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Harnessing the massive amounts of energy that reaches the Earth from the Sun is a challenging but necessary step to ensure that the future energy needs of the planet can be met. Photovoltaic cells composed of various semiconductor materials are springing up all over the world to convert light energy directly into electricity with zero emissions.
Overview of Solar Cells
When light reaches a solar panel or photovoltaic (PV) cell, it can either be reflected, absorbed or pass right through it. At the heart of a solar cell is a semiconductor layer, which is unequivocally the most important part of the cell. This material combines the properties of metals and insulators to yield a substance uniquely skilled at converting sunlight to electricity. When the semiconductor absorbs light, photons transfer their energy to electrons which flow through the material as an electrical current towards metal contacts above and below the semiconductor layer, from where it can travel to the power grid.
The efficiency of a PV cell is defined as the amount of electrical power divided by the energy from sunlight in. The amount of electricity is dependent on the quality of light offered – it’s intensity and wavelengths – and the performance characteristics of the cell.
Among the most efficient and by far the most common semiconductor used is silicon which is found in approximately 90% of modules sold. It was first used in solar cells in 1956 and is considered a key material in solar energy production. Silicon atoms form a crystal lattice – an organised structure that makes the conversion of light to electricity more effective – and are doped with phosphorous and boron to form a semiconductor. Solar cells constructed from crystalline silicon are highly efficient, low cost and long lasting with a lifespan of around 25 years.
Thin-film PVs are becoming more prevalent. They are created by depositing one or more thin layers of a PV material – cadmium telluride (CdTe) or copper indium gallium diselinide (CIGS) – onto a supporting material such as glass, plastic or metal. CdTE is the second most common PV after silicon and while such films enable low-manufacturing they are not quite as efficient.
Organic PV cells consist of carbon-rich polymers and can be tailor-made to enhance a specific function of the cell, sensitivity to a certain wavelength of light for example. They have the theoretical potential to provide electricity at a lower cost – they are less expensive to make in high volumes - but they are about half as efficient as silicon cells and can have a shorter operating lifetime.
Other cells focus sunlight onto PV materials using mirrors or lenses. These concentration PVs (CPV) require less material as the light is focussed on a comparatively small area and have the highest overall efficiency because the light becomes concentrated on one spot. However, the materials to make them are expensive, as is the manufacturing process.
Use of Perovskites in Solar Cells
So, what does the future hold? Many researchers are working with a material called perovskite. Perovskites have a particular crystalline structure and can consist different materials but lead is an especially common choice. This material is currently placed over conventional silicon solar cells to boost power output by converting certain parts of the solar spectrum into electricity more efficiently than silicon. However, perovskites have a limited lifespan and alternative to the toxic lead are necessary.
Solar power is a popular alternative energy choice. It directly harnesses the power of the sun to make electricity – it is that simple. It doesn’t require any turbines or generators, it needs no other fuels, has no moving parts and so has low maintenance costs, and releases no emissions. Semiconductor materials are key to their functioning, and while efficiency can still be improved, solar panels are likely to continue popping up all over the place.
References and Further Reading