D.R. Lillington

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

The path to 1 GW of concentrator photovoltaics using multijunction solar cells

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

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

This paper presents an overview of the status of the high-concentration photovoltaic (HCPV) module technology and discusses the steps required to take it from to the production of gigawatts in the near future. The paper discusses the impact of the recent advances in multijunction cell technology on the economics of concentrator system.

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

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

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>>

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.

32.3% efficient triple junction GaInP/sub 2//GaAs/Ge concentrator solar cells

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.

Production and qualification status of GaAs/Ge top/bottom contact solar cells

This paper describes progress toward achieving high efficiency, multijunction solar cells for cost effective application in terrestrial PV concentrator systems. Small area triple junction GaInP/sub 2//GaAs/Ge solar cells have been fabricated with an efficiency of over 32% when measured by NREL under an AM1.5D spectrum at 47 suns concentration. Small changes to the device design can achieve similar efficiencies at concentration ratios of about 500 suns, resulting in cell costs of

GaAs on Ge cell and panel technology for advanced space flight application

0.5 to