A solar cell is an electronic device made of semiconductors in layers, just as com­puter chips are, but much larger and simpler. Since each cell produces less than 1 V, cells have to be connected in a series to give a useful voltage, like 12 V. Flashlight batteries generate 1.5 V, and we use two of them in series to get the 3 V required

by the bulb. Solar panels, about half a square meter in size, contain many cells connected together by transparent wires. The difference among conductors (like metals), insulators (like glass), and semiconductors arises from quantum mechanics, which mandates that energy levels in a solid are quantized. That means that electrons cannot have any old energy but must have an energy on one of the allowed levels. Furthermore, no two electrons can be on the same level. This situa­tion is shown in Fig. 3.30. Energy levels occur in bands, two of which are shown, each containing seven energy levels. There are, of course, zillions of levels in actuality. In an insulator, the levels in the lower band are all filled, one electron in each level. This material cannot conduct electricity, because the electrons cannot move. To move, they would have to gain a little energy, but there is no level close enough for them to move up to. In a conductor, the lower band is filled, but the material has some electrons in the upper band, which is not full. Those electrons can conduct electricity because there are levels above that they can move up to. In a semiconductor, the lower band is full, but the bandgap is small, so if the topmost electron gets a big enough kick (from sunlight, for instance), it can jump up to the upper band, where it can move. So a semiconductor conducts sometimes.

The most common semiconductor is silicon. The bandgap in silicon is 1.1 eV. It is not important at this point to know how much energy an eV is; it will be explained amply in Chap. 4. The “kick” that the electrons get from sunlight to cross the bandgap depends on the color of the light that hits it. Sunlight contains a range of colors, as we know by separating them with a prism (Fig. 3.31), giving rise to the proverbial sequence violet, indigo, blue, green, yellow, orange, and red. Light can be considered as a stream of photons, which are particles with energy but no mass. No, they do not follow E=mc2! Each color corresponds to photons of a certain energy. Those at the blue end of the spectrum have more energy, and those at the red end have less. For a photon to make a semiconductor conduct, it must have an energy of at least 1.1 eV. That means that the part of sunlight redder than that will be lost. For silicon PV, the idea is to add semiconductors with other bandgaps that can capture the other parts of the solar spectrum.

After a photon kicks an electron into the conduction band, what happens next? This is shown in Fig. 3.32. This is the semiconductor part of Fig. 3.30, but showing only the electrons on the top level. After an electron is kicked into the conduction band, it leaves a hole in the valence band. What we have not shown is that the electrons actually belong to atoms consisting of a positive nucleus surrounded by enough electrons to make the whole atom uncharged. These atoms are locked into a crystal lattice. In Fig. 3.32a, an electron has been knocked out of one atom into the conduction band. It leaves behind an atom with a missing electron and therefore

has a charge +1. That atom, shown in white, has a “hole” in it; that is, a place where an electron should fit but is missing. An electron can then jump from a neighboring atom, thus filling the hole but leaving a hole in the neighboring atom. As shown in B, the hole can move like a positive electron! If an electric field is applied, the electron in the conduction band will move one way, and the hole in the valence band will move the opposite way. These electron-hole pairs will conduct electricity, and now we have to see how the current is collected.

The electrons and holes cannot be collected directly with a copper plate con­nected to a wire because these charges cannot cross the interface between these very different materials. A buffer layer has to be added. These buffer layers are made of “doped” silicon. Here, doping is legal. By adding a few “impurities,” which are specially chosen atoms with one more or one less electron than silicon has, we can make n-type or p-type highly conductive semiconductors. The former has a net nega­tive charge, and the latter a net positive charge. We can then make a sandwich of three layers to form the basic unit of a solar cell (Fig. 3.33). Opposite charges attract, so when solar photons create electron-hole pairs in the silicon, the electrons are attracted to the p-type layer at the bottom, and the holes to the n-type layer at the top. Since they are negative, the electrons carry a current in the opposite direction to their motion. The buffer layer allows them to flow into wires carrying the current to the load (the appliance or battery that uses the juice). When the electrons reach the n-type layer, they fill the holes that had migrated there. The voltage generated is the bandgap voltage. The larger the bandgap, the higher the voltage. This makes sense, since only the energetic photons can push an electron across a large bandgap.