This page describes amorphous solar panels and practical ways to use them.
An amorphous solar cell is a type of solar cell that is relatively cheap to produce and widely available. They are named so because of their composition at the microscopic scale. Amorphous means "without shape". When the term is applied to solar cells it means that the silicon material that makes up the cell is not highly structured or crystalized.
Amorphous solar cells are usually created by applying doped silicon material to the back of a plate of glass. The cells usually appear dark brown on the sun-facing side and silvery on the conductive side. When produced as a solar panel (a collection of many solar cells) it will appear to have several thin parallel lines running across its surface. These thin lines are actually breaks in the N and P layers of the silicon substrate and they create the boundaries of individual cells in the panel. Amorphous solar panels usually come without any obvious hook-up points or wires. It can be very puzzling to figure out how to use them!
The first amorphous solar panels that I ever owned I purchased from Jameco (part #106702. Page 65 in their latest catalog). Years later I bought more panels from a surplus outlet near me. They all appear to be of the same construction: amorphous silicon layered on glass.
When the first panel arrived, I had no idea how to hook it up. After doing some researching and asking around, I found that you have to attach wires to the silvery surface on the back. This can't be done unless you have conductive tape or conductive epoxy. Soldering just doesn't work.
At first, having no epoxy, I chose conductive tape. I've found that aluminum tape from a hardware store works just fine. (If possible, take an ohmmeter with you to test things out). However, I began to fear that the tape might eventually pull the backing off the panel and ruin it. So I switched to conductive epoxy.
The backs of these panels look like this
+---------------+ |-=============-| |-=============-| |-=============-| |-=============-| |-=============-| |-=============-| |-=============-| +---------------+ |
There are many minute lines crossing the back surface. Each line defines the boundary of a solar cell. If you look closely, you'll see there are two types of lines. There are those which look silvery, and those which look brown. These lines are in fact breaks in the two layers of N and P doped silicon used to make the panel. If you looked at the whole panel through a microscopic cross section, it would look like this
(A) (B)
,-------. ,------------. .------------. ,-----/ /-----. .------------,
| P | | P | | P | | / / P | | P |
`-----+-+-+--------+-+-+-+--------+-+-+-+-----/ /-+---+-+-------+----'
| N | | N | | N / / | N |
+-----+------------+-+------------+-+---------/ /-+-------------+----+
| G L A S S / / |
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As you can see, the breaks in the independent N and P layers are staggered. Each brown line is a break in the P layer on the top, while each silver line is a break in the N layer on the bottom. You can measure the voltage between each cell simply by placing probe tips on the back of a cell and one adjacent to it. If you hold one probe in place and move the other father away from it, you will see the voltage increase as you cross each cell boundary.
To get the most out of the panel, you want to attach wires to the backs of the cells at A and B. If you use conductive tape, be sure that no part of the tape crosses a brown line boundary. If it does, you will effectively convert the two cells that it crosses into a parallel circuit, increasing your available current slightly, but decreasing your available voltage as well. (This could in fact be desirable if you find the panel voltage is far too high for your liking).
When I switched to epoxy, I was afraid of epoxying a wire directly to the panel back because permanently attached wires always break off after a while. So instead, I took some heavy gauge wire and made "U" style bends out of them. I then epoxied the ends of the wire to the panel, leaving the bent part to hang over the edge, where alligator clips or some other connection mechanism can be fastened. Be sure to keep the epoxy from crossing a cell boundary, just as you would if you were using conductive tape.
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Amorphous solar cells have a relatively high output impedance when you compare them to something like a Ni-Cad battery or power supply. This means that the more current you draw from them, the lower their output voltage appears to be.
Most manufacturers will give you an 'Open Circuit' rating on their cells. This open circuit rating defines the maximum voltage that cell will attain when it is in bright sunlight and completely unloaded. While it may not seem like this information is of any use because it doesn't tell you what the voltage will be once you start extracting current from the cell, it is important when you consider using the cells to power something like a voltage regulator. At no point in time do you want the input voltage on an unloaded voltage regulator to exceed its rating.
If you are lucky, the manufacturer will also give you another rating. This rating will state another, lower voltage, and the amount of current that will flow at that voltage. My panels came with an 18V open circuit rating and a 12V/0.141A rating, which means that in bright sunlight, if I draw 141mA from the panel there will still be 12V of potential between its positive and negative ends. To find the voltage at another load you need only interpolate. For example, if I were to draw 70mA of current off of the panel, its remaining voltage would be approximately 15V (half way between 12V and 18V).
Jeremy Cooper, KE6JJJ