CIGS2 Thin Film Solar Cells on Stainless Steel Foil
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CIGS2 Thin Film Solar Cells On Stainless Steel Foil Neelkanth G. Dhere and Shantinath R. Ghongadi Florida Solar Energy Center 1679 Clearlake Road, Cocoa, FL 32922-5703 ABSTRACT CuIn1-xGaxS2 (CIGS2) thin-film solar cells are of interest for space power applications because of the near optimum bandgap for AM0 solar radiation in space. CuIn1-xGaxSe2-ySy (CIGS) and CIGS2 solar cells are expected to be superior to Si and GaAs solar cells for the space missions especially in terms of the performance at the end of low earth orbit (LEO) mission. Ultra-lightweight thin-film solar cells deposited on flexible stainless steel (SS) foils have a potential for achieving high specific power. Magnetron-sputter-deposition parameters of molybdenum back-contact layer were optimized so as to minimize residual stress. Cu-rich Cu-Ga/In layers were sputter-deposited on unheated Mo-coated SS foils from CuGa(22%) and In targets. Well-adherent, large (3 µm), compact-grain Cu-rich CIGS2 films were obtained by sulfurization in a Ar:H2S 1:0.04 mixture and argon flow rate of 650 sccm, at the maximum temperature of 475o C for 60 minutes with intermediate 30 minute annealing step at 120o C. p-type CIGS2 thin films were obtained by etching away the Curich layer segregated at the surface in a dilute KCN solution. XRD analysis of a CIGS2 film on SS foil revealed growth of chalcopyrite CIGS2 phase having ao = 5.519 C and co = 11.125 C and {112} preferred orientation. Positive SIMS depth profile of CIGS2 film showed gallium concentration increasing toward the back contact. Solar cells were completed by deposition of CdS heterojunction partner layer by chemical bath deposition, transparent-conducting ZnO/ZnO:Al window bilayer by RF sputtering, and vacuum deposition of Ni/Al contact fingers through metal mask. PV parameters of a CIGS2 solar cell on SS flexible foil measured under AM 0 conditions at the NASA GRC were: Voc = 802.9 mV, Jsc = 25.07 mA/cm2, FF = 60.06%, and 0 = 8.84%. For this cell, AM 1.5 PV parameters measured at NREL were: Voc = 788 mV, Jsc = 19.78 mA/cm2, FF = 59.44%, 0 = 9.26%. Quantum efficiency curve showed a sharp QE cutoff equivalent to CIGS2 bandgap of ~1.50 eV, fairly close to the optimum value for efficient AM0 PV conversion in the space.
INTRODUCTION CuIn1-xGaxS2 (CIGS2) thin-film solar cells are of interest for space power applications because of the near optimum bandgap for AM0 solar radiation in space [1,2]. Ultra-lightweight thin-film solar cells deposited on flexible stainless steel (SS) foils have a potential for achieving high specific power. Future space missions would include very large and very small satellites [3]. Solar power satellites were proposed in 1969 [4]. NASA and the Department of Energy have studied options of providing a reference system consisting of two 5-GW satellites. Some long-term plans envisage swarms of distributed, autonomous, small satellites termed microsats or even nanosats H3.4.1
to perform specific tasks [3]. Some missions will use solar electric propulsion (SEP) instead of rockets [5]. This
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