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      Highly Absorbing Lead-Free Semiconductor Cu 2AgBiI 6 for Photovoltaic Applications from the Quaternary CuI–AgI–BiI 3 Phase Space

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          Abstract

          Since the emergence of lead halide perovskites for photovoltaic research, there has been mounting effort in the search for alternative compounds with improved or complementary physical, chemical, or optoelectronic properties. Here, we report the discovery of Cu 2AgBiI 6: a stable, inorganic, lead-free wide-band-gap semiconductor, well suited for use in lead-free tandem photovoltaics. We measure a very high absorption coefficient of 1.0 × 10 5 cm –1 near the absorption onset, several times that of CH 3NH 3PbI 3. Solution-processed Cu 2AgBiI 6 thin films show a direct band gap of 2.06(1) eV, an exciton binding energy of 25 meV, a substantial charge-carrier mobility (1.7 cm 2 V –1 s –1), a long photoluminescence lifetime (33 ns), and a relatively small Stokes shift between absorption and emission. Crucially, we solve the structure of the first quaternary compound in the phase space among CuI, AgI and BiI 3. The structure includes both tetrahedral and octahedral species which are open to compositional tuning and chemical substitution to further enhance properties. Since the proposed double-perovskite Cs 2AgBiI 6 thin films have not been synthesized to date, Cu 2AgBiI 6 is a valuable example of a stable Ag +/Bi 3+ octahedral motif in a close-packed iodide sublattice that is accessed via the enhanced chemical diversity of the quaternary phase space.

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          Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber.

          Organic-inorganic perovskites have shown promise as high-performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence-quenching measurements to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH3NH3PbI(3-x)Cl(x)) and triiodide (CH3NH3PbI3) perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron-hole diffusion lengths of ~100 nanometers. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a critical parameter to optimize for future perovskite absorber development.
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            Detailed Balance Limit of Efficiency of p-n Junction Solar Cells

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              Solar cells. Electron-hole diffusion lengths > 175 μm in solution-grown CH3NH3PbI3 single crystals.

              Long, balanced electron and hole diffusion lengths greater than 100 nanometers in the polycrystalline organolead trihalide compound CH3NH3PbI3 are critical for highly efficient perovskite solar cells. We found that the diffusion lengths in CH3NH3PbI3 single crystals grown by a solution-growth method can exceed 175 micrometers under 1 sun (100 mW cm(-2)) illumination and exceed 3 millimeters under weak light for both electrons and holes. The internal quantum efficiencies approach 100% in 3-millimeter-thick single-crystal perovskite solar cells under weak light. These long diffusion lengths result from greater carrier mobility, longer lifetime, and much smaller trap densities in the single crystals than in polycrystalline thin films. The long carrier diffusion lengths enabled the use of CH3NH3PbI3 in radiation sensing and energy harvesting through the gammavoltaic effect, with an efficiency of 3.9% measured with an intense cesium-137 source.
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                Author and article information

                Journal
                J Am Chem Soc
                J Am Chem Soc
                ja
                jacsat
                Journal of the American Chemical Society
                American Chemical Society
                0002-7863
                1520-5126
                08 March 2021
                17 March 2021
                : 143
                : 10
                : 3983-3992
                Affiliations
                []University of Liverpool , Department of Chemistry, Crown Street, Liverpool L69 7ZD, U.K.
                []University of Oxford , Clarendon Laboratory, Department of Physics, Parks Road, Oxford OX1 3PU, U.K.
                [§ ]University College London , Institute for Materials Discovery, Torrington Place, London WC1E 7JE, U.K.
                []University of Cambridge , Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
                Author notes
                Article
                10.1021/jacs.1c00495
                8041282
                33684283
                be28e24a-5aef-43ca-8c5d-7a5fbd92e255
                © 2021 The Authors. Published by American Chemical Society

                Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained ( https://creativecommons.org/licenses/by/4.0/).

                History
                : 14 January 2021
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                Chemistry
                Chemistry

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