CHEMISTRY GLOSSARY

Gratzel solar cell    →   Gratzelova sunčeva ćelija

Grätzel solar cell is photoelectrochemical cell, developed by Michael Grätzel and collaborators, simulates some characteristics of the natural solar cell, which enables photosynthesis take place. In natural solar cell the chlorophyll molecules absorb light (most strongly in the red and blue parts of the spectrum, leaving the green light to be reflected). The absorbed energy is sufficient to knock an electron from the excited chlorophyll. In the further transport of electron, other molecules are involved, which take the electron away from chlorophyll. In Grätzel cell, the tasks of charge-carrier generation and transport are also assigned to different species.

His device consists of an array of nanometre-sized crystallites of the semiconductor titanium dioxide, welded together and coated with light-sensitive molecules that can transfer electrons to the semiconductor particles when they absorb photons. So, light-sensitive molecules play a role equivalent to chlorophyll in photosynthesis. In Grätzel cell, the light-sensitive molecule is a ruthenium ion bound to organic bipyridine molecules, which absorb light strongly in the visible range; titanium dioxide nanocrystals carry the received photoexcited electrons away from electron donors. On the other hand, a donor molecule must get back an electron, so that it can absorb another photon. So, this assembly is immersed in a liquid electrolyte containing molecular species (dissolved iodine molecules) that can pick up an electron from an electrode immersed in the solution and ferry it to the donor molecule. These cells can convert sunlight with efficiency of 10 % in direct sunlight and they are even more efficient in diffuse daylight.

nuclear reactor    →   nuklearni reaktor

Nuclear reactor is an assembly of fissionable material (uranium-235 or plutonium-239) designed to produce a sustained and controllable chain reaction for the generation of electric power.

The essential components of a nuclear reactor are:

1. The core, metal rods containing enough fissionable material to maintain a chain reaction at the necessary power level (as much as 50 t of uranium may be required).
2. A source of neutrons to initiate the reaction (such as a mixture of polonium and beryllium)
3. A moderator to reduce the energy of fast neutrons for more efficient fission (material such as graphite, beryllium, heavy water, and light water are used)
4. A coolant to remove the fission-generated heat (water, sodium, helium, and nitrogen may be used)
5. A control system such as rods of boron or cadmium that have high capture cross sections (to absorb neutrons)
6. Adequate shielding, remote-control equipment, and appropriate instrumentation are essential for personnel safety and efficient operation.

Peltier effect    →   Peltierov efekt

Peltier effect is the absorption or generation of heat (depending on the current direction) which occurs when an electric current is passed through a junction between two materials.

photochemical reaction    →   fotokemijska reakcija

Photochemical reactions are those reactions which are conducted under the influence of light that is under the influence of ultraviolet, visible and infrared part of the light spectrum. Some systems can be influenced only by radiation that is absorbed by that system. Photochemical reactions are for example photosynthesis, creation of photography, generation of phosgene, creation of hydrochloride etc.

solar cell    →   sunčeva ćelija

Solar cell, or photovoltaic cell, is a device that captures sunlight and transforms it directly to electricity. All solar cells make use of photovoltaic effect, so often they are called photovoltaic cells. Almost all solar cells are built from solid-state semiconducting materials, and in the vast majority of these the semiconductor is silicon.

The photovoltaic effect involves the generation of mobile charge carriers-electrons and positively charged holes-by the absorption of a photon of light. This pair of charge carriers is produced when an electron in the highest filled electronic band of a semiconductor (the valence band) absorbs a photon of sufficient energy to promote it into the empty energy band (the conduction band). The excitation process can be induced only by a photon with an energy corresponding to the width of the energy gap that separates the valence and the conduction band. The creation of an electron-hole pair can be converted into the generation of an electrical current in a semiconductor junction device, wherein a layer of semiconducting material lies back to back with a layer of either a different semiconductor or a metal. In most photovoltaic cells, the junction is p-n junction, in which p-doped and n-doped semiconductors are married together. At the interface of the two, the predominance of positively charged carriers (holes) in the p-doped material and of negatively charged carriers (electrons) in the n-doped material sets up an electric field, which falls off to either side of the junction across a space-charge region. When absorption of a photon in this region generates an electron-hole pair, these charge carriers are driven in opposite directions by the electric field, i.e. away from the interface and toward the top and bottom of the two-layer structure, where metal electrodes on these faces collect the current. The electrode on the top layer (through which light is absorbed) is divided into strips so as not to obscure the semiconducting layers below. In most widely used commercial solar cells, the p-doped and n-doped semiconductive layers are formed within a monolithic piece of crystalline silicon. Silicon is able to absorb sunlight at those wavelengths at which it is most intense-from the near-infrared region (wavelengths of around 1200 nm) to the violet (around 350 nm).