Narration script

Welcome to SOL 102, Materials Science Foundations for Solar. In SOL 101 we studied light. Now we study matter. Unit one is about atoms. The big idea is simple: a material's behaviour is decided by its outermost electrons, the valence electrons. Silicon has four, so every silicon atom can bond to four neighbours. That is the root of its crystal structure and its semiconducting behaviour. And the elements one column to the left and right of silicon are the dopants that make solar cells work.

By the end, you should be able to describe an atom, write silicon's electron configuration, use the periodic table to predict trends, locate every element important to solar and say why, and convert between grams, moles and numbers of atoms.

Dopant choice, absorber chemistry, passivation, and contamination are all periodic-table reasoning. Cell process engineers talk fluently about group three and group five dopants and about metal contamination, and by the end of this unit, so will you.

An atom has a tiny, dense nucleus of positive protons and neutral neutrons, surrounded by negative electrons. The number of protons, the atomic number, defines the element. Silicon is number fourteen. Atoms of the same element with different numbers of neutrons are isotopes. Natural silicon is about ninety-two percent silicon twenty-eight, with small amounts of twenty-nine and thirty, giving an average atomic mass of about twenty-eight point oh nine.

Quantum mechanics allows electrons only certain energies. They fill shells, numbered one, two, three, and so on, which are divided into orbitals labelled s, p, d and f, holding two, six, ten and fourteen electrons. Filling from the lowest energy up gives silicon's configuration: a neon-like core of ten electrons, then three s two, three p two. The ten inner electrons are tightly bound and chemically inert. The four outer electrons do all the chemistry, and all the electronics.

This is the shell picture of silicon. It is a cartoon, but a useful one: two electrons in the first shell, eight in the second, and four valence electrons in the third. Those four will form four bonds in the crystal.

The periodic table arranges elements by atomic number. Rows are periods, which tell you the outermost shell. Columns are groups, which share the same number of valence electrons and so behave alike chemically.

Here is a solar edition of the periodic table. Silicon, germanium and carbon sit in group four. To the left, in group three, boron, aluminium and gallium are p-type dopants. To the right, in group five, phosphorus and arsenic are n-type dopants. You can also see cadmium and tellurium, for cadmium telluride thin films; copper, indium and selenium for C I G S; lead, tin, iodine, bromine and caesium for perovskites; silver for contacts; iron, the classic lifetime killer; and oxygen, nitrogen and hydrogen, used in passivation layers.

This table summarises each group's role. Group three, with three valence electrons, gives p-type dopants and aluminium for rear contacts and aluminium oxide passivation. Group four is the absorber family. Group five gives phosphorus, the n-type dopant, and nitrogen in silicon nitride. Group six gives oxygen, both useful in oxides and harmful as an impurity, plus selenium and tellurium.

Why do dopants work? A phosphorus atom sitting in a silicon site uses four electrons to bond with its neighbours, and has a fifth left over, only loosely held. It is easily freed to conduct. That is n-type. Boron has only three, leaving a missing bond, a hole. That is p-type.

Three trends are worth knowing. Atomic radius grows down a group and shrinks across a period. Ionisation energy, the energy to remove an electron, does the opposite. And electronegativity, how strongly an atom pulls on shared electrons, grows toward the upper right of the table. These trends explain why the band gap shrinks going down group four, from carbon at five point five electron-volts, to silicon at one point one two, to germanium at zero point six six. Bigger atoms make longer, weaker bonds.

Silicon is the second most abundant element in Earth's crust, about twenty-eight percent by mass, after oxygen. It is almost all locked up as silica and silicates, in sand, quartz and granite. So abundance is not the bottleneck. Purity and energy are. Compare tellurium and indium, which are rare by-products of other mining, a supply issue for some thin-film technologies.

Finally, counting atoms. A mole is six point oh two two times ten to the twenty-three particles, and its mass in grams equals the atomic mass. So a mole of silicon weighs about twenty-eight grams.

A standard one hundred and eighty-two millimetre wafer, one hundred and thirty micrometres thick, has a volume of about four point three cubic centimetres and weighs about ten grams. That is about a third of a mole, or roughly two times ten to the twenty-three atoms.

That ten gram wafer becomes a cell of about eight watts, so about one point two grams of wafer silicon per watt. Including sawing losses and ingot offcuts, the industry used about two grams of polysilicon per watt in twenty twenty-four, according to Fraunhofer I S E, down from about sixteen grams in two thousand and four.

And a sense of scale for purity. Silicon has five times ten to the twenty-two atoms per cubic centimetre. One part per billion is still fifty trillion atoms per cubic centimetre. A typical wafer doping level is just zero point one parts per million. Semiconductors are controlled at astonishingly low concentrations.

Misconceptions. Electrons do not orbit like planets; the shells are bookkeeping. Silicon is not purified because it is rare; it is purified because tiny impurity levels ruin its electronic properties. And dopants are not harmful impurities; they are deliberate, and they make silicon useful.

In the factory: dopant sources like phosphorus oxychloride and boron tribromide come straight from groups three and five. Solar polysilicon is specified at nine nines purity, ninety-nine point nine nine nine nine nine nine nine percent. And material supply questions about silver, indium and tellurium come up in roadmaps and interviews alike.

Try the review questions. Next unit: how these atoms bond, and why the type of bond decides almost everything about a material.