David E. Joslin

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).

Metamorphic GaInP/GaInAs/Ge 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.

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.

Application of infrared reflecting (IRR) coverglass on multijunction III-V solar cells

It is well known that the Ge subcell in multijunction GaInP/GaAs/Ge based solar cells produces a significantly higher photogenerated current (nearly 2x) than the other two subcells connected in series. The excess current is converted into heat, and as a result, increases the cell operating temperature. Because the solar cell efficiency decreases with higher temperatures, it is desirable to maintain a lower cell operating temperature. This can be achieved by rejecting a part of the incident sunlight that would otherwise be absorbed and converted into heat by the Ge subcell. For many space applications, coverglass incorporated with infrared reflecting (IRR) coatings can be applied to these solar cells for the purpose of lowering the cell operating temperature and/or improving the power output from the solar arrays. Achieving higher power output requires an appropriate IRR coating design that carefully balances the reduction in the cell absorptance against the Ge subcell current output. In this paper, this key issue is discussed. Also, preliminary IRR coating designs have been evaluated by applying them on high efficiency 3-junction solar cells, and the performance data are used to help predict optimal designs

The effects of electron irradiation on triple-junction Ga/sub 0.5/In/sub 0.5/P/GaAs/Ge solar cells

Ga/sub 0.5/In/sub 0.5/P/GaAs/Ge solar cells have been fabricated at Spectrolab under the Multijunction Solar Cell Manufacturing Technology (Mantech) program, sponsored by the US Air Force and NASA. The cells were irradiated with increasing 1 MeV electron fluences, and the degradation of their PV parameters was characterized using light I-V and external QE. Analysis of the PV parameters of the GaInP top subcell showed little degradation, and was not a limitation for triple junction (3J) cell performance. Furthermore, the radiation degradation of the Ge subcell PV parameters was almost negligible. The GaAs subcell I/sub sc/, however, did limit the device performance as is traditionally documented. The final-to-initial maximum power ratio (P/P/sub 0/) of 3J cells was near 0.833 at a fluence of 1/spl times/10/sup 15/ e/sup -//cm/sup 2/, and matches Spectrolabs presently established value of 0.83 for standard production of space-qualified dual-junction cells.

Fabrication and electrical characterization of 0.55eV N-on-P InGaAs TPV devices

Results are presented on the characterization and testing of lattice-mismatched 0.55 eV InGaAs/InP thermophotovoltaic (TPV) cells. A robust cell fabrication technique amenable to high throughput production is presented. A versatile light and dark I-V set up capable of fast screening of the TPV cells and an innovative approach for screening high performance cells are presented. We also report on the effect of lattice-matched InAsP and InAlAs back surface field layers on the performance of the TPV cells.