What are Solar Cells?

Solar cells are devices which convert solar energy directly into electricity, either directly via the photovoltaic effect, or indirectly by first converting the solar energy to heat or chemical energy.

The most common form of solar cells are based on the photovoltaic (PV) effect in which light falling on a two layer semi-conductor device produces a photovoltage or potential difference between the layers. This voltage is capable of driving a current through an external circuit and thereby producing useful work.
 

The Origins of Solar Cells

Although practical solar cells have only been available since the mid 1950s, scientific investigation of the photovoltaic effect started in 1839, when the French scientist, Henri Becquerel discovered that an electric current could be produced by shining a light onto certain chemical solutions.

The effect was first observed in a solid material (in this case the metal selenium) in 1877. This material was used for many years for light meters, which only required very small amounts of power. A deeper understanding of the scientific principles, provided by Einstein in 1905 and Schottky in 1930, was required before efficient solar cells could be made. A silicon solar cell which converted 6% of sunlight falling onto it into electricity was developed by Chapin, Pearson and Fuller in 1954, and this kind of cell was used in specialised applications such as orbiting space satellites from 1958.

Today's commercially available silicon solar cells have efficiencies of about 18% of the sunlight falling on to them into electricity, at a fraction of the price of thirty years ago. There is now a variety of methods for the practical production of silicon solar cells (amorphous, single crystal, polycrystalline), as well as solar cells made from other materials (copper indium diselenide, cadmium telluride, etc).
 

The Need for Solar Cells

The development of solar cell use in Australia has been stimulated by:

• the need for low maintenance, long lasting sources of electricity suitable for places remote from both the main electricity grid and from people; eg satellites, remote site water pumping, outback telecommunications stations and lighthouses;
• the need for cost effective power supplies for people remote from the main electricity grid; eg Aboriginal settlements, outback sheep and cattle stations, and some home sites in grid connected areas.
• the need for non polluting and silent sources of electricity; eg tourist sites, caravans and campers
• the need for a convenient and flexible source of small amounts of power; eg calculators, watches, light meters and cameras;
• the need for renewable and sustainable power, as a means of reducing global warming.

Together, these needs have produced a growing market for photovoltaics which has stimulated innovation. As the market has grown, the cost of cells and systems has declined, and new applications have been discovered.
 

How are Solar Cells made?

Silicon solar cells are made using either single crystal wafers, polycrystalline wafers or thin films.

Single crystal wafers are sliced, (approx. 1/3 to 1/2 of a millimeter thick), from a large single crystal ingot which has been grown at around 1400 °C, which is a very expensive process. The silicon must be of a very high purity and have a near perfect crystal structure (see figure 1 (a)).

Polycrystalline wafers are made by a casting process in which molten silicon is poured into a mold and allowed to set. Then it is sliced into wafers (see figure 1 (b)).As polycrystalline wafers are made by casting they are significantly cheaper to produce, but not as efficient as monocrystalline cells. The lower efficiency is due to imperfections in the crystal structure resulting from the casting process.

Almost half the silicon is lost as saw dust in the two processes mentioned above.

Amorphous silicon, one of the thin film technologies, is made by depositing silicon onto a glass substrate from a reactive gas such as silane (SiH₄) (see figure 1 (c)). Amorphous silicon is one of a number of thin film technologies. This type of solar cell can be applied as a film to low cost substrates such as glass or plastic. Other thin film technologies include thin multicrystalline silicon, copper indium diselenide/cadmium sulphide cells, cadmium telluride/cadmium sulphide cells and gallium arsenide cells. There are many advantages of thin film cells including easier deposition and assembly, the ability to be deposited on inexpensive substrates or building materials, the ease of mass production, and the high suitability to large applications.

In solar cell production the silicon has dopant atoms introduced to create a p-type and an n-type region and thereby producing a p-n junction. This doping can be done by high temperature diffusion, where the wafers are placed in a furnace with the dopant introduced as a vapor. There are many other methods of doping silicon. In the manufacture of some thin film devices the introduction of dopants can occur during the deposition of the films or layers.

A silicon atom has 4 relatively weakly bound (valence) electrons, which bond to adjacent atoms. Replacing a silicon atom with an atom that has either 3 or 5 valence electrons will therefore produce either a space with no electron (a hole) or one spare electron that can move more freely than the others, this is the basis of doping. P-type doping, the creation of excess holes, is achieved by the incorporation into the silicon of atoms with 3 valence electrons, most often boron and n-type doping, the creation of extra electrons is achieved by incorporating an atom with 5 valence electrons, most often phosphorus (see figure 2).

Once a p-n junction is created, electrical contacts are made to the front and the back of the cell by evaporating or screen printing metal on to the wafer. The rear of the wafer can be completely covered by metal, but the front only has a grid pattern or thin lines of metal otherwise the metal would block out the sun from the silicon and there would not be any output from the incident photons of light.

How do Solar Cells Work ?

To understand the operation of a PV cell, we need to consider both the nature of the material and the nature of sunlight. Solar cells consist of two types of material, often p-type silicon and n-type silicon. Light of certain wavelengths is able to ionise the atoms in the silicon and the internal field produced by the junction separates some of the positive charges ("holes") from the negative charges (electrons) within the photovoltaic device. The holes are swept into the positive or p-layer and the electrons are swept into the negative or n-layer. Although these opposite charges are attracted to each other, most of them can only recombine by passing through an external circuit outside the material because of the internal potential energy barrier. Therefore if a circuit is made (see figure 3) power can be produced from the cells under illumination, since the free electrons have to pass through the load to recombine with the positive holes.

The amount of power available from a PV device is determined by;

• the type and area of the material;
•  the intensity of the sunlight; and
•  the wavelength of the sunlight.

Single crystal silicon solar cells, for example cannot currently convert more than 25% of the solar energy into electricity, because the radiation in the infrared region of the electromagnetic spectrum does not have enough energy to separate the positive and negative charges in the material.

Polycrystalline silicon solar cells have an efficiency of less than 20% at this time and amorphous silicon cells, are presently about 10% efficient, due to higher internal energy losses than single crystal silicon.

A typical single crystal silicon PV cell of 100 cm² will produce about 1.5 watts of power at 0.5 volts DC and 3 amps under full summer sunlight (1000Wm^-2). The power output of the cell is almost directly proportional to the intensity of the sunlight. (For example, if the intensity of the sunlight is halved the power will also be halved).

An important feature of PV cells is that the voltage of the cell does not depend on its size, and remains fairly constant with changing light intensity. However, the current in a device is almost directly proportional to light intensity and size. When people want to compare different sized cells, they record the current density, or amps per square centimeter of cell area.

The power output of a solar cell can be increased quite effectively by using a tracking mechanism to keep the PV device directly facing the sun, or by concentrating the sunlight using lenses or mirrors. However, there are limits to this process, due to the complexity of the mechanisms, and the need to cool the cells. The current output is relatively stable at higher temperatures, but the voltage is reduced, leading to a drop in power as the cell temperature is increased. More information on PV concentrators can be found later in this information file.

Other types of PV materials which show commercial potential include copper indium diselenide (CuInSe₂) and cadmium telluride (CdTe) and amorphous silicon as the basic material.

PV Panels

As single PV cells have a working voltage of about 0.5 V, they are usually connected together in series (positive to negative) to provide larger voltages. Panels are made in a wide range of sizes for different purposes. They generally fall into one of three basic categories:

• Low voltage/low power panels are made by connecting between 3 and 12 small segments of amorphous silicon PV with a total area of a few square centimeters for voltages between 1.5 and 6 V and outputs of a few milliwatts. Although each of these panels is very small, the total production is large. They are used mainly in watches, clocks and calculators, cameras and devices for sensing light and dark, such as night lights.
   
• Small panels of 1 - 10 watts and 3 - 12 V, with areas from 100cm2 to 1000cm2 are made by either cutting 100cm2 single or polycrystalline cells into pieces and joining them in series , or by using amorphous silicon panels. The main uses are for radios, toys, small pumps, electric fences and trickle charging of batteries.
   
• Large panels, ranging from 10 to 60 watts, and generally either 6 or 12 volts, with areas of 1000cm2 to 5000cm2 are usually made by connecting from 10 to 36 full-sized cells in series. They are used either separately for small pumps and caravan power (lights and refrigeration) or in arrays to provide power for houses, communications pumping and remote area power supplies (RAPS).
   

Arrays and Systems

If an application requires more power than can be provided by a single panel, larger systems can be made by linking a number of panels together. However, an added complexity arises in that the power is often required to be in greater quantities and voltage, and at a time and level of uniformity than can be provided directly from the panels. In these cases, PV systems are used, comprised of the following parts (see figure 5):

(a) a PV panel array, ranging from two to many hundreds of panels;
(b) a control panel, to regulate the power from the panels;
(c) a power storage system, generally comprising of a number of specially designed batteries;
(d) an inverter, for converting the DC to AC power (eg 240 V AC)
(e) backup power supplies such as diesel startup generators (optional)
- framework and housing for the system
- trackers and sensors (optional);

Arrays generally run the panels in series/parallel with each other, so that the output voltage is limited to between 12 and 50 volts, but with higher amperage (current). This is both for safety and to minimize power losses. Panels currently cost about $3 - 6 per Watt. That is, a 50 Watt panel presently costs about $200. Eight years ago, this same ‘standard’ panel would have cost about $500 at a cost of about $8 - 10 per Watt.

Arrays of panels are being increasingly used in building construction where they serve the dual purpose of providing a wall or roof as well as providing electric power for the building. Eventually as the prices of solar cells fall, building integrated solar cells may become a major new source of electric power.

The daily energy output from PV panels will vary depending on the orientation, location, daily weather and season. On average, in summer, a panel will produce about five times its rated power output in watt hours per day, and in winter about two times that amount. For example, in summer a 50 watt panel will produce an average of 250 watt-hours of energy, and in winter about 100 watt-hours. These figures are indicative only, and professional assistance should be sought for more precise calculations.

Trackers are used to keep PV panels directly facing the sun, thereby increasing the output from the panels. Trackers can nearly double the output of an array (see figure 7). Careful analysis is required to determine whether the increased cost and mechanical complexity of using a tracker is cost effective in particular circumstances. A variety of trackers, which will take about 10 panels, are manufactured in Australia.