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.
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.
Spectroscopic‐grade single crystal detectors can register the energies of individual X‐ray interactions enabling photon‐counting systems with superior resolution over traditional photoconductive X‐ray detection systems. Current technical challenges have limited the preparation of perovskite semiconductors for energy‐discrimination X‐ray photon‐counting detection. Here, this work reports the deployment of a spectroscopic‐grade CsPbBr3 Schottky detector under reverse bias for continuum hard X‐ray detection in both the photocurrent and spectroscopic schemes. High surface barriers of ≈1 eV are formed by depositing solid bismuth and gold contacts. The spectroscopic response under a hard X‐ray source is assessed in resolving the characteristic X‐ray peak. The methodology in enhancing X‐ray sensitivity by controlling the X‐ray energies and flux, and voltage, is described. The X‐ray sensitivity varies between a few tens to over 8000 μC Gyair−1 cm−2. The detectable dose rate of the CsPbBr3 detectors is as low as 0.02 nGyair s−1 in the energy discrimination configuration. Finally, the unbiased CsPbBr3 device forms a spontaneous contact potential difference of about 0.7 V enabling high quality of the CsPbBr3 single crystals to operate in “passive” self‐powered X‐ray detection mode and the X‐ray sensitivity is estimated as 14 μC Gyair−1 cm−2. The great potential of spectroscopic‐grade CsPbBr3 devices for X‐ray photon‐counting systems is anticipated in this work.
The nature of the organic cation in two-dimensional (2D) hybrid lead iodide perovskites tailors the structural and technological features of the resultant material. Herein, we present three new homologous series of (100) lead iodide perovskites with the organic cations allylammonium (AA) containing an unsaturated CC group and iodopropylammonium (IdPA) containing iodine on the organic chain: (AA)2MA n –1Pb n I3n+1 (n = 3–4), [(AA) x (IdPA)1–x ]2MA n –1Pb n I3n+1 (n = 1–4), and (IdPA)2MA n –1Pb n I3n+1 (n = 1–4), as well as their perovskite-related substructures. We report the in situ transformation of AA organic layers into IdPA and the incorporation of these cations simultaneously into the 2D perovskite structure. Single-crystal X-ray diffraction shows that (AA)2MA2Pb3I10 crystallizes in the space group P21/c with a unique inorganic layer offset (0, <1/2), comprising the first example of n = 3 halide perovskite with a monoammonium cation that deviates from the Ruddlesden–Popper (RP) halide structure type. (IdPA)2MA2Pb3I10 and the alloyed [(AA) x (IdPA)1–x ]2MA2Pb3I10 crystallize in the RP structure, both in space group P21/c. The adjacent I···I interlayer distance in (AA)2MA2Pb3I10 is ∼5.6 Å, drawing the [Pb3I10]4– layers closer together among all reported n = 3 RP lead iodides. (AA)2MA2Pb3I10 presents band-edge absorption and photoluminescence (PL) emission at around 2.0 eV that is slightly red-shifted in comparison to (IdPA)2MA2Pb3I10. The band structure calculations suggest that both (AA)2MA2Pb3I10 and (IdPA)2MA2Pb3I10 have in-plane effective masses around 0.04m 0 and 0.08m 0, respectively. IdPA cations have a greater dielectric contribution than AA. The excited-state dynamics investigated by transient absorption (TA) spectroscopy reveal a long-lived (∼100 ps) trap state ensemble with broad-band emission; our evidence suggests that these states appear due to lattice distortions induced by the incorporation of IdPA cations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.