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Multijunction photovoltaic cells literature review

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literature review[edit]

A review of snow and ice albedo and the development of a new physically based broadband albedo parameterization[1][edit]

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.

Concentrator multijunction solar cell characteristics under variable intensity and temperature[2][edit]

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. They use a High-Intensity Pulsed Solar Simulator as the illumination source, and then use reference "isotype" single-junction cells to calibrate the simulator output. They find that the voltage temperature coefficients exhibit the expected decrease with concentration. This bodes well for future, high-concentration systems and implies a performance cells in these operating ranges.

Current-matched triple-junction solar cell reaching 41.1% conversion efficiency under concentrated sunlight[3][edit]

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).

Evolution of the high efficiency triple junction solar cell for space power[4][edit]

Abstract
Emcore is presently qualifying the fourth generation triple junction (3J) solar cell under the Air Force ManTech program. This cell referred to as the ZTJ, is designed to achieve 30% conversion efficiency under 1 sun AM0 illumination. In addition a companion cell with a monolithically integrated protection diode referred to as the ZTJM is under qualification at Emcore with a target AM0 efficiency of 29.5%. We present performance results of each of these 3J cells from pilot line production. In addition, modeling results of various 3J solar cell designs are discussed to motivate the use of a new 3J architecture that relieves the constraint of an active germanium subcell. This new architecture known as inverted metamorphic multi-junction (IMM) allows one to more easily realize the optimum set of band gaps in monolithically connected n-junction solar cell devices. Finally, recent performance results of our fifth generation 3J cell known as the 3J IMM are reported here.

Experimental Results From Performance Improvement and Radiation Hardening of Inverted Metamorphic Multijunction Solar Cells[5][edit]

Abstract
This paper discusses results from continued development of inverted metamorphic multijunction (IMM) solar cells with air mass zero (AM0) conversion efficiencies greater than 34%. An experimental best four-junction IMM (IMM4J) design is presented. In an effort to improve IMM performance in space radiation environments, 1-MeV electron irradiation studies are conducted on the individual IMM4J subcells. These data are used to engineer an IMM4J structure with beginning of life AM0 conversion efficiency of approximately 34% and an end of life (EOL) remaining factor greater than 82%, where EOL is defined as performance after exposure to 1-MeV electron irradiation at 1E15 e/cm$^2$ fluence. Next-generation IMM designs are explored and an avenue toward AM0 conversion efficiencies of greater than 35% is presented.

Progress and challenges for next-generation high-efficiency multijunction solar cells[6][edit]

Abstract
The demand for higher efficiency motivates the development of ultrahigh-performance next-generation multijunction cells. They use two approaches to obtain optimal bandgaps while maintaining near-ideal materials quality: either use lattice-mismatched combinations of the AlGaInAsP existing materials and develop ways to mitigate the effects of dislocation arsing from the mismatch strain, or develop novel lattice-matched materials not included in the conventional AlGaInAsP materials set. They find that lattice-mismatched paths provide a more flexible way of achieving the desired bandgaps including control of dislocations; minimizing the required thickness of the graded composition layer; residual strain effects on process yield; and for the inverted approach. Several new generations of multijunction cells get the efficiency above 40%. It is likely that 45% efficiency will be demonstrated with the next couple of years.

High-efficiency quadruple junction solar cells using OMVPE with inverted metamorphic device structures[7][edit]

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[8][edit]

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

The path to 1 GW of concentrator photovoltaics using multijunction solar cells[9][edit]

Abstract
This paper presents an overview of the status of the High-Concentration Photovoltaic 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. They find that large-scale development of HCPV systems will depend on demonstrating reliable field operation. Module reliability must be coupled to the definition and execution of a qualifications test produce that takes into account the peculiarities of the HCPV systems. They also obtain that the similarities between the concentrating PV systems and wind energy systems suggest that a fast transition is possible to take HCPV module production from the 1MW to over 1GW in the near future.

Transmission of solar radiation by clouds over snow and ice surfaces a parameterization in terms of optical depth solar zenith angle and surface albedo[10][edit]

Abstract
A multilevel spectral radiative transfer model is used to develop simple but accurate parameterizations for cloud transmittance as a function of cloud optical depth, solar zenith angle, and surface albedo, for use over snow, ice, and water surfaces. The same functional form is used for broadband and spectral transmittances, but with different coefficients for each spectral interval. When the parameterization is applied to measurements of “raw” cloud transmittance (the ratio of downward irradiance under cloud to downward irradiance measured under clear sky at the same zenith angle), an “effective” optical depth τ is inferred for the cloud field, which may be inhomogeneous and even patchy. This effective optical depth is only a convenient intermediate quantity, not an end in itself. It can then be used to compute what the transmittance of this same cloud field would be under different conditions of solar illumination and surface albedo, to obtain diurnal and seasonal cycles of cloud radiative forcing. The parameterization faithfully mimics the radiative transfer model, with rms errors of 1%–2%. Lack of knowledge of cloud droplet sizes causes little error in the inference of cloud radiative properties. The parameterization is applied to pyranometer measurements from a ship in the Antarctic sea ice zone; the largest source of error in inference of inherent cloud properties is uncertainty in surface albedo.

40% efficient metamorphic GaInP/GaInAs/Ge multijunction solar cells[11][edit]

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.

Band-Gap-Engineered Architectures for High-Efficiency Multijunction Concentrator Solar Cells[12][edit]

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.

Multijunction Photovoltaic Technologies for High-Performance Concentrators[13][edit]

Abstract
Multijunction solar cells provide high-performance technology pathways leading to potentially low-cost electricity generated from concentrated sunlight. The National Center for Photovoltaics at the National Renewable Energy Laboratory has funded different III-V multijunction solar cell technologies and various solar concentration approaches. Within this group of projects, III-V solar cell efficiencies of 41% are close at hand and will likely be reported in these conference proceedings. Companies with well-developed solar concentrator structures foresee installed system costs of $3/watt-half of today's costs-within the next 2 to 5 years as these high-efficiency photovoltaic technologies are incorporated into their concentrator photovoltaic systems. These technology improvements are timely as new large-scale multi-megawatt markets, appropriate for high performance PV concentrators, open around the world.

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

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.

Spectral response and energy output of concentrator multijunction solar cells[15][edit]

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.

III–V compound multi-junction solar cells: present and future[16][edit]

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.

High-efficiency space and terrestrial multijunction solar cells through bandgap control in cell structures[17][edit]

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[18][edit]

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.

Modeling the effect of varying spectra on multi junction A-SI solar cells[19][edit]

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.

Super high-efficiency multi-junction and concentrator solar cells[20][edit]

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 400 W/m2 for terrestrial applications and high-efficiency, light-weight and low-cost MJ solar cells for space applications.

Super-High Efficiency Multijunction Photovoltaics For Solar Energy Harvesting[21][edit]

Abstract
Energy harvesting and alternative renewable energy techniques are currently some of the most sought after research topics for engineers and scientists. Global warming has forced the researchers to abandon the rampant use of coal technology and find out alternative ways to harvest energy whether it maybe solar, wind, water, tides, geothermal heat, ocean waves, bio-fuel etc. Sunlight is the most abundant renewable energy source with an intensity of approximately 0.1W/cm² and over 1.5×10²²J (15,000 Exajoules) reaching earth’s surface everyday. This enormous energy is 10,000 times greater than the daily consumption of 1.3EJ of the world. The single junction solar PV cells have produced very small solar conversion efficiency. The normally available photovoltaics have the conversion efficiency in the range of 8% -12%. This limitation has led to cutting edge researches in the photovoltaic area giving rise to the concept of multijunction solar photovoltaic cells. Multijunction solar cells direct the sunlight towards matched spectral sensitivity by splitting the spectrum into smaller slices. The main challenge in the photovoltaic industry is to make the modules more cost effective. The high efficiency multijunction photovoltaics have played a very significant role in reducing the cost through concentrator photovoltaic systems being implemented around the world. National Renewable Energy Laboratory (NREL) and US Department of Energy have funded several III-IV multijunction solar cell projects. In our research we have introduced a new three layer multijunction photovoltaic material based on InP/InGaAs/InGaSb and four-layers PV comprised of AlGaAs/GaAs/InGaAs/InGaSb and AlGaAs/InP/InGaAs/InGaSb and have drawn a comparison of solar energy absorption, reflection and transmission with existing single-junction and multijunction cells. We discovered that the inclusion of InGaSb layer in the design has made a significant difference in absorption in the spectral range of 598nm-800nm, contributing to a higher efficiency of the solar cell.

Plasmonics for improved photovoltaic devices[22][edit]

Abstract
The emerging field of plasmonics has yielded methods for guiding and localizing light at the nanoscale, well below the scale of the wavelength of light in free space. Now plasmonics researchers are turning their attention to photovoltaics, where design approaches based on plasmonics can be used to improve absorption in photovoltaic devices, permitting a considerable reduction in the physical thickness of solar photovoltaic absorber layers, and yielding new options for solar-cell design. In this review, we survey recent advances at the intersection of plasmonics and photovoltaics and offer an outlook on the future of solar cells based on these principles.

Towards high-efficiency multi-junction solar cells with biologically inspired nanosurfaces[23][edit]

Abstract
Multi-junction solar cells offer extremely high power conversion efficiency with minimal semiconductor material usage, and hence are promising for large-scale electricity generation. However, suppressing optical reflection in the UV regime is particularly challenging due to the lack of adequate dielectric materials. In this work, bio-inspired antireflective structures are demonstrated on a monolithically grown Ga0.5In0.5P/In0.01Ga0.99As/Ge triple-junction solar cell, which overcome the limited optical response of reference devices. The fabricated device also exhibits omni-directional enhancement of photocurrent and power conversion efficiency, offering a viable solution to concentrated illumination with large angles of incidence. A comprehensive design scheme is further developed to tailor the reflectance spectrum for maximum photocurrent output of tandem cells.

Utilization of Direct and Diffuse Sunlight in a Dye-Sensitized Solar Cell Silicon Photovoltaic Hybrid Concentrator System[24][edit]

Abstract
The concept of a tandem hybrid concentrator solar module was demonstrated from a dye-sensitized TiO2 solar cell (DSSCs) and a silicon p−n junction solar cell. The test system employed DSSC and Si cells with indoor AM1.5G efficiencies of 9.1 and 18.1%, respectively. Two different optical filters were used to selectively reflect and concentrate near-infrared light from the DSSC onto the Si cell. On the basis of outdoor testing in a 2× concentrator−reflector arrangement, the tandem system generated 93 and 96% of the output power of directly illuminated Si cells under altostratus/cirrostratus and clear sky irradiances, respectively, despite a DSSC-to-Si active area ratio of only 0.92. Similar performance is expected at higher (5−10×) concentration ratios. The hybrid arrangement of visible- and IR-absorbing solar cells addresses the problem of lower performance of conventional concentrators under diffuse irradiance conditions. These proof-of-concept results suggest that system level efficiencies approaching 20% should be achievable.

Nanostructures for photovoltaics[25][edit]

Abstract
The use of various nanostructures in new solar cell designs and modes of enhancing conventional solar cells are described. The cell designs and enhancements are categorized by the type of nanostructure utilized. These include: (a) bulk nanostructured materials [3D]; (b) quantum wells [2D]; (c) nanowires [1D]; and (d) quantum dots/nanoparticles [0D]. The methods of fabricating such structures are first described, followed by examples from the literature of how they have been utilized in a photovoltaic application. Scientific challenges associated with nanostructured photovoltaic devices are also discussed, followed by the prospects for use in real applications.

Multi-junction solar cells and novel structures for solar cell applications[26][edit]

Abstract
The present status of R&D program for super-high efficiency 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 confinements, and lattice matching between active cell layers and substrate, InGaP/InGaAs/Ge monolithic cascade 3-junction cells with an efficiency of 31.7% at 1-sun AM1.5 and InGaP/GaAs//InGaAs mechanically stacked 3-junction cells with the highest (world-record) efficiency of 33.3% at 1-sun AM1.5 have been realized. As an approach for low-cost and high-efficiency cells, better radiation resistance of GaAs thin-film solar cells with novel structures fabricated on Si substrates has also been demonstrated. Novel structures such as Bragg reflector and super-lattice structures are found to show a better initial cell performance and radiation resistance since those layers act as buffer layers to reduce dislocations, and act as a back-surface field and back-surface reflector layers.

Compound Semiconductor Nanowire Solar Cells[27][edit]

Abstract
There have been many recent developments in compound semiconductor nanowire photovoltaic devices. Of these, advances in nanowire synthesis and performance enable nanowires to be implemented for efficient and low-cost solar-energy-harvesting devices. On the other hand, many challenges in device fabrication must be resolved in order for nanowires to assure a role at the forefront of solar cell technology.

Japanese R&D Activities of High Efficiency III-V Compound Multi-Junction and Concentrator Solar Cells[28][edit]

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.

Performance analysis of a novel concentrating photovoltaic combined system[29][edit]

Abstract
In the present study, a novel Concentrating Photovoltaic Combined System (CPVCS) based on the spectral decomposing approach is introduced, modeled, tested experimentally and evaluated thermodynamically and economically. In this study, energy and exergy analyses of the system have been evaluated, economical analysis has been performed and the experimental results have been compared to data obtained by the control system. As a result, energy efficiencies of concentrator, vacuum tube and overall CPVCS have been determined to be 15.35%; 49.86%; and 7.3% respectively. Similarly the second law (exergy) efficiencies of concentrator, vacuum tube and overall CPVCS are 12.06%; 2.0%; and 1.16% respectively. The cost of energy production has been stated as 6.37 $/W and it is predicted that this value could be decreased by improving the system performance.

Efficiency Enhancement of Organic and Thin-Film Silicon Solar Cells with Photochemical Upconversion[30][edit]

Abstract
The efficiency of thin-film solar cells with large optical band gaps, such as organic bulk heterojunction or amorphous silicon solar cells, is limited by their inability to harvest the (infra)red part of the solar spectrum. Photochemical upconversion based on triplet–triplet annihilation (TTA-UC) can potentially boost those solar cells by absorbing sub-bandgap photons and coupling the upconverted light back into the solar cell in a spectral region that the cell can efficiently convert into electrical current. In the present study we augment two types of organic solar cells and one amorphous silicon (a-Si:H) solar cell with a TTA-upconverter, demonstrating a solar cell photocurrent increase of up to 0.2% under a moderate concentration (19 suns). The behavior of the organic solar cells, whose augmentation with an upconverting device is so-far unreported, is discussed in comparison to a-Si:H solar cells. Furthermore, on the basis of the TTA rate equations and optical simulations, we assess the potential of TTA-UC augmented solar cells and highlight a strategy for the realization of a device-relevant current increase by TTA-upconversion.

Impact of spectral effects on the electrical parameters of multijunction amorphous silicon cells[31][edit]

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 red/infra-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.

Multi-junction III–V solar cells: current status and future potential[32][edit]

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.

Analysis of thermoelectric characteristics of AlGaN and InGaN semiconductors[33][edit]

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 temperature-dependent operation[34][edit]

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.

Novel materials for high-efficiency III–V multi-junction solar cells[35][edit]

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.

Theoretical performance of multi-junction solar cells combining III-V and Si materials[36][edit]

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.

Four-junction spectral beam-splitting photovoltaic receiver with high optical efficiency[37][edit]

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[38][edit]

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[39][edit]

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. All 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[40][edit]

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.

Radiative coupling effects in GaInP/GaAs/Ge multijunction solar cells[41][edit]

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.

InGaP/GaAs-based multijunction solar cells[42][edit]

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.

Variation in spectral irradiance and the consequences for multi-junction concentrator photovoltaic systems[43][edit]

Abstract
The most fundamental figure of merit for a solar collector is its power rating under a standard solar spectrum. For concentrator systems that employ highly efficient multi-junction solar cells, the rating of the panel becomes sensitive to the spectrum of the direct beam sunlight. By considering typical spectral conditions at a measurement site, we show that the power rating can vary by up to 16%. Further, we consider to what extent a single reference spectrum can be used to characterize the irradiance at a particular location. Electrical energy yields produced using both synthesized and standard reference spectra are compared, with standardized references shown to be unsuited for accurate energy yield predictions under realistic spectral conditions.

What is an air mass 1.5 spectrum?[44][edit]

Abstract
The origin of the AM 1.5 spectra, how they are related to actual outdoor spectral distributions, and the implications for outdoor PV (photovoltaic) performance predictions are explained. It is pointed out that the AM 1.5 spectra provide a reference point corresponding to a particular set of atmospheric conditions and a specific air mass. One can expect variations in outdoor PV device performance for different atmospheric conditions and air masses. The uncertainty in using AM 1.5 spectra to predict field performance depends on the particular PV device design and climate. The wavelength distribution of photon flux varies with respect to conditions such as water vapor and air mass, and this in turn influences current densities in PV devices, depending on such device characteristics as bandgap(s). Therefore, PV device design (e.g. optimization) should be based on a range of spectra representing various atmospheric conditions and air masses.

Lattice-mismatched approaches for high-performance, III-V photovoltaic energy converters[45][edit]

Abstract
We discuss lattice-mismatched (LMM) approaches utilizing compositionally step-graded layers and buffer layers that yield III-V photovoltaic devices with performance parameters equaling those of similar lattice-matched (LM) devices. Our progress in developing high-performance, LMM, InP-based GaInAs/InAsP materials and devices for thermophotovoltaic (TPV) energy conversion is highlighted. A novel, monolithic, multi-bandgap, tandem device for solar PV (SPV) conversion involving LMM materials is also presented along with promising preliminary performance results.

Theoretical analysis of the optimum energy band gap of semiconductors for fabrication of solar cells for applications in higher latitudes locations[46][edit]

Abstract
In this work some results of theoretical analysis on the selection of optimum band gap semiconductor absorbers for application in either single or multijunction (up to five junctions) solar cells are presented. For calculations days have been taken characterized by various insolation and ambient temperature conditions defined in the draft of the IEC 61836 standard (Performance testing and energy rating of terrestrial photovoltaic modules) as a proposal of representative set of typical outdoor conditions that may influence performance of photovoltaic devices. Besides various irradiance and ambient temperature ranges, these days additionally differ significantly regarding spectral distribution of solar radiation incident onto horizontal surface. Taking these spectra into account optimum energy band gaps and maximum achievable efficiencies of single and multijunction solar cells made have been estimated. More detailed results of analysis performed for double junction cell are presented to show the effect of deviations in band gap values on the cell efficiency.

Prediction of energy effects on photovoltaic systems due to snowfall events[47][edit]

Abstract
The accurate prediction of yields from photovoltaic systems (PV) is critical for their proper operation and financing, and in northern latitudes the effects of snowfall on yield can become significant. This work provides methods for identifying snowfall effects from commonly collected performance data, and recommends a model to allow for prediction of these effects based solely on meteorological time series. The model was validated with data from two large-scale (>;8MW) operational PV plants. For the low tilt angles most affected by snowfall, this analysis was able to accurately predict both daily and mean values of snow effects. This methodology will enable system operators to utilize performance data to accurately identify and predict snowfall losses, and will assist system designers to optimize for the effects of snowfall on new system designs.

Efficiency of multijunction photovoltaic systems[48][edit]

Abstract
In this paper, an expression for the PV system efficiency is derived that can be used in conjunction with measured device performance and detailed numerical modeling to analyze PV system performance. Such an analysis will help identify design trade-offs and also help to identify which system and cell design changes will be of greatest benefit to the enhancement of PV system performance.

Effects of snow cover on UV irradiance and surface albedo: A case study[49][edit]

Abstract
A heavy snowfall, followed by several days of cloudless skies before significant snow melt had occurred, enabled a quantitative study of the effects of snow on downwelling UV spectral irradiances at the National Institute for Water and Atmospheric Research UV measurements site in Lauder, New Zealand. The largest UV enhancements (>70%) were seen during partly cloudy conditions immediately after the snowfall. A radiative transfer model was used to quantify the enhancements due to the snow cover and the spectral albedo of the snow under clear-sky conditions. The first cloudless day on which the radiative transfer model could be used with confidence occurred 7 days after the snowfall. By this time, the maximum enhancements due to snow at solar zenith angle (SZA) 70° were approximately 22% in the UV-A region. In the UV-B region, the enhancements were approximately 28% and tended to increase slightly at larger SZA. The corresponding surface albedo was 0.62±0.08, and comparison with supplementary measurements indicated that the albedo decayed with time. Any spectral or SZA dependencies in the enhancements were below the measurement uncertainties in the UV region. Comparisons with supplementary data indicated that the albedo immediately after the snow was greater than 0.8.

Spectral effects on PV-device rating[50][edit]

Abstract
A recently developed spectral model “SEDES2” is applied to study the effect of variations in solar spectral irradiance on the efficiency of seven particular solar cells. As a new feature, SEDES2 calculates hourly solar spectral irradiance for clear and cloudy skies from readily available site-specific meteorological data. Based on these hourly spectra, monthly and yearly efficiencies for the solar cells are derived. As a key result the efficiencies of amorphous silicon cells differ by 10% between winter and summer months because of spectral effects only. A second intention of this study is to analyse the sensitivity of power and energy rating methods to spectral irradiance but also to total irradiance and cell temperature. As an outcome, a multi-value energy rating scheme applying the concept of “critical operation periods” is proposed.

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