Chris Fetzer

III-V multijunction solar cells for concentrating photovoltaics

Concerns about the changing environment and fossil fuel depletion have prompted much controversy and scrutiny. One way to address these issues is to use concentrating photovoltaics (CPV) as an alternate source for energy production. Multijunction solar cells built from III–V semiconductors are being evaluated globally in CPV systems designed to supplement electricity generation for utility companies. The high efficiency of III–V multijunction concentrator cells, with demonstrated efficiency over 40% since 2006, strongly reduces the cost of CPV systems, and makes III–V multijunction cells the technology of choice for most concentrator systems today. In designing multijunction cells, consideration must be given to the epitaxial growth of structures so that the lattice parameter between material systems is compatible for enhancing device performance. Low resistance metal contacts are crucial for attaining high performance. Optimization of the front metal grid pattern is required to maximize light absorption and minimize I2R losses in the gridlines and the semiconductor sheet. Understanding how a multijunction device works is important for the design of next-generation high efficiency solar cells, which need to operate in the 45%–50% range for a CPV system to make better economical sense. However, the survivability of solar cells in the field is of chief concern, and accelerated tests must be conducted to assess the reliability of devices during operation in CPV systems. These topics are the focus of this review.

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.

High-efficiency multijunction photovoltaics for low-cost solar electricity

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.

Production ready 30% efficient triple junction space solar cells

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 (NPmp) 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 Spectrolabs 28% BOL efficient UTJ. Finally, XTJ engineering confidence thermal cycle testing and qualification status to the AIAA S-111-2005 standard will be elaborated.

Multijunction Solar Cell Technology for Mars Surface Applications

Solar cells used for Mars surface applications have been commercial space qualified AMO 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 AMO 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

Final qualification test results of XTJ triple junction space solar cell to AIAA - S-111 - 2005 and Spectrolab test standards

Spectrolab successfully completed the qualification of its latest and final triple junction space solar cell, 30% class XTJ (neXt Triple Junction), per AIAA S-111-2005 and Spectrolab test standards. The final qualification and characterization test results are presented in this paper. XTJ exhibits a 4.2 % power gain over Spectrolabs current space PV, UTJ, at both beginning of life and end of life. Moreover, nearly 3% lower solar absorptance of XTJ allows for a cooler operating temperature in a typical GEO mission. XTJ showed excellent front and backside welding contact integrities, insensitivity to ESDS, and negligible loss from humidity and GEO thermal cycle tests. Currently Spectrolabs large area XTJ (59.65cm2) is undergoing qualification per Spectrolab specified test standards.

Mars optimized solar cell technology (MOST)

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.

GaInP/GaAs Tandem Solar Cells With InGaAs/GaAsP Multiple Quantum Wells

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.

Tandem InGaP/GaAs-quantum well solar cells and their potential improvement through phosphorus carry-over management in multiple quantum well structures

InGaP/GaAs/Ge multijunction solar cell (MJSC) efficiency can be increased through improved current matching among the subcells with multiple quantum wells (MQWs) being promising for this purpose. In this study we show that InGaAs/GaAsP QWs utilizing high phosphorus composition barriers can be successfully incorporated into the GaAs subcell of an InGaP/GaAs tandem solar cell. This InGaP/GaAs-MQW device has an enhanced short circuit current density when compared to that of a standard InGaP/GaAs tandem device with minimal impact on either GaAs or InGaP subcell open circuit voltage. Additionally, phosphorus carry-over in the MQW structure is investigated through the use of photoluminescence (PL). It is demonstrated that the phosphorus carry-over can be overcome through the utilization of thick GaAs transition layers at the GaAsP→InGaAs interfaces, resulting in a MQW with an extended absorption edge.

Qualification status of 30% efficient next triple junction (XTJ) GaInP 2 /GaAs/Ge solar cells to AIAA S-111-2005 standard

Spectrolab presents the qualification status of its latest triple junction space solar cell, neXt Triple Junction (XTJ). The beginning of life efficiency averages 29.9% at maximum power (AM0, 28°C, 135.3 mW/cm2). The qualification in progress complies with the AIAA S-111-2005 standards for bare cell, CIC, and coupon level tests. Thermal cycle GEO coupons built for engineering confidence prior to the qualification showed excellent mechanical and electrical performance. Interim qualification status and test results are discussed.