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.

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 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.

ELECTRONICS WEEKLY NEWS ON SOLAR CELLS

Latest News

• Spelling out solar cell stories
• Chip design firms develop tools for solar cell production
• Solar cell, semi industry investments equal by 2010 - iSuppli
• Cardiff solar cell plant gets further $30m
• The Electronics Weekly news roundup - Nokia and Qualcomm, TI DSPs...

Previous news stories to note

MIT turns to photosynthesis for unlimited solar power
The key component in the process is a catalyst - cobalt metal, phosphate and an electrode, placed in water - that produces oxygen gas from water, while another catalyst produces hydrogen gas.

UK universities in £6m solar cell project
Durham University is to lead a £6.3m university collaboration to develop thin-film solar cells.

High efficiency silicon solar cell developed at IMEC
Belgium research organisation IMEC has reported 17.4 per cent efficiency in thin multi-crystalline silicon solar cells.

IBM recycles silicon wafers for solar cell use
IBM is repurposing scrap semiconductor wafers for use in silicon-based solar panel manufacturing. The company has detailed a reclamation process that was developed at its Vermont manufacturing facility which uses a specialized pattern removal technique.

Plastic solar cells project gets £5m government funding
The University of Cambridge and The Technology Partnership are to develop plastic solar cells in a project sponsored with £5m by the Government-funded Carbon Trust.

Durham University improves thin-film solar cells
Durham University has made significant improvements in thin-film solar cells. "We have done a lot of work measuring recombination at grain boundaries and on the control and increase of grain size," principal investigator Professor Ken Durose told Electronics Weekly.

Sharp builds for 10th-generation glass substrates
Sharp is boasting of its plans to build state-of-the-art production facilities for LCD panels and solar cells. They feature the use of 10th-generation glass substrates, producing panels for large-screen LCD TVs, up to the 60-inch class.

UK gets €30m plastic electronics centre
Work is about to start on the €30m Plastic Electronics Technology Centre (PETeC) at NETPark in County Durham.

Solar cells allow optical fibre to carry power
Getting power to a sensor is easy. Just use a 20mA current loop, or a battery at the far end, or a solar cell. The options are almost endless.

OLED project examines science for lighting uses
The University of Bath is to head an international consortium aiming to prepare OLEDs for lighting and other consumer applications.

Plastic solar cell has record-breaking power efficiency
Researchers have succeeded in obtaining an efficiency of over six per cent from plastic solar cells.

High efficiency solar cells are 3D
Three-dimensional solar cells which can capture nearly all of the light which strikes them have been developed by the Georgia Tech Research Institute in the US.

Flexible solar power available for iPods
Flexible solar device charger development is heating up with South African firm Flexopower releasing its Solar Uno and a European project under way.

Mobile phones to get solar panels
European mobile phone firms are looking at techniques for designing mini solar panels into handsets to extend standby times to a week or more.

Imperial College points to lower cost flexible displays
Imperial College in London has invented techniques for mass-producing solution-processed organic semiconductors on flexible substrates, and has won £250,000 to develop manufacturing prototypes.

Applied to build advanced thin film solar line
Semiconductor and thin film equipment leader Applied Materials said it has been selected by Moser Baer India to develop and install what it believes is the world’s first Generation (Gen) 8.5 thin film solar module production line in New Delhi, India.

Hoku to build polysilicon plant to supply Sanyo solar cells
Hoku Materials, which specialises in the manufacture of polysilicon used in the fabrication of solar cells, plans to build a plant capable of producing 2,000 metric tons of polysilicon per year.

ST makes solar cell development move
STMicroelectronics is to collaborate with Taiwanese solar cell specialists in the field of photovoltaic power generation technology.

High efficiency solar cells demonstrated by Kyocera and SunPower
Kyocera says it has achieved a record 18.5% energy conversion efficiency for a 15x15cm multi-crystalline silicon solar cell.

ELECTRONICS WEEKLY ANALYSIS OF SOLAR CELLS

Evergreen Solar signs $200m wafer deal
In a show of the explosive growth in the solar power industry, solar power product supplier Evergreen Solar has signed a sales agreement with Maryland-based SunEdison for its low-cost “String Ribbon” wafer technology.

Solar cells catching the sun's rays
To help solar cells catch more of the sun's rays, researchers guided by the National Science Foundation have created an anti-reflective coating that allows light to travel through it, but lets almost none bounce off its surface.

OTHER RESOURCES ON SOLAR CELLS

"Self-assembling" solar cells developed (New Scientist)

Pliable solar cells are on a roll (New Scientist)

BP Solar

Sharp Solar System

Sharp Electronics (UK)

Solar Cells (Wikipedia)