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.

Manganese­(II)-doped cesium–lead–chloride (Mn2+:CsPbCl3) perovskite nanocrystals have recently been developed as promising luminescent materials and attractive candidates for white-light generation. One approach to tuning the luminescence of these materials has involved anion exchange to incorporate Br–, but the effects of anion exchange on Mn2+ speciation in doped metal-halide perovskites is not well understood at a microscopic level. Here, we use a combination of X-band electron paramagnetic resonance (EPR) and photoluminescence spectroscopies to monitor the Mn2+ dopants in Mn2+:CsPbCl3 nanocrystals during Cl– → Br– anion exchange. Analytical measurements show that the nanocrystals retain their Mn2+ over the course of Cl– → Br– anion exchange and they continue to show strong Mn2+ d–d luminescence but, surprisingly, the Mn2+ EPR intensities all but vanish. Further results suggest that Mn2+ ions migrate during anion exchange to form clusters that are still luminescent but show no EPR signal due to antiferromagnetic superexchange coupling. Monte Carlo simulation and analysis of the Mn2+:CsPb­(Cl1–x Br x )3 lattice at various halide compositions (x) bolsters this interpretation by indicating a propensity for Mn2+–Cl– units to cluster as the Br– content increases, increasing the probability of the nearest-neighbor Mn2+–Mn2+ interactions. The driving force for this clustering is retention of the stronger Mn–Cl bonds compared to Mn–Br bonds. In addition, modeling predicts spinodal decomposition to form Mn2+-enriched domains even at the end point compositions of x = 0 and 1, with Mn2+ ordering in next-nearest-neighbor positions driven by Coulomb interactions and lattice-strain minimization. These results have important implications for both fundamental studies and applications of doped and alloyed metal-halide perovskites.

Delayed luminescence involving charge-carrier trapping and detrapping has recently been identified as a widespread and possibly universal phenomenon in colloidal quantum dots. Its near-power-law decay suggests a relationship with blinking. Here, using colloidal CuInS 2 and CdSe quantum dots as model systems, we show that short (ns) excitation pulses yield less delayed luminescence intensity and faster delayed luminescence decay than observed with long (ms) square-wave excitation pulses. Increasing the excitation power also affects the delayed luminescence intensity, but the delayed luminescence decay kinetics appear much less sensitive to excitation power than to excitation pulse width. An idealized four-state kinetic model reproduces the major experimental trends and highlights the very slow approach to steady state during photoexcitation, stemming from extremely slow detrapping of the metastable chargeseparated state responsible for delayed luminescence. The impacts of these findings on proposed relationships between delayed luminescence and blinking are discussed.

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