Mixed Halide Hybrid Perovskites Research Articles

Tuning Thermal Conductivity of Hybrid Perovskites through Halide Alloying.

Tuning the thermal transport properties of hybrid halide perovskites is critical for their applications in optoelectronics, thermoelectrics, and photovoltaics. Here, an effective strategy is demonstrated to modulate the thermal transport property of hybrid perovskites by halide alloying. A highly tunable thermal conductivity of mixed-halide hybrid perovskites is achieved due to halide-alloying and structural distortion. The experimental measurements show that the room temperature thermal conductivity of MAPb(BrxI1- x)3 (x = 0─1) can be largely modulated from 0.27 ± 0.07W m-1 K-1 (x = 0.5) to 0.47 ± 0.09W m-1 K-1 (x = 1). Molecular dynamics simulations further demonstrate that the thermal conductivity reduction of hybrid halide perovskites results from the suppression of the mean free paths of the low-frequency acoustic and optical phonons. It is found that halide alloying and the induced structural distortion can largely increase the scatterings of optical and acoustic phonons, respectively. The confined diffusion of MA+ cations in the octahedra cage is found to act as an additional thermal transport channel in hybrid perovskites and can contribute around 10-20% of the total thermal conductivity. The findings provide a strategy for tailoring the thermal transport in hybrid halide perovskites, which may largely benefit their related applications.

Unravelling the Interfacial Dynamics of Bandgap Funneling in Bismuth-Based Halide Perovskites.

An environmentally friendly mixed-halide perovskite MA3 Bi2 Cl9- x Ix with a bandgap funnel structure has been developed. However, the dynamic interfacial interactions of bandgap funneling in MA3 Bi2 Cl9- x Ix perovskites in the photoelectrochemical (PEC) system remain ambiguous. In light of this, single- and mixed-halide lead-free bismuth-based hybrid perovskites-MA3 Bi2 Cl9- y Iy and MA3 Bi2 I9 (named MBCl-I and MBI)-in the presence and absence of the bandgap funnel structure, respectively, are prepared. Using temperature-dependent transient photoluminescence and electrochemical voltammetric techniques, the photophysical and (photo)electrochemical phenomena of solid-solid and solid-liquid interfaces for MBCl-I and MBI halide perovskites are therefore confirmed. Concerning the mixed-halide hybrid perovskites MBCl-I with a bandgap funnel structure, stronger electronic coupling arising from an enhanced overlap of electronic wavefunctions results in more efficient exciton transport. Besides, MBCl-I's effective diffusion coefficient and electron-transfer rate demonstrate efficient heterogeneous charge transfer at the solid-liquid interface, generating improved photoelectrochemical hydrogen production. Consequently, this combination of photophysical and electrochemical techniques opens up an avenue to explore the intrinsic and interfacial properties of semiconductor materials for elucidating the correlation between material characterization and device performance.

Improving the properties of MA-based wide-bandgap perovskite by simple precursor salts engineering for efficiency and ambient stability improvement in solar cells

The morphology and optoelectronic properties of wide-bandgap halide perovskites have profound influence on the performance of tandem solar cells in which these materials hold a great potential to push the efficiency beyond the maximum in single junction devices. Typically, alkali metals such as Cs+ are employed to overcome photo-instability in mixed-halide hybrid perovskites. However, while using the popular CsI in precursor solutions, small grain size (~250 nm) along with high densities of grain boundaries and their associated trap states are usually observed in polycrystalline films, leading to low photovoltaic performance. Here, we develop a non-stoichiometric one-step solution casting method to prepare wide-bandgap CsXMA1-XPb(I0·6Br0.4)3 films with reduced grain boundaries and high crystallinity using CsCl which plays a double role of Cs + source and crystallization controller. The resulting films exhibit improved carrier lifetime and mobility, and reduced carrier trap states. Elemental analysis revealed that some Cl ions remains in the final films, passivating the defect states. All these benefits led to high open circuit voltages and improved the power conversion efficiency (PCE) of Cs0.1MA0.9Pb(I0·6Br0.4)3 perovskite solar cells from 13.20% to 15.16%. Moreover, the developed method is also found to enhance the hydrophobicity and moisture resistance of CsXMA1-XPb(I0·6Br0.4)3 films, which enabled the improvement in the device ambient stability.

Anomalous Structural Evolution and Glassy Lattice in Mixed-Halide Hybrid Perovskites.

Hybrid halide perovskites have emerged as highly promising photovoltaic materials because of their exceptional optoelectronic properties, which are often optimized via compositional engineering like mixing halides. It is well established that hybrid perovskites undergo a series of structural phase transitions as temperature varies. In this work, the authors find that phase transitions are substantially suppressed in mixed-halide hybrid perovskite single crystals of MAPbI3-x Brx (MA = CH3 NH3 + and x= 1 or 2) using a complementary suite of diffraction and spectroscopic techniques. Furthermore, as a general behavior, multiple crystallographic phases coexist in mixed-halide perovskites over a wide temperature range, and a slightly distorted monoclinic phase, hitherto unreported for hybrid perovskites, is dominant at temperatures above 100 K. The anomalous structural evolution is correlated with the glassy behavior of organic cations and optical phonons in mixed-halide perovskites. This work demonstrates the complex interplay between composition engineering and lattice dynamics in hybrid perovskites, shedding new light on their unique properties.

Study on the halide effect of MA4PbX6·2H2O hybrid perovskites – From thermochromic properties to practical deployment for smart windows

Thermochromic smart windows empowered by the unique material properties and reversible thermochromism of dihydrated methylammonium lead halide hybrid perovskites (MA4PbX6·2H2O; X: halide) have risen as novel yet promising candidates for thermochromic smart windows, in which the halide plays a crucial role to the functionality and performance. In this study, the effects of halide types and mixing ratios on the thermochromic properties and practical deployment of MA4PbX6·2H2O halide hybrid perovskites are comprehensively studied by extensive fabrication and characterization processes as well as fundamental and applicational investigation tools. The potential perovskite compositions for thermochromic smart window applications are identified with respect to the desired optical properties (i.e. luminous transmittance and solar modulation ability) and transitional properties (i.e. transition temperature, hysteresis width and transition time). Moreover, thermal-regulating performance of the applicable candidate mixed halide hybrid perovskite MA4PbI5Br1·2H2O is demonstrated in a model-house field investigation. This work provides not only an inclusive database of the thermochromic properties of MA4PbX6·2H2O halide hybrid perovskites, but also an application manual about their deployment for thermochromic smart windows, facilitating the development of smart windows and sustainable buildings.

Insight into the structural, chemical, optical and conductivities of (C6H10N2)PbBr2I2 mixed-halide hybrid perovskite produced entirely in ambient air condition

A mixed-halide hybrid perovskite (C6H10N2)PbBr2I2 has been prepared under ambient air condition by mixing 2-amino(methyl)pyridine (2-AMP) and lead bromide (PbBr2) in hydroiodide (HI) acidic solution by reflux method. The existence of NH3+, pyridinium and Ar-H ions in the synthesized sample was confirmed by Fourier Transform Infrared (FTIR) spectroscopy analysis. X-ray diffraction (XRD) and ultraviolet–visible (UV–Vis) spectroscopies were used to examine the structural and optical band gap of the mixed-halide hybrid perovskite. With an optical band gap of 2.47 eV, the mixed-halide hybrid perovskite was polycrystalline. The half-cell consisting active layers TiO2/(C6H10N2)PbBr2I2 was made without a hole-selective layer, using a spin-coating method to build a layer onto indium-tin-oxide (ITO) covered glass substrates. Electrochemical impedance spectroscopy (EIS) analysis revealed that the cell’s bulk conductivity was in the range of 10-8 to 10-6 S cm−1 with various thickness of the cell. Interestingly, by altering the mixed bromide-iodide and improving the perovskite material’s stability, the optical band gap achieved under visible light may be modified to lower the energy band gap of the studied compound.

Spatial microheterogeneity in the valence band of mixed halide hybrid perovskite materials.

The valence band of lead halide hybrid perovskites with a mixed I/Br composition is investigated using electronic structure calculations and complementarily probed with hard X-ray photoelectron spectroscopy. In the latter, we used high photon energies giving element sensitivity to the heavy lead and halide ions and we observe distinct trends in the valence band as a function of the I : Br ratio. Through electronic structure calculations, we show that the spectral trends with overall composition can be understood in terms of variations in the local environment of neighboring halide ions. From the computational model supported by the experimental evidence, a picture of the microheterogeneity in the valence band maximum emerges. The microheterogeneity in the valence band suggests that additional charge transport mechanisms might be active in lead mixed halide hybrid perovskites, which could be described in terms of percolation pathways.

Distinguishing Models for Mixed Halide Lead Perovskite Photosegregation via Terminal Halide Stoichiometry

Photoinduced halide segregation in mixed halide hybrid perovskites [i.e., APb(I1–xBrx)3] represents an intrinsic instability that impedes their commercialization. Resolving this issue requires deve...

Revealing Dynamic Effects of Mobile Ions in Halide Perovskite Solar Cells Using Time-Resolved Microspectroscopy.

Halide perovskites are promising candidate materials for the next generation high-efficiency optoelectronic devices. Since perovskites are electronic-ionic mixed conductors, ion dynamics have a critical impact on the performance and stability of perovskite-based applications. However, comprehensively understanding ionic dynamics is challenging, particularly on nanoscale imaging of ionic dynamics in perovskites. In this review, mobile ion dynamics in halide perovskites investigated via luminescence spectroscopy combined with confocal microscopy are discussed, including mobile ion induced fluorescence quenching, phase segregation in mixed halide hybrid perovskite, and mobile ion accumulation at the interface in perovskite devices. Steady-state and time-resolved luminescence imaging techniques, combined with confocal microscopy, are unique tools for probing ionic dynamics in perovskites, providing invaluable insights on ionic dynamics in nanoscale resolution, along with a wide temporal range from picoseconds to hours. The works in this review are not only for understanding mobile ions to improve the design of perovskite-based devices but also foster the development of microspectroscopic methodologies in a broader solid-state physics context of investigating ionic transports in polycrystalline materials.

Role of Alkali-Metal Cations in Electronic Structure and Halide Segregation of Hybrid Perovskites.

The ability to control or prevent phase segregation in perovskites is crucial to realizing stable and tunable mixed-halide optoelectronic devices. In this work, we systematically examine the impact of alkali-metal-cation (Cs+ and K+) concentration on the band structure, chemical composition, phase segregation, and polycrystalline microstructure on formamidinium-dominated mixed-halide mixed-cation perovskite films. It was found that the incorporation of Cs+ and K+ cations decreases the work function and the core levels of all components shift toward higher binding energy consistent with n-doping the perovskite film, which facilitates electron transfer to the electron transport layer TiO2. A concentration-dependent film structure was observed by X-ray photoemission spectroscopy and grazing incidence wide-angle X-ray scattering where the halides and cations are distributed evenly across perovskite films at low metallic cation concentration (5%). A high metal-cation ratio (20%) leads to halide segregation within the perovskite film and the surface becomes bromide-poor, whereas the bromide and metal cations diffuse more deeply within the film. These differences in electronic properties, element distribution, and film morphology were reflected in the device performance where the power conversion efficiency of low-metallic-cation concentration (5% of Cs+ and K+) perovskite solar cells is ≈5% higher than the high-concentration ones (20%). This study provides valuable chemical and physical insight into the underlying trade-offs in the careful tuning of electrical properties and film structure to optimize multication and mixed-halide hybrid perovskites.

Routes toward Long-Term Stability of Mixed-Halide Perovskites

In the recent anniversary issue of Trends in Chemistry, Brennan et al. review halide segregation in mixed-halide perovskites, discussing multiple perspectives on the underlying origins and reported routes to retard halide segregation. Here, we argue that only slowing down the segregation may be insufficient to achieve long-term stability.

Stability Improvement and Performance Reproducibility Enhancement of Perovskite Solar Cells Following (FA/MA/Cs)PbI3–xBrx/(CH3)3SPbI3 Dimensionality Engineering

Mixed halide hybrid perovskites are strong candidates for fabrication of efficient, stable and reproducible perovskite solar cells (PSCs). To restrain intrinsic volatility and ionic migration effec...

Photo detector based on graded band gap perovskite crystal

In this work, millimeter sized graded band gap mixed halide hybrid perovskite crystal have been synthesized by utilizing reversible halide exchange property of hybrid perovskites. Optoelectronic performance of photo diodes fabricated using the graded band gap crystal has been studied in details. Band gap engineering has formed a smoother path for carrier transport by aligning HOMO and LUMO levels with the charge collection layers. This is reflected in good diode quality and fast photo response has been achieved. On the other hand, effect from both bulk and surface recombination of photo generated carriers are observed in the plateau type spectral response curve with a peak near band edge.

Rashba Triggered Electronic and Optical Properties Tuning in Mixed Cation–Mixed Halide Hybrid Perovskites

The inherent spin-orbit coupling (SOC) effect in non-centrosymmetric crystal structure has laid the foundation of Rashba splitting phenomena. This Rashba splitting directly governs the charge carri ...

Ultrafast Carrier Dynamics and Terahertz Photoconductivity of Mixed-Cation and Lead Mixed-Halide Hybrid Perovskites**Supported by the National Natural Science Foundation of China under Grant Nos 11604202, 11674213, 61735010 and 51603119, the Young Eastern Scholar under Grant Nos QD2015020 and QD2016027, the Shanghai Rising-Star Program under Grant No 18QA1401700, the ‘Chen Guang’ Project under Grant Nos 16CG45 and 16CG46, the Shanghai Municipal

Using time-dependent terahertz spectroscopy, we investigate the role of mixed-cation and mixed-halide on the ultrafast photoconductivity dynamics of two different methylammonium (MA) lead-iodide perovskite thin films. It is found that the dynamics of conductivity after photoexcitation reveals significant correlation on the microscopy crystalline features of the samples. Our results show that mixed-cation and lead mixed-halide affect the charge carrier dynamics of the lead-iodide perovskites. In the (5-AVA)0.05(MA)0.95PbI2.95Cl0.05/spiro thin film, we observe a much weaker saturation trend of the initial photoconductivity with high excitation fluence, which is attributed to the combined effect of sequential charge carrier generation, transfer, cooling and polaron formation.

Vacancy-Mediated Anion Photosegregation Kinetics in Mixed Halide Hybrid Perovskites: Coupled Kinetic Monte Carlo and Optical Measurements

Solution-processed mixed halide perovskites are excellent materials for multijunction solar cells. Unfortunately, light-induced halide phase segregation has prevented their effective integration into working devices. In this study, we rationalize and quantify anion photosegregation in stoichiometric and halide-deficient MAPb(I1–xBrx)3 thin films through kinetic Monte Carlo simulations and complementary optical measurements. Our study reveals that segregation rates are dictated by halide vacancy hopping barriers and are modulated by vacancy concentrations. The simulations further suggest that near-ubiquitous emission energies, which converge on that for MAPb(I0.8Br0.2)3 (i.e., x ≈ 0.2) following photosegregation, arise from the existence of kinetically trapped Br– within nucleated I-rich domains. An established photosegregation excitation intensity threshold is independent of the number of vacancies and instead depends critically on parameters such as carrier diffusion length, lifetime, and bandgap tunabil...

Optoelectronic Dichotomy of Mixed Halide CH3NH3Pb(Br1–xClx)3 Single Crystals: Surface versus Bulk Photoluminescence

In this work, the spatially dependent recombination kinetics of mixed-halide hybrid perovskite CH3NH3Pb(Br1- xCl x)3 (0 ≤ x ≤ 0.19) single crystals are investigated using time-resolved photoluminescence spectroscopy with one- and two-photon femtosecond laser excitation. The introduction of chloride by substituting a fraction of the bromide leads to a decreased lattice constant compared to pure bromide perovskite ( x = 0) and a higher concentration of surface defects. The measured kinetics under one-photon excitation (1PE) shows that increasing the chloride addition quenches the photoluminescence (PL) lifetimes, due to substitution-induced surface defects. In stark contrast, upon 2PE, the PL lifetimes measured deeper in the bulk become longer with increasing chloride addition, until the halide substitution reaches the critical concentration of ∼19%. At x = 19% Cl concentration, a significant reversal of this behavior is observed indicating a change in crystal structure beyond the continuous trends observed at lower percentages of halide substitution ( x ≤ 11%). The observed opposing trends, based on 1PE versus 2PE, highlight a dichotomy between extrinsic (surface) and intrinsic (bulk) effects of chloride substitution on the carrier dynamics in lead bromide perovskites. We discuss the physical relation between halide exchange and bulk carrier lifetimes in CH3NH3PbBr3 in terms of the Rashba effect. We propose that the latter is suppressed at the surface due to disorder in the alignment of the MA and that it increases in the bulk with Cl concentration because of the reduction in lattice parameters, which compresses the space available for the MA orientational degrees of freedom.

Effect of Light Illumination on Mixed Halide Lead Perovskites: Reversible or Irreversible Transformation

One intriguing aspect of mixed halide lead perovskites (e.g., CH3NH3PbBrxI3–x) is halide ion segregation when subjected to visible illumination, but this aspect gives rise to the concerns regarding the influence of halide ion movement on the long–term stability and applications. Here, the impact of light illumination on mixed halide hybrid perovskite films was investigated by exposing such films to continuous wavelength laser (25 mW/cm2). It turns out that under illumination the segregated iodide-rich perovskite species further decompose and ultimately form metallic lead (Pb0). The chemical changes in decomposed films were found to cause an irreversible shift of the absorption band edge and a significant change in film morphology, while no decomposition of MAPbI3 and MAPbBr3 films was observed after exposure to the same illumination condition. Our results unambiguously demonstrate the deleterious effect of phase segregation on material stability for mixed halide perovskites.

Solvent-Free Solid-State Synthesis of High Yield Mixed Halide Perovskites for Easily Tunable Composition and Band Gap

We report the preparation of mixed halide hybrid perovskites (CH3NH3PbI3-xBrx) using a solvent-less mechanosynthesis route. Compositional analysis using EDXRF microscopy has confirmed the production of the desired materials. As relative abundance of halogen (x) increases, the tetragonal phase gradually transforms to cubic phase and the band gap tunability of the material is observed.

Electronic band structure and carrier concentration of formamidinium–cesium mixed cation lead mixed halide hybrid perovskites

The electronic structure of hybrid perovskite compositions of FA0.83 Cs0.17 PbI3−xBrx (x = 0.0, 0.5, 1.0, 1.5, 2.0, and 2.5) is determined using ultraviolet photoelectron spectroscopy (UPS) and UV–Vis–NIR absorption spectroscopy. With the help of UPS, ionization potential and Fermi energy are determined, and using absorption measurements, bandgap values are obtained. It is observed that for FA0.83 Cs0.17 PbI3−xBrx, as the Br content increases, the bandgap increases. The UPS measurements confirm the n-type nature of all compositions. Additionally, the Hall measurements were carried out for the selected compositions and the n-type carrier concentrations were determined.