What are the material properties of Poly- or multicrystalline silicon?

Question

Why is it so suitable to make solar panels?

note:not the technology,but the properties.structures etc.

Answer 1

I'll answer the question, but I know you're not going to like the answer!

First, some material science:

Liquids are amorphous, i.e. have no particular orderly pattern for the location of the atoms/molecules. They just all slip slide all over the place. Amorphose solids occur when you cool down a liquid very quickly, and the atoms/molecules just stay where they happen to be. Real amorphose solids are fairly rare, but many glasses and plastics are amorphous. They are really just supercooled liquids, like syrup in the fridge, they are so thick they run only very slowly. Old houses have glass that is much thicker at the bottom than the top, because over the years, the glass has been running downhill in super slow motion.

If you cool down metals, the atoms start grabbing each other and forming crystals, a process called nucleation.  The crystals are neat, orderly 3D patterns, like a giant 3D tinkertoy network. Crystals are stong and hard, but often tend to be brittle and break. When most materials cool, they have many neucleation sites that start little crystals in random orientations. The crystrals grow until they run into each other and can't grow anymore. They can't join together well because they have different orientations. Each of these tiny crystals are called a grain, and the structure is referred to as polycrystaline. Look at a piece of granite, or google it, to see this grain structure. Most metals are this way.

Mono-crystaline means the atoms/molecules are arranged neatly in a single 3D matrix, with all the atoms/molecules lined up perfectly in the desired orientation. One way to do this, the CZ method, is to lower a small seed of perfect crystal silicon into a bath of barely molten silicon. It is slowly pulled out, and cooled, so that the crystal grows as the seed is slowly lifted. This grows an entire single crystaline ingot, which looks like a huge long cylinder, maybe like a SCUBA tank. To make integrated circuits, they slice the cylinder into thin disks called wafers.

Now, one more issue: semiconductors.

Metals are generally conductors. They are generally crystaline, so the atoms are stuck in place. However, they have electrons in the outer shells that are not tightly attached. These break free of their original atom, and move throughout the metal. Chemistry books say metals are basically "a sea of electrons", because they move all over the place. These electrons move both heat and electricity, and are the reason the metals are all conductors.

Resistors are materials that allow the electrons to move a little, but with difficulty. Examples are carbon (graphite) and pastes made with certain metals that don't conduct so well.

Insulators are like glass or plastic, where the electrons are firmly tied down by their atoms. Since they can't hardly move around in the solid, it is an insulator.

Finally, semiconductors are materials that are somewhere between metals, insulators, and resistors. They can conduct pretty well, just a little like a resistor, or pretty much not at all, like an insulator.

Now, when you look at the periodic table, metals are to the bottom left, and non-metals to the top right. Sort of on a diagonal between the metals and non-metals are the semiconductors. Carbon, silicon germanium, and the ones near them.

These other semiconductor or almost semiconductor materials can be inserted into the silicon matrix to change its electrical properties, and are called dopants. N-type dopants have loose electrons like a metal; p-type have missing electrons like a non-metal. These missing electrons are called "holes" and act like positive charges. Where n-type and p-type materials are in direct contact, they form a semiconductor junction. This is how diodes, transistors, and other semiconductor devices are made.

Finally!, now on to the properties:

Polysilicon has the same properties as single crystaline silicon, but it has smaller crystals. Lightly doped poly (i.e. a very small percentage of certain other similar metals are put in) is used to grow cylindrical ingots of monocrystaline silicon. Heavily doped polysilicon is used to interconnect transistors on integrated circuits. This is done to improve electrical conductivity of poly or monocrystaline silicon.

Like monocrystaline silicon, poly is extremely brittle. Similar conductivity in bulk, but somewhat less overall due to resistance at dislocations and cystal grain boundaries.

Main feature of monocrystaline silicon is that the crystal is oriented and continuous. Thus, you can make optimum semiconductor (p-n and n-p) junctions. Main difference with poly is that the poly is cheaper and requires much less energy and processing to make, and that it is not oriented, and has many small grains. This means it is poorly suited to use for p-n junctions in general.

Solar cells are largely made of monocrystaline silicon. I did not know thay had polysilicon PV's at all until I saw the BP Solar plant referenced in the link. It has lower cost than monocrystalline, but poorer performance. Poly is not ideally suited at all.

Try some of the links below:

I think the material you are looking for is amorphous silicon. This is currently more costly than monocrystaline, but is coming down, and has variants that give higher efficiencies than monocrystalline.

I can't get it to take links, but they are all in en.wikipedia:

Solar_cell

Amorphous_silicon

BP_Solar

Photovoltaics

Solar_Panel