How a Photovoltic Cell Works

How a Photovoltic Cell Works

How a Photovoltic Cell Works.

The "photovoltaic effect" is the basic physical process through which a PV cell converts sunlight into electricity. Sunlight is composed of photons, or particles of solar energy. These photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum.

When photons strike a PV cell, they may be reflected or absorbed, or they may pass right through. Only the absorbed photons generate electricity. When this happens, the energy of the photon is transferred to an electron in an atom of the cell (which is actually a semiconductor).

With its newfound energy, the electron is able to escape from its normal position associated with that atom to become part of the current in an electrical circuit. By leaving this position, the electron causes a "hole" to form. Special electrical properties of the PV cell-a built-in electric field-provide the voltage needed to drive the current through an external load (such as a light bulb).

P-Types, N-Types, and the Electric Field

p-Types, n-Types, and the Electric Field
p-Types, n-Types, and the Electric Field. Courtesy of Department of Energy

To induce the electric field within a PV cell, two separate semiconductors are sandwiched together. The "p" and "n" types of semiconductors correspond to "positive" and "negative" because of their abundance of holes or electrons (the extra electrons make an "n" type because an electron actually has a negative charge).

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.

When the p-type and n-type semiconductors are sandwiched together, the excess electrons in the n-type material flow to the p-type, and the holes thereby vacated during this process flow to the n-type. (The concept of a hole moving is somewhat like looking at a bubble in a liquid. Although it's the liquid that is actually moving, it's easier to describe the motion of the bubble as it moves in the opposite direction.) Through this electron and hole flow, the two semiconductors act as a battery, creating an electric field at the surface where they meet (known as the "junction"). It's this field that causes the electrons to jump from the semiconductor out toward the surface and make them available for the electrical circuit. At this same time, the holes move in the opposite direction, toward the positive surface, where they await incoming electrons.

Absorption and Conduction

Absorption and Conduction.

In a PV cell, photons are absorbed in the p layer. It's very important to "tune" this layer to the properties of the incoming photons to absorb as many as possible and thereby free 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 the cell.

To do this, we design the material so that the electrons are freed as close to the junction as possible, so that the electric field can help send them through the "conduction" layer (the n layer) and out into the electric circuit. By maximizing all these characteristics, we improve the conversion efficiency* of the PV cell.

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

Continue > Making N and P Material

Making N and P Material for a Photovoltic Cell

Silicon has 14 Electrons.

Introduction - How a Photovoltic Cell Works

The most common way of making p-type or n-type silicon material is to add an element that has an extra electron or is lacking an electron. In silicon, we use a process called "doping."

We'll use silicon as an example because crystalline silicon was the semiconductor material used in the earliest successful PV devices, it's still the most widely used PV material, and, although other PV materials and designs exploit the PV effect in slightly different ways, knowing how the effect works in crystalline silicon gives us a basic understanding of how it works in all devices

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

An Atomic Description of Silicon

All matter is composed of atoms. Atoms, in turn, are composed of positively charged protons, negatively charged electrons, and neutral neutrons. The protons and neutrons, which are of approximately equal size, comprise the close-packed central "nucleus" of the atom, where almost all of the mass of the atom is located. The much lighter electrons orbit the nucleus at very high velocities. Although the atom is built from oppositely charged particles, its overall charge is neutral because it contains an equal number of positive protons and negative electrons.

An Atomic Description of Silicon - The Silicon Molecule

The Silicon Molecule.

The electrons orbit the nucleus at different distances, depending on their energy level; an electron with less energy orbits close to the nucleus, whereas one of greater energy orbits farther away. The electrons farthest from the nucleus interact with those of neighboring atoms to determine the way solid structures are formed.

The silicon atom has 14 electrons, but their natural orbital arrangement allows only the outer four of these to be given to, accepted from, or shared with other atoms. These outer four electrons, called "valence" electrons, play an important role in the photovoltaic effect.

Large numbers of silicon atoms, through their valence electrons, can bond together 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, then, consists of basic units of five silicon atoms: the original atom plus the four other atoms with which it shares its valence electrons. 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.

The solid silicon crystal, then, is composed of a regular series of units of five silicon atoms. This regular, fixed arrangement of silicon atoms is known as the "crystal lattice."

Phosphorous as a Semiconductor Material

Phosphorous as a Semiconductor Material.

The process of "doping" introduces an atom of another element into the silicon crystal to alter its electrical properties. The dopant has either three or five valence electrons, as opposed to silicon's four.

Phosphorus atoms, which have five valence electrons, are used for doping n-type silicon (because phosphorous provides its fifth, free, electron).

A phosphorus atom occupies the same place in the crystal lattice that was occupied formerly by the silicon atom it replaced. Four of its valence electrons take over the bonding responsibilities of the four silicon valence electrons that they replaced. But the fifth valence electron remains free, without bonding responsibilities. When numerous phosphorus atoms are substituted for silicon in a crystal, many free electrons become available.

Substituting a phosphorus atom (with five valence electrons) for a silicon atom in a silicon crystal leaves an extra, unbonded electron that is relatively free to move around the crystal.

The most common method of doping is to coat the top of a layer of silicon with phosphorus and then heat the surface. This allows the phosphorus atoms to diffuse into the silicon. The temperature is then lowered so that the rate of diffusion drops to zero. Other methods of introducing phosphorus into silicon include gaseous diffusion, a liquid dopant spray-on process, and a technique in which phosphorus ions are driven precisely into the surface of the silicon.

Boron as a Semiconductor Material

Boron as a Semiconductor Material.

Of course, n-type silicon cannot form the electric field by itself; it's also necessary to have some silicon altered to have the opposite electrical properties. So, boron, which has three valence electrons, is used for doping p-type silicon. Boron is introduced during silicon processing, where silicon is purified for use in PV devices. When a boron atom assumes a position in the crystal lattice formerly occupied by a silicon atom, there is a bond missing an electron (in other words, an extra hole).

Substituting a boron atom (with three valence electrons) for a silicon atom in a silicon crystal leaves a hole (a bond missing an electron) that is relatively free to move around the crystal.

Other Semiconductor Materials

Polycrystalline thin-film cells have a heterojunction structure, in which the top layer is made of a different semiconductor material than the bottom semiconductor layer.

Like silicon, all PV materials must be made into p-type and n-type configurations to create the necessary electric field that characterizes a PV cell. But this is done a number of different ways, depending on the characteristics of the material. For example, amorphous silicon's unique structure makes an intrinsic layer (or i layer) necessary. This undoped layer of amorphous silicon fits between the n-type and p-type layers to form what is called a "p-i-n" design.

Polycrystalline thin films like copper indium diselenide (CuInSe2) and cadmium telluride (CdTe) show great promise for PV cells. But these materials can't be simply doped to form n and p layers. Instead, layers of different materials are used to form these layers. For example, a "window" layer of cadmium sulfide or similar material is used to provide the extra electrons necessary to make it n-type. CuInSe2 can itself be made p-type, whereas CdTe benefits from a p-type layer made from a material like zinc telluride (ZnTe).

Gallium arsenide (GaAs) is similarly modified, usually with indium, phosphorous, or aluminum, to produce a wide range of n- and p-type materials.

Conversion Efficiency of a PV Cell

*The conversion efficiency of a PV cell is the proportion of sunlight energy that the cell converts to electrical energy. This is very important when discussing PV devices, because improving this efficiency is vital to making PV energy competitive with more traditional sources of energy (e.g., fossil fuels). Naturally, if one efficient solar panel can provide as much energy as two less-efficient panels, then the cost of that energy (not to mention the space required) will be reduced. For comparison, the earliest PV devices converted about 1%-2% of sunlight energy into electric energy. Today's PV devices convert 7%-17% of light energy into electric energy. Of course, the other side of the equation is the money it costs to manufacture the PV devices. This has been improved over the years as well. In fact, today's PV systems produce electricity at a fraction of the cost of early PV systems.