Solar Cells

PV cells can be made of many different semiconductors. But we'll use crystalline silicon as an example, for three reasons. First, crystalline silicon was the material used in the earliest successful PV devices. Second, and more important, it's still the most widely used PV material. And third, although other PV materials and designs exploit the photoelectric effect in slightly different ways, if you know how the effect works in crystalline silicon, then you'll have a basic understanding of how it works in all PV mediums.

An Atomic Description of Silicon

All matter is composed of atoms, which are made up of positively charged protons, negatively charged electrons, and neutral neutrons. Protons and neutrons, which are about the same size, are in the close-packed, central nucleus of the atom. The much lighter electrons orbit the nucleus. Although atoms are built of oppositely charged particles, their overall charge is neutral because they contain an equal number of positive protons and negative electrons whose charges offset one another.

As depicted in this simplified diagram, silicon has 14 electrons. The four electrons that orbit the nucleus in the outermost "valence" energy level are given to, accepted from, or shared with other atoms.

Electrons orbit at different distances from the nucleus, depending on their energy level. For example, an electron with less energy orbits close to the nucleus, whereas one with greater energy orbits farther away. The higher energy electrons farthest from the nucleus are the ones that interact with neighboring atoms to form solid structures.

In the basic unit of a crystalline silicon solid, a silicon atom shares each of its four valence electrons with each of four neighboring atoms.

A silicon atom has 14 electrons, but their natural orbital arrangement allows only the outermost four electrons to be given to, accepted from, or shared with other atoms. These outermost four electrons, called valence electrons, play a very important role in the photoelectric effect.

Large numbers of silicon atoms bond with each other by means of their valence electrons to form a crystal. In a crystalline solid, each silicon atom normally shares one of its four valence electrons in a covalent bond with each of four neighboring silicon atoms. The solid thus consists of basic units of five silicon atoms: the original atom plus the four other atoms with which it shares valence electrons.

The solid silicon crystal is thus made up of a regular series of units of five silicon atoms. This regular, fixed arrangement of silicon atoms is known as the crystal lattice.

Bandgap Energies of Semiconductors and Light

When light shines on crystalline silicon, electrons within the crystal lattice may be freed. But not all photons (the name for 'packets' of light energy) are created equal. Only photons with a certain level of energy can free electrons in the semiconductor material from their atomic bonds to produce an electric current.

This level of energy, known as the "bandgap energy," is the amount of energy required to dislodge an electron from its covalent bond and allow it to become part of an electrical circuit. To free an electron, the energy of a photon must be at least as great as the bandgap energy. However, photons with more energy than the bandgap energy will expend that extra amount as heat when freeing electrons. So, it's important for a PV cell to be "tuned"—through slight modifications to the silicon's molecular structure—to optimize the photon energy. A key to obtaining an efficient PV cell is to convert as much sunlight as possible into electricity.

Built-In Electric Field

Light shining on crystalline silicon may free electrons within the crystal lattice. But for these electrons to do useful work—as in providing electricity to light a light bulb—they must be separated and directed into an electrical circuit. To separate the electrical charges, the silicon solar cell must have a built-in electric field.

To create this electric field within a photovoltaic (PV) cell, two separate semiconductors are sandwiched together. P-type (or "positive") semiconductors have an abundance of positively charged holes, whereas n-type (or "negative") semiconductors have an abundance of negatively charged electrons.
When n- and p-type silicon come into contact, excess electrons move from the n-type side to the p-type side. The result is a buildup of positive charge along the n-type side of the interface and a buildup of negative charge along the p-type side.

Because of the flow of electrons and holes, the two semiconductors behave like a battery, creating an electric field at the surface where they meet—what we call the p/n junction. The electrical field causes the electrons to move from the semiconductor toward the negative surface, making them available for the electrical circuit. At the same time, the holes move in the opposite direction, toward the positive surface, where they await incoming electrons.

Although both materials are electrically neutral, n-type silicon has excess electrons and p-type silicon has excess holes. Sandwiching these together creates a p/n junction at their interface, thereby creating an electric field.

Absorption and Conduction

In a PV cell, photons are absorbed in the p-layer. And it's very important to "tune" this layer to the properties of incoming photons to absorb as many as possible, and thus, to free up as many electrons as possible. Another challenge is to keep the electrons from meeting up with holes and recombining with them before they can escape from the PV cell. To do all this, we design the material to free the electrons as close to the junction as possible, so that the electric field can help send the free electrons through the conduction layer (the n-layer) and out into the electrical circuit. By optimizing all these characteristics, we improve the PV cell's conversion efficiency, which is how much of the light energy is converted into electrical energy by the cell.

To make an efficient solar cell, we try to maximize absorption, minimize reflection and recombination

Electrical Contacts

Electrical contacts are essential to a photovoltaic (PV) cell because they bridge the connection between the semiconductor material and the external electrical load, such as a light bulb.

The back contact of a cell — on the side away from the incoming sunlight — is relatively simple. It usually consists of a layer of aluminum or molybdenum metal. But the front contact — on the side facing the sun — is more complicated. When sunlight shines on the PV cell, a current of electrons flows all over its surface. If we attach contacts only at the edges of the cell, it will not work well because of the great electrical resistance of the top semiconductor layer. Only a small number of electrons would make it to the contact.

To collect the most current, we must place contacts across the entire surface of a PV cell. This is normally done with a "grid" of metal strips or "fingers." However, placing a large grid, which is opaque, on the top of the cell shades active parts of the cell from the sun. The cell's conversion efficiency is thus significantly reduced. To improve the conversion efficiency, we must minimize these shading effects.

Another challenge in cell design is to minimize the electrical resistance losses when applying grid contacts to the solar cell material. These losses are related to the solar cell material's property of opposing the flow of an electric current, which results in heating the material.

Therefore, in designing grid contacts, we must balance shading effects against electrical resistance losses. The usual approach is to design grids with many thin, conductive fingers spreading to every part of the cell's surface. The fingers of the grid must be thick enough to conduct well (with low resistance), but thin enough not to block much of the incoming light. This kind of grid keeps resistance losses low while shading only about 3% to 5% of the cell's surface.

Grids can be expensive to make and can affect the cell's reliability. To make top-surface grids, we can either deposit metallic vapors on a cell through a mask or paint them on via a screen-printing method. Photolithography is the preferred method for the highest quality, but has the greatest cost. This process involves transferring an image via photography, as in modern printing.

An alternative to metallic grid contacts is a transparent conducting oxide (TCO) layer such as tin oxide (SnO2). The advantage of TCOs is that they are nearly invisible to incoming light, and they form a good bridge from the semiconductor material to the external electrical circuit.

TCOs are very useful in manufacturing processes involving a glass superstrate, which is the covering on the sun-facing side of a PV module. Some thin-film PV cells, such as amorphous silicon and cadmium telluride, use superstrates. In this process, the TCO is generally deposited as a thin film on the glass superstrate before the semiconducting layers are deposited. The semiconducting layers are then followed by a metallic contact that will actually be the bottom of the cell. As you can see, the cell is actually constructed "upside down," from the top to the bottom.

But the construction technique isn't the only thing that determines whether a metallic grid or TCO is best for a certain cell design. The sheet resistance of the semiconductor is also an important consideration. In crystalline silicon, for example, the semiconductor carries electrons well enough to reach a finger of the metallic grid. Because the metal conducts electricity better than a TCO, shading losses are less than losses associated with using a TCO. Amorphous silicon, on the other hand, conducts very poorly in the horizontal direction. Therefore, it benefits from having a TCO over its entire surface. - Reference U.S. Department of Energy

Grid contacts on the top surface of a typical cell are designed to have many thin, conductive fingers spreading to every part of the cell's surface.

Anti-reflective Coating

Silicon is a shiny gray material and can act as a mirror, reflecting more than 30% of the light that shines on it. To improve the conversion efficiency of a solar cell, we want to minimize the amount of light reflected so that the semiconductor material can capture as much light as possible to use in freeing electrons.

Two techniques are commonly used to reduce reflection. The first technique is to coat the top surface with a thin layer of oxide from silicon, titanium or other elements. A single layer reduces surface reflection to about 10%, and a second layer can lower the reflection to less than 4%.

A second technique is to texture the top surface. Chemical etching creates a pattern of cones and pyramids, which capture light rays that might otherwise to deflected away from the cell. Reflected light is redirected down into the cell, where it has another chance to be absorbed.