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====[http://apl.aip.org/resource/1/applab/v92/i12/p123502_s1 Internal voltages in GaInP/GaInAs/Ge multijunction solar cells determined by electroluminescence measurements]<ref> Thomas Kirchartz, Uwe Rau, Martin Hermle, Andreas W. Bett, Anke Helbig, and Jürgen H. Werner, "Internal voltages in GaInP/GaInAs/Ge multijunction solar cells determined by electroluminescence measurements," Appl. Phys. Lett., Volume 92, Issue 12, Pages 123502, March 2008.</ref>====
====[http://apl.aip.org/resource/1/applab/v92/i12/p123502_s1 Internal voltages in GaInP/GaInAs/Ge multijunction solar cells determined by electroluminescence measurements]<ref> Thomas Kirchartz, Uwe Rau, Martin Hermle, Andreas W. Bett, Anke Helbig, and Jürgen H. Werner, "Internal voltages in GaInP/GaInAs/Ge multijunction solar cells determined by electroluminescence measurements," Appl. Phys. Lett., Volume 92, Issue 12, Pages 123502, March 2008.</ref>====
'''Abstract''': We analyze electroluminescence spectra of a GaInP/GaInAs/Ge triple-junction solar cell at different injection currents. Using the reciprocity theorem between electroluminescent emission and external quantum efficiency of solar cells allows us to derive the current/voltage curves and the diode quality factors of all individual subcells.
'''Abstract''': We analyze electroluminescence spectra of a GaInP/GaInAs/Ge triple-junction solar cell at different injection currents. Using the reciprocity theorem between electroluminescent emission and external quantum efficiency of solar cells allows us to derive the current/voltage curves and the diode quality factors of all individual subcells.
====[http://onlinelibrary.wiley.com/doi/10.1002/pip.514/abstract Spectral response measurements of monolithic GaInP/Ga(In)As/Ge triple-junction solar cells: Measurement artifacts and their explanation]<ref> M. Meusel, C. Baur, G. Létay, A. W. Bett, W. Warta, and E. Fernandez, "Spectral response measurements of monolithic GaInP/Ga(In)As/Ge triple-junction solar cells: Measurement artifacts and their explanation," Prog. Photovoltaics, Volume 11, Issue 8, Pages 499–514, December 2003.</ref>====
'''Abstract''':
Procedures for measuring the spectral response of multi-junction cells in general require variation of the bias spectrum and voltage biasing. It is shown that a refined procedure including optimization of bias spectrum and voltage is necessary to minimize a measurement artifact, which appears if the subcell under test has non-ideal properties, such as a low shunt resistance or a low reverse breakdown voltage.
This measurement artifact is often observed on measuring the spectral response of the Ge bottom cell of GaInP/Ga(In)As/Ge triple-junction cells. The main aspect of the measurement artifact is that the response of another subcell is simultaneously measured, while at the same time the signal of the Ge subcell is too low. Additionally, the shape of the spectral response curve is influenced under certain measurement conditions. In this paper the measurement artifact is thoroughly discussed by measurement results and simulation. Based on this analysis, a detailed procedure for the spectral response measurement of multi-junction cells is developed, specially designed to minimize such measurement artifacts.


==References==
==References==
<references/>
<references/>

Revision as of 19:53, 4 February 2013

This page describes selected literature available on Band gap Engineering for PV material optimization.

Analysis of thermoelectric characteristics of AlGaN and InGaN semiconductors[1]

Abstract: The thermoelectric properties of AlGaN and InGaN semiconductors are analyzed. In Author(s) analysis, the thermal conductivities, electrical conductivities, Seebeck coefficients, and figure of merits (Z*T) of AlGaN and InGaN semiconductors are computed. The electron transports in AlGaN and InGaN alloys are analyzed by solving Boltzmann transport equation, taking into account the dominant mechanisms of energy-dependent electron scatterings. Virtual crystal model is implemented to simulate the lattice thermal conductivity from phonon scattering for both AlGaN and InGaN alloys. The calculated Z*T is as high as 0.15 for optimized InGaN alloy at temperature around 1000 K. For optimized AlGaN composition, the Z*T of 0.06-0.07 can be achieved. The improved thermoelectric performance of ternary alloys over binary alloys can be attributed to the reduced lattice thermal conductivity.

High-efficiency multi-junction solar cells:Current status and future potential[2]

AlGaAs tunnel junction for high efficiency multi-junction solar cells: Simulation and measurement of[3]

Abstract: AlGaAs tunnel junctions are shown to be well-suited to concentrated photovoltaics where temperatures and current densities can be dramatically higher than for 1-sun flat-panel systems. Detailed comparisons of AlGaAs/AlGaAs tunnel junction experimental measurements over a range of temperatures expected during device operation in concentrator systems are presented. Experimental and simulation results are compared in an effort to decouple the tunnel junction from the overall multi-junction solar cell. The tunnel junction resistance is experimentally studied as a function of the temperature to determine its contribution to overall efficiency of the solar cell. The current-voltage behavior of the isolated TJ shows that as the temperature is increased from 25°C to 85°C, the resistance decreases from ~4.7×10-4 Ω∙cm2 to ~0.3×10-4 Ω∙cm2 for the operational range of a multi-junction solar cell under concentration.

Impact of spectral effects on the electrical parameters of multijunction amorphous silicon cells[4]

Abstract: The influence of spectral variation on the efficiency of single-, double- and triple-junction amorphous silicon cells has been investigated. The average photon energy (APE) proves to be a useful device-independent environmental parameter for quantifying the average hue of incident spectra. Single-junction devices increase in efficiency as light becomes blue shifted, because more of the incident spectrum lies within the absorption window and less in the redlinfra-red tail; this is denoted the primary spectral effect. Double- and triple-junction devices also exhibit a secondary spectral effect due to mismatch between the device structure and the incident spectrum. These both reach a maximum efficiency, which drops off as light is red or blue shifted. The effect is more pronounced for triple-junction than double-junction devices, as mismatch between junctions is statistically more likely.

Modeling the effect of varying spectra on multi junction A-SI solar cells[5]

Abstract: The performance of multijunction amorphous silicon cells has been investigated for outdoor solar spectral radiation, using long term measurement for existing data at CREST, Loughborough University. It is a further study of the solar system, destined to analyze the outdoor performance of the amorphous silicon cells. The short circuit current for each subcells have been modeled and implemented into a computer program to calculate the mismatched short circuit current of the whole device.

Japanese R&D Activities of High Efficiency III-V Compound Multi-Junction and Concentrator Solar Cells[6]

Abstract: This paper reviews Japanese R&D activities of III-V compound multi-junction (MJ) and concentrator solar cells. As a result of advanced technologies development for high efficiency cells and discovery of superior radiation-resistance of InGaP based materials, InGaP-based MJ solar cells have been commercialised for space use in Japan. A new world-record efficiency of 35.8% at 1 sun has been achieved with InGaP/GaAs/InGaAs 3-junction solar cell. MJ solar cells composing of multi-layers with different bandgap energies have the potential for achieving high conversion efficiencies of over 50% and are promising for space and terrestrial applications due to wide photo response. In order to solve the problems of difficulties in making high performance and stable tunnel junctions, a double hetero (DH) structure tunnel junction was found to be useful for preventing diffusion from the tunnel junction and improving the tunnel junction performance by the authors. An InGaP material instead of AlGaAs for the top cell was proposed by NREL. As a result of advanced technologies development for high efficiency cells and discovery of superior radiation-resistance of InGaP-based materials by the authors, InGaP-based MJ solar cells have been commercialised for space use even in Japan since 2002. Most recently, world-record efficiency (35.8%) at 1-sun AM1.5G has been realised with inverted epitaxial grown InGaP/GaAs/InGaAs 3-junction cells by Sharp. Since the concentrator modules have been demonstrated to produce roughly 1.7 to 2.6 times more energy per area per annum than the 14 % multicrystalline silicon modules in most cities in Japan, concentrator PV Photovoltaics) as the 3rd PV technologies in addition to the 1st crystalline Si PV and the 2nd thin-film PV technologies are expected to contribute to electricity cost reduction for widespread PV applications.

Low-dimensional Systems and Nanostructures - Multi-junction solar cells and novel structures for solar cell applications[7]

Abstract: The present status of R& D program for super-high e*ciency III–V compound multi-junction solar cells in the New Sunshine Project in Japan is presented. As a result of InGaP top cell material quality improvement, development of optically and electrically low-loss double-heterostructure InGaP tunnel junction, photon and carrier con5nements, and lattice matching between active cell layers and substrate, InGaP=InGaAs=Ge monolithic cascade 3-junction cells with an e*ciency of 31.7% at 1-sun AM1.5 and InGaP=GaAs==InGaAs mechanically stacked 3-junction cells with the highest (world-record) e*ciency of 33.3% at 1-sun AM1.5 have been realized. As an approach for low-cost and high-e*ciency cells, better radiation resistance of GaAs thin-5lm solar cells with novel structures fabricated on Si substrates has also been demonstrated. Novel structures such as Bragg re=ector and super-lattice structures are found to show a better initial cell performance and radiation resistance since those layers act as bu>er layers to reduce dislocations, and act as a back-surface 5eld and back-surface re=ector layers.

Multi-junction III–V solar cells: current status and future potential[8]

Abstract: Our recent R&D activities of III–V compound multi-junction (MJ) solar cells are presented. Conversion efficiency of InGaP/InGaAs/Ge has been improved up to 31–32% (AM1.5) as a result of technologies development such as double hetero-wide band-gap tunnel junction, InGaP–Ge hetero-face structure bottom cell, and precise lattice-matching of InGaAs middle cell to Ge substrate by adding indium into the conventional GaAs layer. For concentrator applications, grid structure has been designed in order to reduce the energy loss due to series resistance, and world-record efficiency InGaP/InGaAs/Ge 3-junction concentrator solar cell with an efficiency of 37.4% (AM1.5G, 200-suns) has been fabricated. In addition, we have also demonstrated high-efficiency and large-area (7000 cm2) concentrator InGaP/InGaAs/Ge 3-junction solar cell modules of an outdoor efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing high thermal conductivity modules. Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd generation solar cells in addition to 1st generation crystalline Si solar cells and 2nd generation thin-film solar cells. We are now developing low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m2 for terrestrial applications.

Novel materials for high-efficiency III–V multi-junction solar cells[9]

Abstract: As a result of developing wide bandgap InGaP double hetero structure tunnel junction for sub-cell interconnection, InGaAs middle cell lattice-matched to Ge substrate, and InGaP-Ge heteroface structure bottom cell, we have demonstrated 38.9% efficiency at 489-suns AM1.5 with InGaP/InGaP/Ge 3-junction solar cells by in-house measurements. In addition, as a result of developing a non-imaging Fresnel lens as primary optics, a glass-rod kaleidoscope homogenizer as secondary optics and heat conductive concentrator solar cell modules, we have demonstrated 28.9% efficiency with 550-suns concentrator cell modules with an area of 5445 cm2. In order to realize 40% and 50% efficiency, new approaches for novel materials and structures are being studied. We have obtained the following results: (1) improvements of lattice-mismatched InGaP/InGaAs/Ge 3-junction solar cell property as a result of dislocation density reduction by using thermal cycle annealing, (2) high quality (In)GaAsN material for 4- and 5-junction applications by chemical beam epitaxy, (3)11.27% efficiency InGaAsN single-junction cells, (4) 18.27% efficiency InGaAs/GaAs potentially modulated quantum well cells, and (5) 7.65% efficiency InAs quantum dot cells.

III–V compound multi-junction solar cells: present and future[10]

Abstract: As a result of top cell material quality improvement, development of optically and electrically low-loss double-hetero structure tunnel junction, photon and carrier confinements, and lattice-matching between active cell layers and substrate, the last 15 years have seen large improvements in III–V compound multi-junction (MJ) solar cells. In this paper, present status of R&D program for super-high-efficiency MJ cells in the New Sunshine Project in Japan is presented. InGaP/InGaAs/Ge monolithic cascade 3-junction cells with newly recorded efficiency of 31.7% at AM1.5 (1-sun) were achieved on Ge substrates, in addition to InGaP/GaAs//InGaAs mechanically stacked 3-junction cells with world-record efficiency of 33.3%. Future prospects for realizing super-high-efficiency and low-cost MJ solar cells are also discussed. r 2002 Published by Elsevier Science B.V.

Super high-efficiency multi-junction and concentrator solar cells[11]

Abstract: III–V compound multi-junction (MJ) (tandem) solar cells have the potential for achieving high conversion efficiencies of over 50% and are promising for space and terrestrial applications. We have proposed AlInP–InGaP double hetero (DH) structure top cell, wide-band gap InGaP DH structure tunnel junction for sub cell interconnection, and lattice-matched InGaAs middle cell. In 2004, we have successfully fabricated world-record efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cells with an efficiency of 37.4% at 200-suns AM1.5 as a result of widening top cell band gap, current matching of sub cells, precise lattice matching of sub cell materials, proposal of InGaP–Ge heteroface bottom cell, and introduction of DH-structure tunnel junction. In addition, we have realized high-efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cell modules (with area of 7000 cm2) with an out-door efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing low thermal conductivity modules. Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd-generation solar cells in addition to 1st-generation crystalline Si solar cells and 2nd generation thin-film solar cells. We are now challenging to develop low-cost and high output power concentrator MJ solar cell modules with an output power of 400W/m2 for terrestrial applications and high-efficiency, light-weight and low-cost MJ solar cells for space applications.

Theoretical performance of multi-junction solar cells combining III-V and Si materials[12]

Abstract: A route to improving the overall efficiency of multi-junction solar cells employing conventional III-V and Si photovoltaic junctions is presented here. A simulation model was developed to consider the performance of several multi-junction solar cell structures in various multi-terminal configurations. For series connected, 2-terminal triple-junction solar cells, incorporating an AlGaAs top junction, a GaAs middle junction and either a Si or InGaAs bottom junction, it was found that the configuration with a Si bottom junction yielded a marginally higher one sun efficiency of 41.5% versus 41.3% for an InGaAs bottom junction. A significant efficiency gain of 1.8% over the two-terminal device can be achieved by providing an additional terminal to the Si bottom junction in a 3-junction mechanically stacked configuration. It is shown that the optimum performance can be achieved by employing a four-junction series-connected mechanically stacked device incorporating a Si subcell between top AlGaAs/GaAs and bottom In0.53Ga0.47As cells.

Progress and challenges for next-generation high-efficiency multijunction solar cells[13]

Abstract: Multijunction solar cells are the most efficient solar cells ever developed with demonstrated efficiencies above 40%, far in excess of the performance of any conventional single-junction cell. This paper describes paths toward next-generation multijunction cells with even higher performance. Starting from fundamental multijunction concepts, the paper describes the desired characteristics of semiconductor materials for multijunction cells; the corresponding challenges in obtaining these characteristics in actual materials; and materials and device architectures to overcome these challenges.

Future technology pathways of terrestrial III–V multijunction solar cells for concentrator photovoltaic systems[14]

Abstract: Future terrestrial concentrator cells will likely feature four or more junctions. The better division of the solar spectrum and the lower current densities in these new multijunction cells reduce the resistive power loss (I2R) and provide a significant advantage in achieving higher efficiencies of 45–50%. The component subcells of these concentrator cells will likely utilize new technology pathways such as highly metamorphic materials, inverted crystal growth, direct-wafer bonding, and their combinations to achieve the desired bandgaps while maintaining excellent device material quality for optimal solar energy conversion. Here, we report preliminary results of two technical approaches: (1) metamorphic ∼1 eV GaInAs subcells in conjunction with an inverted growth approach and (2) multijunction cells on wafer-bonded, layer-transferred epitaxial templates.

Four-junction spectral beam-splitting photovoltaic receiver with high optical efficiency[15]

Abstract: A spectral beam-splitting architecture is shown to provide an excellent basis for a four junction photovoltaic receiver with a virtually ideal band gap combination. Spectrally selective beam-splitters are used to create a very efficient light trap in form of a 45° parallelepiped. The light trap distributes incident radiation onto the different solar cells with an optical efficiency of more then 90%. Highly efficient solar cells including III–V semiconductors and silicon were fabricated and mounted into the light trapping assembly. An integrated characterization of such a receiver including the measurement of quantum efficiency as well as indoor and outdoor I–V measurements is shown. Moreover, the optical loss mechanisms and the optical efficiency of the spectral beam-splitting approach are discussed. The first experimental setup of the receiver demonstrated an outdoor efficiency of more than 34% under unconcentrated sunlight.

Fluctuations of the peak current of tunnel diodes in multi-junction solar cells [16]

Abstract: Interband tunnel diodes are widely used to electrically interconnect the individual subcells in multi-junction solar cells. Tunnel diodes have to operate at high current densities and low voltages, especially when used in concentrator solar cells. They represent one of the most critical elements of multi-junction solar cells and the fluctuations of the peak current in the diodes have an essential impact on the performance and reliability of the devices. Recently we have found that GaAs tunnel diodes exhibit extremely high peak currents that can be explained by resonant tunnelling through defects homogeneously distributed in the junction. Experiments evidence rather large fluctuations of the peak current in the diodes fabricated from the same wafer. It is a challenging task to clarify the reason for such large fluctuations in order to improve the performance of the multi-junction solar cells. In this work we show that the large fluctuations of the peak current in tunnel diodes can be caused by relatively small fluctuations of the dopant concentration. We also show that the fluctuations of the peak current become smaller for deeper energy levels of the defects responsible for the resonant tunnelling.

Performance of amorphous silicon double junction photovoltaic systems in different climatic zones [17]

Abstract: To date the. majority of investigations into the performance of amorphous silicon photovoltaic systems have been limited to single sites, and therefore the conclusions from such studies are unlikely to be as generic as they might at first appear. This paper compares data collected from different systems across the world in Brazil, Hong Kong, Spain, Switzerland, and the United Kingdom. Ail systems have been operating for a number of years, and are employing double junction amorphous silicon devices of a similar age manufactured by RWE Solar. The data are analysed for performance variations reflecting the different climatic zones, and the variations are explained on the basis of operating temperature, incident irradiation and seasonal spectral shift.

Multijunction solar cell technologies - high efficiency, radiation resistance, and concentrator applications[18]

Abstract: The conversion efficiency of InGaP/(In)GaAs/Ge-based multijunction solar cells has been improved up to 29-30% (AM0) and 31-32% (AM1.5 G) by technologies, such as double-hetero wide band-gap tunnel junctions, combination with Ge bottom cell with the InGaP first layer, and precise lattice-matching to Ge substrate by adding 1% indium to the conventional GaAs lattice-match structure. Employing a 1.96 eV AlInGaP top cell should improve efficiency further. For space use, radiation resistance has been improved by technologies such as introducing of an electric field in the base layer of the lowest-resistance middle cell, and EOL current matching of sub-cells to the highest-resistance top cell. A grid structure has been designed for concentrator applications in order to reduce the energy loss due to series resistance, and 36% (AM1.5 G, 100-500 suns) efficiency has been demonstrated.

InGaP/GaAs-based multijunction solar cells[19]

Abstract: The conversion efficiency of InGaP/(In)GaAs/Ge -based multijunction solar cells has been improved up to 29–30% (AM0) and 31–32% (AM1·5G) by technologies, such as double-hetero wide band-gap tunnel junctions, combination with Ge bottom cell with the InGaP first hetero-growth layer, and precise lattice-matching to Ge substrate by adding 1% indium to the conventional GaAs lattice-match structure. Employing a 1·95 eV AlInGaP top cell should improve efficiency further. For space use, radiation resistance has been improved by technologies such as introducing of an electric field in the base layer of the lowest-resistance middle cell, and EOL current matching of sub-cells to the highest-resistance top cell. A grid structure and cell size have been designed for concentrator applications in order to reduce the energy loss due to series resistance, and 38% (AM1·5G, 100–500 suns) efficiency has been demonstrated. Furthermore, thin-film structure which is InGaP/GaAs dual junction cell on metal film has been newly developed. The thin-film cell demonstrated high flexibility, lightweight, high efficiency of over 25% (AM0) and high radiation resistance.

Current-matched triple-junction solar cell reaching 41.1% conversion efficiency under concentrated sunlight[20]

Abstract: A metamorphic Ga0.35In0.65P/Ga0.83In0.17As/Ge triple-junction solar cell is shown to provide current-matching of all three subcells and thus composes a device structure with virtually ideal band gap combination. We demonstrate that the key for the realization of this device is the improvement of material quality of the lattice-mismatched layers as well as the development of a highly relaxed Ga1−yInyAs buffer structure between the Ge substrate and the middle cell. This allows the metamorphic growth with low dislocation densities below 106 cm−2. The performance of the approach has been demonstrated by a conversion efficiency of 41.1% at 454 suns (454 kW/m2, AM1.5d ASTM G173–03).

Concentrator multijunction solar cell characteristics under variable intensity and temperature[21]

Abstract: The performance of multijunction solar cells has been measured over a range of temperatures and illumination intensities. Temperature coefficients have been extracted for three-junction cell designs that are in production and under development. A simple diode model is applied to the three-junction performance as a means to predict performance under operating conditions outside the test range. These data may be useful in guiding the future optimization of concentrator solar cells and systems.

High-efficiency quadruple junction solar cells using OMVPE with inverted metamorphic device structures[22]

Abstract: We have produced a monolithically grown, two-terminal, series connected, quadruple junction III–V solar cell with a 1 sun AM0 conversion efficiency of 33.6%. The device epitaxial layers were grown using organometallic vapor phase epitaxy in an inverted order with a 1.91 eV GaInP subcell grown lattice-matched to a GaAs substrate followed by the growth of a lattice-matched 1.42 eV GaAs subcell, a metamorphic 1.02 eV GaInAs subcell, and a metamorphic 0.7 eV GaInAs subcell. This combination of bandgap energies is nearly ideal in that the current generation in each of the four subcells is nearly identical with absorption limited subcell thicknesses. We will discuss the motivation and development for a particular embodiment of the quadruple junction as well as the outlook for future improvements.

Multi-Junction Solar Cell Spectral Tuning with Quantum Dots[23]

Abstract: We have theoretically analyzed the potential efficiency improvement to multi-junction solar cell efficiencies which are available through the incorporation of quantum dot using detailed balance calculations. We have also experimentally investigated the Stranski-Krastanov growth of self-organized InAs quantum dots and quantum dot arrays on lattice-matched GaAs by metallorganic vapor phase epitaxy (MOVPE). The morphology of the quantum dots were investigated as a function of their growth parameters by atomic force microscopy (AFM). Photoluminescence and optical absorption measurements have demonstrated that the incorporation of InAs quantum dots (QD) into a GaAs structure can provide sub-GaAs bandgap electronic states

A review of snow and ice albedo and the development of a new physically based broadband albedo parameterization[24]

Abstract: We present a computationally simple, theoretically based parameterization for the broadband albedo of snow and ice that can accurately reproduce the theoretical broadband albedo under a wide range of snow, ice, and atmospheric conditions. Depending on its application, this parameterization requires between one and five input parameters. These parameters are specific surface area of snow/ice, concentration of light-absorbing carbon, solar zenith angle, cloud optical thickness, and snow depth. The parameterization is derived by fitting equations to albedo estimates generated with a 16-stream plane-parallel, discrete ordinates radiative transfer model of snow and ice that is coupled to a similar model of the atmosphere. Output from this model is also used to establish the physical determinants of the spectral albedo of snow and ice and evaluate the characteristics of spectral irradiance over snow-covered surfaces. Broadband albedo estimates determined from the radiative transfer model are compared with results from a selection of previously proposed parameterizations. Compared to these parameterizations, the newly proposed parameterization produces accurate results for a much wider range of snow, ice, and atmospheric conditions.

Band-Gap-Engineered Architectures for High-Efficiency Multijunction Concentrator Solar Cells[25]

Abstract: Beginning with maximum theoretical efficiencies from detailed balance calculations, we evaluate the real-world energy loss mechanisms in a variety of high-efficiency multijunction cell architectures such as inverted metamorphic 3- and 4-junction cells, as a step toward closing the gap between theory and experiment. Experimental results are given on band-gap-engineered lattice-matched and metamorphic 3-junction cells, and on 4-junction terrestrial concentrator cells. A new world record 41.6%-efficient solar cell is presented, the highest efficiency yet demonstrated for any type of solar cell.

Spectral response and energy output of concentrator multijunction solar cells[26]

Abstract: The spectral response of concentrator multijunction solar cells has been measured over a temperature range of 25–75°C. These data are combined with reference spectra representing the AM1·5 standard as well as annual spectral irradiance at representative geographical locations. The results suggest that higher performance in the field may be obtained if multijunction cells are designed for an effective air mass higher than AM1·5.

Radiative coupling effects in GaInP/GaAs/Ge multijunction solar cells[27]

Abstract: Direct measurements of radiative coupling effects in GaInP/GaAs/Ge multijunction solar cells are presented. Radiative coupling between the GaInP and GaAs cells is observed by using isotype cells as well as specially fabricated 3-terminal device structures. Spectral response measurements of the GaAs cell in both isotype and 3-terminal approaches are shown to exhibit enhanced quantum efficiency in the short wavelength region under favorable radiative coupling conditions. Additionally, electroluminescence of the GaInP cell is shown to enhance the current output from the GaAs cell using a 3-terminal device structure. One consequence of this effect is the possible influence on the measured J-ratio of a multijunction cell. Consideration of radiative coupling may become increasingly important as multijunction III-V based solar cells - including 4- and 5- junction cells - continue to develop and improve in performance.

High-efficiency space and terrestrial multijunction solar cells through bandgap control in cell structures[28]

Abstract: Using the energy bandgap of semiconductors as a design parameter is critically important for achieving the highest efficiency multijunction solar cells. The bandgaps of lattice-matched semiconductors that are most convenient to use are rarely those which would result in the highest theoretical efficiency. For both the space and terrestrial solar spectra, the efficiency of 3-junction GaInP/GaAs/Ge solar cells can be increased by a lower bandgap middle cell, as for GaInAs middle cells, as well as by using higher bandgap top cell materials. Wide-bandgap and indirect-gap materials used in parasitically absorbing layers such as tunnel junctions help to increase transmission of light to the active cell layers beneath. Control of bandgap in such cell structures has been instrumental in achieving solar cell efficiencies of 29.7% under the AMO space spectrum (0.1353 W/cm2, 28°C) and 34% under the concentrated terrestrial spectrum (AM1.5G, 150-400 suns, 25°C), the highest yet achieved for solar cells built on a single substrate.

Advances in High-Efficiency III-V Multijunction Solar Cells[29]

Abstract: The high efficiency of multijunction concentrator cells has the potential to revolutionize the cost structure of photovoltaic electricity generation. Advances in the design of metamorphic subcells to reduce carrier recombination and increase voltage, wide-band-gap tunnel junctions capable of operating at high concentration, metamorphic buffers to transition from the substrate lattice constant to that of the epitaxial subcells, concentrator cell AR coating and grid design, and integration into 3-junction cells with current-matched subcells under the terrestrial spectrum have resulted in new heights in solar cell performance. A metamorphic Ga0.44In0.56P/Ga0.92In0.08As/ Ge 3-junction solar cell from this research has reached a record 40.7% efficiency at 240 suns, under the standard reporting spectrum for terrestrial concentrator cells (AM1.5 direct, low-AOD, 24.0 W/cm2, 25∘C), and experimental lattice-matched 3-junction cells have now also achieved over 40% efficiency, with 40.1% measured at 135 suns. This metamorphic 3-junction device is the first solar cell to reach over 40% in efficiency, and has the highest solar conversion efficiency for any type of photovoltaic cell developed to date. Solar cells with more junctions offer the potential for still higher efficiencies to be reached. Four-junction cells limited by radiative recombination can reach over 58% in principle, and practical 4-junction cell efficiencies over 46% are possible with the right combination of band gaps, taking into account series resistance and gridline shadowing. Many of the optimum band gaps for maximum energy conversion can be accessed with metamorphic semiconductor materials. The lower current in cells with 4 or more junctions, resulting in lower I2R resistive power loss, is a particularly significant advantage in concentrator PV systems. Prototype 4-junction terrestrial concentrator cells have been grown by metal-organic vapor-phase epitaxy, with preliminary measured efficiency of 35.7% under the AM1.5 direct terrestrial solar spectrum at 256 suns.

40% efficient metamorphic GaInP/GaInAs/Ge multijunction solar cells[30]

Abstract: An efficiency of 40.7% was measured and independently confirmed for a metamorphic three-junction GaInP/GaInAs/Ge cell under the standard spectrum for terrestrial concentrator solar cells at 240 suns (24.0 W/cm2, AM1.5D, low aerosol optical depth, 25 °C). This is the initial demonstration of a solar cell with over 40% efficiency, and is the highest solar conversion efficiency yet achieved for any type of photovoltaic device. Lattice-matched concentrator cells have now reached 40.1% efficiency. Electron-hole recombination mechanisms are analyzed in metamorphic GaxIn1−xAs and GaxIn1−xP materials, and fundamental power losses are quantified to identify paths to still higher efficiencies.

Regional climate consequences of large-scale cool roof and photovoltaic array deployment [31]

Abstract: Modifications to the surface albedo through the deployment of cool roofs and pavements (reflective materials) and photovoltaic arrays (low reflection) have the potential to change radiative forcing, surface temperatures, and regional weather patterns. In this work we investigate the regional climate and radiative effects of modifying surface albedo to mimic massive deployment of cool surfaces (roofs and pavements) and, separately, photovoltaic arrays across the United States. We use a fully coupled regional climate model, the Weather Research and Forecasting (WRF) model, to investigate feedbacks between surface albedo changes, surface temperature, precipitation and average cloud cover. With the adoption of cool roofs and pavements, domain-wide annual average outgoing radiation increased by 0.16 ± 0.03 W m − 2 (mean ± 95% C.I.) and afternoon summertime temperature in urban locations was reduced by 0.11–0.53 °C, although some urban areas showed no statistically significant temperature changes. In response to increased urban albedo, some rural locations showed summer afternoon temperature increases of up to + 0.27 °C and these regions were correlated with less cloud cover and lower precipitation. The emissions offset obtained by this increase in outgoing radiation is calculated to be 3.3 ± 0.5 Gt CO2 (mean ± 95% C.I.). The hypothetical solar arrays were designed to be able to produce one terawatt of peak energy and were located in the Mojave Desert of California. To simulate the arrays, the desert surface albedo was darkened, causing local afternoon temperature increases of up to + 0.4 °C. Due to the solar arrays, local and regional wind patterns within a 300 km radius were affected. Statistically significant but lower magnitude changes to temperature and radiation could be seen across the domain due to the introduction of the solar arrays. The addition of photovoltaic arrays caused no significant change to summertime outgoing radiation when averaged over the full domain, as interannual variation across the continent obscured more consistent local forcing.

Internal voltages in GaInP/GaInAs/Ge multijunction solar cells determined by electroluminescence measurements[32]

Abstract: We analyze electroluminescence spectra of a GaInP/GaInAs/Ge triple-junction solar cell at different injection currents. Using the reciprocity theorem between electroluminescent emission and external quantum efficiency of solar cells allows us to derive the current/voltage curves and the diode quality factors of all individual subcells.

Spectral response measurements of monolithic GaInP/Ga(In)As/Ge triple-junction solar cells: Measurement artifacts and their explanation[33]

Abstract: Procedures for measuring the spectral response of multi-junction cells in general require variation of the bias spectrum and voltage biasing. It is shown that a refined procedure including optimization of bias spectrum and voltage is necessary to minimize a measurement artifact, which appears if the subcell under test has non-ideal properties, such as a low shunt resistance or a low reverse breakdown voltage. This measurement artifact is often observed on measuring the spectral response of the Ge bottom cell of GaInP/Ga(In)As/Ge triple-junction cells. The main aspect of the measurement artifact is that the response of another subcell is simultaneously measured, while at the same time the signal of the Ge subcell is too low. Additionally, the shape of the spectral response curve is influenced under certain measurement conditions. In this paper the measurement artifact is thoroughly discussed by measurement results and simulation. Based on this analysis, a detailed procedure for the spectral response measurement of multi-junction cells is developed, specially designed to minimize such measurement artifacts.

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