CIGS2 thin‐film solar cells on flexible foils for space power

Abstract

Culnl_×Ga×S2 (CIGS2) thin-film solar cells are of interest for space power applications because of the near optimum bandgap for AM0 solar radiation in space. CIGS2 thin film solar cells on flexible stainless steel (SS) may be able to increase the specific power by an order of magnitude from the current level of 65 Wkg -1. CIGS solar cells are superior to the conventional silicon and gallium arsenide solar cells in the space radiation environment.This paper presents research efforts for the development of CIGS2 thin-film solar cells on 127 pm and 20 pm thick, bright-annealed flexible SS foil for space power. A large-area, dual-chamber, inline thin film deposition system has been fabricated. The system is expected to provide thickness uniformity of +2% over the central 5" width and +3% over the central 6" width. During the next phase, facilities for processing larger cells will be acquired for selenization and sulfurization of metallic precursors and for heterojunction CdS layer deposition both on large area. Small area CIGS2 thin film solar cells are being prepared routinely.Curich Cu-Ga/In layers were sputter-deposited on unheated Mo-coated SS foils from CuGa (22%) and In targets. Well-adherent, large-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 475°C for 60 minutes with intermediate 30 minutes annealing step at 120°C. Samples were annealed at 500°C for 10 minutes without H2S gas flow. The intermediate 30 minutes annealing step at 120°C was changed to 135°C. p-type CIGS2 thin films were obtained by etching the Cu-rich layer segregated at the surface using dilute KCN solution. Solar cells were completed by deposition of CdS heterojunction partner layer by chemical bath deposition, transparent-conducting ZnO/ZnO: AI window bilayer by RF sputtering, and vacuum deposition of Ni/AI contact fingers through metal mask. PV parameters of a CIGS2 solar cell on 127pm thick SS flexible foil measured 2 o under AM 0 conditions at NASA GRC were: Voo = 802.9 mV, Jso= 25.07 mA/cm , FF = 60.06 Yo,and efficiency 0 = 8.84%. For this cell, AM 1.5 PV parameters measured at NREL were: Voo = 788 mV, Jso = 19.78 mA/cm 2, FF = 59.44%, efficiency 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.

Sulphurisation of gallium-containing thin-film precursors analysed in-situ

“…At a temperature of about 400 K the maxima assigned to the latter phases disappear and Cu 11 In 9 appears to be present for a short period. After that metals are transforming into a phase which we assume to be Cu 9 (In,Ga) 4 [9,10]. This is followed by the formation of CuInS 2 .…”

“…1. The maximum of the phase Cu:Ga in the precursor spectra cannot be clearly separated from the maxima of the subsequent phases Cu 11 In 9 and Cu 9 (In,Ga) 4 . Therefore the intensity of these signals are plotted as a single curve.…”

“…In this work we aim for a straightforward modification, replacing the Cu target by a Cu:Ga alloy target and restricting further modifications of the process to a minimum. This also implies the use of sulphur vapour for rapid thermal annealing even though H 2 S gas appears to have some advantages especially in connection with the introduction of gallium [4,5]. Two-step preparation of chalcopyrite thin films is governed to a large extent by phase formation kinetics which have to be known in detail to optimise the process.…”

Cu(In,Ga)Se2 Thin Film Preparation from a Cu(In,Ga) Metallic Alloy and Se Nanoparticles by an Intense Pulsed Light Technique

“…The most common method of fabricating a CIGS thin film is the sequential evaporation of metals. 2,3 Several researchers have attempted various techniques other than coevaporation to develop CIGS absorber layers, for example, sequential sputtering of precursors, 4 reactive sputtering in H 2 Se, 5 hybrid sputtering and evaporation, 6 rapid thermal processing (RTP), 7 stacked elemental layer (SEL) technique, 8 selenization of amorphous CuIn-O precursors, 9 sprayed metal oxide precursors followed by selenization, 10,11 electrodeposition, 12 and CIGS thin film preparation from nanoparticle precursors. 13 However, in all the above-mentioned methods, toxic selenization and vacuum treatment are unavoidable.…”

CIGSS Thin Film Solar Cells: Final Subcontract Report, 10 October 2001-30 June 2005

“…As of now, CIGSS/CdS thin film solar cells are being prepared by rapid thermal processing and conventional selenization using diethylselenide (DESe) as selenium source [6,7] and H S as sulfur source [8], in combination with DC/RF magnetron sputtering and chemical bath deposition technique. Sulfurization of metallic precursors is a well-developed process to produce a high bandgap (1.55eV) absorber.…”