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[NREL] NREL Develops ZnSiP2 for Silicon-Based Tandem Solar Cells

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Combining an Earth-abundant chalcopyrite with a silicon layer could significantly boost conversion efficiency above that of single-junction silicon solar cells.

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.

Solar Cells

II-IV-V2 chalcopyrite compounds such as ZnSiP2 are closely related to III-V zincblende compounds. ZnSiP2, which is lattice matched to Si, has been formed as single crystals (top). It has bright photoluminescence emission (middle) and forms an epitaxial interface with Si (bottom).

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

Achievement

NREL researchers have synthesized bulk single-crystalline ZnSiP2 and then characterized it by structural and optical techniques.

Key Result

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.

Potential Impact

NREL predicts an increase in efficiency of up to 12% greater than that of single-junction  silicon for a Si/ZnSiP2 tandem cell.

Year: 2014

Technical Contact: Adele Tamboli, adele.tamboli@nrel.gov

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.

Source: NREL

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