![Ifa San'at Gharb :: Online Shopping & Showcase [Home]](includes/templates/classic/images/logo.gif "Ifa San'at Gharb :: Online Shopping & Showcase [Home]"){kind=logo}
Latest Physics & Astronomy Stories |
||||||||||||||||
|
||||||||||||||||
| click here for more physics news | ||||||||||||||||
| Coming In Strong On Your AM Dial by Gene Mascoli and ScienceIQ.com |
||
The ionosphere is relatively thin and affected by a steady stream of solar particles and radiation that pelt it day and night. During the day, it is not very reflective and AM radio waves, tend to bounce along the ground up to 100 miles (160.9 km) from the broadcast tower. The further away from the transmitter, the weaker the radio signal. At night though, it is a different story. The ionosphere becomes highly reflective. This allows AM waves to bounce off the ionosphere and travel great distances through the atmosphere, hundreds of miles in fact. Without some night-time curbs on AM stations, a Boston station could be knocked out by a stronger station in Chicago, or farther. Why aren't FM radio waves affected the same way? The answer simply is that FM radio waves are much shorter than AM waves. An FM radio wave may be as small at 10 feet (3 m), while AM waves can range from 600 to 1,800 feet (182.9 to 548.64 km). So FM waves scatter and never even come close to the ionosphere. |
||
This research group is well known for work on the characterisation of efficiency limiting effects in thin film solar cells. You can find out more by looking at our publications page, searching the published litertature or by contacting 'ken.durose@durham.ac.uk , the solar cell materials group leader.
The material below is intended as an introductory tutorial for some of the concepts of thin film solar cells and was written by Paul Edwards.
Background to photovoltaics
Faced with ever-increasing demand, the earth's sources of non-renewable energy are not expected to last long. Among the many contenders vying to replace fossil fuels, photovoltaic solar cells offer many advantages, including needing little maintenance and being relatively "environmentally-friendly"; the major drawback to date has been cost. In order for photovoltaics to be viable for large-scale energy conversion, their efficiency must be improved whilst making them cheaper.Principle of p-n junction solar cell
In its simplest form, the solar cell consists of a junction formed between n-type and p-type semiconductors, either of the same material (homojunction) or different materials (heterojunction). The bandstructure of the two differently doped sides with respect to their Fermi levels can be seen in Figure 1.
Figure 1: Band structure of differently-doped semiconductors
When the two halves are brought together, the Fermi levels on either side are forced in to coincidence, causing the valence and conduction bands to bend (Figure 2).
Figure 2: Heterojunction band-bending
These bent bands represent a built-in electric field over what is referred to as the depletion region. When a photon, with an energy greater than the bandgap of the semiconductor, passes through the solar cell, it may be absorbed by the material. This absorption takes the form of a band-to-band electronic transition, so an electon/hole pair is produced. If these carriers can diffuse to the depletion region before they recombine, then they are separated by the electric field, causing one quantum of charge to flow through an external load. This is the origin of the solar cell's photocurrent, and is shown in Figure 3.
Figure 3: Principle of photovoltaic device
The CdS/CdTe solar cell
Advantages of CdS/CdTe
Currently, the semiconductor most widely used in solar cells is single-crystal silicon. Because of the cost involved in producing the bulk material, cells produced by this method are prohibitively expensive for all but the smallest scale or most specialised applications (such as on calculators and satellites). Higher efficiencies have been produced by using single-crystal III-V semiconductors and more elaborate constructions (e.g. multi-quantum wells), but this advantage has always been more than offset by the resultant increase in cost.The thin-film cadmium telluride / cadmium sulphide solar cell has for several years been considered to be a promising alternative to the more widely used silicon devices. It has several features which make it especially attractive:
- The cell is produced from polycrystalline materials and glass, which is a potentially much cheaper construction than bulk silicon.
- The chemical and physical properties of the semiconductors are such that the polysilicon thin-films can be deposited using a variety of different techniques (see below).
- CdTe has a bandgap which is very close to the theoretically-calculated optimum value for solar cells under unconcentrated AM1.5 sunlight.
- CdTe has a high absorption coefficient, so that approximately 99% of the incident light is absorbed by a layer thickness of only 1µm (compared with around 10µm for Si), cutting down the quantity of semiconductor required.
Cell construction
The CdTe/CdS solar cell is based around the heterojunction formed between n-type CdS and p-type CdTe. The basic composition of the cell can be seen in Figure 4.
Figure 4: CdS/CdTe solar cell (not to scale)
The functions of the different layers are as follows:
- Glass The solar cell is produced on a substrate of ordinary window glass, because it is transparent, strong and cheap. Typically around 2-4 mm thick, this protects the active layers from the environment, and provides all the device's mechanical strength. The outer face of the pane often has an anti-reflective coating to enhance its optical properties.
- Transparent conducting oxide Usually of tin oxide or indium tin oxide (ITO), this acts as the front contact to the device. It is needed to reduce the series resistance of the device, which would otherwise arise from the thinness of the CdS layer.
- Cadmium sulphide The polycrystalline CdS layer is n-type doped (as CdS invariably is), and therefore provides one half of the p-n junction. Being a wide band gap material (Eg ~ 2.4 eV at 300K) it is transparent down to wavelengths of around 515 nm, and so is referred toas the window layer. Below that wavelength, some of the light will still pass through to the CdTe, due the thinness of the CdS layer (~ 100 nm).
- Cadmium telluride The CdTe layer is, like the CdS, polycrystalline, but is p-type doped. Its energy gap (1.5 eV) is ideally suited to the solar spectrum, and it has a high absorption coefficient for energies above this value. It acts as an efficient absorber and is used as the p side of the junction. Because it is less highly doped than the CdS, the depletion region is mostly within the CdTe layer. This is therefore the active region of the solar cell, where most of both the carrier generation and collection occur. The thickness of this layer is typically around 10 µm.
- Back contact Usually of gold or aluminium, the back contact proves a low resistance electrical connection to the CdTe. P-type CdTe is a notoriously difficult material on which to produce an ohmic contact, and so the junction will inevitably display some Schottky diode (rectifying) characteristics. Due to its high conductivity, the metal layer needs only be a few tens of nanometres in thickness.
Deposition techniques
The polycrystalline layers of CdS and CdTe can deposited by a number of different methods, including, amongst others, those outlined below:- Physical vapour deposition (PVD) (or evaporation) involves the vaporisation in a vacuum of a source of either the compound (CdS or CdTe) or the separate elements (Cd + S or Cd + Te). The resulting vapours recombine on the surface of the substrate (which can be heated, but is still much cooler than the source) to deposit the required polycrystalline material. The stoichiometry of the deposited layer is difficult to control accurately, as it depends strongly on the equilibrium vapour pressures of the elements, as well as the stoichiometry of the source material .
- Close-space sublimation (CSS), which has been used to produce the highest efficiency cells so far , is based on the reversible dissociation of the materials at high temperatures e.g.
2CdTe(s) = Cd(g) + Te2(g) The source is of a large area and is positioned close to the substrate. The substrate is maintained at a high temperature (but below that of the source) such that the elemental vapours will not become deposited on the substrate but the compound form will, due to its lower equilibrium vapour pressure. - Chemical vapour deposition (CVD) can also be used to deposit both semiconductors. It involves chemical reactions between vapours to produce the required species which then condense on the substrate to form the compound. One variation of this method, Metal-Organic CVD (MOCVD), uses metallo-organic precursors: this is an especially widespread technique, as it produces thin films with very good optical and electronic properties.
- Chemical bath deposition is sometimes used for depositing CdS films, and involves producing the required ions in a solution by chemical means, which combine and precipitate out onto the substrate if the required equilibrium conditions are met. For example, cadmium ions can be produced by the hydrolysis of Cd(OH)2:
Cd(OH)2 = Cd2+ + 2(OH)- and sulphide ions from an alkaline aqueous solution of thiourea:
(NH2)2CS + OH- = CH2N2 + H2O + HS-
HS- + OH- = H2O + S2-However, this method can not used for CdTe, due to the difficulty of synthesising tellurides.
- Electrodeposition may also be used to deposit many semiconductor materials at low temperature from solution.
