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.

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The alloy GaInAsN has great potential as a lower-band-gap material lattice matched to GaAs, but there is little understanding of what causes its poor optoelectronic properties and why these improve with annealing. This study provides information about the structural changes that occur when GaInAsN is annealed. The Fourier transform infrared spectra exhibit two primary features: a triplet at ∼470 cm−1 (Ga–N stretch) and two or three bands at ∼3100 cm−1 (N–H stretch). The change in the Ga–N stretch absorption can be explained if the nitrogen environment is converted from NGa4 to NInGa3 after annealing. The N–H stretch is also changed after annealing, implying a second, and unrelated, structural change.

The potential for new 4‐, 5‐, and 6‐junction solar cell architectures to reach 50% efficiency is highly leveraging for the economics of concentrator photovoltaic (CPV) systems.The theoretical performance of such next‐generation cells, and experimental results for 3‐ and 4‐junction CPV cells, are examined here to evaluate their impact for real‐world solar electricity generation. Semiconductor device physics equations are formulated in terms of the band gap‐voltage offset Woc  (Eg/q) − Voc, to give a clearer physical understanding and more general analysis of the multiple subcell band gaps in multijunction cells. Band gap‐voltage offset is shown experimentally to be largely independent of band gap Eg for a wide range of metamorphic and lattice‐matched semiconductors from 0.67 to 2.1 eV. Its theoretical Eg dependence is calculated from that of the radiative recombination coefficient, and at a more fundamental level using the Shockley‐Queisser detailed balance model, bearing out experimental observations. Energy production of 4‐, 5‐, and 6‐junction CPV cells, calculated for changing air mass and spectrum over the course of the day, is found to be significantly greater than for conventional 3‐junction cells. The spectral sensitivity of these next‐generation cell designs is fairly low, and is outweighed by their higher efficiency. Lattice‐matched GaInP/GaInAs/Ge cells have reached an independently confirmed efficiency of 41.6%, the highest efficiency yet demonstrated for any type of solar cell. Light I‐V measurements of this record 41.6% cell, of next‐generation upright metamorphic 3‐junction cells with 40% target production efficiency, and of experimental 4‐junction CPV cells are presented. Copyright © 2010 John Wiley & Sons, Ltd.

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/cm 2 ) (AM1.5D, ASTM G173-03, 25 C), the highest demonstrated for a cell of this type requiring a single metalorganic vapor-phase epitaxy growth run.

Spectrolab has fabricated a direct semiconductor bonded space solar cell with an efficiency of 3S.8% under the AMO space spectrum. Using a similar technology, Spectrolab has achieved as-junction (SJ) direct bonded terrestrial cell with a record efficiency of 38.8% under the one-sun AM1.SG terrestrial spectrum. Efforts to further improve the SJ cell efficiency have focused on development of the top 3 junctions (T3J) grown on GaAs. Experiments with top 3J isotype cells have yielded an improvement of 1 % in current and 100 mV in voltage for the T3J. Spectrolab has also made significant improvements in its direct bonding process. The improved process has increased bond strengths by more than a factor of S and eliminated issues with large voids.

Spectrolab has demonstrated a 2.2/1.7/1.4/1.05/ 0.73 eV 5J cell with an efficiency of 37.8% under 1 sun AM1.5G spectrum and 35.1% efficiency for 1 sun AM0. The top three junctions and bottom two junctions were grown on GaAs and InP substrates, respectively, by metal organic vapor phase epitaxy. The GaAs-and InP-based cells were then direct bonded to create a lowresistance, high-transmissive interface. Both the space and terrestrial cells have high 1 sun V o c between 4.75 and 4.78 V. Initial tests of the terrestrial cells at concentration are promising with efficiencies increasing up to 10× concentration to a maximum value close to 41%.

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 metamorphicGa0.44In0.56P/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, 24.0W/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 lowerI2Rresistive 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.