I. Lead-in:In what way are the following words connected with solar cells: arrays, photovoltaic, solar panels, crystalline, space-based?
1. Solar cells, whether used in a central power station, a satellite, or a calculator, have the same basic structure. Light enters the device through an optical coating, or antireflection layer, that minimizes the loss of light by reflection; it effectively traps the light falling on the solar cell by promoting its transmission to the energy-conversion layers below. The antireflection layer is typically an oxide of silicon, tantalum, or titanium that is formed on the cell surface by spin-coating or a vacuum deposition technique. 2. The three energy-conversion layers below the antireflection layer are the top junction layer, the absorber layer, which constitutes the core of the device, and the back junction layer. Two additional electrical contact layers are needed to carry the electric current out to an external load and back into the cell, thus completing an electric circuit. The electrical contact layer on the face of the cell where light enters is generally present in some grid pattern and is composed of a good conductor such as a metal. Since metal blocks light, the grid lines are as thin and widely spaced as is possible without impairing collection of the current produced by the cell. The back electrical contact layer has no such diametrically opposed restrictions. It need simply function as an electrical contact and thus covers the entire back surface of the cell structure. Because the back layer also must be a very good electrical conductor, it is always made of metal. 3. Since most of the energy in sunlight and artificial light is in the visible range of electromagnetic radiation, a solar cell absorber should be efficient in absorbing radiation at those wavelengths. Materials that strongly absorb visible radiation belong to a class of substances known as semiconductors. Semiconductors in thicknesses of about one-hundredth of a centimetre or less can absorb all incident visible light; since the junction-forming and contact layers are much thinner, the thickness of a solar cell is essentially that of the absorber. Examples of semiconductor materials employed in solar cells include silicon, gallium arsenide, indium phosphide, and copper indium selenide. 4. When light falls on a solar cell, electrons in the absorber layer are excited from a lower-energy “ground state,” in which they are bound to specific atoms in the solid, to a higher “excited state,” in which they can move through the solid. In the absence of the junction-forming layers, these “free” electrons are in random motion, and so there can be no oriented direct current. The addition of junction-forming layers, however, induces a built-in electric field that produces the photovoltaic effect. In effect, the electric field gives a collective motion to the electrons that flow past the electrical contact layers into an external circuit where they can do useful work. 5. The materials used for the two junction-forming layers must be dissimilar to the absorber in order to produce the built-in electric field and to carry the electric current. Hence, these may be different semiconductors (or the same semiconductor with different types of conduction), or they may be a metal and a semiconductor. The materials used to construct the various layers of solar cells are essentially the same as those used to produce the diode s and transistor s of solid-state electronics and microelectronics. Solar cells and microelectronic devices share the same basic technology. In solar cell fabrication, however, one seeks to construct a large-area device because the power produced is proportional to the illuminated area. In microelectronics the goal is, of course, to construct electronic components of ever smaller dimensions in order to increase their density and operating speed within semiconductor chips, or integrated circuits.
6. Since solar cells obviously cannot produce electric power in the dark, part of the energy they develop under light is stored, in many applications, for use when light is not available. One common means of storing this electrical energy is by charging electrochemical storage batteries. This sequence of converting the energy in light into the energy of excited electrons and then into stored chemical energy is strikingly similar to the process of photosynthesis.
II. Reading. Which paragraph speaks about the following? (Sometimes more than one answer is possible). Give your reasons.
A. The relation of power to the illuminated area.
B. The semiconductor materials employed in solar cells.
C. The structure of a solar cell.
D. The special properties of an absorber.
E. The principle of operation of a solar cell.
F. The similarity of the process of converting energy to some natural phenomenon.
G. The materials used for making the constituent parts of a solar cell.
III. Speak about the functions of the following constituent components/ elements of a solar cell: the anti-reflection layer, the top/ back junction layers, the absorber layer; the grid lines, integrated circuits, storage batteries.
IV. Speaking
A. Get ready with theabstractof the article (5 – 6 lines).
B. Speak in detail about:
- the structure of a solar cell;
- the principle of its operation;
- the materials used for it.
C. In what way does the process of converting energy resemble the process of photosynthesis?
D. Supplementary Questions (this part can require some extra information)
Speak in more detail about:
1. The possible applications of solarls cells;
2. The advantages of using a solar panel;
3. The structure of a solar-panel;
4. The cost of materials and efficiency.
E. Discussion
Situation I. Advertise a solar panel. Draw out a list of its advantages. Try to sound convincing and scientific.
Situation II. Try to convince the local authorities to let solar panels into the local market.
