Lichtenberg was the first Payload Specialist. He flew on Spacelab-1 (STS-9) mission for ten days in 1983, conducted multiple experiments in life sciences, materials sciences, Earth observations, astronomy and solar physics, upper atmosphere and plasma physics. His second flight was ATLAS-1 (STS-45) Spacelab mission for nine days in 1992; conducted 13 experiments in Atmospheric sciences and astronomy. He flew 310 orbits, and logged 468 hours in space.

Adapted from the Wikipedia article Byron K. Lichtenberg, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Since it was formed, the MSSL has worked on a number of different solar physics hardware projects. Its earliest involvement came with an experiment on [http://www.nasm.si.edu/research/dsh/artifacts/SS-ariel1.htm Ariel-I] that made the first spectroscopic X-ray observations of solar flares. Other instruments were later flown on the [http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/oso4.html OSO-4], [http://imagine.gsfc.nasa.gov/docs/sats_n_data/missions/esro2b.html ESRO-II], [http://imagine.gsfc.nasa.gov/docs/sats_n_data/missions/oso5.html OSO-5], and [http://imagine.gsfc.nasa.gov/docs/sats_n_data/missions/oso6.html OSO-6] missions. Instruments built have also flown on the Solar Maximum Mission (SMM), the Coronal Helium Abundance Spacelab Experiment (CHASE) which was part of the [http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/spacelab2.html Spacelab-2] missions. More recently, MSSL has played a significant role in the [http://swift.gsfc.nasa.gov/docs/swift/swiftsc.html Swift] mission. MSSL engineers participated in building and testing the [http://www.mssl.ucl.ac.uk/pages/general/projects/swift/uvot.htm Ultraviolet Optical Telescope] (UVOT) instrument on Swift.

In the future MSSL plans to participate in the [http://msslxr.mssl.ucl.ac.uk:8080/SolarB/ Solar-B] and STEREO missions. For Solar-B, MSSL is leading a consortium building the EUV Imaging Spectrometer (EIS) [http://msslxr.mssl.ucl.ac.uk:8080/SolarB/] which will provide plasma diagnostic in the solar chromosphere, transition region and corona.

Adapted from the Wikipedia article Mullard Space Science Laboratory, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The following discussion is based primarily on Mayer et al.’s review, cited below. Organic photovoltaics are made of electron donor and electron acceptor materials rather than semiconductor p-n junctions. The molecules forming the electron donor region of organic PV cells, where exciton electron-hole pairs are generated, are generally conjugated polymers possessing delocalized π electrons that result from carbon p orbital hybridization. These π electrons can be excited by light in or near the visible part of the spectrum from the molecule’s highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), denoted by a π -π* transition. The energy bandgap between these orbitals determines which wavelength of light can be absorbed.

Unlike in an inorganic crystalline PV cell material, with its band structure and delocalized electrons, excitons in organic photovoltaics are strongly bound with an energy between 0.1 and 1.4 eV. This strong binding occurs because electronic wavefunctions in organic molecules are more localized, and electrostatic attraction can thus keep the electron and hole together as an exciton. The electron and hole can be dissociated by providing an interface across which the chemical potential of electrons decreases. The material that absorbed the photon is the donor, and the material acquiring the electron is called the acceptor. In Fig. 2, the polymer chain is the donor and the fullerene is the acceptor. After dissociation has taken place, the electron and hole may still be joined as a geminate pair and an electric fiel

After exciton dissociation, the electron and hole must be collected at contacts. However, charge carrier mobility now begins to play a major role: if mobility is not sufficiently high, the carriers will not reach the contacts, and will instead recombine at trap sites or remain in the device as undesirable space charges that oppose the drift of new carriers. The latter problem can occur if electron and hole mobilities are highly imbalanced, such that one species is much more mobile than the other. In that case, space-charge limited photocurrent (SCLP) hampers device performance.

As an example of the processes involved in device operation, organic photovoltaics can be fabricated with an active polymer and a fullerene-based electron acceptor. The illumination of this system by visible light leads to electron transfer from the polymer chain to a fullerene molecule. As a result, the formation of a photoinduced quasiparticle, or polaron (P+), occurs on the polymer chain and the fullerene becomes an ion-radical C60- Polarons are highly mobile along the length of the polymer chain and can diffuse away. Both the polaron and ion-radical possess spin ”S”= ½, so the charge photoinduction and separation processes can be controlled by the Electron Paramagnetic Resonance method.

Adapted from the Wikipedia article Polymer solar cell, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The Physical Research Laboratory holds various seminars and public lectures. It has a workshop, computer centre, library and various other laboratories. It also offers a five-year doctoral programme in Physics, with specializations in Theoretical Physics and Complex Systems, Space and Atmospheric Sciences, Quantum optics and Quantum Information, Astronomy and Astrophysics (Infrared, Sub-mm and Radio Astronomy, Solar Physics, Planetary and Geosciences). The admission is through a written test/interview.

Adapted from the Wikipedia article Physical Research Laboratory, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The fundamental difference between multi-junction solar cells and c-Si solar cells is that there are several pn junctions connected in series instead of one, as illustrated on figure C(a). In order to better cover the solar spectrum AM1.5 (terrestrial reference spectrum for photovoltaic performance evaluation), we must choose suitable materials for each pn junction.

First, their band gaps E_g have to be significantly different so that photons of different wavelength lambda = frac{h c}{e E_g} approx frac{1.24 cdot 10{E_g} are absorbed. The figure C(b) illustrates this fact by plotting spectral irradiance G(lambda), which is the source power density at a given wavelength lambda. Secondly, E_g must decrease from top cell to bottom cell so that photons would not all be absorbed by the top cell. Thirdly, the layers have to be electrically optimal for high performances and be stacked on each other. Thereby, we have to find materials with better absorption coefficients alpha(lambda), higher minority carrier lifetimes tau_{minority}, higher mobilities mu and similar lattice constants l_c. The values in Table below justifie the choice of the usual materials used for multi-junction solar cells : the top cell is generally made in InGaP (E_g = 1.86 eV), the middle cell in InGaAs (E_g = 1.4 eV) and the bottom cell in Ge (E_g = 0.65 eV). The use of Germanium Ge is mainly due to its robustness, its low cost and its facility in production. In addition to the benefits listed above, the use of GaAs instead of Si allows to reach higher efficiencies because the efficiency of GaAs cells is 25.1% w

Because the different layers are closely lattice-matched, the fabrication of the device is usually obtained by using the Metal-Organic Chemical Vapour Deposition (MOCVD). This technique is preferable to the Molecular Beam Epitaxial growth (MBE) because it ensures high crystal quality and large scale production .

{| class=”wikitable”

|-

! Material

! E_g [eV]

! l_c [nm]

! absorption (lambda = 0.8 mu m) [mum}{}{}{}{}{

The different layers of the structure

Metallic contacts

The metallic contacts in aluminium are low resistivity electrodes that make contact with the semiconductor layer in GaAs. They are disposed on the two sides of the structure but mainly on the backwards face so that the shadowing on the lightning surface is reduced.

Anti-Reflective (AR) Coating

The Anti-Reflective (AR) coating is generally composed of several layers in the case of MJ solar cells. The top AR layer has usually a NaOH surface texturation with several pyramids in order to increase the transmittion coefficient T, the trapping of the light in the material (because photons cannot easily get out the MJ structure due to pyramids) and therefore, the path length of photons in the material . On the one hand, the thickness of each AR layer is chosen to get destructive interferences. Therefore, the reflection coefficient R decreases to 1%. In the case of two AR layers L_1 (the top layer usually in SiO2) and L_2 (usually in TiO2), we must have n_{L2} = n_{AlInP}^{1/2} cdot n_{L1} to get the same amplitudes for reflected fields and n_{L1} cdot d_{L1} = 4 lambda_{min}, n_{L2} cdot d_{L2} = frac{lambda{4} to have opposite phase for reflected fields . On the other hand, the thickness of each AR layer is also chosen to minimize the reflectance at wavelengths for which the photocurrent is the lowest. Consequently, this maximizes J_{SC} by matching currents of the three subcells . As example, because the current generated by the bottom cell is greater than the currents generated by the other cells, we ajust the thickness of AR layers so that the Infra-Red (IR) transmission (which corresponds to the bottom cell) is degraded while the UV transmission (which corresponds to the top cell) is upgraded. Particularly, an AR coating is very important at low wavelengths because, without it, T would be strongly reduced to 70%.

Tunnel Junctions

The main goal of tunnel junctions is to provide a low electrical resistance and optically low-loss connection between two subcells . Without it, the p-doped region of the top cell would be directly connected with the n-doped region of the middle cell. Hence, a pn junction with opposite direction to the others would appear between the top cell and the middle cell. Consequently, the photovoltage would be lower than if there would be no parasitic diode. In order to decrease this effect, we use a tunnel junction . It's simply a wide band gap highly doped diode. The high doping reduces the length of the depletion region because l_{depl} = sqrt{fra}{}{}{. Hence, electrons can easily tunnel through the depletion region as illustrated in figure [TunnelJunction:]. Now, the J-V characteristic of the tunnel junction is very important because it explains why tunnel junctions can be used to have a low electrical resistance connection between two pn junctions. The figure D shows three different regions : the tunneling region, the negative differential resistance region and the thermal diffusion region. The region where electrons can tunnel through the barrier is called the tunneling region. There, the voltage must be low enough so that energy of some electrons who are tunneling is equal to energy states available on the other side of the barrier. Consequently, current density through the tunnel junction is high (with maximum value of J_P, the peak current density) and the slope near the origin is therefore steep. Then, the resistance is extremely low and consequently, the voltage too . This is why tunnel junctions are ideal for connecting two pn junctions without having a voltage drop. When voltage is higher, electrons cannot cross the barrier because energy states are no longer available for electrons. Therefore, the current density decreases and the differential resistance is negative. The last region, called thermal diffusion region, corresponds to the J-V characteristic of the usual diode : J = J_S (exp(frac{qV}{kT}) – 1). In order to avoid the reduction of the MJ solar cell performances, tunnel junctions must be transparent to wavelengths absorbed by the next photovoltaic cell, the middle cell, i.e. E_{gTunnel} > E_{gMiddleCell}.

Window Layer and Back-Surface Field (BSF)

In order to reduce the surface recombination velocity S, we insert a window layer. Similarly, we insert a Back-Surface Field (BSF) layer to reduce the scattering of carriers towards the tunnel junction. The structure of these two layers is the same : it’s a heterojunction which catches electrons (resp. holes). Indeed, despite the electric field E_d, these one cannot jump above the barrier formed by the heterojunction because they don’t have enough energy, as illustrated in figure E. Hence, electrons (resp. holes) cannot recombine with holes (resp. electrons) and cannot diffuse through the barrier. By the way, window and BSF layers must be transparent to wavelengths absorbed by the next pn junction i.e. E_{gWindow} > E_{gEmitter} and E_{gBSF} > E_{gEmitter}. Furthermore, the lattice constant must be close to the one of InGaP and the layer must be highly doped (n geq 10^{18} cm^{-3}) .

J-V Characteristic

To obtain the maximum efficiency, the functioning point of each subcell has to be the point where the power supply is maximum. But, currents at these points are not necessarily equal for each subcell. If they are different, the total current through the solar cell is the lower of the three. By approximation , we get the same relationship for the short-circuit current of the MJ solar cell : J_{SC} = min { J}{}{ where J_{SCi}(lambda) is the short-circuit current density at a given wavelength lambda for the subcell i.

Because of the impossibility to obtain J_{SC1}, J_{SC2}, J_{SC3} directly from the total J-V characteristic, we use the Quantum Efficiency QE(lambda). It simply measures the ratio between the amount of electron-hole pairs created and the incident photons at a given wavelength lambda as illustrated in figure F. Let phi_i(lambda) be the photon flux of corresponding incident light in subcell i and let QE_i(lambda) be the quantum efficiency of the subcell i. By definition, we get : QE_i(lambda) = frac{J{q phi_i(lambda)} Rightarrow J_{SCi} = int_{0}^{lambda2} q phi_i(lambda) QE_i(lambda) d lambda

The value of QE_i(lambda) is obtained by linking it with the absorption coefficient alpha(lambda), i.e. the number of photons absorbed per unit of length by a material. If we assume that each photon absorbed by a subcell creates an electron/hole pair (which is a good approximation), we can write : QE_i(lambda) = 1 – e^{-alpha(lambda) d_i} where d_i is the thickness of the subcell i and e^{-alpha(lambda) d_i} is the percentage of incident light which is not absorbed by the subcell i.

Similarly, because V = sum_{i=1}^3 V_i, we can write the following approximation : V_{OC} = sum_{i=1}^3 V_{OCi}. The values of V_{OCi} are then given by the J-V diode equation : J_i = J_{0i} (e^{fra}{-1)-J_{SCi} Rightarrow V_{OCi} approx frac{kT}{q} ln(frac{J{J)

Adapted from the Wikipedia article Multijunction solar cell, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Peter Andrew Sturrock (born 1924) is a British scientist.

An emeritus professor of applied physics at Stanford University, much of Sturrock’s career has been devoted to astrophysics, plasma physics, and solar physics, but Sturrock is interested in other fields, including ufology, scientific inference and in the history of science and philosophy of science. Sturrock has been awarded many prizes and honors, and has written or co-authored many scientific articles and textbooks.

Adapted from the Wikipedia article Peter A. Sturrock, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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While studying at Caltech, Gibson was a research assistant in the field of jet propulsion and classical physics. His technical publications are in the fields of plasma physics and solar physics. He was senior research scientist with the Applied Research Laboratories of Philco Corporation at Newport Beach, California, from June 1964 until coming to NASA. While at Philco, he did research on lasers and the optical breakdown of gases. Subsequent to joining NASA in 1965, he wrote a textbook in solar physics entitled ”The Quiet Sun.” Gibson’s training and data acquisition as science-pilot on the last Skylab mission were in the areas of solar physics, comet observations, stellar observations, earth resources studies, space medicine and physiology, and flight surgeon activities.

He has logged more than 4,300 hours flying time—2,270 hours in jet aircraft.

Adapted from the Wikipedia article Edward Gibson, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Sami Khan Solanki (born 1958) is Professor at the Institute of Astronomy at the Eidgenössische Technische Hochschule Zürich (Swiss Federal Institute of Technology in Zürich) and is the Director for the Sun-Heliosphere Department of the Max Planck Institute for Solar System Research, and a scientific member of the Max Planck Society. and a Chair (and spokesperson) of the International Max Planck Research School on Physical Processes in the Solar System and Beyond at the Universities of Braunschweig and Göttingen.. He is the editor-in-chief of the ”Living Reviews in Solar Physics” an online review journal for solar physics and related fields.

Solanki’s main topics of research are:

* Solar and heliospheric physics, in particular solar magnetism and Sun-Earth relations

* Stellar astrophysics, mainly stellar activity and magnetism

* Astronomical tests of theories of gravitation

* Atomic and molecular physics of astronomical interest

* Protoplanetary discs and extrasolar planets

* Radiative transfer of polarized light

He has contributed to SOHO Virgo, STEREO Secchi, SDO HMI projects.

Adapted from the Wikipedia article Sami Solanki, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Solar Probe+ ”(Solar Probe Plus)”, previously NASA Solar Probe, is a planned robotic spacecraft to probe the outer corona of the sun. It will approach to within 8.5 solar radii (0.04 astronomical units or 5.9 million kilometers or 3.67 million miles) to the surface of the sun. The project was announced as a new mission start in the fiscal 2009 budget year. On May 1, 2008 Johns Hopkins University Applied Physics Laboratory announced to design and build the spacecraft, on a schedule to launch in 2015.

Adapted from the Wikipedia article Solar Probe+, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Karl-Otto Kiepenheuer (10 November 1910 – 23 May 1975) was a German astronomer and astrophysicist. His research focused on the Sun, and for that purpose he initiated construction of several solar telescopes and founded the Kiepenheuer Institute for Solar Physics.

Adapted from the Wikipedia article Karl-Otto Kiepenheuer, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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