asbestos "eternit" tiles that make up the siding of the house. Not visible, of course, is the insulation in the walls, which is nearly non-existant in my 1910 house. Also not visible are the solar panels. That's s right - I don't have any! Despite a 25+ year career making a better solar cell, I have never actually owned my own. It's not for lack of interest - solar cell research has always been a passion and never just a job. I suppose I could blame it partly on my nomadic lifestyle, always expecting to pick up and move to some new town, some new country. But that's not a good reason. I've actually been in Sweden for 10 years and before that in Bologna, Italy for 11.

Another reason would seem to be that I live in Sweden, SWEDEN!! Solar in Sweden?? But the reality is that Sweden gets about 30% as much of the sun's energy as does Sahara, even if it does mostly arrive in those incredibly long summer days. Being fairly certain that the lack of solar panels isn't entirely due to my love of procrastination, I'd say that the reason (blame) lies elsewhere. Ultimately I'm just another solar inspired consumer looking for market incentives and a reasonable payback time to buy and install a system. And for this, Sweden is not another Germany or Japan or even the U.S. where solar incentives abound.

So what do these countries, Germany in particular, offer that Sweden doesn't? The list is long, but the one that's most important is the ability of the home energy producer to sell energy back to "the grid". To date, Swedish power companies are not required to buy any electricity from their wanna-be-suppliers aka customers and some have even issued declarations to indicate they will refuse any such requests. Not good news for the poor home producer whose solar installation merrily provides electricity for free all day long to the power companies, and, who, on returning home in the evening from a day's work at the office, discovers the need to buy electricity from the same power company because the sun is low or gone. This is the current situation in Sweden. Talk about a disincentive! And while the occasional political mumble has been heard to address this issue, so far it's been about as effective as a horse's snort. On an aside, I might say the horses have made more headway, having recently snorted effectively about their rights to socialize such that it is now written into law that they must have the company of at least one other horse.

However, the power companies have started to acquiesce: they will now buy electricity once a number of prohibitely expensive conditions have been met and the electricity producer makes enough noise about it. In a report issued by Elforsk http://www.elforsk.se/solel/ (in Swedish, of course!), an organization that researches electricity issues and is owned by the governmentally owned Svenska Kraftnät http://www.svk.se/, the company that controls Swedish electricity (phew, that was a long intro), it would require about 45 square meters of solar panels to reach "break even". The costs involve metering measurements and reporting - one of the requirements is that energy production be reported hourly!

Well, let's see - back to my solar installation. The roof of my house in the pic is facing west - I'd better choose one of my sheds that has both southern exposure and isn't shaded. A quick measurement reveals that the shed in the picture - it once was the goat house under a former owner - has a roof length of 8 meters and is about 3 meters wide. Covering it entirely, that would give me 24 square meters, or about half the area needed for break even. So I can forget about selling electricity! And, darn it, I'm not at home during the day when the electricity is being produced, thus leaving me no choice but to give it away for free or to sell it at a loss. I could, of course, get very expensive batteries to store it, but that defeats the whole purpose or being grid connected. Or I could get inventive about consuming electricity while not at home. Hmmm... maybe I could get an electric car, which is expensive, leave it at home to charge during the day, then drive around with it all night. Or not.

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.

Solar cells, modules and panels

I've written about a solar module, so this time I'll write about how solar cells are related to solar modules. First of all, what's the difference?
The short answer is that a module is made up of cells that are electrically connected together. In an electrical circuit, each solar cell acts as a temporary battery that operates while the sun is shining on it. There are essentially two ways that cells can be connected together. A series connection has the cells all connected in a row, just like a flashlight can have batteries connected in a row. If a single battery has 1.5 volts, then 4 batteries in a row will have 4 x 1.5V = 6V where the batteries are all touching each other end to end and the external connections are made to one end of the first battery and the other end of the last battery. Likewise for solar cells. Very roughly, a solar cell has half a volt or a bit more. Surprisingly, this is true of most kinds of solar cells, whether thin film or crystalline silicon or any other material - this is by design, not coincidence, but I won't go into the reasons here (ok, it has to do with making the most efficient use of the solar spectrum). If the solar cells are series connected the total voltage of the system can be increased. A typical setup might be to string 24 cells together for an output of 24 x 0.5V = 12 volts.
There's another way to connect cells together. Instead of lining them up end to end, they can be connected in parallel by placing them in a row with the all positive terminals connected together and all the negative terminals connected together. The voltage of this assembly will be just that of a single cell. However, each cell contributes current and the currents will all add up. If you remember from my description of the solar module, the bigger it is, the more current it generates. Connecting cells in parallel is just as if a single cell got bigger.
To get both more voltage and more current, cells can be both parallel and series connected. A string of series connected cells can be parallel connected to other strings of series connected cells. In this way, cells can be assembled to produce a desired current and voltage. Just as a refresher, the electrical power output of a module is the voltage multiplied by the current and is measured in watts.
When you're in the solar store looking at solar panels, what you're seeing is a packaged module with a supporting frame and a little box of circuitry. The circuitry deals with compatibility issues when many panels are interconnected but not performing equally, such as when some panels are under full sun and some are shaded. Partial shading of solar installations can be a real problem, but I'm not going to discuss that this time. In fact, I think I'll stop here, now that I hope I've explained the relationship between solar cells, solar modules and solar panels.

A solar module

As long as this blog lacks a sense of direction, I expect that I'll be jumping from topic to topic at random with no organization. So today I thought I would show a small solar module that I made at the Ångström Laboratory, which is where I work. The module is a copper-indium-gallium-diselenide cadmium-free device, but I'm not going to explain that at this time. This particular module is about three years old, and unfortuneately, this picture wasn't taken until the device had already started to oxidize. That's why it looks like a map of the earth's land masses was printed on it. I had glued the protective glass on it - encapsulated, in the solar world - rather sloppily. It originally had a satiny black appearance, which is the look any self-respecting solar module should strive for since it's the color that absorbs the most sunlight.
The outer dimensions of the glass are 12.5 by 12.5 cm, but only the black area between the electrical contacts actively contributes to electricity generation. And this is where it starts to get technical. The graph I've included namely shows the module's electrical operating characteristics when under sunlight, although, to be precise, I faked the sunlight by using a light bulb whose intensity and color spectrum more or less imitate that of your basic 'standard' sun. The word 'standard' should, of course, make your eyes roll - maybe I'll say something about it some other day. Anyway, back to the graph: the vertical axis is current, the horizontal one is voltage. There are three little x's on the curve. One marks the point where the device is short-circuited such that the voltage is zero and the current is maximized, noted as Isc in the table. Another is when the circuit is open such that the current is zero and the voltage is maximized, noted as Voc in the table. The third is the one of most interest. It lies in the middle of the curve at the maximum power point where the product of I times V - power, that is - is maximized. Any external circuit that tries to be powered by a solar module should try to draw just the right amount of current at the maximum power point if it wants the most power that can be produced.
The table, besides showing the active area of the module, Isc and Voc, also calculates two other things: fill factor FF, and efficiency. The fill factor is a measure of how 'square' the curve is and is used to calculate the efficiency of a device, although fill factor has a different meaning to my beer drinking friends who are contemplating their beer glasses at our regular Friday night beer gatherings (this could be a topic for another day). The efficiency measures, in percent, how much of the sun's power can be delivered by the device in the form of electricity and is calculated by multiplying Isc times Voc times FF. This particular module is 14.8% efficient. In terms of power output, it can produce 14.8% of the sun's power, which, for a 'standard' sun just happens to be 1000W per square meter. In other words, if this module were as big as 1 square meter it could produce 148W of power (if the sun were 'standard', ha ha). But this particular little module is only 76.8 square centimeters (=0.00768 square meters) so it can produce 0.00768 times 148 equals to 1.14W, a result that makes me glow with pride.
Oh, by the way, I should mention that the I-V curve above was measured at NREL, the National Renewable Energy Lab in Colorado, a former employer of mine where they do such internationally recognized efficiency measurements of solar devices to keep people like me honest.