Solar Cells - How do they work?
Solar energy may be converted into electrical energy by a thermal route using heated vapour to drive a turbine/generator set, much like that of a conventional power station but without the need to burn a chemical fuel to generate steam.
Alternatively the heating process may be eliminated by using direct conversion of sunlight into electricity in a solar cell.
This solid–state device operates by the photovoltaic effect, using the internal electric field of the cell to separate positive and negative charges that have been created by the absorption of light.
An effective cell comprises an opaque material that absorbs the incoming light, an electric field that arises from the difference in composition between the semiconducting layers comprising the absorber, and two electrodes to carry the positive and negative charges to the electrical load.
Designs of solar cells differ in detail but all must include the above features.
Solar Cells – Efficiencies and Materials
The first commercial cells used crystalline silicon as the semiconducting absorber, because it can absorb a significant fraction of the solar spectrum and the technology of 'doping' silicon to form two layers that have a built-in electric field between them was becoming established.
Since those early days, technological developments have been driven to increasing the power conversion efficiency of sunlight to electricity, restricted only by the thermodynamic limit of ~32%.
Crystal silicon cells have attained ~24%, but with increased complexity in their structure and at greater manufacturing cost. The recent efforts on perovskite solar cells have already achieved efficiencies of this order in the laboratory and these materials are being combined with silicon in a tandem arrangement that is being field-tested. Such advanced cells offer even higher efficiency than present-day panels but are not suitable for flexible substrates and the best formulations contain lead as one of the constituents.
Other developments in semiconductor science and technology have produced much cheaper thin-film non-crystalline silicon cells that achieve about half this efficiency.
Extensive research into organic materials (especially polymers) has promised even cheaper solar absorbing materials, but efficiencies for large-area cells have been restricted to ~5% and they have had limited durability. Promising developments such as a combination of polymer and fullerene or non-fullerene materials on a flexible plastic sheet offer an efficiency of over 12% combined with several years lifetime, so far for small area cells. Organic solar cells on plastic foil are commercially available for trials, having a few percent efficiency, but have limited lifetime compared with mainstream silicon cells.
Other crystalline semiconductors (such as cadmium telluride) have the potential for greater conversion efficiency than silicon and in thinner cells, but as with crystalline silicon, they generally use high processing temperatures and in their thin-film form therefore require an appropriate and expensive high temperature substrate.
Depending on the location, a low cost cell with only moderate efficiency may generate electrical energy at a similar cost to an expensive cell with high efficiency.
Solar Cells – Power, Current & Voltage
The power developed by a solar cell depends on the product of current and voltage: to operate any load will require a certain voltage and current, and is tailored by adding cells in series and/or parallel, just as with conventional batteries.
The output voltage is between 0.5 and 1.0 volts from many types of simple solar cell, and is provided by the electric field built in to the semiconductor: it varies only slightly with illumination conditions.
Multiple junction cells have several different semiconductor layers stacked together during manufacture, producing a greater voltage per cell than from a single junction cell. In contrast, the output current depends on the cell area: high currents require large areas of cells. There is a stronger dependence of current (than for the voltage) on the illumination intensity and colour.
Solar Cells – Absorbing the light
When a semiconductor absorbs light shorter than the threshold wavelength specific to that material, electrons are untied from their chemical bonding and may move through the material to the external contacts.
If the threshold is too far into the blue end of the spectrum, most sunlight (red and infrared) will pass through, unabsorbed.
If the threshold is too far towards the red end of the spectrum then more sunlight will be absorbed, but the blue wavelengths will be absorbed less effectively and the output voltage will be low.
The threshold for a simple solar cell to convert most of the solar spectrum to electricity lies in the red. (Multiple junction cells can improve on this at the expense of a more complex structure that allows different layers to absorb different wavebands.)
Amorphous silicon is a better match to the solar spectrum than is single-crystal silicon, and by using additional coatings or textured surfaces, the absorption may be enhanced further. In our technology, we are also exploiting the deposition of nanocrystalline silicon, which is expected to provide much greater stability and durability. In addition, the texture of a woven fabric should lead to enhanced absorption of long wavelength radiation that is otherwise reflected out of the cell, by utilising the scattering of this light from the rough surface.