Solar cell applications

Applications for solar cells are varied, but often involve instances where normal power sources are not available, for example in space probes. More prosaically, they are also used in calculators and wrist watches.

When used in combination - solar modules, or photovoltaic arrays - they can help provide alternative power sources in combination with the electricity grid.

Solar Cell history
The photovoltaic effect was first discovered by Edmond Becquerel in 1839, and the likes of Albert Einstein continued his work. In 1921 Einstein was awarded the Nobel Prize in Physics "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect".

The first silicon p-n junction (a combination of N-type and P-type semiconductors) solar cell was made at Bell Labs in 1954, with solar cells first being used to power satellites, such as the Vanguard I, in 1958.


In the following, we bring together resources from Electronics Weekly and UK and EU governmental bodies to provide detailed reference information about solar cells.

Manufacturing solar cells

In general, solar cells are made from thin flat layers of semiconductor that include a p-n junction across the whole area of the cell.

A photon hitting the structure creates an electron-hole carrier pair (an exciton) which is separated by the junction. This develops a potential difference between the front and the back of the cell which can produce a current externally.

There is a relationship between the energy in the photon and the bandgap of the structure which governs the success rate of photon to exciton conversion.
Essentially: a particular semiconductor will only efficiently convert one colour of light - or more accurately, light between two wavelengths with sensitivity peaking somewhere near the middle.

For example, 'single junction' silicon solar cells can only absorb the near-infra red part of the sun's spectrum and have a light to electricity conversion efficiency somewhere around 20 per cent.
GaAs and other compound semiconductors can be used to form junctions with other bandgaps, and these junctions can be stacked to harvest a broader range of wavelengths - or photon energies - depending on whether you are thinking classically or in quantum terms.

Providing they are stacked in the right order so the top layers pass unused light through to lower layers, efficiencies of around 40 per cent can be achieved. The current record is almost 50 per cent.

Double and triple junction cells very expensive, and are found on satellites and transcontinental solar racing cars.

Single crystal junctions achieve the efficiencies mentioned above, but are not the cheapest way to produce solar cells.

Amorphous silicon deposited on glass offers around six per cent efficiency at far lower cost than single crystal silicon, and is frequently seen on solar-powered calculators.
Polysilicon on glass is between amorphous and single crystal silicon in both cost and efficiency.

It is widely believed that, depending on latitude, a minimum efficiency of 10 per cent is required to make cost-effective large-scale solar power installations - and the raw materials will have to be cheap.

Researchers are looking at alternative materials to achieve these aims.

For example: single crystal solar cells are made from IC-grade silicon wafers, whereas less pure silicon could be used with negligible loss in efficiency.

The question of whether an industry will form to produce these less pure wafers remains to be answered.

Organic semiconductors can be used to make solar cells, dopes with materials including carbon nanotubes.

Currently efficiency is a few per cent at most and the cells degrade rapidly in sunlight, but some predict organic solar technology will be the one to take off. In the mean time, these cells are likely to develop enough to be used in solar-powered portable electronics.

Out in the far field are solar cells based on structures that mimic photosynthesis, and various liquid and solid bulk technologies, such as the dye-sensitised solar cells in which the excitons form in dye; titanium dioxide pulls away the electrons; and an electrolyte takes away the holes.

With some forms of organic solar cell, as well as solid dye-sensitised cells, it may be possible to make large areas very cheaply on printing presses.

As the manufacture of nano-scale material powders becomes possible, researchers are not only reviewing existing solar cell types, but looking at schemes in which the light is absorbed by particles of similar size to its wavelength.

Solar Cell in spotlight

Taking light energy and converting it into electrical energy, the solar cell is an ecological device. The light absorbing material of a solar cell will lead to photogeneration of charge carriers and a conductive contact will carry off the electrons into another wire or circuit.

Solar cells are made up of thin layers of silicon, and when sunlight strikes a cell's light absorbing material, chemical reactions release electrons, generating an electric current.

For example, they can be constructed with sequential layers of thin film semiconductor materials, which are usually only micrometers thick. According to Sharp Electronics, a specialist in this area, such thin-film technologies account for around 12 percent of all solar modules sold worldwide.

The manufacturers of solar cells boast that they are cost-effective, quiet, safe, and reliable, and only require minimal maintenance over a long operational life.

Note that the term photovoltaic cell is sometimes used when the cell's light source is not explicitly sunlight. Also, the study of solar cells is known as photovoltaics.

Developing Technologies: Electrochemical PV cells

Unlike the crystalline and thin film solar cells that have solid-state light absorbing layers, electrochemical solar cells have their active component in a liquid phase. They use a dye sensitizer to absorb the light and create electron-hole pairs in a nanocrystalline titanium dioxide semiconductor layer. This is sandwiched in between a tin oxide coated glass sheet (the front contact of the cell) and a rear carbon contact layer, with a glass or foil backing sheet.

Some consider that these cells will offer lower manufacturing costs in the future because of their simplicity and use of cheap materials. The challenges of scaling up manufacturing and demonstrating reliable field operation of products lie ahead. However, prototypes of small devices powered by dye-sensitised nanocrystalline electrochemical PV cells are now appearing (120cm2 cells with an efficiency of 7%).

Developing Technologies: Concentrators

Solar cells usually operate more efficiently under concentrated light. This has led to the development of a range of approaches using mirrors or lenses to focus light on to specially designed cells and use heat sinks, or active cooling of the cells, to dissipate the large amount of heat that is generated. Unlike conventional flat plate PV arrays, concentrator systems require direct sunlight (clear skies) and will not operate under cloudy conditions. They generally follow the sun's path through the sky during the day using single-axis tracking. To adjust to the sun's varying height in the sky through the seasons, two-axis tracking is sometimes used.
Concentrators have not yet achieved widespread application in photovoltaics, but solar concentration has been widely used in solar thermal electricity generation technology where the generated heat is used to power a turbine.

Crystalline silicon solar cells

Historically, crystalline silicon (c-Si) has been used as the light-absorbing semiconductor in most solar cells, even though it is a relatively poor absorber of light and requires a considerable thickness (several hundred microns) of material. Nevertheless, it has proved convenient because it yields stable solar cells with good efficiencies (11-16%, half to two-thirds of the theoretical maximum) and uses process technology developed from the huge knowledge base of the microelectronics industry.

wo types of crystalline silicon are used in the industry. The first is monocrystalline, produced by slicing wafers (up to 150mm diameter and 350 microns thick) froma high-purity single crystal boule. The second is multicrystalline silicon, made by sawing a cast block of silicon first into bars and then wafers. The main trend in crystalline silicon cell manufacture is toward multicrystalline technology.

For both mono- and multicrystalline Si, a semiconductor homojunction is formed by diffusing phosphorus (an n-type dopant) into the top surface of the boron doped (p-type) Si wafer. Screen-printed contacts are applied to the front and rear of the cell, with the front contact pattern specially designed to allow maximum light exposure of the Si material with minimum electrical (resistive) losses in the cell.

The most efficient production cells use monocrystalline c-Si with laser grooved, buried grid contacts for maximum light absorption and current collection.

Some companies are productionizing technologies that by-pass some of the inefficiencies of the crystal growth/casting and wafer sawing route. One route is to grow a ribbon of silicon, either as a plain two-dimensional strip or as an octagonal column, by pulling it from a silicon melt.

Another is to melt silicon powder on a cheap conducting substrate. These processes may bring with them other issues of lower growth/pulling rates and poorer uniformity and surface roughness.

Each c-Si cell generates about 0.5V, so 36 cells are usually soldered together in series to produce a module with an output to charge a 12V battery. The cells are hermetically sealed under toughened, high transmission glass to produce highly reliable, weather resistant modules that may be warrantied for up to 25 years.

Thin film solar cells

The high cost of crystalline silicon wafers (they make up 40-50% of the cost of a finished module) has led the industry to look at cheaper materials to make solar cells.

The selected materials are all strong light absorbers and only need to be about 1micron thick, so materials costs are significantly reduced. The most common materials are amorphous silicon (a-Si, still silicon, but in a different form), or the polycrystalline materials: cadmium telluride (CdTe) and copper indium (gallium) diselenide (CIS or CIGS).

Each of these three is amenable to large area deposition (on to substrates of about 1 meter dimensions) and hence high volume manufacturing. The thin film semiconductor layers are deposited on to either coated glass or stainless steel sheet.

The semiconductor junctions are formed in different ways, either as a p-i-n device in amorphous silicon, or as a hetero-junction (e.g. with a thin cadmium sulphide layer) for CdTe and CIS. A transparent conducting oxide layer (such as tin oxide) forms the front electrical contact of the cell, and a metal layer forms the rear contact.

Thin film technologies are all complex. They have taken at least twenty years, supported in some cases by major corporations, to get from the stage of promising research (about 8% efficiency at 1cm2 scale) to the first manufacturing plants producing early product.

Amorphous silicon is the most well developed of the thin film technologies. In its simplest form, the cell structure has a single sequence of p-i-n layers. Such cells suffer from significant degradation in their power output (in the range 15-35%) when exposed to the sun.

The mechanism of degradation is called the Staebler-Wronski Effect, after its discoverers. Better stability requires the use of a thinner layers in order to increase the electric field strength across the material. However, this reduces light absorption and hence cell efficiency.

This has led the industry to develop tandem and even triple layer devices that contain p-i-n cells stacked one on top of the other. In the cell at the base of the structure, the a-Si is sometimes alloyed with germanium to reduce its band gap and further improve light absorption. All this added complexity has a downside though; the processes are more complex and process yields are likely to be lower.

In order to build up a practically useful voltage from thin film cells, their manufacture usually includes a laser scribing sequence that enables the front and back of adjacent cells to be directly interconnected in series, with no need for further solder connection between cells.