Highly Absorbing Lead-Free Semiconductor Cu₂AgBiI₆ for Photovoltaic Applications from the Quaternary CuI–AgI–BiI₃ Phase Space

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 2 AgBiI 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 3 NH 3 PbI 3 . Solution-processed Cu 2 AgBiI 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 2 AgBiI 6 thin films have not been synthesized to date, Cu 2 AgBiI 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.

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

“…CuBiI 4 does not fit the trend but we previously found it to be metastable at room temperature. 2 (b) The same relationship shown in the CuI–AgI–BiI 3 phase space, where the color map and red contour lines represent total Oct site occupancy. We note that the Oct site occupancy in (b) is not representative of materials which contain tetrahedral Ag + such as the room temperature structure of AgI.…”

“… Reported structures in the CuI–AgI–BiI 3 phase space: binaries CuI, AgI, and BiI 3 ; 15 − 17 ternaries Ag 3 BiI 6 , Ag 2 BiI 5 , AgBiI 4 , AgBi 2 I 7 , Ag 2 Bi 3 I 11 , CuBiI 4 , and Cu 2 BiI 5 ; 1 , 18 − 22 and quaternary Cu 2 AgBiI 6 . 2 We have also added the compound we report here, CuAgBiI 5 . As discussed in the text, all these compounds consist of a close-packed iodide sublattice with varying arrangements of the cations filling the octahedral and tetrahedral interstitial sites to form the structures shown.…”

“…(b) The two Cu + sites in the small trigonal unit cell (Cu 2 AgBiI 6 and CuBiI 4 (CdCl 2 )) showing layered ordering. 2 Also shown are the connectivities of the Tet sites, which give a 3D Tet network. (c) The layered ordering of Cu + sites in CuAgBiI 5 means they are only in layers with Oct interstitial occupancy and do not occupy all the sites associated with tetrahedral site 2 in CuBiI 4 (spinel).…”

“…However, the unstable nature of the phase compared to a mixture of CuI and BiI 3 at room temperature can be associated with the outlier Oct site occupancy. 2 This means that CuAgBiI 5 and Cu 2 AgBiI 6 are examples of phase stable Cu-containing compounds attained by increasing total Oct site occupancy compared to CuBiI 4 .…”

Chemical Control of the Dimensionality of the Octahedral Network of Solar Absorbers from the CuIAgIBiI3 Phase Space by Synthesis of 3D CuAgBiI5

“…We have previously reported the optimised procedure to synthesise Cu2AgBiI6 powder without going in to detail of the exploratory synthesis. 14 We showed there that it was found to be crucial to quench the Cu2AgBiI6 powder from 350°C, and avoiding temperatures which would melt the compound, to avoid the compositional inhomogeneity, and the same is true here for CuAgBiI5, which leads to our optimised CuAgBiI5 synthesis.…”