- By Navid S. Fatemi, Howard E. Pollard, Hong Q. Hou, and Paul R. Sharps, Emcore Photovoltaics -
This paper describes a trade study between state-ofthe-art, commercially-available very high-efficiency III-V
multi-junction solar cells and advanced high-efficiency silicon cells at the bare cell and panel levels. The solar
cell technologies in this comparison will be high-efficiency rad-hard 3-mil Si, dual-junction InGaP/GaAs (on Ge), and triple-junction InGaP/GaAs/Ge, with the beginning-of-life (BOL) efficiencies of 17%, 23%, and 26%, respectively.
Two different typical orbits will be considered: geosyncronous (GEO) and low-earth (LEO) orbits. It will be assumed that the end-of-life (EOL) conditions for GEO and LEO will be equivalent to degradation due to 1-MeV
electrons at 5E14 and 1E15 e/cm2, respectively.
Parameters critical to conventional rigid solar arrays such as specific power/mass (W/Kg), specific mass/area (Kg/m2), specific power/area (W/m2), and normalized end-of-life (EOL) $/W will be compared for these cell
Historically, the need for power in space has been dominantly provided by silicon solar cells. In the past several years, however, high-volume manufacturing of high-efficiency multi-junction solar cells has enabled the use of this alternative technology for space power generation.
Compared to Si, multi-junction cells are more radiation resistant and have greater energy conversion efficiencies, but they are also heavier (higher density and thickness) and cost more. When the need for very high power or smaller solar arrays are paramount in a spacecraft, multi-junction cells are often used instead of, or in hybrid combinations with, Si to reduce the array size.
Very large solar arrays, for example, pose a difficult challenge for the attitude control systems onboard a typical satellite. Several trade studies have been published in the past comparing the performance, cost, weight, area, etc. of different solar cell technologies for space applications.
Since then however, both multi-junction and silicon cell technologies have improved in three significant ways: conversion efficiency, radiation resistance, and cost. High-efficiency rad-hard 3-mil Si solar cells, for example, exhibit beginning-of-life (BOL) efficiencies averaging about 17%, under one-sun, airmass zero (AM0) illumination conditions.
Commercially-available dual- and triple-junction InGaP/GaAs/Ge solar cells, on the other hand, have demonstrated minimum average BOL AM0 efficiencies, as high as, 23% and 26%, respectively. These cell technologies are also more radiation hard than Si space cells, and their cost has decreased significantly in the past two years. In fact, the EOL cost per unit power ($/W) for the multi-junction cell technologies are only now becoming competitive with high-efficiency Si technologies.
The following is a trade comparison at the cell and panel levels between high-efficiency Si and multi-junction
space solar cells.