Aula anterior Apresentação de Paulo sobre Condutividade em Sólidos

Apresentação em tema: "Aula anterior Apresentação de Paulo sobre Condutividade em Sólidos"— Transcrição da apresentação:

1 Aula anterior Apresentação de Paulo sobre Condutividade em Sólidos
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2 Aula anterior Figuras de mérito dos detectores: NEP, R, tc, D
Dois tipos de detectores: Fotônicos Térmicos Detectores fotônicos: Fotoemissivos: tubos a vácuo, efeito fotoelétrico, channeltron, PMT Photoemissive Detectors 6 ratings | 3.00 out of 5 Print | PDF Overview Photoemissive detectors are either evacuated or gas-filled tubes containing a cathode and one or more anodes. When photons impinge on the cathode, the electrons are ejected from the cathode surface and are accelerated toward the anode that is at a positive potential with respect to the cathode. The photoelectric current increases proportionally to the intensity of the illumination. This design is the simplest diode version of the detector. In order to increase the sensitivity, several more electrodes (dynodes) are added to the construction of the detector. In this device, called a photomultiplier, the electrons that are ejected from the cathode are focused on one of the dynodes. When the surface of the dynode is struck, an increased number of electrons are liberated. The electrons flow to the next dynode, where the process is repeated with a progressively increasing number of electrons. The dynodes have sequentially higher positive potentials with respect to each other. As a result, the output current has a significantly increased magnitude, which defines the high sensitivity of the detector. Photoemissive cell (a) and photomultiplier (b). L = light flux, Eo through E4 = differences in potentials between cathode and one of electrodes, 1 and 2 = cathodes, 3 and 4 = anodes, 5, 6, and 7 = dynodes, 8 and 9 = glass envelopes. ===================================================== Capítulo 7 do livro Detectors of Light Photoemission refers to a process in which the absorption of a photon by a sample of material results in ejection of an electron. If the electron can be captured, this process can be used to detect light. Photoemissive detectors use electric or magnetic fields or both to accelerate the ejected electron into an amplifier. At the amplifier output, the photon stream can be detected as a current or even as an individual particle. These detectors are capable of very high time resolution (up to 10−9 s) even with sensitive areas several centimeters in diameter. They can also provide excellent spatial resolution either with electronic readouts or by displaying amplified versions of the input light pattern on their output screens. They have moderately good quantum efficiencies of 10–40% in the visible and near infrared; in some cases, significantly higher values apply in the ultraviolet. They are unmatched in sensitivity at room temperature or with modest cooling, leading to many important applications. In addition, they provide unequalled performance in the ultraviolet. They can be readily manufactured with 106 or more pixels. If the photon arrival rate is low enough that they can distinguish individual photons, the detectors are extremely linear. 7.1 General description A photoemissive detector is basically a vacuum tube analog of a photodiode; in fact, the simplest form of such a detector is a vacuum photodiode, illustrated in Figure 7.1. The device is biased by placing a negative potential from its cathode (analogous to the p-type region of the diode), to its anode (analogous to the n-type region). The vacuum maintained in the tube between cathode and anode is depleted of everything, not just free charge carriers; it corresponds to the depletion region of the diode. A photon releases an electron from the photocathode; it is accelerated by the electric field maintained by the voltage supply and is collected at the anode. The resulting current is a measure of the level of illumination of the cathode. This kind of detector performs extremely well even at room temperature because of the very high impedance of the physical vacuum that forms its depletion region. dispoptic 2013

3 Continuação aula sobre detectores
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4 Detectores fotoemissivos
Baseados no efeito fotoelétrico e geração de portadores de carga. Fotodiodos à vácuo e à gás, PMT e Photo-channeltron dispoptic 2013

5 Fotomultiplicadora - PMT
Impacto inicial sobre material fotoemissivo Posterior emissão secundaria de elétrons a través de dinodos Multiplicação considerável de elétrons Ganho de 109 elétrons no anodo por fotoelétron (pulso) Ganho de 107 em modo continuo E.g. pulso de 2ns no anodo com 109 fotoelétrons gera 4V numa RL de 50 Contador de fótons D* até 1016 cm Hz1/2 W-1, só o olho humano é capaz de detectar 10 fótons no azul que se aproxima a esse valor. dispoptic 2013

6 Características essenciais na montagem de PMT
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7 Corte transversal de uma PMT
Um getter (termo oriundo de apanhador, capturador ou ainda absorvente em língua inglesa) é um depósito de material reativo que é deliberadamente colocado dentro do sistema de vácuo, com o propósito de completar e manter o vácuo. Quando as moléculas colidem com o material do gett, combinam-se com ele quimicamente ou por absorção. Então o getter remove pequenas quantidades de gás do espaço evacuado. dispoptic 2013

8 Algumas características dos dinodos
Muitos materiais emitem, em média, d novos elétrons por cada elétron que colide na sua superfície. Se a energia cinética do elétron incidente for suficientemente energética, entre 100 e 200 eV, então d > 1, teremos amplificação Assim teremos para N dinodos a geração de dN elétrons dispoptic 2013

9 dispoptic 2013

10 Superfícies fotoemissivas de PMT’s
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11 Outros detectores dispoptic 2013

12 Multiplicador de elétrons ou channeltron
A channeltron is effectively a photomultiplier with an infinite number of dynodes. It consists of a hollow, evacuated tube (the 'channel'), generally curved, of a few mm section and a few cm long, made of semiconducting glass: its inner surface acts as both, the photocathode and a continuous dynode. Channeltrons are used to detect optical, UV, X-ray light, electrons and ions (e.g. they were used as X-ray detectors on the Copernicus satellite, and in early MSSL magnetospheric experiments). Basically, when light, or charged particles, strike the channeltron's inner surface close to the entrance, electrons are expelled from it. An electric field, established by applying a positive voltage of several kV between the ends of the tube, accelerates the secondary electrons emitted at each point, they hit the walls again and again generating more and more secondary electrons; finally, the cloud of electrons is collected at the anode. The channeltron is curved to prevent 'ion feedback': this takes place when residual gas molecules, still present even if the device is run in vacuum, are ionised, becoming positive ions, are accelerated opposite to the electrons, reach the front end and release electrons which may produce spurious signals. dispoptic 2013

13 channeltron Voltagem de entrada -3000 V Defletores +- 400V
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14 http://www. olympusmicro
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15 Outra forma do Channeltron
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16 E a superfície para emissão?
Microchanneltron? Shown below is a cross section of the active surface of a Channeltron detector. Secondary emission occurs within the first 200 angstrom of the surface. Below this is a conductive layer that can be several thousands of angstrom thick. Surface & Plasma Technology Research Group of the Institut für Allgemeine Physik A variety of x-ray light sources is used in XPS studies, comprising the standard MgKα/AlKα twin anode laboratory source (shown in the Figure), (monochromated) rotating anode sources, UV lamps, or synchrotron light sources. The standard laboratory source shown in the Figure contains an Al/Mg twin-anode which is bombarded by electrons emitted from a filament that is held at ground potential. The anode is positively biased at ~15keV, in this way accelerating nearby electrons towards it. These electrons produce core holes in the anode target materials by electron impact ionization. The vacancies can relax by emission of characteristic x-rays that illuminate the sample. By switching the filament, AlKα (Ex=1485eV, ΔEFWHM= 0.85eV) or MgKα (Ex=1254eV, ΔEFWHM= 0.7eV). The observed line width is a consequence of the different x-ray lines contributing. For most line sources like the one shown in the Figure, the emission of Bremsstrahlung together with the desired characteristic x-ray line cannot be avoided and produces a broad (but weak) background in the spectrum. A thin Al-window prevents secondary and backscattered electrons to reach the sample. Since considerable power is dissipated in the anode ( W), the anode needs to be water cooled and must therefore be a good heat conductor (e.g. a metal). The anode top is silver coated to prevent Cu x-rays ghost lines from complicating the spectra when the anode material gets thin in the course of time. For high resolution studies of chemical states the quoted linewidths of the exciting radiation of ~1eV are sometimes prohibitively large. In such cases, monochromatized sources must be used. One way to achieve monochromatization of x-rays is to disperse the x-ray spectrum in a diffraction grating (a crystal). Since selection of a small energy band in a monochromator implies discarding a considerable fraction of the x-ray intensity, the anode must be operated at high power. Thus, cooling becomes a critical issue. For this reason, rotating anode x-ray sources are used in monochromated laboratory x-ray sources. The laboratory x-ray sources adressed above emit photons at a fixed energy. Very often it is desirable to be able to tune the energy of the incident light in a photoelectron emission experiment. The most convenient way to produce light with frequencies tunable over a certain range is to use an intense source with a broad continuous spectrum and employing a monochromator to select the desired frequency. Such a source is realized in a synchrotron for energies between UV and hard x-rays (~30keV). When a charged particle in a storage ring is deflected (accelerated) to maintain its trajectory within the torus, it emits Bremsstrahlung. This process is particularly effective for electrons owing to their small resting mass. In a synchrotron, electrons are accelerated to velocities approaching the speed of light, and an intense Bremsstrahlungsspectrum is emitted. It extends down in energy to the UV-regime, while the intensity falls off rapidly above a maximum energy determined by the critical energy of the synchrotron-a design parameter. The emitted spectrum is highly collimated to within a fraction of a mrad along the tangential direction of the torus. Therefore such a light beam is ideally suited for monochromatization. Indeed, the intensity of photon beams (after monochromatization) produced in this way may exceed those of laboratory sources by up to 10 orders of magnitude. Furthermore, different polarization states of the light can be produced (linear polarization near the center of the beam in the central plane of the torus, circular polarization off-plane). Another useful feature of synchrotron light is that it is produced by bunches of electrons orbiting the torus (bunch duration~100ps) and therefore the light has a well defined time structure that can be advantageous in e.g. coincidence experiments. Common electron guns in electron spectroscopy can produce a beam of medium energy electrons (~50eV-50keV) with a beam current of typically some nanoamperes and an energy spread less than an eV. The two most popular types of electron guns are the thermionic emission gun (shown schematically in the figure) and the field emission gun. In thermionic emission the mean energy of the electron gas in the solid is increased by resistively heating the filament providing a certain fraction of the loosely bound electrons with an energy exceeding the work function barrier. The cathode material in this case must have a high melting point and a possibly low work function. These requirements are met by Tungsten that is often used as filament. Thoria coated metals as well as LaB6 single crystals have still a lower work function and are also used. At the core of each apparatus for electron spectrometry is a component that performs the actual selection of the electron energy or speed. The most common analyzers in use today are electrostatic analyzers: the cylindrical mirror analyzer (CMA) and the hemispherical mirror analyzer (HMA) that is sometimes referred to as concentrical hemispherical analyzer (CHA). The retarding field analyzer (RFA) that is employed to acquire low energy electron diffraction data is sometimes also used for Auger electron spectroscopy, mainly for qualitative purposes of checking sample cleanliness but is not optimally suited for electron spectroscopy in general. The Figure schematically shows the CHA and CMA type analyzers. In a CHA, an electrostatic field is applied between an inner and outer hemisphere Electrons with an energy equal to the pass energy that are injected through the entrance slit tangentially to the median hemispherical surface are transferred from the entrance slit to the detector. They then describe a circular trajectory on the median hemispherical surface The potential difference between the hemispheres determines the pass energy. The energy resolution of the CHA is determined by the radius and the width of the slits at the analyzer entrance and exit. A lens is used to transfer electrons from the sample to the entrance slit. The electron optics in the transfer lens assembly also contain an element that retards or accelerates electrons of a given energy to the pass energy. The polar opening angle $\Delta\alpha$ is of the order of several degrees and the solid angle is therefore rather small. For example for a 12° full polar opening angle the solid angle is approximately ΔΩ= 2πxcos 12 °=0.01 x 4 π. Therefore the best attainable transmission of such an analyzer would be ~1%. The analyzer shown schematically in panel b of the figure is a double pass CMA. It consists of two concentric cylinders. The inner cylinder is held at ground potential while the outer cylinder is at a certain negative potential. The electric field between the cylinders forces the electrons entering it to describe trajectories with a radius depending on their energy and the field in the analyzer. Only those electrons with a given energy, the so-called pass energy, are focussed onto the electron detector and are registered by the acquisition electronics. The entrance aperture of the CMA is annular in shape.The aperture is usually located at a polar angle of θa=42°.15'.5''. For this special geometry a second order focus is formed at the exit slit of each stage of the analyzer. In this case the pass energy is related to the potential V at the outer cylinder and the radii of the cylinders. The (half polar) angular width of the annulus can be made as wide as Δα=6° without noticeably deteriorating the energy resolution. Therefore, the solid angle of acceptance of a CMA is quite large, ΔΩ~ 2π(cos 36-cos 48)=0.14 x 4π. In consequence, the transmission of a CMA is relatively high and would optimally amount to ~14%. This may constitute a major advantage over the CHA if sensitivity is a critical issue. Two drawbacks of the CMA should be mentioned: the intensity as well as the energy calibration depend strongly on the sample analyzer distance making very careful adjustment of the sample necessary. This poses significant geometrical limitations to an apparatus containing a CMA for energy selection: while in the case of a CHA a lens can be used to transfer the signal electrons to the spectrometer entrance so that the distance between the analyzer and the sample can be of the order of some tens of cm, this is strictly impossible in the case of the CMA. Recording of an electron energy spectrum, i.e. the energy distribution of particles leaving the specimen, can be performed in two modes, the so-called constant analyzer energy (CAE) and constant retard ratio (CRR) mode. In the CAE mode, the electric field in the analyzer, and thus the analyzer pass energy, is kept constant, and a variable retarding voltage is applied to retardation grids (or the decelaration element in the transfer lens of a CHA). In this way, only electrons that leave the target with an energy E can pass through the analyzer before they are detected. By recording the number of detected electrons as a function of the retarding voltage the electron spectrum is obtained. Alternatively, an electron energy distribution may be obtained by variation of the pass energy, keeping the ratio between the pass energy and the retardation voltage constant. This is done in the CRR mode. Since the spatial divergence of the electron trajectories in the analyzer increases with decreasing pass energy, the energy resolution in the CRR mode is proportional to the detected energy Δ E α E, whereas in the CAE mode the energy resolution is constant over the entire spectrum. The CAE mode is therefore usually used in XPS since a constant energy resolution over the entire spectrum is desirable in this case. For AES, on the other hand, energy resolution of the spectrometer is not crucial since the Auger peaks generally exhibit a rather large intrinsic linewidth of >1eV. Furthermore, for electron excited AES, the peaks are superimposed on a broad and intense background of inelastically backscattered primary electrons rising monotonically with kinetic energy. Therefore, the important issue in electron excited AES is the signal to background ratio being important for the sensitivity rather than energy resolution. Then it is advantageous to operate the spectrometer in the CRR mode. Generally, when electron excited AES uses fine focussed electron beams to perform nanoanalysis, the signal may originate only from a very small volume. This again emphasizes the importance of high transmission spectrometers for AES. Since furthermore neither energy nor angular resolution is essential in most applications, a CMA is commonly used for AES. Recently, however, CHAs with special transfer lens systems have become available for which the transmission is comparable to that of the CMA. For XPS, the situation is just the opposite: usually a good energy resolution is crucial, sometimes angular resolution is desired, but sensitivity is much less an issue than in AES since the area of the sample from which the signal electrons originate is always rather large while the signal to background ratio is almost always much better than for electron excited AES where the background of inelastically backscattered primaries dominates the entire spectrum. A channeltron, shown in panel a of the figure is often used as electron detector. This is a bent tube that is coated with an insulator with a high secondary electron coefficient. Over this tube, a potential of about 2.5kV is applied. When an electron strikes the mouth of the tube, a number of secondaries is produced that is accelerated in the channeltron. These accelerated secondaries in turn produce secondaries and so on. In effect, a large number of secondaries is produced. The energy required to produce these secondaries is supplied by the channeltron voltage supply. In this way a pulse is produced that indicates the arrival of an electron in the detector. This pulse is then shaped and ultimately registered by the acquisition electronics. In a similar way, pulses of electrons are produced in the microchannels of a microchannel plate (MCP) (panel b) When this device is used in combination with a position sensitive anode, electrons can be detected with a lateral resolution of some tens of a micron. In view of the high surface sensitivity of techniques like AES, it is necessary to conduct experiments at low ambient pressures (UHV, ultra high vacuum p~ mbar) in order to avoid contamination of the surface under study by absorption of ambient atoms or molecules. Ambient pressures in this range can be attained routinely in a modern UHV system. Such a system generally consists of a stainless steel vessel to which all the required components are attached through flanges that are leak sealed by means of copper gaskets. The apparatus must be evacuated down to the desired pressure by a system of pumps. Since the walls of the apparatus are always covered with water vapour when the system has been to air and the desorption of this vapour is a very slow process (of the order of weeks at room temperature), the entire system must be baked to above the boiling point of water overnight. Monitoring of the pressure is performed by means of various pressure gauges and sometimes a quadrupole mass spectrometer is used to study the composition of the resiudual gas in detail. The Figure shows the pressure ranges in which the different types of pumps and pressure gauges can be operated. Contents » E a superfície para emissão? dispoptic 2013

17 channeltron dispoptic 2013

18 Aplicação? dispoptic 2013

19 dispoptic 2013 MCP = MicroChannel Plate
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20 Aplicação dispoptic 2013

21 Aplicação? Visão noturna E.I.T. = Enhanced Image Tube dispoptic 2013

22 Espectroscopia emissiva de elétrons - com raios-X ou canhão de ions
Modo Espectral – Área analisada de 2x1mm até 15 mícrons de diâmetro. –energia: 5 to 160 eV –8 channeltron multi-detector Hemispherical Electron Energy Analyser ( ) dispoptic 2013

23 Fotodiodo de silício dispoptic 2013

24 Detectores fotovoltaicos
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25 Resposta espectral de D* de detectores fotovoltaicos
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26 O mesmo principio é usado na obtenção de células solares
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27 http://www.specmat.com/Overview of Solar Cells.html
A = encapsulamento que sela a célula do ambiente externo, pode ser vidro ou plástico B = grade de contato, bom material condutor, que pode servir como coletor de elétrons C = Camada Anti-Refletora, com índice de refração e espessura apropriados, guia a luz para dentro da célula, evitando que a luz incidente seja refletida para fora da célula D = silício tipo -n (e.g. P ou As como impurezas) E = silício tipo -p (e.g. B) F = contato metálico recobrindo toda a base da célula HOW SOLAR CELLS WORK - SOLAR CELL OVERVIEW SOLAR CELLS, How They Work The solar cell offers a limitless and environmentally friendly source of electricity. The solar cell, is able to create electricity directly from photons. A photon can be thought of as a packet of light and the amount of energy in a photon is proportional to the wavelength of light. A. Encapsulate - The encapsulate, made of glass or other clear material such clear plastic, seals the cell from the external environment. B. Contact Grid- The contact grid is made of a good conductor, such as a metal, and it serves as a collector of electrons. C. The Antireflective Coating (AR Coating)- Through a combination of a favorable refractive index, and thickness, this layer serves to guide light into the solar cell. Without this layer, much of the light would simply bounce off the surface. D. N-Type Silicon - N-type silicon is created by doping (contaminating) the Si with compounds that contain one more valence electrons* than Si does, such as with either Phosphorus or Arsenic. Since only four electrons are required to bond with the four adjacent silicon atoms, the fifth valence electron is available for conduction. E. P-Type Silicon- P-type silicon is created by doping with compounds containing one less valence electrons* than Si does, such as with Boron. When silicon (four valence electrons) is doped with atoms that have one less valence electrons (three valence electrons), only three electrons are available for bonding with four adjacent silicon atoms, therefore an incomplete bond (hole) exists which can attract an electron from a nearby atom. Filling one hole creates another hole in a different Si atom. This movement of holes is available for conduction. F. Back Contact - The back contact, made out of a metal, covers the entire back surface of the solar cell and acts as a conductor. *[ A valence electron is an electron found in the outermost electron shell. An element containing more valence electrons will try to donate valence electrons to an element containing fewer valence electrons.] * A photon's  path through the solar cell. After a photon makes its way through the encapsulate it encounters the antireflective layer. The antireflective layer channels the photon into the lower layers of the solar cell. Click on the following link if you would like to learn about SPECMAT's novel room temperature wet chemical growth antireflective layer (RTWCG - AR). Once the photon passes the antireflective layer, it will either hit the silicon surface of the solar cell or the contact grid metallization. The metallization, being opaque, lowers the number of photons reaching the Si surface. The contact grid must be large enough to collect electrons yet cover as little of the solar cell's surface, allowing more photons to penetrate. A Photon causes the Photoelectric Effect*. The photon's energy transfers to the valence electron of an atom in the n-type Si layer. That energy allows the valence electron to escape its orbit leaving behind a hole. In the n-type silicon layer, the free electrons are called majority carriers whereas the holes are called minority carriers. As the term "carrier" implies, both are able to move throughout the silicon layer of the solar cell, and so are said to be mobile. Inversely, in the p-type silicon layer, electrons are termed minority carriers and holes are termed majority carriers, and of course are also mobile. *[ The photoelectric effect is simply defined as an experimentally measurable effect where a metal emits electrons when hit by photons..] * The p-n junction. The region in the solar cell where the n-type and p-type Si layers meet is called the p-n junction. As you may have already guessed, the p-type silicon layer contains more positive charges, called holes, and the n-type silicon layer contains more negative charges, or electrons. When p-type and n-type materials are placed in contact with each other, current will flow readily in one direction (forward biased) but not in the other (reverse biased). An interesting interaction occurs at the p-n junction of a darkened solar cell. Extra valence electrons in the n-type layer move into the p-type layer filling the holes in the p-type layer forming what is called a depletion zone. The depletion zone does not contain any mobile positive or negative charges. Moreover, this zone keeps other charges from the p and n-type layers from moving across it. So, to recap, a region depleted of carriers is left around the junction, and a small electrical imbalance exists inside the solar cell. This electrical imbalance amounts to about 0.6 to 0.7 volts. So due to the p-n junction, a built in electric field is always present across the solar cell. P = V × I When photons hit the solar cell, freed electrons (-) attempt to unite with holes on the p-type layer. The p-n junction, a one-way road, only allows the electrons to move in one direction. If we provide an external conductive path, electrons will flow through this path to their original (p-type) side to unite with holes. The electron flow provides the current ( I ), and the cell's electric field causes a voltage ( V ). With both current and voltage, we have power ( P ), which is just the product of the two. Therefore, when an external load (such as an electric bulb) is connected between the front and back contacts, electricity flows in the cell, working for us along the way. dispoptic 2013

28 Numa maneira ilustrativa
HOW SOLAR CELLS WORK - SOLAR CELL OVERVIEW SOLAR CELLS, How They Work The solar cell offers a limitless and environmentally friendly source of electricity. The solar cell, is able to create electricity directly from photons. A photon can be thought of as a packet of light and the amount of energy in a photon is proportional to the wavelength of light. A. Encapsulate - The encapsulate, made of glass or other clear material such clear plastic, seals the cell from the external environment. B. Contact Grid- The contact grid is made of a good conductor, such as a metal, and it serves as a collector of electrons. C. The Antireflective Coating (AR Coating)- Through a combination of a favorable refractive index, and thickness, this layer serves to guide light into the solar cell. Without this layer, much of the light would simply bounce off the surface. D. N-Type Silicon - N-type silicon is created by doping (contaminating) the Si with compounds that contain one more valence electrons* than Si does, such as with either Phosphorus or Arsenic. Since only four electrons are required to bond with the four adjacent silicon atoms, the fifth valence electron is available for conduction. E. P-Type Silicon- P-type silicon is created by doping with compounds containing one less valence electrons* than Si does, such as with Boron. When silicon (four valence electrons) is doped with atoms that have one less valence electrons (three valence electrons), only three electrons are available for bonding with four adjacent silicon atoms, therefore an incomplete bond (hole) exists which can attract an electron from a nearby atom. Filling one hole creates another hole in a different Si atom. This movement of holes is available for conduction. F. Back Contact - The back contact, made out of a metal, covers the entire back surface of the solar cell and acts as a conductor. *[ A valence electron is an electron found in the outermost electron shell. An element containing more valence electrons will try to donate valence electrons to an element containing fewer valence electrons.] * A photon's  path through the solar cell. After a photon makes its way through the encapsulate it encounters the antireflective layer. The antireflective layer channels the photon into the lower layers of the solar cell. Click on the following link if you would like to learn about SPECMAT's novel room temperature wet chemical growth antireflective layer (RTWCG - AR). Once the photon passes the antireflective layer, it will either hit the silicon surface of the solar cell or the contact grid metallization. The metallization, being opaque, lowers the number of photons reaching the Si surface. The contact grid must be large enough to collect electrons yet cover as little of the solar cell's surface, allowing more photons to penetrate. A Photon causes the Photoelectric Effect*. The photon's energy transfers to the valence electron of an atom in the n-type Si layer. That energy allows the valence electron to escape its orbit leaving behind a hole. In the n-type silicon layer, the free electrons are called majority carriers whereas the holes are called minority carriers. As the term "carrier" implies, both are able to move throughout the silicon layer of the solar cell, and so are said to be mobile. Inversely, in the p-type silicon layer, electrons are termed minority carriers and holes are termed majority carriers, and of course are also mobile. *[ The photoelectric effect is simply defined as an experimentally measurable effect where a metal emits electrons when hit by photons..] * The p-n junction. The region in the solar cell where the n-type and p-type Si layers meet is called the p-n junction. As you may have already guessed, the p-type silicon layer contains more positive charges, called holes, and the n-type silicon layer contains more negative charges, or electrons. When p-type and n-type materials are placed in contact with each other, current will flow readily in one direction (forward biased) but not in the other (reverse biased). An interesting interaction occurs at the p-n junction of a darkened solar cell. Extra valence electrons in the n-type layer move into the p-type layer filling the holes in the p-type layer forming what is called a depletion zone. The depletion zone does not contain any mobile positive or negative charges. Moreover, this zone keeps other charges from the p and n-type layers from moving across it. So, to recap, a region depleted of carriers is left around the junction, and a small electrical imbalance exists inside the solar cell. This electrical imbalance amounts to about 0.6 to 0.7 volts. So due to the p-n junction, a built in electric field is always present across the solar cell. P = V × I When photons hit the solar cell, freed electrons (-) attempt to unite with holes on the p-type layer. The p-n junction, a one-way road, only allows the electrons to move in one direction. If we provide an external conductive path, electrons will flow through this path to their original (p-type) side to unite with holes. The electron flow provides the current ( I ), and the cell's electric field causes a voltage ( V ). With both current and voltage, we have power ( P ), which is just the product of the two. Therefore, when an external load (such as an electric bulb) is connected between the front and back contacts, electricity flows in the cell, working for us along the way. dispoptic 2013

29 Cálculo de célula solar
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30 Eficiência da célula solar
Como era considerado: Material Efficiency in lab (%) Efficiency of production cell (%) monocrystalline silicon about 24 14-17 polycrystalline silicon about 18 13-15 amorphous silicon about 13 5-7 É um tema atual, pode-se dizer que existe até publicações indicando até 100% de eficiência? The material is comprised of a hybrid of plastics, molybdenum and titanium. Outro: dispoptic 2013

31 Eficiência de células solares
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32 Será que parou nos ~40% de eficiência?
New Solar Cell Technology Greatly Boosts Efficiency ScienceDaily (Apr. 29, 2011) — With the creation of a 3-D nanocone-based solar cell platform, a team led by Oak Ridge National Laboratory's Jun Xu has boosted the light-to-power conversion efficiency of photovoltaics by nearly 80 percent. The technology substantially overcomes the problem of poor transport of charges generated by solar photons. These charges -- negative electrons and positive holes -- typically become trapped by defects in bulk materials and their interfaces and degrade performance. "To solve the entrapment problems that reduce solar cell efficiency, we created a nanocone-based solar cell, invented methods to synthesize these cells and demonstrated improved charge collection efficiency," said Xu, a member of ORNL's Chemical Sciences Division. The new solar structure consists of n-type nanocones surrounded by a p-type semiconductor. The n-type nanoncones are made of zinc oxide and serve as the junction framework and the electron conductor. The p-type matrix is made of polycrystalline cadmium telluride and serves as the primary photon absorber medium and hole conductor. With this approach at the laboratory scale, Xu and colleagues were able to obtain a light-to-power conversion efficiency of 3.2 percent compared to 1.8 percent efficiency of conventional planar structure of the same materials. "We designed the three-dimensional structure to provide an intrinsic electric field distribution that promotes efficient charge transport and high efficiency in converting energy from sunlight into electricity," Xu said. Key features of the solar material include its unique electric field distribution that achieves efficient charge transport; the synthesis of nanocones using inexpensive proprietary methods; and the minimization of defects and voids in semiconductors. The latter provides enhanced electric and optical properties for conversion of solar photons to electricity. Because of efficient charge transport, the new solar cell can tolerate defective materials and reduce cost in fabricating next-generation solar cells. "The important concept behind our invention is that the nanocone shape generates a high electric field in the vicinity of the tip junction, effectively separating, injecting and collecting minority carriers, resulting in a higher efficiency than that of a conventional planar cell made with the same materials," Xu said. Research that forms the foundation of this technology was accepted by this year's Institute of Electrical and Electronics Engineers photovoltaic specialist conference and will be published in the IEEE Proceedings. The papers are titled "Efficient Charge Transport in Nanocone Tip-Film Solar Cells" and "Nanojunction solar cells based on polycrystalline CdTe films grown on ZnO nanocones." The research was supported by the Laboratory Directed Research and Development program and the Department of Energy's Office of Nonproliferation Research and Engineering. Other contributors to this technology are Sang Hyun Lee, X-G Zhang, Chad Parish, Barton Smith, Yongning He, Chad Duty and Ho Nyung Lee. dispoptic 2013

33 51,8% Towards an optimized all lattice-matched InAlAs/InGaAsP/InGaAs multijunction solar cell with efficiency >50%. Appl. Phys. Lett. 102, (2013). (Phys.org)—Scientists have designed a new multijunction solar cell that, in simulations, can achieve an efficiency of 51.8%. This high performance exceeds the current goal of 50% efficiency in multijunction solar cell research as well as the current world record of 43.5% for a 3-junction solar cell. dispoptic 2013

34 Entretanto o girassol 1.291 mirrored heliostats and a 54 story high tower the World's largest solar power tower plant located near Seville in Spain in now on line generating 20 megawatts (MW) of electricity, enough to supply 10,000 homes. dispoptic 2013

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