James H. Ermer

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

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.

Advances in High-Efficiency III-V Multijunction Solar Cells

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 Ga 0 .44 In 0 .56 P / Ga 0.92 In 0.08 As/ 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/cm 2 , 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 I 2 R 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.

Advanced III-V Multijunction Cells for Space

III-V solar cells have become the dominant power generation technology in space, due to their unparalleled high efficiency, reliability in the space environment, and ability to be integrated into very lightweight panels. As remarkable as these attributes are, new types of space III-V solar cells are continually reaching new heights in performance. Commercially-available multijunction solar cells with 30% conversion efficiency under the AM0 space spectrum are just around the corner. Understanding of radiation resistance and thermal cycling reliability has reached levels never before attained, and is resulting in new standards of reliability. A flurry of research activity has resulted in very-thin, flexible, and extremely lightweight space solar cells and panels in several groups around the world, capable of being folded or rolled into a smaller stowage volume for launch than has been possible to date. This approach combines the very high efficiency and reliability of III-V multijunction cells with the thin, flexible PV blanket functionality normally associated only with thin-film polycrystalline or amorphous PV technology. This paper discusses the latest developments in III-V space solar cell technology, and explores opportunities for still higher performance in the future

Evolution of Multijunction Solar Cell Technology for Concentrating Photovoltaics

Multijunction solar cells have evolved from their original development for space missions to displace silicon cells in high concentrating photovoltaic (CPV) systems. Todays three-junction lattice-matched production cells have efficiency of 39–39.5% under high concentration, and there appears to be little opportunity for further efficiency gain with this three-junction technology. Future generations of CPV cells will exploit more than three junctions, with metamorphic subcells, or both technical approaches to achieve efficiencies >45%. As new designs seek closer current matching and further spectral splitting, atmospheric variability will necessitate careful modeling to optimize energy output. These new cells will also be higher cost, which will favor higher CPV system concentration.

Ultra triple-junction high-efficiency solar cells

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.

The ultra triple-junction cell: high-efficiency solar cell for space applications

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.