Band gap engineering literature review
 This page describes selected literature available on Band gap Engineering for PV material optimization.
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.
 AlGaAs tunnel junction for high efficiency multi-junction solar cells: Simulation and measurement of
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
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.
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
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
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.
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.
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.
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.
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.
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.
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
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.
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.
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 
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
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.
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
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
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
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.
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
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
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.
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.
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
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.
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.
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 
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
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
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.
Abstract: Procedures at NREL and Fraunhofer ISE for evaluating multijunction cells are detailed with a triple-junction GaInP/GaAs/Ge concentrator cell designed and grown at Spectrolab and processed at NREL, and a tandem Fraunhofer ISE Ga0.35In0.85P/Ga0.83 In0.17As cell as examples. The one-sun efficiency and I sc for the triple-junction device measured at Fraunhofer ISE and NREL agreed within 0.2%, well below the ±6% uncertainty estimated by NREL. The procedures for determining the one-sun characteristics involve determining the quantum efficiency and using it for spectral correction during the I-V characterization. The characteristics under concentration are evaluated with a flash simulator
 Spectral mismatch correction and spectrometric characterization of monolithic III–V multi-junction solar cells
Abstract: III–V monolithic multi-junction (MJ) solar cells reach efficiencies exceeding 30% (AM 1.5 global) and have applications in space and in terrestrial concentrator systems. The subcells of monolithic MJ cells are not accessible separately, which presents a challenge to measurement systems and procedures. A mathematical approach is presented which enables a fast way of spectral mismatch correction for MJ cells, thereby significantly reducing the time required for calibration. Moreover, a systematic investigation of the I–V parameters of a MJ solar cell with variation of the incident spectrum is possible, herein called ‘spectrometric characterization’. This analysis method visualizes the effects of current limitation and shifting of the operating voltage, and yields precise information about the current-matching of the subcells. MJ cells can hereby be compared without the need to match the current of the structures to a reference spectrum in advance. Further applications of the spectrometric characterization are suggested, such as for the determination of the radiation response of the subcells of MJ space solar cells or for the prediction of the annual power output of terrestrial MJ concentrator cells.
Abstract: The commercialization of multi-junction solar cells for both space and terrestrial applications has increased the need to accurately determine cell performance using typical solar simulators and test equipment. This paper describes specific test methods recently applied in characterizing the performance of both tandem and triple-junction solar cells. Methods applied included: current-voltage measurements in forward and reverse bias using a xenon-arc solar simulator; absolute spectral response measurements of separate junctions using both light and voltage bias; a device simulation model; and a spectral mismatch calculation procedure tailored to multi-junction cells. Procedures are illustrated using measurements for GaInP-GaAs tandem cells, GaInP-GaAs-Ge triple-junction cells, and Ge cells supplied by different manufacturers.
Abstract: Procedures at NREL and Fraunhofer ISE for evaluating multijunction cells are detailed with a triple-junction GaInP/GaAs/Ge concentrator cell designed and grown at Spectrolab and processed at NREL, and a tandem Fraunhofer ISE Ga0.35In0.85P/Ga0.83 In0.17As cell as examples. The one-sun efficiency and I sc for the triple-junction device measured at Fraunhofer ISE and NREL agreed within 0.2%, well below the ±6% uncertainty estimated by NREL. The procedures for determining the one-sun characteristics involve determining the quantum efficiency and using it for spectral correction during the I-V characterization. The characteristics under concentration are evaluated with a flash simulator.
Abstract: The authors demonstrate a thin, Ge-free III–V semiconductor triple-junction solar cell device structure that achieved 33.8%, 30.6%, and 38.9% efficiencies under the standard 1 sun global spectrum, space spectrum, and concentrated direct spectrum at 81 suns, respectively. The device consists of 1.8 eV Ga0.5In0.5P, 1.4 eV GaAs, and 1.0 eV In0.3Ga0.7As p-n junctions grown monolithically in an inverted configuration on GaAs substrates by organometallic vapor phase epitaxy. The lattice-mismatched In0.3Ga0.7As junction was grown last on a graded GaxIn1−xP buffer. The substrate was removed after the structure was mounted to a structural “handle.” The current-matched, series-connected junctions produced a total open-circuit voltage over 2.95 V at 1 sun.
Abstract: This paper presents an overview of the status of the high-concentration photovoltaic (HCPV) module technology and discusses the steps required to take it from to the production of gigawatts in the near future. The paper discusses the impact of the recent advances in multijunction cell technology on the economics of concentrator system.
Abstract: In this paper, we look at the question "how high can solar cell efficiency go?" from both theoretical and experimental perspectives. First-principle efficiency limits are analyzed for some of the main candidates for high-efficiency multijunction terrestrial concentrator cells. Many of these cell designs use lattice-mismatched, or metamorphic semiconductor materials in order to tune subcell band gaps to the solar spectrum. Minority-carrier recombination at dislocations is characterized in GaInAs inverted metamorphic solar cells, with band gap ranging from 1.4 to 0.84 eV, by light I-V, electron-beam-induced current (EBIC), and cathodoluminescence (CL). Metamorphic solar cells with a 3-junction GaInP/ GaInAs/ Ge structure were the first cells to reach over 40% efficiency, with an independently confirmed efficiency of 40.7% (AM1.5D, low-AOD, 240 suns, 25°C). The high efficiency of present III-V multijunction cells now in high-volume production, and still higher efficiencies of nextgeneration cells, is strongly leveraging for low-cost terrestrial concentrator PV systems.
Abstract: This paper discusses the approach taken by Concentrating Technologies (CT), Spectrolab, and Arizona Public Service (APS) to demonstrate a High Concentration Photovoltaic (HCPV) module using Spectrolab's GaInP/GaInAs/Ge triple-junction solar cells. This module is currently connected to an inverter, feeding electricity into the grid at the Solar Test & Research (STAR) facility of APS in Tempe, AZ. Although the module output is small, under 1 kW AC, it is the world's first demonstration of a grid-connected utility module using the same triple-junction solar cell technology that have been used to power satellites and other spacecrafts. This paper also discusses the next steps required to increase module efficiency and enhance its reliability. The economics of the MJ receivers in the context of this module are also discussed.
 Criteria for the design of GaInP/GaAs/Ge triple-junction cells to optimize their performance outdoors
Abstract: This paper investigates which reference spectrum should be used to design GalnP/GaAs/Ge triple-junction cells (at 300 K) in order to optimize their performance outdoors (at elevated temperatures). The outdoor performance is simulated using direct spectra from the recently proposed Module Energy Rating Procedure. We find that triple-junction cells designed for AM1.5D, low-AOD and AM1.5G standard spectra at 300 K all work well for maximizing daily energy production at elevated temperatures. AM1.5G cells are the best choice for midday power production, whereas AM1.5D cells are the best choice for power production during the morning and evening. Performance of cells optimized for a newly proposed Low-AOD spectrum is intermediate between these two extremes.
 A Theoretical Analysis on the Energy Production of III-V Multi-Junction Solar Cells under Realistic Spectral Conditions
Abstract: In this paper we present a methodology which uses the detailed balance method to determine the optimum bandgap combination of III-V triple-junction solar cells for the highest yearly energy production. As an example for the methodology we analyse two geographical locations on Earth with distinct spectral conditions. For these places the monthly average of the measured aerosol optical depth and the precipitable water are used to calculate direct solar spectra with a discretisation of one spectrum per hour. The model is used to analyse the spectral sensitivity of the bandgap design of four practical III-V triple-junction solar cell structures. In addition, the impact of the designated operating temperature is investigated. Furthermore, the ideal bandgap combination for a maximal energy harvest is calculated for each location. It is shown that structures optimized for the standard AM1.5d reference spectrum yield nearly optimal energy harvesting efficiencies at geographical locations with “red-rich” spectral conditions. However, the choice of the right bandgap combination is essential. By contrast, structures should be re-optimized for locations with a high share of blue light.
 Performance and Reliability of Multijunction III-V Modules for Concentrator Dish and Central Receiver Applications
Abstract: Over the last 15 years, Solar Systems have developed a dense array receiver PV technology for 500X concentrator reflective dish applications. This concentrator PV technology has been successfully deployed at six different locations in Australia, counting for more than 1 MWp of installed peak power. A new Multijunction III-V receiver to replace the current silicon Point-Contact solar cells has recently been developed. The new receiver technology is based on high-efficiency (>32%) Concentrator Ultra Triple Junction (CUTJ) solar cells from Spectrolab, resulting in system power and energy performance improvement of more than 50% compared to the silicon cells. The 0.235 m2 concentrator PV receiver, designed for continuous 500X operation, is composed of 64 dense array modules, and made of series and parallel-connected solar cells, totaling approximately 1,500 cells. The individual dense array modules have been tested under high intensity pulsed light, as well as with concentrated sunlight at the Solar Systems research facility and at the National Renewable Energy Laboratory's High Flux Solar Furnace. The efficiency of the dense array modules ranges from 30% to 36% at 500X (50 W/cm2, AM1.5D low AOD, 21C). The temperature coefficients for power, voltage and current, as well as the influence of Air Mass on the cell responsivity, were measured. The reliability of the dense array multijunction III-V modules has been studied with accelerated aging tests, such as thermal cycling, damp heat and high-temperature soak, and with real-life high-intensity exposure. The first 33 kWp multijunction III-V receiver was recently installed in a Solar Systems dish and tested in real-life 500X concentrated sunlight conditions. Receiver efficiencies of 30.3% and 29.0% were measured at Standard Operating Conditions and Normal Operating Conditions respectively.
Abstract: A major step forward has been made towards cost reduction of terrestrial PV. World-record multijunction III-V solar cells have been integrated into a commercial concentrator photovoltaic (CPV) system. A dense array of high-efficiency solar cells in the receiver of a high-intensity (~500X) concentrator system has been identified as a viable, cost-effective system. Concentrator ultra triple junction (CUTJ) cells have been developed for use in the Solar Systems CS500 solar electric power generator. The cell is designed for efficient conversion of the specific solar spectrum delivered to the system receiver while minimizing cell cost. Cells are optimized for maximum active area in a Solar Systems dense-array cell module. Solar Systems modules using CUTJ dense-array cells have demonstrated module efficiencies of over 35%. Field testing of CUTJ dense-array cells in a CS500 CPV dish unit at the Hermannsburg solar power plant in Australia was initiated in December 2005. A full multi-junction receiver in a CS500 dish has delivered over 30kW with an efficiency of almost 30%. Following qualification, these systems are slated for entry into the terrestrial market in 2006.
Abstract: Solar-cell efficiencies have exceeded 40% in recent years. The keys to achieving these high efficiencies include: 1) use of multiple materials that span the solar spectrum, 2) growth of these materials with near-perfect quality by using epitaxial growth on single-crystal substrates, and 3) use of concentration. Growth of near-perfect semiconductor materials is possible when the lattice constants of the materials are matched or nearly matched to that of a single-crystal substrate. Multiple material combinations have now demonstrated efficiencies exceeding 40%, motivating incorporation of these cells into concentrator systems for electricity generation. The use of concentration confers several key advantages.
Abstract: Champion concentrator cell efficiencies have surpassed 40% and now many are asking whether the efficiencies will surpass 50%. Theoretical efficiencies of >60% are described for many approaches, but there is often confusion about “the” theoretical efficiency for a specific structure. The detailed balance approach to calculating theoretical efficiency gives an upper bound that can be independent of material parameters and device design. Other models predict efficiencies that are closer to those that have been achieved. Changing reference spectra and the choice of concentration further complicate comparison of theoretical efficiencies. This paper provides a side-by-side comparison of theoretical efficiencies of multi-junction solar cells calculated with the detailed balance approach and a common one-dimensional-transport model for different spectral and irradiance conditions. Also, historical experimental champion efficiencies are compared with the theoretical efficiencies.
 40.8% efficient inverted triple-junction solar cell with two independently metamorphic junctions
Abstract: A photovoltaic conversion efficiency of 40.8% at 326 suns concentration is demonstrated in a monolithically grown, triple-junction III–V solar cell structure in which each active junction is composed of an alloy with a different lattice constant chosen to maximize the theoretical efficiency. The semiconductor structure was grown by organometallic vapor phase epitaxy in an inverted configuration with a 1.83 eV Ga.51In.49P top junction lattice-matched to the GaAs substrate, a metamorphic 1.34 eV In.04Ga.96As middle junction, and a metamorphic 0.89 eV In.37Ga.63As bottom junction. The two metamorphic junctions contained approximately 1×105 cm−2 and 2–3×106 cm−2 threading dislocations, respectively.
Abstract: Besides the efficiency, the radiation hardness of a solar cell is one of the key parameters for space applications. Today's GaInP/GaInAs/Ge triple-junction solar cells achieve remaining factors for pmpp of 88 % after 1 MeV electron irradiation at a fluence of 1015 cm-2. The degradation is dominated by the GaInAs middle cell. New solar cell structures with 5 pn-junctions have been developed to further improve the radiation resistance and excellent remaining factors of 95 % for Voc and 93 % for pmpp are reported in this paper. The structure consists of AlGaInP, GaInP, AlGaInAs, GaInAs and Ge active pn-junctions. A 1.1 mum thin Ga0.99In0.01As 4th subcell with a radiation hard layer structure was developed. This subcell has now a remaining factor for Jsc of 95 %. This proves the high radiation hardness of the 5-junction space solar cell concept.
Abstract: Monolithic multi-junction III-V solar cells with five active junctions, namely AlGaInP, GaInP, AlGaInAs, GaInAs and Ge have been realized for the first time. An open circuit voltage of 4826 mV was measured. The cells are expected to benefit of a superior radiation hardness as the thickness of each individual subcell is only in the range of 120-1600 nm. The measurement of important parameters like the IV-characteristics or the quantum efficiency of individual subcells is extremely challenging but first measurements were successful and are discussed in this paper.
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- Wolfgang Guter, Jan Schöne, Simon P. Philipps, Marc Steiner, Gerald Siefer, Alexander Wekkeli, Elke Welser, Eduard Oliva, Andreas W. Bett, and Frank Dimroth,“Current-matched triple-junction solar cell reaching 41.1% conversion efficiency under concentrated sunlight,”Appl. Phys. Lett.,Volume 94, pp. 223504, June 2009
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- Alex S. Gardner, Martin, “A review of snow and ice albedo and the development of a new physically based broadband albedo parameterization,” Journal of Geophysical Research: Earth Surface, Volume 115, Issue F1, March 2010.
- R.R.King, A.Boca, W.Hong, X.-Q.Liu, D.Bhusari, D.Larrabee, K.M.Edmondson, D.C.Law, C.M.Fetzer, S.Mesropian, N.H.Karam, "Band-Gap-Engineered Architectures for High-Efficiency Multijunction Concentrator Solar Cells," 24th European Photovoltaic Solar Energy Conference, Pages 21-25, September 2009.
- Geoffrey S. Kinsey1, Kenneth M.Edmondson, "Spectral response and energy output of concentrator multijunction solar cells," Progress in Photovoltaics: Research and Applications, Volume 17, Issue 5, pp. 279–288, August 2009.
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- R.R.King, "High-efficiency space and terrestrial multijunction solar cells through bandgap control in cell structures," Photovoltaic Specialists Conference, 2002. Conference Record of the Twenty-Ninth IEEE, Pages 776-781, May 2002.
- Richard R.King, Daniel C.Law, Kenneth M.Edmondson, Christopher M.Fetzer, Geoffrey S.Kinsey, Hojun Yoon, Dimitri D.Krut, James H.Ermer, Raed A.Sherif, and Nasser H.Karam, "Advances in High-Efficiency III-V Multijunction Solar Cells," Advances in OptoElectronics, Volume 2007, Article ID 29523, 8 pages, September 2007.
- R.R.King, D.C.Law, K.M.Edmondson, C.M.Fetzer, G.S.Kinsey, H.Yoon, R.A.Sherif, and N.H.Karam, "40% efficient metamorphic GaInP/GaInAs/Ge multijunction solar cells," Applied Physics Letters, Volume 90, Issue 18, Lett 90, Pages 183516, May 2007.
- Dev Millstein and Surabi Menon, "Regional climate consequences of large-scale cool roof and photovoltaic array deployment," Environ. Res. Lett., Volume 06, Issue 03, Pages 034001, July 2011.
- 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.
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- J. F. Geisz, Sarah Kurtz, M. W. Wanlass, J. S. Ward, A. Duda, D. J. Friedman, J. M. Olson, W. E. McMahon, T. E. Moriarty, and J. T. Kiehl, "High-efficiency GaInP/GaAs/InGaAs triple-junction solar cells grown inverted with a metamorphic bottom junction," Applied Physics Letters, Volume 91, Issue 2, pp. 023502, July 2007.
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