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Lattice-matched multiple quantum wells (MQWs) consisting of InxGa1-xAs wells with very thin GaAs0.2P0.8 barriers have been incorporated into a GaInP/GaAs tandem solar cell. InGaAs/GaAsP MQWs increase the short-circuit current of the GaAs cell by extending the absorption range, with minimal impact on an open-circuit voltage, thus alleviating current matching restrictions placed by the GaAs cell on multijunction solar cells. MQWs with very thin, tensile strained, high phosphorus content GaAsP barriers allow tunneling to dominate carrier transport across the MQWs and balance the compressive strain of the InGaAs wells such that material quality remains high for subsequent top cell growth. We show that the addition of the QW layers enhances the GaAs cell, does not degrade the performance of the GaInP top cell, and leads to potential efficiency enhancements.

Multijunction III-V concentrator cells of several different types have demonstrated solar conversion efficiency over 40% since 2006, and represent the only third-generation photovoltaic technology to enter commercial power generation markets so far. The next stage of solar cell efficiency improvement, from 40% to 50%-efficient production cells, is perhaps the most important yet, since it is in this range that concentrator photovoltaic (CPV) systems can become the lowest cost option for solar electricity, competing with conventional power generation without government subsidies. The impact of 40% and 50% cell efficiency on cost-effective geographic regions for CPV systems is calculated in the continental US, Europe, and North Africa. We take a systematic look at a progression of multijunction cell architectures that will take us up to 50% efficiency, using modeling grounded in well-characterized solar cell materials systems of today's 40% cells, discussing the theoretical, materials science, and manufacturing considerations for the most promising approaches. The effects of varying solar spectrum and current balance on energy production in 4-junction, 5-junction, and 6-junction terrestrial concentrator cells are shown to be noticeable, but are far outweighed by the increased efficiency of these advanced cell designs. Production efficiency distributions of the last five generations of terrestrial concentrator solar cells are discussed. Experimental results are shown for a highly manufacturable, upright metamorphic 3-junction GaInP/GaInAs/Ge solar cell with 41.6% efficiency independently confirmed at 484 suns (48.4 W/cm2) (AM1.5D, ASTM G173-03, 25 °C), the highest demonstrated for a cell of this type requiring a single metal-organic vapor-phase epitaxy growth run. Copyright © 2012 John Wiley & Sons, Ltd.

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 (I^(2)R) 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.

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. Copyright © 2008 John Wiley & Sons, Ltd.

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.

Recent advances in Spectrolab's high efficiency III-V multi-junction space solar cells are reported. The latest generation GaInP/GaAs/Ge solar cells under production-the Ultra Triple Junction (UTJ)-have an average efficiency at maximum power of 28.0% (AMO, 28°C, 135.3 m W/cm 2 ) at beginning of life (BOL), and 22.6% at 60°C at end of life (EOL, 15 years in GEO orbit). This represents a 5.5% relative higher efficiency at EOL, compared with the previous generation Improved Triple Junction (ITJ) cells. The P/P 0 -for 1 MeV electrons after 10 15 cm -2 fluence has also improved to 0.86, compared with 0.84 for the ITJ cells. These and other performance results of UTJ solar cells are reviewed in comparison with ITJ solar cells.

This paper focuses on the metal-organic vapor-phase epitaxy growth of 3-junction (3J) solar cells where the epitaxial Ga 0.44 In 0.56 P top and Ga 0.92 In 0.08 As middle subcells are grown lattice-mismatched on a Ge substrate. Single-junction metamorphic devices with 8% and 12%-In, GaInAs are grown on 100 mm dia. (0 0 1) Ge substrates and evaluated in comparison to approximately lattice-matched GaAs and Ga 0.99 In 0.01 As subcells. Layers are observed to be nearly 100% relaxed by high-resolution X-ray diffraction. Threading dislocation densities of ∼2×10 5 cm −2 in the 8%-In layers are observed by electron beam induced current and cathodoluminescence. Single-junction devices show a constant offset between open-circuit voltage and bandgap of ∼380 mV. Building upon these results, 3J metamorphic Ga 0.44 In 0.56 P/Ga 0.92 In 0.08 As/Ge solar cells are fabricated. Very high performances of small area devices are reported with 28.8% efficiency under the AM0 spectrum and 31.3% efficiency under the AM1.5G 1-sun terrestrial spectrum.

Inverted metamorphic (IMM) solar cells are being pursued for space applications. This work examines their extended performance under key environmental conditions. Electron irradiation with 1 MeV electrons found 81% of the power remained after exposure to 1e15/ cm2 fluence. Test strings mounted on rigid panels were subjected to 11,000 thermal cycles without electrical degradation. Thermal soak testing at 250 °C identified 1% of efficiency loss, equivalent to production devices. No performance loss was found after 30 days of humidity exposure. These tests verify the integrity of IMM devices, while matching measurements by NREL confirm the quality of Spectrolab's IMM testing method.

Dual-junction Ga 0.5 In 0.5 P/GaAs solar cells on Ge substrates have rapidly gone from small, high-efficiency laboratory cells, to large-area, high-efficiency cells manufactured at Spectrolab in high volume. Over 500,000 of these dual-junction (DJ) cells with 27-cm 2 area have been produced, with average AM0 load point efficiency of 21.4%. The next step in the evolution of this type of multijunction solar cell has been taken, with the development of triple-junction (TJ) Ga 0.5 In 0.5 P/GaAs/Ge cells. The addition of the germanium third junction, plus several significant improvements in the device structure, have led to a measured efficiency of 27.0% (AM0, 28°C) at Spectrolab on large-area (>30 cm 2 ) TJ cells. The TJ cell is now in production at Spectrolab. Ga 0.5 In 0.5 P/GaAs/Ge cells are viable not only for non-concentrating space applications, but also for terrestrial and space concentrator systems. Efficiencies up to 32.3% at 47 suns under the terrestrial AM1.5D spectrum have been achieved.

Recent progress in III-V multijunction sp ace solar cell has led to Spectrolab’s GaIn P/GaAs/Ge triple-junction, XTJ, cells with average 1-sun efficiency of 29% (AM0, 28oC) for cell size ranging from 59 to 72-cm 2 . High-efficiency inverted metamorphic (IMM) multijunction cells are developed as the next space solar cell architecture. Spectrolab’s large-area IMM3J and IMM4J cells have achieved 33% and 34% 1-sun, AM0 efficiencies, respectively. The IMM3J and the IMM4J cells have both demonstrated normalized power retention of 0.86 at 5x10 14 e - /cm 2 fluence and 0.83 and 0.82 at 1x10 15 e - /cm 2 fluence post 1-MeV electron radiation, respectively. The IMM cells were further assembled into coverglass-interconnect-cell (CIC) strings and affixed to typical rigid aluminum honeycomb panels for thermal cycling characterization. Preliminary temperature cycling data of two coupons populated with IMM cell strings showed no performance degradation. Spectrolab has also developed semiconductor bonded technology (SBT) where high-performance component subcells were grown on GaAs and InP substrates separately then bonded directly to form the final multijunction cells. Large-area SBT 5-junction cells have achieved a 35.1% efficiency under 1-sun, AM0 condition. Keywords: space solar cells, III-V multijunction, inverted me tamorphic, semiconductor bonding technology.

High efficiency Inverted Metamorphic (IMM) multi-junction solar cells have been under development at Spectrolab for use in space and near space applications This paper reviews the present state-of-the-art of this technology at Spectrolab with an emphasis on performance characterization data at in-flight operating conditions. Large area IMM3J and IMM4J solar cells with 1X AM0 efficiency greater than 32% at 28 °C have been fabricated and characterized. Degradation factors after exposure to 1 MeV electron irradiation for both IMM3J and IMM4J technologies is presented. A coupon utilizing large area, IMM solar cells has been assembled and subjected to thermal cycling. Pre-and post thermal cycling data have been collected. Preliminary temperature cycling data indicate that a small coupon populated with strings of these cells suffered no degradation.

Solar cells suitable for the space environment must combine high-efficiency, high energy density, and radiation hardness in a manufacturable design. As improvement in one performance parameter usually results in degradation in one or more of the remaining parameters, careful optimization is required to enhance overall performance. The ultra triple-junction cell developed builds upon the established success of the fully qualified improved triple-junction cell currently in production. In the ultra triple-junction cell configuration, improved robustness and efficiency after radiation exposure augment a cell design expected to deliver 28% beginning-of-life efficiency in production.

Multijunction solar cells used in concentrating photovoltaic (CPV) systems deliver solar power at low cost in high DNI climates where the solar resource is greatest. Because the cells subdivide the solar spectrum and are used with optical systems of varying spectral response, careful evaluation and optimization of cell performance under real field conditions is important. This paper presents modeled performance of the latest generation Spectrolab cells in systems with PMMA and silicone-on-glass optics, using a simple cell temperature model, TMY3 data, and the SMARTS spectral irradiance model. Results show that the improved efficiency of C4P5 cells compared to the lattice-matched C3P5 cells is more pronounced in systems with PMMA optics.

High-efficiency Inverted Metamorphic (IMM) multi-junction solar cells are being developed at Spectrolab for use in space and near-space applications. Recently, large-area (26-cm2) IMM3J cells achieved a 1-sun, AM0 conversion efficiency of 32% with an open-circuit-voltage of 3.04 V, a short-circuit current-density of 16.7 mA/cm2, and a fill factor of 0.84. In addition, IMM4J cells (1-cm2) reached a 1-sun, AM0 conversion efficiency of 33%. The 4-junction cell achieved an open-circuit-voltage of 3.42 V, a short-circuit current-density of 15.8 mA/cm2, and a fill factor of 0.82. Both the inverted metamorphic 3J and 4J cells reveal excellent component subcell quantum efficiency. Improvements in crystal growth, as well as more optimal subcell current density balance, will further raise the 1-sun, AM0 efficiency of the 3J and the 4J IMM cells to 33% and 35% respectively. The latest status of the 72-cm2 XTJ (based on 150-mm Ge substrate) Supercells will also be discussed.

Prototype Inverted Metamorphic (IMM3J) cells were grown and fabricated and assembled onto an experimental flight coupon for inclusion on the 7th Materials International Space Station Experiment (MISSE-7). This paper examines the first eleven months of on-orbit data of prototype IMM3J solar cells in a low earth orbit (LEO) environment. The prototype IMM3J solar cells show excellent electrical and mechanical stability over the first eleven months in orbit with 97–98% in Jsc and maximum power (Pmp) retention for the four actively measured IMM3J solar cells. The loss in Jsc and Pmp is likely due to etching of the anti-reflective glasscover coating due to atomic oxygen.

High efficiency Inverted Metamorphic (IMM) and Semiconductor Bonded Technology (SBT) multi-junction solar cells have been under development at Spectrolab for use in space and near space applications. This paper will review the present state-of-the-art of this technology at Spectrolab with an emphasis on performance characterization data at operating conditions that these solar cells will experience in flight. Solar cell current-bias characteristics under illumination (LIV) at AM0 28°C are presented along with external quantum efficiency measurements that are used to verify the X-25 solar simulator LIV short circuit current density. A mechanical and thermal stress model has been used to predict mechanical stresses on a ultra-lightweight panel assembly in orbit and will be discussed.

High efficiency multi-junction solar cells utilizing inverted metamorphic1,2 and semiconductor bonding technology3 are being developed at Spectrolab for use in one-sun space and near-space applications. Recently that effort has been extended to include low-concentration space applications. This paper will review the present state-of-the-art cell technologies at Spectrolab, with an emphasis on performance characterization data at both 1-sun and low-concentration operating conditions that these cells will experience in flight‥ A cell coupon utilizing IMM solar cells has been assembled and subjected to thermal cycling. Pre-and post thermal cycling data have been collected and there is no performance degradation or mechanical issues after the test.

Prismatic cover experiments were performed on GaInP/Ga(In)As/Ge triple-junction concentrator solar cells having grid designs with up to 39% metal coverage. The devices were characterized both before and after the application of optically-clear silicone covers patterned with cylindrical-lens microstructures. In solar-simulator LIV measurements at 1 sun and at 500 suns, we observed a boost in the efficiency of prism-covered cells, relative to before covering, of up to 34%, depending on the grid coverage fraction. The spectral response of the cells before and after covering was also characterized, and the observed boost in external quantum efficiency was found to be consistent with the LIV performance gains. Also studied were the lateral tolerances allowed by the prismatic covers, for alignment to the underlying cell grid structure, as a function of the maximum angle of incidence of the light impinging onto the cell. Ray-tracing simulations were performed, and found to be consistent with preliminary angular-dependence spectral response measurements done under monochromatic illumination.

Multijunction solar cells divide the solar spectrum into smaller slices, delivering experimental efficiencies over 40%, and enabling theoretical efficiency over 60%. These high efficiency cells have a powerful effect on the cost effectiveness of new concentrator photovoltaic systems now being deployed around the world, making this technology one of the most viable options for plentiful solar-generated electricity.

The Materials on the International Space Station Experiments (MISSE) present a unique opportunity in space science by offering a low-cost platform to expose materials directly to the space environment on the International Space Station (ISS). MISSE experiments consist of a “suitcase” like package known as the “Passive Experiment Carrier” (PEC) that can be carried by astronauts and mounted externally to the ISS. The 5th MISSE payload (MISSE-5) contained both passive and active experiments. The Forward Technology Solar Cell Experiment (FTSCE) on MISSE-5 measured current-voltage (I–V) characteristics on 36 solar cells of various types. Over 1500 I–V curves were recorded on each cell during a 13-month period. This paper analyses the results for all the III–V multi-junction cells flown, including state-of-the-art space qualified cells and next generation metamorphic cells.

Inverted metamorphic (IMM) solar cells based on III–V materials have the potential to achieve solar conversion efficiencies that are significantly higher than today's state of the art solar cells which are based on the 3-junction GaInP/GaInAs/Ge design. The 3J IMM device architecture based on (Al)GaInP/GaInAs/GaInAs, for example, allows for a higher voltage solar cell by replacing the low bandgap Ge (0.67 eV) from the conventional 3J structure with the higher bandgap (∼1 eV) metamorphic GaInAs. The inverted growth simply allows the lattice-matched junctions (i.e., (Al)GaInP/GaInAs) to be grown first on the growth substrate, thereby minimizing or shielding them from the defects that arise from the metamorphic layers. Spectrolab has demonstrated 30.5% AM0 efficiency based on the 3J IMM cell architecture grown on a Ge substrate, with V oc = 2.963V, J sc = 16.9 mA/cm2, and FF = 82.5%. In addition, 4J IMM cells have been demonstrated with V oc of 4.072 V and AM0 efficiency approaching 25%. With additional development, demonstrating 33% AM0 efficiency is expected in the near future. However, the IMM devices demand more complex processing requirements than conventional solar cells, and we demonstrate the capability to fabricate large area solar cells from standard Ge solar cell substrates.

Solar cells used for all Mars surface applications have been commercially available space qualified AM0 devices optimized for Earth orbiting geosynchronous applications. Due to fine dust circulating in the Martian atmosphere, which reduces the short wavelength light component, these cells are not optimized for Mars surface operation. As a result, these cells operate at less than optimal efficiency. As part of an effort to address Mars applicable technology, sponsored by JPL and funded by NASA, a multi-year program (MOST) was initiated in 2004, to optimize commercial AM0 cells to the Mars surface spectrum. This paper discusses the overall program and performance results for the modified cells. In addition, the performance of the MER solar arrays will be updated to reflect their nearly four (4) year operation. This includes data for the daily peak current and integrated power, the impact of dust deposited on the panels, and the panel temperatures.

Spectrolab presents its next production GaInP/GaAs/Ge space triple junction solar cell, the XTJ space solar cell, averaging 29.8% efficiency at maximum power (AM0, 28°C, 135.3 mW/cm2) at beginning-of-life (BOL) testing of large populations (845 cells) of large-area solar cells without coverglass. Bare cells from this population with area of 26.62 cm2 have been tested to a maximum efficiency of 31.1% under the same AM0 testing. Additional optical and electrical performance characterization will be presented. XTJ is more than a BOL power improvement over heritage products. Bare cell radiation testing shows power retention at max power (NP mp ) of 0.89 and 0.84 after irradiation with 1-MeV energy electrons to a fluence of 5e14 cm−2 and 1e15 cm−2, respectively. Environmental degradation testing of XTJ parts shows only 1.7% degradation for more than 4000 hours at 250°C and no degradation after 90 days at 95% r.h. and 45°C. Together these characteristics enable XTJ to be a 7% improvement in end-of-life cost of power on space flight panel over Spectrolab's 28% BOL efficient UTJ. Finally, XTJ engineering confidence thermal cycle testing and qualification status to the AIAA S-111-2005 standard will be elaborated.

Large-area (26.6 cm2), thin GaInP/GaInAs/Ge triple-junction (TJ) solar cells with thickness as low as 50 mum were demonstrated. The average conversion efficiency of fifty thin TJ cells is 28 %, 1-sun AMO. The thin TJ cells showed nominal performance after welding of interconnects and flexibility test (50 mm curvature radius). Prototype coupons made with these thin TJ cells met flexible solar module bending requirement of 100 mm radius. The thin-cell coupons showed a specific power of 500 W/kg and a power density of 325 W/m2 . Ultra-thin (

Solar cells used for Mars surface applications have been commercial space qualified AM0 optimized devices. Due to the Martian atmosphere, these cells are not optimized for the Mars surface and as a result operate at a reduced efficiency. A multi-year program, MOST (Mars Optimized Solar Cell Technology), managed by JPL Science Mission Directorate (SMD) and funded by NASA Code S, was initiated in 2004, to develop tools to modify commercial AM0 cells for the Mars surface solar spectrum and to fabricate Mars optimized devices for verification. This effort required defining the surface incident spectrum, developing an appropriate laboratory solar simulator measurement capability, and to develop and test commercial cells modified for the Mars surface spectrum. This paper discusses the program, including results for the initial modified cells. Simulated Mars surface measurements of MER cells and Phoenix Lander cells (2007 launch) are provided to characterize the performance loss for those missions. In addition, the performance of the MER rover solar arrays is updated to reflect their more than two (2) year operation.

Metamorphic III-V semiconductor materials offer access to bandgaps that span key portions of the solar spectrum, enabling new bandgap combinations in multijunction solar cells, and increasing both theoretical and practical efficiency limits for terrestrial concentrator cells. Experimental results are given for the quantum efficiency of metamorphic GaInAs solar cells with bandgap from 1.1 to 1.4 eV, and for metamorphic GaInP with both ordered and disordered group-III sublattices. Variable intensity Jsc vs. Voc measurements are used to compare recombination components due to n = 1 and n = 2 mechanisms in metamorphic and lattice-matched GaInAs, GaInP, and 3-junction solar cells. A record efficiency metamorphic GaInP/ GaInAs/ Ge 3-junction solar cell has been produced with 38.8% efficiency independently confirmed (241 suns, AM1.5D, low-AOD, 25°C), essentially equaling the performance of a lattice-matched 3-junction cell with 39.0% efficiency, the highest efficiency yet demonstrated and verified for a solar photovoltaic conversion device. With the combination of high-quality metamorphic materials that are increasingly less controlled by recombination at dislocations, and the higher efficiency limits afforded by freedom of lattice constant selection, practical terrestrial concentrator cell efficiencies well over 40% are expected in the near future.

Thin high-efficiency triple junction GaInP/GaAs/Ge solar cells have been incorporated into prototype flexible blanket coupons with a coupon efficiency of 28% for AM0 (0.1353 W/cm2). Prototype coupons have a specific power density from 360 - 500 W/kg and may well approach > 1000 W/kg with further thin cell and blanket fabrication improvements. Initial LIV results are shown for a thin flexible three-cell and twelve-cell coupon utilizing fully active triple-junction solar cells. This paper will outline the fabrication of prototype three and twelve cell flexible coupons, and initial illuminated I-V (LIV) test results under a large area pulsed solar simulator (LAPSS) at Spectrolab and discusses the advantages and applications of thin flexible GaInP/GaAs/Ge solar blankets. Thin flexible triple-junction solar cells have been demonstrated and incorporated into electrically active prototype blankets with specific power of 500 W/kg.

Solar cells suitable for the space environment must combine high efficiency, high energy density, and radiation hardness in a manufacturable design. As improvement in one performance parameter usually results in degradation in one or more of the remaining parameters, careful optimization is required to enhance overall performance. The ultra triple-junction cell developed by Spectrolab builds upon the established success of the fully qualified improved triple-junction cell currently in production. In the ultra triple-junction cell configuration, improved robustness and efficiency after radiation exposure augment a cell design that is expected to deliver 28% beginning-of-life efficiency in production.

The performance of a series of metamorphic GaInP and GaInAs solar cells grown on Ge with lattice-mismatch ranging from 0% to 2.4% is reported, with emphasis on device structures with 0.5% and 1.6% mismatch. Dual-junction cells with moderately lattice-mismatched (0.2% and 0.5%) structures have already reached electrical performance comparable to lattice-matched devices, at about 26% efficiency under AM0, 1-sun condition. Development efforts to date on highly lattice-mismatched (1.6% mismatch) structures have resulted in 22.6% efficiency dual-junction cells, with many improvements still possible. Spectral response measurements reveal excellent quantum efficiency (QE) for metamorphic GaInP and GaInAs materials, with a measured internal QE of over 90%. The offsets between the bandgap voltage (E/sub g//q) and the open-circuit voltage (V/sub OC/) of GaInP and GaInAs metamorphic cells were kept below 550 mV and 450 mV, respectively. Experimental results indicate that lattice-mismatched GalnP/GalnAs dual-junction cells can achieve higher energy conversion efficiency than lattice-matched GaInP/GaInAs dual-junction solar cells.

We demonstrate a 40% efficient multijunction (MJ) cell capable of being used to make a 33% efficient solar concentrator PV module using the new Cassegrainian PV module design. This very high efficiency module can then produce cost competitive solar electric power.

GaInAsN cells are measured to retain 93 /spl plusmn/ 3% and 89 /spl plusmn/ 4% of their original efficiency after exposure to 5 /spl times/ 10/sup 14/ and 1 /spl times/ 10/sup 15/ cm/sup -2/ 1-MeV electrons, respectively. The rate of degradation is not correlated with the performance at beginning of life (BOL). The depletion width remains essentially unchanged, increasing by < 1%. Temperature-coefficient data for GaInAsN cells are also presented. These numbers are used to project the efficiency of GaInAsN-containing multijunction cells. The GaInAsN junction is not currently predicted to increase the efficiencies of the multijunction cells. Nevertheless, GaInAsN-containing multijunction cell efficiencies are predicted to be comparable to those of the conventional structures, and even small improvements in the GaInAsN cell may lead to higher multijunction cell efficiencies, especially for high-radiation applications and when cell operating temperature is low.

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/cm/sup 2/, 28/spl deg/C) and 34% under the concentrated terrestrial spectrum (AM1.5G, 150-400 suns, 25/spl deg/C), the highest yet achieved for solar cells built on a single substrate.

Four-junction GaInP/GaAs/GaInNAs/Ge solar cells are a widely-pursued route toward AM0 efficiencies of 35% and above, and terrestrial efficiencies of up to 40%. Extensive research into the new material system of GaInNAs has so far yielded subcells with AM0 current densities far below the /spl sim/17 mA/cm/sup 2/ needed to current match the other subcells in the stack. A new multijunction structure, a 5-junction GaInP/GaInP/GaAs/GaInNAs/Ge cell, divides the solar spectrum more finely in order to relax this current matching requirement, by using an optically thin, high-bandgap GaInP top subcell, with an additional thick, low-bandgap GaInP subcell beneath it, in combination with a GaInNAs subcell. In this way, the 5-junction cell design allows the practical use of GaInNAs subcells to increase the efficiency of multijunction cells. Light I-V and external quantum efficiency measurements of the component subcells of such 5-junction cells are discussed. Experimental results are presented for the first time on GaInP/GaInP/GaAs/GaInNAs/Ge cells with the top four junctions active, with measured V/sub oc/ of 3.90 V.

Next-generation solar cell approaches such as AlGaInP/GaAs/GaInNAs/Ge 4-junction cells, lattice-mismatched GaInP/GaInAs/Ge, concentrator cells, and improved 3-junction device structures hold the promise of greater efficiency than even today's highly successful multijunction cells. Wide-bandgap tunnel junctions, improved heterointerfaces, and other device structure improvements have resulted in several record-efficiency GaInP/GaAs/Ge cell results. Triple-junction (3J) cells grown in this work have demonstrated 29.3% efficiency for space (AMO, 1 sun). Space concentrator 3J cells have efficiency up to 30.0% at low concentration (AMO, 7.6 suns), and terrestrial concentrator cells grown at Spectrolab and processed at NREL have reached 32.3% (AM1.5D, 440 suns).

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 (24.0W∕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.