Novel Solution Process for Fabricating Ultra-Thin-Film Absorber Layers in Fe 2 SiS 4 and Fe 2 GeS 4 Photovoltaics
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Novel Solution Process for Fabricating Ultra-Thin-Film Absorber Layers in Fe2SiS4 and Fe2GeS4 Photovoltaics Samuel A. Orefuwa1, Cheng-Yu Lai1, Kevin Dobson2, Chaoying Ni3, Daniela Radu1* 1 Department of Chemistry, Delaware State University, 1200 N. DuPont Highway,Dover, DE, U.S.A. 2 Institute of Energy Conversion, University of Delaware, 451 Wyoming Road, Newark, DE 19716, U.S.A. 3 University of Delaware, Newark, DE 19716, U.S.A. * Corresponding author
ABSTRACT Fe2SiS4 and Fe2GeS4 crystalline materials posses direct bandgaps of ~1.55 and ~1.4 eV respectively and an absorption coefficient larger than 105 cm–1; their theoretical potential as solar photovoltaic absorbers has been demonstrated. However, no solar devices that employ either Fe2SiS4 or Fe2GeS4 have been reported to date. In the presented work, nanoprecursors to Fe2SiS4 and Fe2GeS4 have been fabricated and employed to build ultra-thin-film layers via spray coating and rod coating methods. Temperature-dependent X-Ray diffraction analyses of nanoprecursors coatings show an unprecedented low temperature for forming crystalline Fe2SiS4 and Fe2GeS4. Fabricating of ultra-thin-film photovoltaic devices utilizing Fe2SiS4 and Fe2GeS4 as solar absorber material is presented. INTRODUCTION Three forms of thin-film solar panels have been developed and commercialized in the last decade by identifying materials that are both efficient absorbers of solar power and costeffective for manufacturer and consumer; these are: amorphous silicon (a-Si), cadmium telluride (CdTe) and CIGS (copper indium gallium sulfo-selenide). Although they operate effectively in thin-film (1-3 microns), there are both environmental and economic concerns for the cost and sustainability of the materials and processes employed in these approaches. An alternative solution was seen in pursuing sustainable PV materials composed of Earthabundant elements such as Cu2ZnSn(S, Se)4 (copper zinc tin sulfide–CZTS or sulfo-selenide CZTSSe) or FeS2 (iron sulfide) for the absorber layer. CZTSSe, benefiting from CIGS similarities, has already proved itself at efficiencies > 12%. 1 However, photovoltaic research using FeS2 absorbers still reports very low efficiencies (~2%) despite material’s potential comparable to a-Si, CdS and CIGS (>20%). The use of Fe in PV was proposed more than 25 years ago in the form of FeS2. FeS2 (also called pyrite or “fool’s gold”) is an indirect band gap semiconductor with sustainable
composition of abundant elements. Unfortunately, the performance problems associated to this material as PV absorber are not fully understood.2 The appeal of FeS2, in addition to material’s low cost and abundance, was that that exhibits a useful band gap (Eg = 0.9 eV) and an absorption coefficient above 105 at Eg + 0.1 eV. This high absorption coefficient makes FeS2 unique among inorganic materials allowing downsizing the thickness of the absorber layer to lower than 0.1 µm in a solar cell able to capture most of the incident solar radiation. The attractiveness of this thickness is visible when compared
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