Mechanochemical synthesis of the lead-free double perovskite Cs₂[AgIn]Br₆ and its optical properties

Abstract

Hitting hard on the binary halides yields in the formation of Cs 2 [AgIn]Br 6 . The lead-free double perovskite marks, although not usable itself, a further step forward in finding sustainable and durable perovskite materials for photovoltaic applications. Cs 2 [AgIn]Br 6 is one of the prominent examples of double perovskites materials that have been suggested to circumvent the use of lead compounds in perovskite solar cells. We herein report the successful synthesis of the material as bulk powder using a mechanochemical approach. It crystallises in an elpasolite-type structure, an ordered perovskite superstructure, with a cell parameter of a = 11.00 Å. However, the compound exhibits a relatively large optical bandgap of 2.36 eV and is unstable under illumination, which impedes its use as solar absorber material at this early stage. Still, substitution of lead and the potential of this synthesis method are promising as well as the fruitful combination of theoretical considerations with experimental materials discovery.

Ag/In lead‐free double perovskites

“…The refined lattice parameter is a = 10.4796(5) Å (where a is twice the cube edge), in excellent agreement with a previous measurement of 10.47 Å . The calculated lattices are a = 10.39 Å for CAIC and a = 10.93 Å for CAIB, which compares very well with value 11.0 Å found by XRD . In passing, we remark the striking difference with the cubic‐equivalent a ~ 12.5 Å of the methylammonium lead iodide prototype, mostly due to the replacement of the molecular sublattice.…”

“…We obtain for CAIC an “optical gap” of 3.66 eV, thus only a bit larger than the value extracted by Tauc plot. For CAIB the same procedure gives 2.1 eV, in satisfying agreement with the 2.36 eV value extracted from Tauc in Reference . According to calculations, absorption is small but not vanishing below these energy thresholds: at the plateau, just below the threshold, we calculate absorption rates equal to ~1.5 × 10 3 cm −1 and 5 × 10 3 cm −1 for CAIC and CAIB, respectively, thus, about two order of magnitude smaller than, for example, the CH 3 NH 3 PbI 3 (MAPI) absorption at the band gap edge.…”

“…Here, we present a combined theoretical and experimental study headed to recast in a solid framework the most fundamental aspects of this material. On the other hand, CAIB is much less explored than its Cl‐based counterpart . While difficult to synthesize, being on the verge of a thermodynamic instability, our calculations reveal a system with great potential as a lead‐free photovoltaic absorber, with direct band‐gap of 1.17 eV and large conversion efficiency.…”

“…Eventually, what a single cation cannot achieve, can be done, perhaps, by the cooperation of two: this is the idea which pushed several groups to explore the coexistence of two different B‐site cations in the same compound, thus extending the chemical template of the search from that of single perovskites to the double perovskites of the form A 2 BB'X 6 ; in fact, if B and B′ are not too different and size‐compatible with A and X, these materials can grow structurally ordered and highly symmetric, with B and B′ alternating in regular patterns; clearly, the chemical equivalence with Pb 2+ requires a B 1+ and B′ 3+ combination.…”

On the Nitridation of Zn2GeO4

“…Taking lead‐based halide perovskites as an example, the replacement of divalent lead ions by other divalent ions has resulted in the formulation of Sn 2+ ‐ and Ge 2+ ‐based materials that do, however, show a severe instability against oxidation into the tetravalent states . Instead of replacing divalent ions directly by other divalent ions, a mixture of monovalent and trivalent ions would also result in a stable structure, as was shown for double perovskites like Cs 2 [AgIn]Br 6 . Applying the same rationale on copper indium/gallium chalcogenides (CIGS) and replacing the trivalent gallium/indium ions with equimolar amounts of divalent (e.g.…”