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.

A series of Mn 2+ -doped CsPbCl 3 nanocrystals (NCs) was synthesized using reaction temperature and precursor concentration to tune Mn 2+ concentrations up to 14%, and then studied using variable-temperature photoluminescence (PL) spectroscopy. All doped NCs show Mn 2+ 4 T 1g → 6 A 1g d−d luminescence within the optical gap coexisting with excitonic luminescence at the NC absorption edge. Room-temperature Mn 2+ PL quantum yields increase with increased doping, reaching ∼60% at ∼3 ± 1% Mn 2+ before decreasing at higher concentrations. The low-doping regime is characterized by singleexponential PL decay with a concentration-independent lifetime of 1.8 ms, reflecting efficient luminescence of isolated Mn 2+ . At elevated doping, the decay is shorter, multiexponential, and concentration-dependent, reflecting the introduction of Mn 2+ −Mn 2+ dimers and energy migration to traps. A large, anomalous decrease in Mn 2+ PL intensity is observed with decreasing temperature, stemming from the strongly temperature-dependent exciton lifetime and slow exciton-to-Mn 2+ energy transfer, which combine to give a strongly temperature-dependent branching ratio for Mn 2+ sensitization.

Realizing solution-processed heterostructures is a long-enduring challenge in halide perovskites because of solvent incompatibilities that disrupt the underlying layer. By leveraging the solvent dielectric constant and Gutmann donor number, we could grow phase-pure two-dimensional (2D) halide perovskite stacks of the desired composition, thickness, and bandgap onto 3D perovskites without dissolving the underlying substrate. Characterization reveals a 3D–2D transition region of 20 nanometers mainly determined by the roughness of the bottom 3D layer. Thickness dependence of the 2D perovskite layer reveals the anticipated trends for n-i-p and p-i-n architectures, which is consistent with band alignment and carrier transport limits for 2D perovskites. We measured a photovoltaic efficiency of 24.5%, with exceptional stability of T 99 (time required to preserve 99% of initial photovoltaic efficiency) of >2000 hours, implying that the 3D/2D bilayer inherits the intrinsic durability of 2D perovskite without compromising efficiency.

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Doping lanthanide ions into colloidal semiconductor nanocrystals is a promising strategy for combining their sharp and efficient 4f-4f emission with the strong broadband absorption and low-phonon-energy crystalline environment of semiconductors to make new solution-processable spectral-conversion nanophosphors, but synthesis of this class of materials has proven extraordinarily challenging because of fundamental chemical incompatibilities between lanthanides and most intermediate-gap semiconductors. Here, we present a new strategy for accessing lanthanide-doped visible-light-absorbing semiconductor nanocrystals by demonstrating selective cation exchange to convert precursor Yb-doped NaInS nanocrystals into Yb-doped PbInS nanocrystals. Excitation spectra and time-resolved photoluminescence measurements confirm that Yb is both incorporated within the PbInS nanocrystals and sensitized by visible-light photoexcitation of these nanocrystals. This combination of strong broadband visible absorption, sharp near-infrared emission, and long (>400 μs) emission lifetimes in a colloidal nanocrystal system opens promising new opportunities for both fundamental-science and next-generation spectral-conversion applications such as luminescent solar concentrators.