This paper describes a trade study between state-of- the-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) $/W will be compared for these cell technologies.
INTRODUCTION Historically, the need for power in space has been dominantly provided by silicon solar cells. In the past several years, however, high-volume manufacturing of high-efficiency multi-junction solar cells has enabled the use of this alternative technology for space power generation [1-3]. Compared to Si, multi-junction cells are more radiation resistant and have greater energy conversion efficiencies, but they are also heavier (higher density and thickness) and cost more. When the need for very high power or smaller solar arrays are paramount in a spacecraft, multi-junction cells are often used instead of, or in hybrid combinations with, Si to reduce the array size. Very large solar arrays, for example, pose a difficult challenge for the attitude control systems onboard a typical satellite. Several trade studies have been published in the past comparing the performance, cost, weight, area, etc. of different solar cell technologies for space applications [4-7]. Since then however, both multi-junction and silicon cell technologies have improved in three significant ways: conversion efficiency, radiation resistance, and cost. High-efficiency rad-hard 3-mil Si solar cells, for example, exhibit beginning-of-life (BOL)
efficiencies averaging about 17%, under one-sun, air- mass zero (AM0) illumination conditions [8-9]. Commercially-available dual- and triple-junction InGaP/GaAs/Ge solar cells, on the other hand, have demonstrated minimum average BOL AM0 efficiencies, as high as, 23% and 26%, respectively [1,10]. These cell technologies are also more radiation hard than Si space cells, and their cost has decreased significantly in the past two years. In fact, the EOL cost per unit power ($/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.
SOLAR CELL TECHNOLOGY COMPARISONS The solar cell technologies that are considered in this paper are state-of-the-art 3-mil high-efficiency Si, InGaP/GaAs-on-Ge dual-junction (2J), and InGaP/GaAs/Ge-on-Ge triple-junction (3J) solar cells. Only solar cells that are currently in volume production and are available for commercial sales were considered for this study [9-10]. The solar cell characteristics that primarily determine EOL power are particle irradiation and temperature coefficient degradation mechanisms. Other secondary effects such as ultra-violet (UV) and thermal cycling degradation mechanisms also affect EOL power in space. As a rule, the degradation rates for multi-junction cells under particle irradiation are significantly lower (better) than for Si cells. As mentioned earlier, the multi-junction cell technologies offer higher BOL and EOL electrical performance than their Si counterpart. This is illustrated in Table 1, where the BOL minimum average AM0 (one-sun) solar cell efficiency, EOL temperature coefficients, and power remaining factors (P/Po) after 1-MeV electron irradiation under 5E14 and 1E15 e/cm2 fluences for these cells are presented. It should be noted that the electrical performance of these cells normally do not degrade after they are made into CICs (coverglass-interconnected- cells). The data presented in Table 1, therefore, also applies to CICs. Blue-red reflector (BRR) coverglass is sometimes used with Si cells to lower their operating
Presented at the 28th IEEE PVSC, September 17-22, Anchorage, Alaska