J.H. Ermer

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).

Triple-junction solar cell efficiencies above 32%: the promise and challenges of their application in high-conceniration-ratio PV systems

Results from Spectrolab-grown Ga/sub 0.5/In/sub 0.5/P/GaAs/Ge structures optimized for the AM1.5D spectrum are described along with progress toward developing next generation multijunction solar cells for high concentration ratios (X). The epitaxially-grown layers were processed into triple junction cells both at Spectrolab and NREL, and I-V tested vs. X. Cells were tested with efficiencies as high as 32.4% near 372 suns. The FF limited the performance with increasing X as a result of the increased role of the series resistance. The V/sub oc/ vs. X showed its log-linear dependence on I/sub sc/ over 1000 suns. Based on cell improvements for space applications, multijunction cells appear to be ideal candidates for high efficiency, cost effective, PV concentrator systems. Future development of new 1 eV materials for space cells, and further reduction in Ge wafer costs, promises to achieve cells with efficiencies >40% that cost

Status of C3MJ+ and C4MJ Production Concentrator Solar Cells at Spectrolab

0.3/W or less at concentration levels between 300 to 500 suns.

Metamorphic GaInP/GaInAs/Ge solar cells

Multijunction solar cells based on III-V semiconductors, having recently demonstrated 43.5%, remain the worlds most efficient solar cells, and the preferred technology in point-focus and dense-array concentrator photovoltaic (CPV) system architectures. The year 2011 proved to be a pivotal year for CPV technology, with multiple power plant installations in the megawatt to tens of megawatt scale. Spectrolab is working closely with CPV system manufacturers to provide a reliable and well-characterized cell technology, in volumes commensurate with this increasing demand. The evolutionary C3MJ+ and the C4MJ cell technologies are the latest in a sequence of CPV solar cell designs, with conversion efficiencies approaching or greater than 40%. Both technologies have completed detailed characterization and qualification programs, including accelerated laboratory and (for C4MJ) on-sun reliability testing, and have entered into high volume production at Spectrolabs manufacturing facility in Sylmar, CA. The metamorphic C4MJ technology affords new opportunities to optimize cell designs, taking into consideration both the spectral optical transmittance of a particular CPV system and the installation sites average solar resource over a typical meteorological year.

Modeling of electron diffusion length in GaInAsN solar cells

High-efficiency, metamorphic multijunction cells have been fabricated by growing GaInP/GaInAs subcells that are lattice-mismatched to an active Ge substrate, resulting in GaInP/GaInAs/Ge 3-junction (3J) cells. The efficiency dependence of this 3J cell on lattice-constant of the top two cells and on sublattice ordering in the GaInP top cell is presented. A variety of composition-graded buffers have been explored through X-ray diffraction reciprocal space mapping to measure strain in the cell layers, and transmission electron microscopy to minimize misfit and threading dislocations. Quantum efficiency is measured for metamorphic 1.3-eV Ga/sub 0.92/In/sub 0.08/As (8%-ln GaInAs) cells and 1.75-eV Ga/sub 0.43/In/sub 0.57/P cells grown on a Ge substrate, as well as for the 3J cell based on 4%-in GaInAs. Three-junction Ga/sub 0.43/In/sub 0.57/P/Ga/sub 0.92/In/sub 0.08/As/Ge cells with 0.50% lattice-mismatch to the Ge substrate are measured to have AMO efficiency of 27.3% (0.1353 W/cm/sup 2/, 28/spl deg/C), similar to high-efficiency, conventional GaInP/GaAs/Ge 3-junction cells based on the GaAs lattice constant.

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

This paper seeks to quantify the electron diffusion length in p-type GaInAsN and to understand the performance of GaInAsN cells. The usual modeling is complicated because the electron diffusion length is quite short (often <0.1 /spl mu/m) and is usually less than the hole diffusion length. The properties (e.g. absorption and transport) of GaInAsN are both variable and poorly studied, and, because the band gap of GaInAsN is less than the band gap of the substrate, light that is transmitted through the GaInAsN layer may be reflected from the back metallization and make a second pass through the GaInAsN. Layers that are expected to be p-type can sometimes change to n-type, changing the location of the junction and complicating the modeling, but improving the performance of the cell. Internal quantum efficiencies as high as 80% near the GaAs band edge are reported.

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

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.

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

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%.

Ge concentrator cells for III-V multijunction devices

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.

Advancements in GaInP/sub 2//GaAs/Ge solar cells - production status, qualification results and operational benefits

We identify a failure mode due to a photoactive back contact for Ge concentrator solar cells. This problem manifests itself as a leveling off and subsequent decrease of open-circuit voltage (V/sub oc/) as the concentration increases above /spl sim/20 suns. Correction of this problem yields a much improved Ge cell for which V/sub oc/ increases in an almost ideal n=1 manner from 0.2 volts at one sun to 0.4 volts at 1400 suns. This cells fill factor remains at or above its one-sun value up to 500 suns, confirming that this cell is fully suitable for high-concentration use. We show that solving the back-contact problem can significantly improve the high-concentration performance of GaInP/GaAs/Ge three-junction solar cells.