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

Abstract

A newly reported compound, CuAgBiI 5 , is synthesized as powder, crystals, and thin films. The structure consists of a 3D octahedral Ag + /Bi 3+ network as in spinel, but occupancy of the tetrahedral interstitials by Cu + differs from those in spinel. The 3D octahedral network of CuAgBiI 5 allows us to identify a relationship between octahedral site occupancy (composition) and octahedral motif (structure) across the whole CuI–AgI–BiI 3 phase field, giving the ability to chemically control structural dimensionality. To investigate composition–structure–property relationships, we compare the basic optoelectronic properties of CuAgBiI 5 with those of Cu 2 AgBiI 6 (which has a 2D octahedral network) and reveal a surprisingly low sensitivity to the dimensionality of the octahedral network. The absorption onset of CuAgBiI 5 (2.02 eV) barely changes compared with that of Cu 2 AgBiI 6 (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 CuAgBiI 5 powder samples are found to be more sensitive to degradation under solar irradiation compared to Cu 2 AgBiI 6 .

Critical Evaluation of the Photovoltaic Performance of (AgI)x(BiI3)y Thin Films from the Viewpoint of Ultrafast Spectroscopy and Photocurrent Experiments

“…166, lattice constants summarized in Table ), which is consistent with a structure previously reported by Pfitzner and co-workers (ICSD database no. 414635) and also in agreement with the finding of Sansom et al that (AgI) x (BiI 3 ) y crystallizes in small trigonal unit cells over the whole x / y composition space, either as a NaVO 2 or CdCl 2 -structure type …”

“…8,11 These compounds are also good silver ion conductors because of the cation vacancies in the crystal structure. 11,12 Similar to BiI 3 , the Ag x Bi y I x+3y compounds show a strong absorption because of a direct transition below 700 nm, which makes them attractive as panchromatic light harvesters. In contrast to BiI 3 , which has an indirect band gap, 13 there appears to be no consensus if these Ag x Bi y I x+3y compounds have an indirect or direct band gap; previous studies used Tauc plots to extract indirect or direct band gaps in the range of 1.60−1.83 eV and 1.73−2.05 eV, respectively.…”

“…9 The phase diagram of the silver iodobismuthate system is fairly complex. 10,11 Several ternary Ag−Bi−I compounds have been identified, including AgBi 2 I 7 , AgBiI 4 , Ag 2 BiI 5 , and Ag 3 BiI 6 , which feature AgI/BiI 3 molar ratios of 1:2, 1:1, 2:1, and 3:1, respectively, and have a cubic close-packed iodide sublattice. 8,11 These compounds are also good silver ion conductors because of the cation vacancies in the crystal structure.…”

“…10,11 Several ternary Ag−Bi−I compounds have been identified, including AgBi 2 I 7 , AgBiI 4 , Ag 2 BiI 5 , and Ag 3 BiI 6 , which feature AgI/BiI 3 molar ratios of 1:2, 1:1, 2:1, and 3:1, respectively, and have a cubic close-packed iodide sublattice. 8,11 These compounds are also good silver ion conductors because of the cation vacancies in the crystal structure. 11,12 Similar to BiI 3 , the Ag x Bi y I x+3y compounds show a strong absorption because of a direct transition below 700 nm, which makes them attractive as panchromatic light harvesters.…”

Perovskite-inspired Cu2AgBiI6 for mesoscopic indoor photovoltaics under realistic low-light intensity conditions

“…Cu 2 AgBiI 6 (CABI) is a newly proposed PIM with favorable optoelectronic properties that has rapidly received great interest from the solar cell community. 18,[20][21][22][23][24][25] Very recently, we successfully improved the lm morphology and increased the surface coverage of CABI lms. 18 As a result of the reduced interfacial recombination, we achieved IPVs with a promising PCE(i) of 4.7% under 1000 lux WLED illumination, which is already approaching that of commercially available a-Si:H IPVs.…”