In 1951 Franklin joined the King's College Medical Research Council biophysics unit. With Raymond Gosling she conducted X-ray diffraction studies of the molecular structure of DNA. Based on these studies, she at first concluded that the structure was helical (having spiral arms). Later research caused her to change her mind, and it was left to Watson and Crick to develop the double-helix model of the molecule that proved to be consistent with DNA's known properties. Some of the data used by those scientists in their successful effort, however, was first produced by Franklin. She also helped determine the structure of the tobacco mosaic virus.
The simplest version of X-Ray Diffraction takes a thin slice of material and allows the X-Rays to pass through it onto a simple screen. arly experiments were lead by a German physicist called Max von Laue (see picture below). After X-Rays were discovered in 1986 there nature was the subject of mass speculation. After von Laue's experiments X-Rays were found not to be charged particles because they were not deflected by magnetic fields unlike electrons. In 1912 von Laue succeeded in showing that X-Rays were electromagnetic waves.
This is a simple version of the experiment he did:
Von Laue realised that if the spacing between the regularly spaced atoms in a crystal could be used to diffract the X-Rays and cause interference patterns as below:
The central bright spot surrounded by a fainter pattern of spots confirmed that X-rays were waves, and measurements showed that their wavelength was of the order 10 -10 m
Most X-Ray Diffraction analysis is carried out on a powdered sample, which is turned into a rod-shaped specimen either by mixing it with an adhesive or by sealing it in a glass capillary tube. The rod is placed in the cylindrical X-Ray camera and illuminated with a beam of monochromatic X-Rays (see diagram underneath).
As the X-Rays are reflected off successive layers of atoms, they undergo interference, and the film in the camera records a number of bright lines or areas of constructive interference. The distances between these lines give information on the crystal lattice of the crystals present, which then is used to identify the actual composition of the object by comparing the patterns of lines with the lines formed by known substances.
As mentioned above X-Ray diffraction only shows a structure/model of the substance and doesn't identify the main constituents. To do this mass spectrometry and an electron microscope are used.
The next subject I am going to report on is solar cells.
Solar Cells
Solar cells are all around us. You've probably seen calculators that have solar cells - calculators that never need batteries, and in some cases don't even have an off button. As long as you have enough light, they seem to work forever. You may have seen larger solar panels - on emergency road signs or call boxes, on buoys, even in parking lots to power lights. Although these larger panels aren't as common as solar powered calculators, they're out there, and not that hard to spot if you know where to look. You have also seen solar cell arrays on satellites, where they are used to power the electrical systems. In Shrivenham there was an experiment ongoing investigating whether a solar cell or a wind turbine is more effective for this country.
There is an idea at the moment called the "solar revolution". This has been apparent for the last 20 years - the idea that one day we will all use free electricity from the sun. This is a seductive promise - on a bright, sunny day the sun shines approximately 1,000 watts of energy per square meter of the planet's surface, and if we could collect all of that energy we could easily power our homes and offices for free.
- Using Silicon to convert photons to electrons
The solar cells that you see on calculators and satellites are photovoltaic cells. Photovoltaics, as the word implies (photo = light, voltaic = electricity), convert sunlight directly into electricity. Once used almost exclusively in space, photovoltaics are used more and more in less exotic ways. They could even power your house.
- How do these devices work?
Photovoltaic (PV) cells are made of special materials called semiconductors such as silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. PV cells also all have one or more electric fields, which act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off to use externally. For example, the current can power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power that the solar cell can produce.
Here is a basic diagram of a solar cell:
That's the basic process, but there's really much more to it. I am going to take a deeper look into one example of a PV cell: the single crystal silicon cell.
Silicon has some special chemical properties, especially in its crystalline form. An atom of silicon has 14 electrons, arranged in 3 different shells. The first 2 shells, those closest to the centre, are completely full. The outer shell, however, is only half full, having only 4 electrons. A silicon atom will always look for ways to fill up its last shell (which would like to have 8 electrons). To do this, it will share electrons with 4 of its neighbour silicon atoms. It's like every atom holds hands with its neighbours, except that in this case, each atom has 4 hands joined to 4 neighbours. That's what forms the crystalline structure, and that structure turns out to be important to this type of PV cell. What I have described is pure, crystalline silicon. Pure silicon is a poor conductor of electricity because none of its electrons are free to move about as electrons are in good conductors like copper. Instead, the electrons are all locked in the crystalline structure. The silicon in a solar cell is modified slightly so that it will work as a solar cell.
Our cell has silicon with impurities - other atoms mixed in with the silicon atoms, changing the way things work a bit. We usually think of impurities as something undesirable, but in this case, our cell wouldn't work without them. These impurities are actually put there on purpose. Consider silicon with an atom of phosphorous here and there, maybe one for every million silicon atoms. Phosphorous has 5 electrons in its outer shell, not 4. It still bonds with its silicon neighbour atoms, but in a sense, the phosphorous has one electron that doesn't have anyone to hold hands with. It doesn't form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place.
When energy is added to pure silicon, for example in the form of heat, it can cause a few electrons to break free of their bonds and leave their atoms. A hole is left behind in each case where an electron could bond. These electrons then wander randomly around the crystalline lattice looking for another hole to fall into. These electrons are called free carriers, and can carry electrical current. There are so few of them in pure silicon, however, that they aren't very useful. Our impure silicon with phosphorous atoms mixed in is a different story. It turns out that it takes a lot less energy to knock loose one of our "extra" phosphorous electrons because they aren't tied up in a bond - their neighbours aren't holding them back. As a result, most of these electrons do break free, and we have a lot more free carriers than we would have in pure silicon. The process of adding impurities on purpose is called doping, and when doped with phosphorous, the resulting silicon is called n-type (n for negative) because of the prevalence of free electrons. N-type doped silicon is a much better conductor than pure silicon is.
Actually, only part of our cell is n-type. The other part is doped with boron, which has only 3 electrons in its outer shell instead of 4, to become p-type silicon. Instead of having free electrons, p-type silicon (p for positive) has free holes. Holes really are just the absence of electrons, so they carry the opposite (positive) charge. They move around just like electrons do.
The interesting part starts when you put n-type silicon together with p-type silicon. Every PV cell has at least one electric field. Without an electric field, the cell wouldn't work, and this field forms when the n-type and p-type silicon are in contact. Suddenly, the free electrons in the n side, who have been looking all over for holes to fall into, see all the free holes on the p side, and there's a mad rush to fill them in. Before now, the silicon was all electrically neutral. The extra protons in the phosphorous balanced the extra electrons out. The missing electrons (holes) were balanced out by the missing protons in the boron. When the holes and electrons mix at the junction between n-type and p-type silicon, however, that neutrality is disrupted. Do all the free electrons fill all the free holes? No. If they did, then the whole arrangement wouldn't be very useful. Right at the junction, however, they do mix and form a barrier, making it harder and harder for electrons on the n side to cross to the p side. Eventually equilibrium is reached, and we have an electric field separating the two sides. This field acts as a diode, allowing (and even pushing) electrons to flow from the p side to the n side, but not the other way around. It's like a hill - electrons can easily move down the hill (to the n side), but can't climb it (to the p side).
Now, when light, in the form of photons, hits our cell, its energy frees electron-hole pairs. Each photon with enough energy will normally free exactly one electron, and result in a free hole as well. If this happens close enough to the electric field, or if they happen to wander into its range of influence, the field will send the electron to the n side, and the hole to the p side. This causes further disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the path to their original side (the p side) to unite with holes the electric field sent there, doing work for us along the way. The electron flow provides the current, and the cell's electric field causes a voltage. With both current and voltage, we have power, which is the product of the two.
How much sunlight energy does a PV cell absorb? Unfortunately, the most that a simple cell could absorb is around 25%, and more likely is 15% or less. Why so little? Visible light is only part of the electromagnetic spectrum. Electromagnetic radiation is not monochromatic - it is made up of a range of different wavelengths, and therefore energy levels. Light can be separated into different wavelengths, and we can see them in the form of a rainbow. Since the light that hits our cell has photons of a wide range of energies, it turns out that some of them won't have enough energy to form an electron-hole pair. They'll simply pass through the cell as if it were transparent. Still other photons have too much energy. Only a certain amount of energy, measured in electron volts and defined by our cell material (about 1.1 eV for crystalline silicon), is required to knock an electron loose. We call this the band gap energy of a material. If a photon has more energy than the required amount, then the extra energy is lost (unless a photon has twice the required energy, and can create more than one electron-hole pair, but this effect is not significant). These 2 effects alone account for the loss of around 70% of the radiation energy incident on a cell.
Why can't scientists choose a material with a really low band gap, so we can use more of the photons? Unfortunately, the band gap also determines the strength (voltage) of the electric field, and if it's too low, than what we make up in extra current (by absorbing more photons), we lose by having a small voltage. Remember that power is voltage times current. The optimal band gap, balancing these two effects, actually is around 1.4 eV for a cell made from a single material.
There are other losses as well. The electrons have to flow from one side of the cell to the other through an external circuit. We can cover the bottom with a metal, allowing for good conduction, but if we completely cover the top, then photons can't get through the opaque conductor and we lose all our current (in some cells transparent conductors are used on the top surface, but not in all). If we put our contacts only at the sides of our cell, then the electrons have to travel an extremely long distance (for an electron) to reach the contacts. Remember, silicon is a semiconductor - it's not nearly as good as a metal for transporting current. Its internal resistance (called series resistance) is fairly high, and high resistance means high losses. To minimize these losses, the cell is covered by a metallic contact grid that shortens the distance that electrons have to travel while covering only a small part of the cell surface. Even so, some photons are blocked by the grid, which can't be too small or its own resistance will be too high.
There are a few more steps left before we can really use the cell. Silicon happens to be a very shiny material, which means that it is very reflective. Photons that are reflected can't be used by the cell. For that reason, an antireflective coating is applied to the top of the cell to reduce reflection losses to below 5%. The final step is the glass cover plate, which protects the cell from the elements. PV modules are made by connecting several cells (usually 36) in series and parallel to achieve useful levels of voltage and current, and putting them in a sturdy frame complete with a cover glass and positive and negative terminals on the back.
Single crystal silicon isn't the only material used in PV cells. Polycrystalline silicon is also used in an attempt to cut manufacturing costs, although resulting cells aren't as efficient as single crystal silicon. Amorphous silicon, which has no crystalline structure, is also used, again in an attempt to reduce production costs. Other materials used include gallium arsenide, copper indium diselenide and cadmium telluride. Since different materials have different band gaps, they seem to be "tuned" to different wavelengths, or photons of different energies. One way efficiency has been improved is to use two or more layers of different materials with different band gaps. The higher band gap material is on the surface, absorbing high-energy photons while allowing lower energy photons to be absorbed by the lower band gap material beneath. This technique can result in much higher efficiencies. Such cells, called multi-junction cells, can have more than one electric field.
That is the end of my report. I have concentrated on two main aspects of Physics which I am particularly interested in. I enjoyed the trip and look forward to furthering my studies.
Appendix
Sources of Information:
- Figures 1,2,3 from Science Explained on the Internet
- Hutchinson’s Encyclopaedia CD-Rom
- Salter’s Physics Textbook
