Dmitri D. Krut

Next-generation, high-efficiency III-V multijunction solar cells

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 todays 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).

Development of advanced space solar cells at Spectrolab

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.

High-voltage, low-current GaInP/GaInP/GaAs/GaInNAs/Ge solar cells

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.

Metamorphic and Lattice-Matched Solar Cells Under Concentration

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, 25degC), 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

Monolithic multi-cell GaAs laser power converter with very high current density

A very high current density 2-Volt Laser Power Photovoltaic GaAs converter has been fabricated to produce over 360 mW of output power with monochromatic illumination of one optical Watt at 810 nm. To build-up the voltage, the converter consists of two interconnected N/P GaAs elements integrated in series on the semi-insulating GaAs substrate. Several technological issues, including, conductive losses through the sheet for the series interconnection of elements, have been successfully addressed. This device operates at a nominal optical power density of 57W/cm/sup 2/, which is equivalent to over 700 suns of AM1.5D illumination, demonstrating feasibility of fabricating high voltage integrated cells for concentrator applications.

Triple-junction GaInP/GaAs/Ge solar cells-production status, qualification results and operational benefits

In 1999 Spectrolab completed design and qualification, and began production on the next generation of multijunction solar cells-a triple-junction GaInP/GaAs/Ge. With over 20% AM0 conversion efficiency at an operating temperature of 60/spl deg/C, this cell provides 8-11% more power than competing dual-junction designs in GEO orbit after 15 years (6/spl times/10/sup 14/ 1-MeV electron equivalence). Spectrolab is currently qualifying an improved triple-junction cell capable of delivering over 22% AM0 conversion efficiency under these same conditions, with a beginning-of-life operating efficiency of 27%.

High concentration testing and performance of multijunction solar cells

The use of the appropriate characterization tools along with the use of single cell assemblies that are robust, are necessary for proper evaluation of device performance and stability tests for GaInP/sub 2//GaAs/Ge triple junction (3J) concentrator solar cells. Certain discrepancies exist between the photovoltaic (PV) values from continuous outdoor illumination measurements at high concentration levels and those from the High Intensity Pulse Solar Simulator (HIPSS) measurements, which are addressed. The series resistance inferred from the current-voltage (IV) curve, however, is similar from both measurement techniques. Outdoor tests of concentrator cells yielded efficiencies near 29% (31% from HIPSS) for an average concentration of 432 suns. Both types of measurements revealed that the fill factor (FF) limited the performance of the cell at high concentrations due to the role of the series resistance. The formation of heat from the nonuniformity in the Fresnel optics also limited the device performance. Before the cell stability can be evaluated, cells are first screened for defects from the 1-sun, indoor, IV measurements. Then, the good cells are incorporated into small assemblies for further test by forward bias injection (FBI). The stability outcome of the FBI determines whether or not the cell can be used for outdoors tests. Outdoor IV measurements are then conducted. Stability test results showed that no degradation occurs in the 1-sun IV data before and after light soak of the concentrator assemblies.

The design and evaluation of dual-junction GaInP/sub 2//GaAs solar cells for space applications

The necessary design considerations for a space-qualified GaInP/sub 2//GaAs cell are addressed. Analyses are presented which indicate that, with the proper active layer thicknesses and doping levels, BOL efficiencies of 27% (AM0, 28 C) can be achieved from a cell with 84% remaining power at EOL ( 1*10/sup 15 /e/cm/sup 2/). Experimental data from 1 MeV electron irradiation of terrestrial GaInP/sub 2//GaAs tandem cells which are in support of these predictions are presented. The results indicate that the thin GaInP/sub 2/ top cell degrades less than 10% and that the GaAs behaves as predicted.<<ETX>>

The development of Ge bottom cell for monolithic and stacked multi-junction applications

Ge solar cells have applications in space solar systems as well as terrestrial concentrator systems. Progress in fabrication of Ge cells to work under GaAs is described. While the fabrication of monolithic GaAs/Ge cells has not led to the theoretically predicted efficiencies, the Ge bottom cell still has applications in the monolithic stack. Currents achieved in Ge are sufficient for it to serve as a bottom cell of the three-junction GaAs/Ge stack. The diffusion length measured in Ge cells is sufficient to fabricate good cells. The effect of a BSR on electrical performance is considered. Radiation performance results for Ge cells are reported.<<ETX>>

Development of terrestrial concentrator modules incorporating high-efficiency multi-junction cells

This paper presents key results of an on-going program to develop terrestrial concentrator modules incorporating high-efficiency multi-junction (MJ) photovoltaic (PV) cell technology. This program evolved from a successful space concentrator module development. Indeed, space mini-concentrators, comprising a stretched-membrane line-focus Fresnel lens and a prism-covered triple-junction solar cell, have been found to perform exceptionally well in the terrestrial environment. In outdoor tests at both ENTECH and NREL, space mini-concentrators have achieved overall module (combined lens and cell) operational solar-to-electric conversion efficiency levels from 25 to 29% under a variety of conditions. The long-term goal of the present program is to incorporate MJ cell technology into existing field-proven modules and two-axis sun-tracking arrays. With twice the efficiency of present silicon-based concentrators, these next-generation MJ concentrators should offer unprecedented economics.