Photoelectric Effect and Production

Today, solar energy is commonly converted directly into electricity using photovoltaic (pv) cells. These are based on the photoelectric effect: when photons of light strike certain substances, electrons are released. When light hits a pv cell, electrons are freed from the atoms that bind it, leaving a “hole” in its place. This hole can accept neighbouring electrons, which in turn create more holes. Electrons will travel to a positive potential and holes will travel to a negative one, establishing an electric current.

To make use of this property, a p-n junction can be applied. One present type uses silicon crystals, which are ideal insulators and have strong molecular bonds. However, by doping the silicon with phosphorous, or another species which has more valence electrons, the lattice structure is preserved, while the substance acquires more electrons than is required to bond atoms together. Electrons in the material are more easily excited and freed, and thus accept more electrons, making it an n-type semiconductor. Similarly, silicon doped with a species containing fewer valence electrons, such as boron, can accept more holes - making it a p-type semiconductor.

In this cell, the n-type layer is above the p-type layer, with the contact area called the p-n junction. Light shining on the cell creates free electrons in the top layer and holes in the bottom. More electrons cross the p-n junction one way than the other and enter an external circuit. (Fig. 1 - click to enlarge)

Design

Figure 1: Simple contemporary pv cells are based on the photoelectric effect.Currently, the most common elements comprising solar panels are Sb, As, Cd, Ga, Ge, In, Pb, P, Si, and Te, which are easily extracted and commercially available. Often their compounds, such as GaAs and InP are used. As for the durability of pv cells, they have roughly a 20-year lifetime13.

Recently scientists have produced cells made of thin films, some less than a micron thick21. This is especially useful to increase efficiency and lower cost, since less material is used.

Solar cells have a very wide range of efficiency, determined by power input and output. The current cells in use today are between 5-25% efficient. The new Ga-I-P, Ga-As, Ge pv cell has the current record for highest efficiency, with 32.3%19.

In the US, pv cells cost $10-$4000 each (depending on efficiency, size, etc.) and the electricity they generate costs around 20-30˘/kWh18. However, for 6-7˘ equivalent electricity can be bought from other sources. Nevertheless, because of availability, pv cells are especially popular in developing countries such as Africa.

Applications

The photovoltaic cell is commonly found in calculators and other small devices. Such applications are ideal because the cell is light and thin, uses a renewable, inexhaustible source of energy, and has a longer life span than their battery-powered counterparts. The efficiencies of current cells are low because they can only convert middle wavelengths of the light spectrum, and energy is lost to heat. The solar cells used in calculators convert 12-18% of the light’s energy, and although inefficient, they still provide sufficient energy to operate. Presently, pv cells are also becoming very popular on rooftops, cars and airplanes.

Nearly all current space structures, including satellites and stations, run on pv cells, since solar power is a renewable, convenient source of energy; batteries and other energy sources would eventually expire. In outer space solar cells receive sunlight of greater intensity, since impediment is negligible, unlike on earth where roughly 70% of light is scattered by the atmosphere7. This allows the solar cell to function more effectively in space than on earth.