Colloidal Nanocrystals of Lead-Free Double-Perovskite (Elpasolite) Semiconductors: Synthesis and Anion Exchange To Access New Materials

Abstract

Concerns about the toxicity and instability of lead-halide perovskites have driven a recent surge in research toward alternative lead-free perovskite materials, including lead-free double perovskites with the elpasolite structure and visible bandgaps. Synthetic approaches to this class of materials remain limited, however, and no examples of heterometallic elpasolites as nanomaterials have been reported. Here, we report the synthesis and characterization of colloidal nanocrystals of CsAgBiX (X = Cl, Br) elpasolites using a hot-injection approach. We further show that postsynthetic modification through anion exchange and cation extraction can be used to convert these nanocrystals to new materials including CsAgBiI, which was previously unknown experimentally. Nanocrystals of CsAgBiI, synthesized via a novel anion-exchange protocol using trimethylsilyl iodide, have strong absorption throughout the visible region, confirming theoretical predictions that this material could be a promising photovoltaic absorber. The synthetic methodologies presented here are expected to be broadly generalizable. This work demonstrates that nanocrystal ion-exchange reactivity can be used to discover and develop new lead-free halide perovskite materials that may be difficult or impossible to access through direct synthesis.

From Lead Halide Perovskites to Lead‐Free Metal Halide Perovskites and Perovskite Derivatives

“…[179][180][181] Greul et al [179] first demonstrated the synthesis of Cs 2 Ag-BiBr 6 films and then used the films to fabricate solar cells in the regular mesoporous structure. [175] The obtained Cs 2 AgBiI 6 nanocrystals have an indirect bandgap of ≈1.75 eV. Recently, Ning et al [180] reported the first Cs 2 AgBiBr 6 -based solar cells using the planar structure, which exhibited an average PCE of over 1%.…”

“…[55] Second, for B(III) cations, the transition metal cations with multiple oxidation states and/or partially occupied d or f orbitals are not desirable for designing photovoltaic absorbers, because they may introduce deep defect states and too localized band edges. Largesize single crystals, [155,[172][173][174] nanocrystals, [175][176][177][178] and highquality films [179][180][181][182] The element Au is half-colored because it exists only in the compounds A 2 Au 2 X 6 with A = Rb, Cs and X = Cl, Br, I. Most of the reported double perovskites are fluorides and chlorides, [158,159] which generally have too large bandgaps for photovoltaic applications.…”

Lead-free hybrid perovskites for photovoltaics

“…16) or by exchanging Cs + cations in Cs 3 BiBr 6 NCs with Ag + (Fig. 16) [202]. The NCs preserve the average size (≈8 nm) and size distribution (Fig.…”

“…16–e) during the ion-exchange conversions, showing a good morphology control provided by this method. The indirect band gap of Cs 2 AgBiX 6 NCs decreases from ≈2.8 eV for X = Cl to ≈2.2 eV for X = Br to ≈1.75 eV for X = I [202]. …”

“…OAc is acetate anion, Et – ethyl, OA – oleic acid, OLA – oleylamine, ODE – octadecene, TMS – trimethylsilyl. Reprinted and adapted with permission from [202], copyright 2018 American Chemical Society.…”