Halide Ion Exchange Research Articles

Prevention of ion migration in lead halide perovskites upon plugging the anion vacancies with PbSe islands.

To circumvent the issue of halide ion exchange in perovskites, we have decorated CsPbBr3 and CsPbI3 nanocrystals with different sized PbSe nanoparticles and demonstrated that it effectively prevents anion exchange reaction in CsPbBr3/CsPbI3 nanoheterostructures (NHSs) as a consequence of halide vacancy passivation by the more covalent selenide anion.

Halide Ion Mixing across Colloidal 2D Ruddlesden-Popper Perovskites: Implication of Spacer Ligand on Mixing Kinetics.

Halide ion exchange seen in metal halide perovskites provide a substantial opportunity to control their halide composition and corresponding optoelectronic properties. Halide ion mixing across colloidal 3D perovskite nanocrystals have been extensively studied while the mixing within colloidal 2D counterparts remain underexplored. In this study, the halide ion exchange kinetics across colloidally stable 2D Ruddlesden-Popper layered bromide (Br) and iodide (I) perovskites using two different spacer ligands such as aromatic phenethylammonium (PEA) versus linear butyammonium (BA) is demonstrated. The halide exchange kinetic rate constant (k), as determined by tracking time-dependent absorbance changes, indicates that Br/I halide mixing in 2D PEA-based perovskites (2.7 × 10-3 min-1 ) occurs at an order of magnitude slower than in 2D BA-based perovskites (3.3 × 10-2 min-1 ). Concentration (≈1mM to 100mM) and temperature-dependent (50 to 80 °C) kinetic studies further allow for the determination of activation barrier for halide ion mixing across the 2D layered perovskites with 75.2 ± 4.4kJ mol-1 (2D PEA) and 57.8 ± 7.8kJ mol-1 (2D BA), respectively. The activation energy reveals that the type of spacer cations plays a crucial role in controlling the halide ion mobility and halide stability due mainly to the internal ligand chemical interaction within 2D structures.

Achieving one-step solution deposition of high quality CsPbBr3 films for efficient solar cells through halide ion exchange

CsPbBr3 perovskite solar cell (PSC) has triggered an increasing interest owing to its high tolerance to thermal and humidity in the open air. The one-step method is preferable for preparing high-quality CsPbBr3 films and thereby highly efficient PSCs. However, the poor solubility of CsBr in common solvents makes it a big challenge to prepare desired CsPbBr3 films using the traditional one-step method. Therefore, researchers have to employ a two-step method to prepare CsPbBr3 films for PSCs. Actually, the one-step solution processed perovskite films is more attractive owing to its compatible advantages to existing large-scale technology. Therefore, it is crucial to develop a one-step method to prepare high-quality CsPbBr3 films. Here, we propose a one-step solution approach to prepare CsPbBr3 films through halide exchange. In this approach, a PbBr2-CsI-DMSO intermediate film is firstly prepared by spin-coating CsPbIBr2/DMSO solution (1.0 M), followed by dropping hydrogen bromide/acetic acid (HBr/HAc) solution while spinning, and subsequently realize the halide ion exchange between Br- and I-, which separately comes from HBr and PbBr2-CsI-DMSO. After completing the halide ions exchange, it forms a new complex of PbBr2-CsBr-DMSO, and eventually converts to CsPbBr3 films through annealing treatment. As a result, high-quality CsPbBr3 films with a thickness of ~260 nm are obtained due to the appropriate concentration of CsPbIBr2/DMSO solution in the initial step. After integrating into an HTL-free carbon-based PSC, it delivers much higher efficiency than that prepared by the traditional one-step method. This work provides a new one-step preparation method for CsPbBr3 films, making it has attractive applications in CsPbBr3-based devices.

Effect of PbSO4-Oleate Coverage on Cesium Lead Halide Perovskite Quantum Dots to Control Halide Exchange Kinetics.

The selective control of halide ion exchange in metal halide perovskite quantum dots (PQDs) plays an important role in determining their band gap and composition. In this study, CsPbX3 (X = Cl−, Br−, and I−) PQDs were self-assembled with PbSO4-oleate to form a peapod-like morphology to selectively control halide ion exchange. Considering the distinct absorption and bright luminescence characteristics of these PQDs, in situ UV-Vis. absorption and fluorescence spectroscopies were employed to monitor the time-dependent band gap and compositional changes of the PQDs. We determined that the halide exchange in the capped PQDs is hindered—unlike the rapid anion exchange in noncapped PQDs—by a reduction in the halide exchange kinetic rate depending on the extent of coverage of the PQDs. Thus, we tracked the halide ion exchange kinetics between CsPbBr3 and CsPbI3 PQDs, depending on the coverage, using in situ UV-Vis. absorption/photoluminescence spectroscopy. We regulated the halide exchange reaction rate by varying the capping reaction temperature of the PQDs. The capping hindered the halide exchange kinetics and increased the activation energy. These results will enable the development of white LEDs, photovoltaic cells, and photocatalysts with alternative structural designs based on the divalent composition of CsPbX3 PQDs.

Temperature Sensitive and Reversible Halide Ion Exchange in Inorganic–Organic Hybrid CH3NH3PbI3−xBrx Mixed-Halide Perovskite

Ion exchange of organic-inorganic hybrid perovskite plays a significant role in controlling the performance of materials ant its devices. In this study, an interesting phenomenon was observed that the precipitate in suspension (CH3NH3PbI3−xBrx in γ-butyrolactone) brought out different colors at high and room temperature. The mechanism study appears that the phenomenon is controlled by temperature sensitive and reversible halide ion exchange in organic-inorganic hybrid CH3NH3PbI3−xBrx mixed-halide perovskite. The results of structure phase, element composition morphology and band gap illustrate that high temperature 55 °C is beneficial to the increasing of I content in MAPbI3−xBrx and room temperature or lower is advantageous to increasing of Br content in MAPbI3−xBrx. Compared with MAPbI0.76Br2.24 precipitate acquired at room temperature, MAPbI1.17Br1.83 precipitate got at high temperature illustrate wider lattice spacing, better crystallinity, better morphology and narrower band gap. The results and findings in this study will prompt the interest of readers or experts in the field of organic-inorganic hybrid perovskite materials and related optoelectronic applications.

Patterning Rainbowlike Amplified Spontaneous Emission Arrays for Full-Color CsPbX3 Quantum Dot Film Displays

Large-area multicolor laser arrays on the nano- to microscale are key components for the development of on-chip nanophotonics and next-generation color display technology, which remain a formidable task owing to the material lattice mismatch and low-resolution patterning techniques. Here, we demonstrated multicolor-amplified spontaneous emission (ASE) arrays on single-chip integration by patterning CsPbX3 (X = Cl, Br, and I) quantum dot films through a rapid vapor-phase halide ion exchange strategy. Stoichiometrically tunable ASEs were achieved across the whole visible spectral range with similar low thresholds. When the multicolor ASE arrays were further miniaturized to microscale, a spatial definition of 12.5 μm could be achieved, corresponding to an image resolution of up to 2030 pixel per inch.

Slow Anion-Exchange Reaction of Cesium Lead Halide Perovskite Nanocrystals in Supramolecular Gel Networks.

Cesium lead halide perovskite nanocrystals are widely studied as among the most attractive emissive nanomaterials because of their high photoluminescence quantum yield and tunable emission wavelengths over the whole visible-light region by the halide ion-exchange reaction. However, the reactions were often observed in solution and generally very fast, which interferes with the fine-tuning capability of the emission properties. Here, we report a novel nanocrystal-organogel hybrid soft material in which the perovskite nanocrystals in a supramolecular gel exhibit extremely slow and inhomogeneous anion-exchange reactions that are different from those in solution. Furthermore, the inhomogeneous emission in the gel became homogeneous over several days due to a slow diffusion.

Polymer Nanoreactors Shield Perovskite Nanocrystals from Degradation.

Halide perovskite nanocrystals (NCs) have shown impressive advances, exhibiting optical properties that outpace conventional semiconductor NCs, such as near-unity quantum yields and ultrafast radiative decay rates. Nevertheless, the NCs suffer even more from stability problems at ambient conditions and due to moisture than their bulk counterparts. Herein, we report a strategy of employing polymer micelles as nanoreactors for the synthesis of methylammonium lead trihalide perovskite NCs. Encapsulated by this polymer shell, the NCs display strong stability against water degradation and halide ion migration. Thin films comprising these NCs exhibit a more than 15-fold increase in lifespan in comparison to unprotected NCs in ambient conditions and even survive over 75 days of complete immersion in water. Furthermore, the NCs, which exhibit quantum yields of up to 63% and tunability of the emission wavelength throughout the visible range, show no signs of halide ion exchange. Additionally, heterostructures of MAPI and MAPBr NC layers exhibit efficient Förster resonance energy transfer (FRET), revealing a strategy for optoelectronic integration.

Bidirectional Halide Ion Exchange in Paired Lead Halide Perovskite Films with Thermal Activation

MAPbBr3 and MAPbI3 films cast onto glass slides and physically paired together undergo halide exchange to form mixed halide films. The change in halide composition in these two ∼130 nm thick films ...

Chemical Cutting of Perovskite Nanowires into Single-Photon Emissive Low-Aspect-Ratio CsPbX3 (X=Cl, Br, I) Nanorods.

Post-synthetic shape-transformation processes provide access to colloidal nanocrystal morphologies that are unattainable by direct synthetic routes. Herein, we report our finding about the ligand-induced fragmentation of CsPbBr3 perovskite nanowires (NWs) into low aspect-ratio CsPbX3 (X=Cl, Br and I) nanorods (NRs) during halide ion exchange reaction with PbX2 -ligand solution. The shape transformation of NWs-to-NRs resulted in an increase of photoluminescence efficiency owing to a decrease of nonradiative decay rates. Importantly, we found that the perovskite NRs exhibit single photon emission as revealed by photon antibunching measurements, while it is not detected in parent NWs. This work not only reports on the quantum light emission of low aspect ratio perovskite NRs, but also expands our current understanding of shape-dependent optical properties of perovskite nanocrystals.

PbSe Quantum Dot Passivated Via Mixed Halide Perovskite Nanocrystals for Solar Cells With Over 9% Efficiency

PbSe quantum dots (QDs) have stronger electronic coupling resulting from a large Bohr exciton radius, suggesting PbSe QDs may be able to achieve superior charge separation and transport in optoelectronic devices compared with PbS QDs. However, PbS QDs solar cell have achieved a certified 12.01% power conversion efficiency (PCE), whereas PbSe QD photovoltaics lag behind at 8.2% PCE. One reason for this difference is that there has been significantly less work done on surface passivation of PbSe QDs. Here, the surface passivation of chlorinated PbSe QDs is optimized via a halide ion exchange treatment using mixed halide CsPb(Br/I)3 perovskite nanocrystals. Champion devices made from treated QDs achieved a PCE of 9.2%, Voc of 0.56 V, Jsc of 25.7 mA cm−2, and fill factor of 64%. Average PCEs for optimized cells are 8.9%. Detailed physical characterizations including capacitance‐voltage (C‐V), Voc, and Jsc as a function of light intensity, transient photovoltage, and photocurrent measurements are all carried out to investigate the mechanism of the improvement in the PCE and to understand the role of the mixed halide perovskites in providing superior surface passivation for PbSe solar cells. At this time, 9.2% is the highest PCE yet reported for PbSe QDs solar cells.

Hierarchical Arrays of Cesium Lead Halide Perovskite Nanocrystals through Electrophoretic Deposition.

The suppression of halide ion exchange between CsPbBr3 and CsPbI3 nanocrystals achieved through capping with PbSO4-oleate has enabled us to deposit different perovskite nanocrystals as aligned arrays on the electrode surfaces without intermixing of species. The electrophoretic deposition of PbSO4-oleate-capped CsPbX3 (X = Cl, Br, I) nanocrystals suspended in hexane solution on mesoscopic TiO2 films allows the design of controlled architecture with single or multiple layers of perovskite films. The hierarchy in the assembly of these nanocrystals is seen first through the linearly organized nanocrystals in hexane followed by the deposition of larger linear rods ∼500 nm in length. Since most of the photophysical properties of nanocrystals are retained in these aligned arrays, we can design films with tunable luminescence including white color. The electrophoretic deposition of layered films of perovskites in a controlled fashion opens up new ways to design tandem perovskite solar cells and tunable display devices.

Spontaneous Self-Assembly of Perovskite Nanocrystals into Electronically Coupled Supercrystals: Toward Filling the Green Gap.

Self-assembly of nanoscale building blocks into ordered nanoarchitectures has emerged as a simple and powerful approach for tailoring the nanoscale properties and the opportunities of using these properties for the development of novel optoelectronic nanodevices. Here, the one-pot synthesis of CsPbBr3 perovskite supercrystals (SCs) in a colloidal dispersion by ultrasonication is reported. The growth of the SCs occurs through the spontaneous self-assembly of individual nanocrystals (NCs), which form in highly concentrated solutions of precursor powders. The SCs retain the high photoluminescence (PL) efficiency of their NC subunits, however also exhibit a redshifted emission wavelength compared to that of the individual nanocubes due to interparticle electronic coupling. This redshift makes the SCs pure green emitters with PL maxima at ≈530-535 nm, while the individual nanocubes emit a cyan-green color (≈512 nm). The SCs can be used as an emissive layer in the fabrication of pure green light-emitting devices on rigid or flexible substrates. Moreover, the PL emission color is tunable across the visible range by employing a well-established halide ion exchange reaction on the obtained CsPbBr3 SCs. These results highlight the promise of perovskite SCs for light emitting applications, while providing insight into their collective optical properties.

To Exchange or Not to Exchange. Suppressing Anion Exchange in Cesium Lead Halide Perovskites with PbSO4–Oleate Capping

The ease of halide ion exchange in metal halide nanocrystals offers an opportunity to utilize them in a layered or tandem fashion to achieve graded bandgap films. We have now successfully suppressed the halide ion exchange by capping CsPbBr3 and CsPbI3 nanocrystals with PbSO4–oleate to create a nanostructure assembly that inhibits the exchange of anions. Absorption measurements show that the nanocrystal assemblies maintain their identity as either CsPbBr3 or CsPbI3 for several days. Furthermore, the effect of PbSO4–oleate capping on the excited state dynamics has also been elucidated. The effectiveness of PbSO4–oleate capping of lead halide perovskite nanocrystals offers new opportunities to overcome the challenges of halide ion exchange and aid toward the tandem design of perovskite light-harvesting assemblies.

From Precursor Powders to CsPbX3 Perovskite Nanowires: One-Pot Synthesis, Growth Mechanism, and Oriented Self-Assembly.

The colloidal synthesis and assembly of semiconductor nanowires continues to attract a great deal of interest. Herein, we describe the single-step ligand-mediated synthesis of single-crystalline CsPbBr3 perovskite nanowires (NWs) directly from the precursor powders. Studies of the reaction process and the morphological evolution revealed that the initially formed CsPbBr3 nanocubes are transformed into NWs through an oriented-attachment mechanism. The optical properties of the NWs can be tuned across the entire visible range by varying the halide (Cl, Br, and I) composition through subsequent halide ion exchange. Single-particle studies showed that these NWs exhibit strongly polarized emission with a polarization anisotropy of 0.36. More importantly, the NWs can self-assemble in a quasi-oriented fashion at an air/liquid interface. This process should also be easily applicable to perovskite nanocrystals of different morphologies for their integration into nanoscale optoelectronic devices.

Photoinduced Anion Exchange in Cesium Lead Halide Perovskite Nanocrystals.

Cesium lead halide (CsPbX3) perovskite nanocrystals (NCs) possess the unique capability of post-synthesis anion exchange providing facile tunability of the optical properties, which is usually achieved by mixing NCs with reactive anion precursors. In this work, we show that the controllable anion exchange can be achieved in a dihalomethane solution of CsPbX3 NC in the absence of any spontaneously reacting anion source using photoexcitation of CsPbX3 NCs as the triggering mechanism for the halide ion exchange. The reaction begins with the photoinduced electron transfer from CsPbX3 NCs to dihalomethane solvent molecules producing halide ions via reductive dissociation, which is followed by anion exchange. The reaction proceeds only in the presence of excitation light and the rate and extent of reaction can be controlled by varying the light intensity. Furthermore, the asymptotic extent of reaction under continuous excitation can be controlled by varying the wavelength of light that self-limits the reaction when light becomes off-resonance with the absorption of NCs. The light-controlled anion exchange demonstrated here can be utilized to pattern the post-synthesis chemical transformation of CsPbX3 NCs, not readily achievable using typical methods of anion exchange.

All-inorganic cesium lead halide perovskite nanocrystals for photodetector applications.

Herein, we describe simple, fast and reproducible halide ion exchange reactions in CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) at room temperature. Through the simple adjustment of the halide ion concentration, the photoluminescence of these NCs can be tuned over the entire visible region (425-655 nm). Photodetector devices based on entirely inorganic CsPbI3 NCs are demonstrated for the first time. The photodetectors exhibited a good on/off photocurrent ratio of 10(5).

Preparations and structures of (π-C 5H 5) 2NbI 2 and (π-C 5H 5) 2NbI 3

Dicyclopentadienylniobium triiodide can be synthesized by halide ion exchange reaction from (π-C 5H 5) 2NbCl 2OH and NH 4I in aqueous solution. This compound, in contrast to the other known (π-C 5H 5) 2NbX 3 compounds (XCl, Br), shows on tendency to hydrolyze. It can be crystallized as thin violet plates from dichloromethane. Dicyclopentadienylniobium diiodie is formed when (π-C 5H 5) 2NbCl 2OH is reduced by benzyl mercaptan in a methanol/benzene solution, in the presence of NH 4I. This compound forms black, lustrous crystals, which are slightly air sensitive, though the compound itself is decomposed rapidly by air in solution. The compound is paramagnetic. The infrared spectrum of (π-C 5H 5) 2NbI 2 is almost identical with the spectrum of the analogous zirconium compound, suggesting similar structures. The infrared spectrum of (π-C 5H 5) 2NbI 3 supports a structure involving parallel or near parallel cyclopentadienyl rings. Models for such a system are proposed which strongly support this structure.

Mechanism of substitution at a saturated carbon atom. Part XXXVII. Halide-ion exchange with a simple alkyl halide in sulphur dioxide solution

Mechanism of substitution at a saturated carbon atom. Part XXXVII. Halide-ion exchange with a simple alkyl halide in sulphur dioxide solution