Combining an Earth-abundant chalcopyrite with a silicon layer could significantly boost conversion efficiency above that of single-junction silicon solar cells.
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A current technological challenge in photovoltaics (PV) is to implement a lattice-matched, optically efficient material to be used in conjunction with silicon for tandem PV cells.
III-V materials currently hold the world-record conversion efficiencies for both single- and multijunction cells. Researchers at the National Renewable Energy Laboratory (NREL), collaborating with the Colorado School of Mines, are investigating materials that have similar properties to the III-V materials, but that are also lattice-matched to silicon. The II-IV-V2 chalcopyrites are a promising class of materials that could satisfy both of these criteria.
NREL researchers have synthesized bulk single-crystalline ZnSiP2 and characterized the material by structural and optical techniques. ZnSiP2 is a member of the II-IV-V2 class of materials and is known to have a bandgap of ~2 eV and a lattice mismatch with silicon of 0.5%. In addition, its elements are Earth abundant.
NREL has grown single crystals of ZnSiP2 by a flux growth technique that results in thin platelets that are up to 1 mm thick, 4 mm wide, and 10 mm long. Structure and phase purity have been confirmed by X-ray diffraction. Initial optical measurements show strong luminescence and confirm the ~2-eV bandgap. The research has shown that there is no sub-bandgap absorption that would be detrimental to the silicon cell performance, and that ZnSiP2 forms an epitaxial interface with Si.
This material, in conjunction with silicon PV, could find an application as a monolithic tandem layer, as well as a passivated
contact or surface-passivation layer.
This work was funded in part by the Renewable Energy Materials Research Science and Engineering Center (REMRSEC) at Colorado School of Mines.
Key Research Results
NREL researchers have synthesized bulk single-crystalline ZnSiP2 and then characterized it by structural and optical techniques.
X-ray diffraction by NREL confirms the structure and phase purity of the synthesized ZnSiP2, and optical measurements show strong luminescence and confirm an ~2-eV bandgap.
NREL predicts an increase in efficiency of up to 12% greater than that of single-junction silicon for a Si/ZnSiP2 tandem cell.
Technical Contact: Adele Tamboli, firstname.lastname@example.org
Reference: Martinez, A.D.; Ortiz, B.R.; Johnson, N.E.; Krishna, L.; Choi, S.; To, B.; Norman, A.G.; Stradins, P.; Stevanovic, V.; Toberer, E.S.; Tamboli, A.C. (2014). “Development of ZnSiP2 for Si-Based Tandem Solar Cells.” IEEE J. Photovoltaics, forthcoming.
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