Abstracted from “A Scientists View of Almost Everything” by Mark M. Green
Electrons are the stuff of electricity, hence the name. And electrons are also what the world is made of. Every grain of sand is full of electrons. Everything is made of electrons together with other unimaginably small particles, which together constitute all the elements in that periodic table you learned about in school. But it’s that grain of sand that’s the foundation of creating electricity from sunlight.
Sand is made from the elements silicon and oxygen. In fact most of the crust of the earth is made from silicon and oxygen. Silicon, symbol Si, exists in sand in a network of atoms in which each Si is surrounded by four oxygen atoms, so that the general formula for sand is SiO2. Each oxygen atom is bonded to a silicon atom in this network so that each half of each of the four oxygen atoms around one silicon atom “belongs” to that atom—hence SiO2. If we could use some of those electrons in sand to make electricity a great deal of electricity could be generated. But the elements in sand hold their electrons too tightly to be wrenched free by the sun’s energy. However, if the oxygen is removed from sand to obtain pure silicon, which can be accomplished by heating sand to very high temperatures with charcoal, then we have a substance in which the electrons can be made into electricity using sunlight.
This chemical reaction makes sense. The product of heating sand with charcoal is a very stable molecule, carbon dioxide, allowing transfer of the oxygen from its connection to silicon to a connection to carbon in CO2. And this process only takes place once to produce a material that makes a great deal of electricity, not as in the burning of fossil fuels where CO2 is continuously produced. And helping this process is the formation of a very disorderly situation, a gas, CO2, at high temperature, from a very orderly situation, the regular arrangement of the silicon and oxygen atoms in sand. Nature loves an increase in disorder, an increase of entropy, which makes the process easier.
Electrons belong to the world of quantum mechanics, a world sort of like Alice’s Adventures in Wonderland. Both the book and the physics were created around the same time, in the late nineteenth century, and in both book and physics, nothing is what it seems. Although electrons (which are estimated to weigh about 10-31 kilograms each) cannot be described in terms that make sense to the world we are familiar with, we know how many of them there are in each kind of chemical element. Each silicon atom, if it be in sand or pure silicon, has fourteen electrons surrounding the nucleus of the atom, four of which are involved in connecting the Si atoms to the four surrounding oxygen atoms in sand or to four surrounding silicon atoms in the network of pure silicon.
But the electrons in pure silicon are still held too tightly to use the sun’s energy to get them to make electricity. To accomplish the objective, a trick is used called doping. Whereas silicon has four electrons available for its connections to other atoms, phosphorus has five electrons. When phosphorus is added in small amounts to the otherwise pure silicon only four of phosphorus’ five electrons are used to fit into the silicon network. One electron is left over. Boron atoms on the contrary only have three electrons available for connections to other atoms so that when boron atoms are added to pure silicon they make holes where electrons are missing. These so-called holes in the boron doped area then allow some of the extra electrons in the phosphorus doped area to move about causing adjacent areas of the silicon network to be positively and negatively charged, like a battery. Movement of electrons stops at this point until light shines on the silicon network, which energizes the electrons to continue to jump from one kind of doped region to another. With the proper conducting wires attached, an electric current is generated creating a solar cell.
Solar cells are serious business for the energy needs of the world. Sun-generated electricity, photovoltaics, is becoming a very important enterprise with large corporations created to advance the field. Huge amounts of money are pouring into this technology causing an inexorable march for solar generated electricity to become as cost-efficient as electricity generated from fossil fuels. These corporations want to collect from us when we plug into their grid or sell us solar cells for our homes to reduce the fare we pay to the grid.
What I’ll have to save for another day is the irony that modern gasoline could not be produced without also using silicon based material in a process called catalytic cracking. But that’s another story about silicon, the second most abundant element in the earth’s crust after oxygen.
