Halide Mixing Research Articles

Tuning the photocatalytic performance of halide perovskites for efficient solar hydrogen production: A DFT study of CsGeCl3-xXx (X= Br, I)

Photoelectrochemical water splitting holds immense potential for sustainable hydrogen production to address mounting energy demands. However, the development of efficient and cost-effective photocatalysts remains a significant challenge. Perovskites, recognized for their cost-effectiveness and tunable characteristics, show potential as viable candidates in this context. Our investigation is the first to mark the photocatalytic efficacy of CsGeCl3. The findings reveal a modest solar-to-hydrogen (STH) efficiency of 0.74 % due to its wide bandgap energy. Halide mixing with iodine and bromine significantly improves the STH efficiencies, reaching approximately 8.47 %. In addition, applying uniaxial compressive stress further boosts the efficiency to 14.6 %. In this study, we employed the density functional theory (DFT) through the WIEN2k software to scrutinize the structural, electronic, optical, photocatalytic, and thermoelectric properties of all the studied structures. Furthermore, the study evaluated the compounds' capacity for CO2 photoreduction, susceptibility to degradation, and the influence of pH on their photocatalytic performance. The insights gained from this work contribute to the development of efficient and cost-effective perovskite-based photocatalysts for renewable hydrogen production.

From Pb‐based MAPbI3−xClx to Pb‐free FASnI3−xClx and CsSbCl4 derivatives fabrication in atmospheric conditions for optoelectronic and solar cell applications

AbstractMAPbI3 is the most attractive perovskite, but toxicity and instability issues hinder its commercial applications. Stability can be improved by halide mixing; however, Pb‐free perovskites are designed to alleviate the toxicity and to enable green photovoltaics (PVs). To this end, MAPbI3‐xClx, FASnI3‐xClx and CsSbCl4 films are deposited by spay pyrolysis technique in atmospheric conditions. SEM images demonstrated that through this process, high quality film fabrication is possible. Color of the precursor solutions changes with stirring time. High crystallinity and existence of mixed‐phases are confirmed by XRD analysis. Compositions greatly impact the morphology and optical properties. Value of α is larger than 105 cm−1 for all films. Band gaps of FASnI3‐xClx and CsSbCl4 are 1.46 eV and 1.52 eV, which are more suitable for PVs, optoelectronic applications than MAPbI3‐xClx (Eg = 1.59 eV). The efficiency was obtained as 16.34%, 9.90%, and 13.08% for deposited MAPbI3‐xClx, FASnI3‐xClx, and CsSbCl4 films. The lower efficiency can further be enhanced by optimizing parameters, and in this study it was found as 20.78%, 11.93%, and 18.02%. Theoretical calculations show the films can easily produce O2 by a strong oxidation process. Thus, the favorable characteristics of FASnI3‐xClx and CsSbCl4 make alternative Pb‐free perovskites for PV, electronic, and optoelectronic applications.image

Halide Mixing in Cs2AgBi(I x Br1-x )6 Double Perovskites: A Pathway to Tunable Excitonic Properties.

Cs2AgBiBr6 is an emerging double perovskite semiconductor with robust stability. However, its potential for photovoltaics is limited by its indirect band gap and localized electronic structure featuring a resonant exciton with a large binding energy. Cs2AgBi(I x Br1-x )6 nanocrystals with iodide concentrations of up to 100% were recently demonstrated, but an atomistic understanding of how halide mixing affects the electronic and excited-state structure is missing. Here, we use first-principles GW and Bethe-Salpeter Equation calculations to show that halide mixing leads to a pronounced change in the band gap and character of optical excitations. Exciton binding energies are reduced by up to a factor of 5, with significantly more delocalized excitons in I-rich compounds. We further show that phase-pure bulk alloys with x ≤ 0.11 can be fabricated using mechanosynthesis and measure a red-shifted absorption in line with our calculations. Our study highlights that halide mixing in double perovskites can not only lead to significant band gap changes but may also be used for tuning excitonic properties.

Halide tuning in derivative organic–inorganic perovskites, MAPb(Br1-xClx)3 (x = 0–1): exploring structural, optical and vibrational characteristics

Organic–inorganic metal halide perovskites (OIMHP’s), particularly CH3NH3PbX3 (X = I, Br, Cl) and their derivatives shows favorable properties for energy harvesting such as high absorption coefficients, adjustable band gaps, and low charge recombination rate. The structure and hence nature of bonding between different atoms of these perovskites is known to affect their properties significantly. Tuning of band gap can be achieved in these systems with the help of compositional variation. These systems are studied extensively in the single halide compositions (MAPbI3, MAPbBr3 and MAPbCl3); while, their derivatives seem to have gained less attention though being important for various applications. So, in this work, halide tuning is achieved in derivative perovskite, MAPb(Br1-xClx)3 (x = 0 to 1) and further studied for structural and optical properties along with vibrational properties using X-ray diffraction (XRD), diffuse reflectance spectroscopy, and Raman spectroscopy techniques, respectively. A decrease in the lattice parameter is observed as the Chlorine content increases in the MAPb(Br1-xClx)3 (x = 0 to 1) perovskite. The substitution of Chlorine with Bromine also results in significant increase in the band gap value. In contrast to previous reports, it was clearly observed that the Urbach energy strongly depends on the composition. For the first time, appearance of two features for torsional mode of methylammonium (MA) is discussed even at room temperature, indicative of disorder. It is observed that although the band gap tuning is achieved with the help of halide mixing (Br and Cl), it is also found to introduce disorder in the intermediate compositions; while, the stability increases toward Chlorine compositions. Interestingly, the information of disorder is found to be contained in both the global as well as local measurements which opens up new pathways for studying these materials. This study will lay down a pathway for better understanding of key properties of these hybrid mix halide perovskites which are promising material for futuristic energy applications.

Perovskite Quantum Heterostructure Constructed by Halide Mixing between a Single CsPbI3 Nanocrystal and an Individual CsPbBr3 Microplate.

Ion migration is significantly enhanced in lead-halide perovskites with a soft crystal lattice, which can promote the formation of a heterogeneous interface between two such materials with different halide-anion compositions. Here we have deposited a single CsPbI3 nanocrystal (NC) on top of an individual CsPbBr3 microplate to create a mixed-halide CsPbBrxI3-x (0 < x < 3) NC by means of the anion exchange process. The formation of a CsPbBrxI3-x/CsPbBr3 heterostructure is confirmed by the much-enlarged geometric volume of the CsPbBrxI3-x NC as compared to the original CsPbI3 one, as well as by its capability of receiving photogenerated excitons from the CsPbBr3 microplate with a larger bandgap energy. The quantum nature of this heterostructure is reflected from single-photon emission of the composing CsPbBrxI3-x NC, which can also be bulk-like during phase segregation to demonstrate a red shift in the photoluminescence peak that is opposite to the common trend observed in smaller-sized mixed-halide NCs.

Structural Evolution and Photoluminescence Quenching across the FASnI3-xBrx (x = 0-3) Perovskites.

One of the primary methods for band gap tuning in metal halide perovskites has been halide (I/Br) mixing. Despite widespread usage of this type of chemical substitution in perovskite photovoltaics, there is still little understanding of the structural impacts of halide alloying, with the assumption being the formation of ideal solid solutions. The FASnI3-xBrx (x = 0-3) family of compounds provides the first example where the assumption breaks down, as the composition space is broken into two unique regimes (x = 0-2.9; x = 2.9-3) based on their average structure with the former having a 3D and the latter having an extended 3D (pseudo 0D) structure. Pair distribution function (PDF) analyses further suggest a dynamic 5s2 lone pair expression resulting in increasing levels of off-centering of the central Sn as the Br concentration is increased. These antiferroelectric distortions indicate that even the x = 0-2.9 phase space behaves as a nonideal solid-solution on a more local scale. Solid-state NMR confirms the difference in local structure yielding greater insight into the chemical nature and local distributions of the FA+ cation. In contrast to the FAPbI3-xBrx series, a drastic photoluminescence (PL) quenching is observed with x ≥ 1.9 compounds having no observable PL. Our detailed studies attribute this quenching to structural transitions induced by the distortions of the [SnBr6] octahedra in response to stereochemically expressed lone pairs of electrons. This is confirmed through density functional theory, having a direct impact on the electronic structure.

High-Efficiency Pure-Red CsPbI3 Quantum Dot Light-Emitting Diodes Enabled by Strongly Electrostatic Potential Solvent and Sequential Ligand Post-treatment Process.

Efficient pure-red emission light-emitting diodes (LEDs) are essential for high-definition displays, yet achieving pure-red emission is hindered by challenges like phase segregation and spectral instability when using halide mixing. Additionally, strongly confined quantum dots (QDs) produced through traditional hot-injection methods face byproduct contamination due to poor solubility of metal halide salts in the solvent octadecene (ODE) at low temperatures. Herein, we introduced a novel method using a benzene-series strongly electrostatic potential solvent instead of ODE to prevent PbI2 intermediates and promote their dissolution into [PbI3]-. Increasing methyl groups on benzene yields precisely sized (4.4 ± 0.1 nm) CsPbI3 QDs with exceptional properties: a narrow 630 nm PL peak with photoluminescence quantum yield (PLQY) of 97%. Sequential ligand post-treatment optimizes optical and electrical performance of QDs. PeLEDs based on optimized QDs achieve pure-red EL (CIE: 0.700, 0.290) approaching Rec. 2020 standards, with an EQE of 25.2% and T50 of 120 min at initial luminance of 107 cd/m2.

Band Gap Alteration of Halide Mixing in Hybrid Perovskites: A First-Principles Study with Statistical Analysis.

Despite numerous studies on the band gap of three-dimensional halide perovskites using the first-principles calculations, there are still significant discrepancies between theoretical and experimental values. Various solutions have been proposed, such as employing a system-specific hybrid functional with varying degrees of exact exchange and explicitly incorporating spin-orbit coupling effects. Our research involved a comprehensive investigation of three typical lead-containing three-dimensional perovskites MAPbI3, MAPbBr3, and MAPbCl3 (MA = CH3NH3). Through a statistical analysis comparing mean absolute error (MAE) with experimental results, we demonstrated that the nonlocal van der Waals (vdW) density functional corrections (i.e., optB86b) yielded the most approximate lattice parameters in comparison to experimental values. Furthermore, based on these lattice parameters, the HSE06 hybrid functional is the optimal estimation of the band gap among all the options. Moreover, we investigated three sets of mixed three-dimensional halide perovskites by varying the halide component. This exploration contributes to the identification of MAPb(Br0.333I0.667)3 and MAPb(Cl0.333I0.667)3 as exhibiting the smallest band gap of 1.315 (1.867) eV and 1.313 (1.885) eV for PBE (HSE06), respectively. These band gaps were determined using the HSE06 method with the optimized lattice by PBE considering the optB86b corrections. The approach employed in this work produced a band gap trend closely aligned with experimental observations, underscoring the importance of adopting a reliable and material-independent computational strategy when screening new halide perovskite materials for optoelectronic applications.

Halide Ion Mobility in Paired 2D Halide Perovskites: Ruddlesden–Popper Versus Dion–Jacobson Phases

Abstract2D lead halide perovskites with tunable optoelectronic properties through modulating inorganic components and organic spacer ligands represent a promising candidate in perovskite optoelectronics. However, an intrinsic soft lattice of perovskites gives rise to a vacancy‐mediated halide ion migration, which further affects the long‐term stability of materials as well as devices. Here, halide ion mobility is evaluated using two different 2D perovskites yielded with butylammonium (BA) for the Ruddlesden–Popper (RP) phase and butane‐1,4‐diamine (BDA) for the Dion–Jacobson (DJ) phase, respectively. By probing the halide ion migration kinetics in the physically paired 2D bromide and iodide perovskite films, the softer lattice (RP) shows a lower activation energy of 50.9 kJ mol−1 as compared to that of more rigid lattice (DJ) of 60.8 kJ mol−1. Given similar halide diffusion pathlength and halide mixing chemical potential, the binding mode (RP and DJ) can indeed dictate the overall halide ion stability. Understanding such halide ion mobility is important in designing perovskites with increased stability and performances at material and device levels.

Strongly Confined and Spectrally Tunable CsPbBr3 Quantum Dots for Deep Blue QD‐LEDs

AbstractDespite recent advances, it is still difficult to fabricate deep blue‐emitting perovskite light‐emitting diodes (LEDs) that are immune to the color instability caused by halide mixing. This is largely because it is still challenging to achieve bright, stable, pure bromide perovskite materials with deep blue emission. Here, a novel strategy is reported for synthesizing ultrasmall CsPbBr3 quantum dots (QDs) under ambient conditions. Precise size control is used to tune the peak emission wavelengths, via the confinement effect, from 433 to 501 nm. These ultrasmall QDs, with sizes of 2–6 nm, exhibit ultralow trap density and stable photoluminescence quantum yields up to 94%, particularly in the deep blue spectral region (440–465 nm). QD‐LEDs fabricated with these materials achieve an external quantum efficiency of 1.0% at 459 nm. The exceptional control of the emission properties demonstrated here suggests a new strategy for building stable, efficient blue QD‐LEDs.

Band-gap engineering in methylammonium bismuth bromide perovskites for less-toxic perovskite solar cells

We present the structure of methylammonium bismuth bromide (DMSO solvate) halide mixing's impact on morphology, structure and band-gap.

To Stop or to Shuttle Halides? The Role of an Ionic Liquid in Thermal Halide Mixing of Hybrid Perovskites

To Stop or to Shuttle Halides? The Role of an Ionic Liquid in Thermal Halide Mixing of Hybrid Perovskites

On the Origin of Energetic Disorder in Mixed Halides Lead Perovskites

AbstractMixed halide perovskites are emerging as promising candidates for wide‐bandgap components in tandem solar cells and color‐tunable light‐emitting diodes. Yet, halide mixing poses a fundamental question of whether the inhomogeneous halide distribution impacts the intrinsic electronic disorder in these materials. To address this point, density functional theory (DFT)‐based molecular dynamics (MD) simulations are performed for pure and mixed halide perovskites, accounting for disorder stemming from inhomogeneous chemical composition associated with the halide component and from finite temperature effects. For pure halide perovskites, finite‐temperature band gap fluctuations from the MD simulations are in good agreement with the broadening measured using photoluminescence. Furthermore, these calculations confirm the natively modest inhomogeneous disorder in the electronic structure of these materials. Most notably, such a low degree of electronic disorder is preserved in models mimicking finely intermixed Br/I solid‐state solutions. In contrast, models featuring halide segregation show comparably wider band gap fluctuations, with a sizable contribution from inhomogeneous (static) broadening, which is associated, at least in part, with structural distortions stemming from lattice mismatch.

Halide Ordering Enables Superior Charge Transport in 3D (NMPDA)Pb2 I4 Br2 Perovskitoid Single Crystal.

Halide composition engineering has been demonstrated as an effective strategy for optical and electronic properties modulation in 3D perovskites. While the impact of halide mixing on the structural and charge transport properties of 3D perovskitoids remains largely unexplored. Herein, it is demonstrated that bromine (Br) mixing in 3D (NMPDA)Pb2 I6 (NMPDA = N-methyl-1,3-propane diammonium) perovskitoid yields stabilized (NMPDA)Pb2 I4 Br2 with specific ordered halide sites, where Br ions locate at the edge-sharing sites. The halide ordered structure enables stronger H-bonds, shorter interlayer distance, and lower octahedra distortion in (NMPDA)Pb2 I4 Br2 with respect to the pristine (NMPDA)Pb2 I6 . These attributes further result in high ion migration activation energy, low defect states density, and enhanced carrier mobility-lifetime product (µτ), as underpinned by the electrical properties investigation and DFT calculations. Remarkably, the parallel configured photodetector based on (NMPDA)Pb2 I4 Br2 single crystal delivers a high on/off current ratio of 3.92×103 , a satisfying photoresponsivity and detectivity of 0.28 AW-1 and 3.05×1012 Jones under 10.94 µWcm-2 irradiation, superior to that of (NMPDA)Pb2 I6 and the reported 3D perovskitoids. This work sheds novel insight on exploring 3D mixed halide perovskitoids toward advanced and stable optoelectronic devices.

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.

Contrasting Charge‐Carrier Dynamics across Key Metal‐Halide Perovskite Compositions through In Situ Simultaneous Probes

AbstractMetal‐halide perovskites have proven to be a versatile group of semiconductors for optoelectronic applications, with ease of bandgap tuning and stability improvements enabled by halide and cation mixing. However, such compositional variations can be accompanied by significant changes in their charge‐carrier transport and recombination regimes that are still not fully understood. Here, a novel combinatorial technique is presented to disentangle such dynamic processes over a wide range of temperatures, based on transient free‐space, high‐frequency microwave conductivity and photoluminescence measurements conducted simultaneously in situ. Such measurements are used to reveal and contrast the dominant charge‐carrier recombination pathways for a range of key compositions: prototypical methylammonium lead iodide perovskite (MAPbI3), the stable mixed formamidinium‐caesium lead‐halide perovskite FA0.83Cs0.17PbBr0.6I2.4 targeted for photovoltaic tandems with silicon, and fully inorganic wide‐bandgap CsPbBr3 aimed toward light sources and X‐ray detector applications. The changes in charge‐carrier dynamics in FA0.83Cs0.17PbBr0.6I2.4 across temperatures are shown to be dominated by radiative processes, while those in MAPbI3 are governed by energetic disorder at low temperatures, low‐bandgap minority‐phase inclusions around the phase transition, and non‐radiative processes at room temperature. In contrast, CsPbBr3 exhibits significant charge‐carrier trapping at low and high temperatures, highlighting the need for improvement of material processing techniques for wide‐bandgap perovskites.

Old trick for new dogs – modification of structural and optoelectronic properties in layered methylhydrazinium lead halide perovskites via halide mixing

Old trick for new dogs – modification of structural and optoelectronic properties in layered methylhydrazinium lead halide perovskites <i>via</i> halide mixing

Potassium tin mixed iodide-bromide KSn(I1-xBrx)3 [x = 0.25,0.5,0.75,1] perovskites for solar cell absorbers: A computational study

Metal-halide perovskite (PVK) materials have been the subject of investigation in the photovoltaic community because of their remarkable optoelectronic characteristics. Nonetheless, the extrinsic and intrinsic instabilities of PVK materials remain a major obstacle to their commercial fabrication prospects. This problem can be resolved through halide mixing. In the current study, we examine the elastic, structural, mechanical, thermal, electronic, optical, and thermoelectric properties of potassium tin mixed iodide-bromide(I-Br) PVK, KSn(I1-xBrx)3 [x = 0.25, 0.5, 0.75, 1] (KSnIBr). With an increase in bromine concentration, KSnIBr’s thermodynamic stability is found to rise. The elastic properties demonstrate the mechanical stability of the PVK materials. The Pugh’s ratio, Cauchy’s pressure, and Poisson’s ratio highlight their ductility. Ductile materials are desirable for the formation of thin films. An extremely high melting point ensures that the PVK materials retain their solid state even at very high temperatures. The high absorption coefficient (αω), low energy loss factor (L(ω)), and low reflectivity(R(ω)) in the visible region are used to pitch their candidature as absorber materials in perovskite solar cells (PSC). A bandgap (Eg) of 1.9 eV, 1.9608 eV, 1.9761 eV, and 2.1285 eV is obtained for KSn(I0.75Br0.25)3, KSn(I0.5Br0.5)3, KSn(I0.25Br0.75)3, and KSnBr3 respectively. A single junction perovskite solar cell utilizing the KSnIBr perovskite is simulated using SCAPS software. An efficiency of 11.31 %, 10.3 %, 9.89 %, and 8.05 % are reported for KSn(I0.75Br0.25)3, KSn(I0.5Br0.5)3, KSn(I0.25Br0.75)3 and KSnBr3 PVK respectively. These wide bandgap perovskite materials in conjunction with other narrow bandgap materials may be beneficial for the design of tandem solar cells in future.

Disorder to order: how halide mixing in MAPbI3-xBrx perovskites restricts MA dynamics.

Mixed-halide lead perovskites are of particular interest for the design of tandem solar cells currently reaching record efficiencies. While halide phase segregation upon illumination of mixed perovskites is extensively studied, the effect of halide disorder on A cation dynamics is not well understood, despite its importance for charge carrier diffusion and lifetime. Here, we study the methylammonium (MA) reorientational dynamics in mixed halide MAPbI3-xBrx perovskites by a combined approach of experimental solid-state NMR spectroscopy and molecular dynamics (MD) simulations based on machine-learning force-fields (MLFF). 207Pb NMR spectra indicate the halides are randomly distributed over their lattice positions, whereas PXRD measurements show that all mixed MAPbI3-xBrx samples are cubic. The experimental 14N spectra and 1H double-quantum (DQ) NMR data reveal anisotropic MA reorientations depending on the halide composition and thus associated disorder in the inorganic sublattice. MD calculations allow us to correlate these experimental results to restrictions of MA dynamics due to preferred MA orientations in their local Pb8I12-nBrn "cages". Based on the experimental and simulated results, we develop a phenomenological model that correlates the 1H dipolar coupling and thus the MA dynamics with the local composition and reproduces the experimental data over the whole composition range. We show that the dominant interaction between the MA cations and the Pb-X lattice that influences the cation dynamics is the local electrostatic potential being inhomogeneous in mixed halide systems. As such, we generate a fundamental understanding of the predominant interaction between the MA cations and the inorganic sublattice, as well as MA dynamics in asymmetric halide coordinations.

Stabilizing Wide Bandgap Triple‐Halide Perovskite Alloy through Organic Gelators

Engineering the chemical composition of metal‐halide perovskites via halide mixing allows a facile bandgap modulation but renders perovskite materials prone to photoinduced halide segregation. Triple‐halide alloys containing Cl, I, and Br were recently reported as a means to stabilize CsyFA1–yPb(BrxI1–x)3 perovskite under illumination. Herein, these triple‐halide alloys are found to be intrinsically less stable with respect to the reference I‐Br in ambient conditions. By exploiting the influence of low‐molecular‐weight organic gelators on the crystallization of the perovskite material, a triple‐halide alloy with improved moisture tolerance and thermal stability at temperatures as high as 120 °C is demonstrated. The hydroxyl‐terminated organic gelators are found to aggregate into nanoscale fibers and promote the gelation of the solvent inducing the formation of a 3D network, positively interfering with perovskite solidification. The addition of a tiny amount of organic gelators imparts a more compact morphology, higher crystallinity, and compositional stability to the resulting triple‐halide polycrystalline films, making them more robust over time without compromising the photovoltaic performance. Overall, this approach offers a solution toward fabrication of active perovskite materials with higher energy gap and improved stability, making these triple‐halide alloys truly exploitable in solar cells.