Solar energy provides light, heat and energy to all living things on Earth. The sun is the most inexhaustible, renewable source of energy known to man. It's free energy for life. Photovoltaic cells convert sunlight directly into electricity. When sunlight strikes a PV cell, electrons are dislodged, creating an electrical current.

A photovoltaic module made from a set of PV cells is a non-mechanical device commonly comprised of silicon alloys. Sunlight is composed of photons, or particles of energy. These photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum. When the photons strike a PV module, they may be reflected, pass through or be absorbed. The absorbed photons provide the energy to generate electricity. When the panels absorb enough sunlight, electrons are excited and dislodge from the material's atoms, creating the flow of electricity used to power anything from small calculators to larger appliances.

The cost of PV has fallen by 90 percent since the early 1970s. Photovoltaic modules are producing electricity for critical loads from the polar ice caps to the tropics to satellites in outer space. There is a strong market today in developing countries to provide rural electrification with solar panels, which replace kerosene lamps, batteries, and wood fires at a far lower cost than the central station power plants.

Photovoltaics are also making inroads as supplementary power for utility customers already served by the grid. Currently costly compared to most conventional choices for grid power, Photovoltaics is still a very small part of the energy make-up of any country. However, more and more individuals, companies, and communities choose PV for reasons other than cost: because of a desire to develop a clean, sustainable energy source, interest in a clean back-up power source, a need for placing power generation right at the source with no fuel, noise or moving parts; and an attraction to a power technology that can be built right into building roofs, facades, canopies and windows.

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.

Making "n" and "p" Material

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.

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.

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.