Chemical Control of the Dimensionality of the Octahedral Network of Solar Absorbers from the CuI-AgI-BiI3 Phase Space by Synthesis of 3D CuAgBiI5.

Harry C Sansom,Laura M Herz,Troy D Manning,John B Claridge,Michael J Pitcher,Leonardo R V Buizza,Henry J Snaith,Vinod R Dhanak,James T Gibbon,Marco Zanella,Matthew S Dyer,Matthew J Rosseinsky

doi:10.1021/acs.inorgchem.1c02773

Harry C Sansom, Laura M Herz + Show 10 more

Open Access

https://doi.org/10.1021/acs.inorgchem.1c02773

Copy DOI

Abstract

A newly reported compound, CuAgBiI5, is synthesized as powder, crystals, and thin films. The structure consists of a 3D octahedral Ag+/Bi3+ network as in spinel, but occupancy of the tetrahedral interstitials by Cu+ differs from those in spinel. The 3D octahedral network of CuAgBiI5 allows us to identify a relationship between octahedral site occupancy (composition) and octahedral motif (structure) across the whole CuI–AgI–BiI3 phase field, giving the ability to chemically control structural dimensionality. To investigate composition–structure–property relationships, we compare the basic optoelectronic properties of CuAgBiI5 with those of Cu2AgBiI6 (which has a 2D octahedral network) and reveal a surprisingly low sensitivity to the dimensionality of the octahedral network. The absorption onset of CuAgBiI5 (2.02 eV) barely changes compared with that of Cu2AgBiI6 (2.06 eV) indicating no obvious signs of an increase in charge confinement. Such behavior contrasts with that for lead halide perovskites which show clear confinement effects upon lowering dimensionality of the octahedral network from 3D to 2D. Changes in photoluminescence spectra and lifetimes between the two compounds mostly derive from the difference in extrinsic defect densities rather than intrinsic effects. While both materials show good stability, bulk CuAgBiI5 powder samples are found to be more sensitive to degradation under solar irradiation compared to Cu2AgBiI6.

Highlights

Ternary and quaternary compounds from the CuI−AgI−BiI3 phase space show huge potential for photovoltaics due to their suitable band gaps (1.67−2.06 eV) with very high absorption coefficients exceeding 105 cm−1 and low excitonic binding energies (∼25 meV) that arise from their stable Bi3+ iodide octahedral network in a close packed iodide sublattice.[1−4] In contrast, the double perovskite Cs2AgBiBr6 does not strongly absorb light at energies below its excitonic absorption peak at 2.8 eV and direct band gap at 3.03 eV,[5,6] and the hypothetical compound Cs2AgBiI6, which would likely have a narrower band gap, is not a stable phase.[7]
It is unlikely to be due to the reduction of photosensitive Ag−I bonds in Cu2AgBiI6 compared to CuAgBiI5, because the more Ag-rich AgBiI4 exposed to the same conditions did not show this decomposition.[2]. We report this instability of CuAgBiI5, it does not necessarily mean that 3D Oct networks are intrinsically less stable than the 2D Oct networks, and the instability may not persist in related systems via chemical substitution
The 3D Oct network has been obtained via chemical tuning, namely the total occupancy of the Ag+ and Bi3+ Oct sites, which allows selectivity between a spinel (3D), CdCl2 (2D), or NaVO2 (3D but with layered ordering) type Oct motif

Summary

■ INTRODUCTION

Ternary and quaternary compounds from the CuI−AgI−BiI3 phase space show huge potential for photovoltaics due to their suitable band gaps (1.67−2.06 eV) with very high absorption coefficients exceeding 105 cm−1 and low excitonic binding energies (∼25 meV) that arise from their stable Bi3+ iodide octahedral network in a close packed iodide sublattice.[1−4] In contrast, the double perovskite Cs2AgBiBr6 does not strongly absorb light at energies below its excitonic absorption peak at 2.8 eV and direct band gap at 3.03 eV,[5,6] and the hypothetical compound Cs2AgBiI6, which would likely have a narrower band gap, is not a stable phase.[7]. BiI3 has layered ordering consisting of a layer phases, in which the cations are tetrahedrally coordinated, 1 2 of the Tet interstitial sites are occupied giving a 3D network of corner-sharing tetrahedra This concept can be extended to the ternary and quaternary compounds but with the added complexities of disorder and nonstoichiometric compositions. In Ag-rich compositions Ag2BiI5 and Ag3BiI6, overall Oct site occupancies are over 50%, where 50% is the maximum which can be occupied by the spinel and CdCl2 Oct motifs.[18,19] In these structures, the excess Ag+ occupies Oct interstitial sites between the layers of a CdCl2 Oct motif

Summary Spacea of Structural

■ CONCLUSION

■ ACKNOWLEDGMENTS

Findings

■ REFERENCES