Mixed Lead Halide Perovskite Research Articles

Br-Induced Suppression of Low-Temperature Phase Transitions in Mixed-Cation Mixed-Halide Perovskites.

Mixed-cation mixed-halide lead perovskites have been shown to be excellent candidates for solar energy conversion. However, understanding the structural phases of these mixed-ion perovskites across a wide range of operating temperatures, including very low temperatures for space applications, is crucial. In this study, we investigated the structure of formamidinium-based Cs y FA1-y Pb(Br x I1-x )3 using low-temperature in situ synchrotron powder X-ray diffraction. Our findings revealed that substituting the I anion with Br in mixed-cation (Cs,FA) perovskites suppressed the phase transformation from tetragonal to orthorhombic at low temperatures. The addition of Br also prevented the formation of nonperovskite secondary phases. We gained fundamental insights into the structural behavior of these materials by creating a low-temperature phase diagram for the compositional set of mixed-cation mixed-halides. This understanding of the structural properties lays the groundwork for designing more robust and efficient energy materials capable of functioning under extreme temperature conditions, including space-based solar energy conversion.

Inhibiting the Appearance of Green Emission in Mixed Lead Halide Perovskite Nanocrystals for Pure Red Emission.

Mixed halide perovskites exhibit promising optoelectronic properties for next-generation light-emitting diodes due to their tunable emission wavelength that covers the entire visible light spectrum. However, these materials suffer from severe phase segregation under continuous illumination, making long-term stability for pure red emission a significant challenge. In this study, we present a comprehensive analysis of the role of halide oxidation in unbalanced ion migration (I/Br) within CsPbI2Br nanocrystals and thin films. We also introduce a new approach using cyclic olefin copolymer (COC) to encapsulate CsPbI2Br perovskite nanocrystals (PNCs), effectively suppressing ion migration by increasing the corresponding activation energy. Compared with that of unencapsulated samples, we observe a substantial reduction in phase separation under intense illumination in PNCs with a COC coating. Our findings show that COC enhances phase stability by passivating uncoordinated surface defects (Pb2+ and I-), increasing the formation energy of halide vacancies, improving the charge carrier lifetime, and reducing the nonradiative recombination density.

Dynamics of glassy state in MAPbBr3-xClx mixed lead halide perovskites

Mixed lead halide perovskites have shown remarkable performance in photovoltaic and optoelectronic applications. Pure hybrid halide perovskites undergo clear phase transitions, however, the phase transitions in mixed perovskites are less studied. We present a contrasting behavior of the phase transition dynamics of MAPbBr3-xClx (MA = CH3NH3+) mixed halide perovskites compared to the pure MAPbX3 (X = Cl, Br) counterparts, which typically exhibit order-disorder phase transitions through a series of distinct structural phases. We have employed several spectroscopic techniques including temperature-dependent PXRD, Raman, dielectric and Brillouin spectroscopy. The results demonstrate that the mixed halide perovskite MAPbBr1.5Cl1.5 does not undergo any structural phase transition typically observed in pure halide perovskites. We observed signatures of glassy behavior in mixed compositions of this system. In addition, we also explored the effects of replacing A-site cation on the phase transition behaviors. The results contribute to the understanding of phase transition mechanisms in perovskites and have implications for their stability and optoelectronic properties.

Exciton–photocarrier interference in mixed lead-halide-perovskite nanocrystals

The use of semiconductor nanocrystals in scalable quantum technologies requires characterization of the exciton coherence dynamics in an ensemble of electronically isolated crystals in which system–bath interactions are nevertheless strong. In this communication, we identify signatures of Fano-like interference between excitons and photocarriers in the coherent two-dimensional photoluminescence excitation spectral lineshapes of mixed lead-halide perovskite nanocrystals in dilute solution. Specifically, by tuning the femtosecond-pulse spectrum, we show such interference in an intermediate coupling regime, which is evident in the coherent lineshape when simultaneously exciting the exciton and the free-carrier band at higher energy. We conclude that this interference is an intrinsic effect that will be consequential in the quantum dynamics of the system and will thus dictate decoherence dynamics, with consequences in their application in quantum technologies.

BMP]+[BF4]--Modified CsPbI1.2Br1.8 Solar Cells with Improved Efficiency and Suppressed Photoinduced Phase Segregation.

With the rapid progress in a power conversion efficiency reaching up to 26.1%, which is among the highest efficiency for single-junction solar cells, organic-inorganic hybrid perovskite solar cells have become a research focus in photovoltaic technology all over the world, while the instability of these perovskite solar cells, due to the decomposition of its unstable organic components, has restricted the development of all-inorganic perovskite solar cells. In recent years, Br-mixed halogen all-inorganic perovskites (CsPbI3-xBrx) have aroused great interests due to their ability to balance the band gap and phase stability of pure CsPbX3. However, the photoinduced phase segregation in lead mixed halide perovskites is still a big burden on their practical industrial production and commercialization. Here, we demonstrate inhibited photoinduced phase segregation all-inorganic CsPbI1.2Br1.8 films and their corresponding perovskite solar cells by incorporating a 1-butyl-1-methylpiperidinium tetrafluoroborate ([BMP]+[BF4]-) compound into the CsPbI1.2Br1.8 films. Then, its effect on the perovskite films and the corresponding hole transport layer-free CsPbI1.2Br1.8 solar cells with carbon electrodes under light is investigated. With a prolonged time added to the reduced phase segregation terminal, this additive shows an inhibitory effect on the photoinduced phase segregation phenomenon for perovskite films and devices with enhanced cell efficiency. Our study reveals an efficient and simple route that suppresses photoinduced phase segregation in cesium lead mixed halide perovskite solar cells with enhanced efficiency.

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites.

Lead halide perovskites are extensively investigated as efficient solution-processable materials for photovoltaic applications. The greatest stability and performance of these compounds are achieved by mixing different ions at all three sites of the APbX3 structure. Despite the extensive use of mixed lead halide perovskites in photovoltaic devices, a detailed and systematic understanding of the mixing-induced effects on the structural and dynamic aspects of these materials is still lacking. The goal of this review is to summarize the current state of knowledge on mixing effects on the structural phase transitions, crystal symmetry, cation and lattice dynamics, and phase diagrams of three- and low-dimensional lead halide perovskites. This review analyzes different mixing recipes and ingredients providing a comprehensive picture of mixing effects and their relation to the attractive properties of these materials.

Stability of Mixed Lead Halide Perovskite Films Encapsulated in Cyclic Olefin Copolymer at Room and Cryogenic Temperatures.

Lead Mixed Halide Perovskites (LMHPs), CsPbBrI2, have attracted significant interest as promising candidates for wide bandgap absorber layers in tandem solar cells due to their relative stability and red-light emission with a bandgap ∼1.7 eV. However, these materials segregate into Br-rich and I-rich domains upon continuous illumination, affecting their optical properties and compromising the operational stability of devices. Herein, we track the microscopic processes occurring during halide segregation by using combined spectroscopic measurements at room and cryogenic temperatures. We also evaluate a passivation strategy to mitigate the halide migration of Br/I ions in the films by overcoating with cyclic olefin copolymer (COC). Our results explain the correlation between grain size, intensity dependencies, phase segregation, activation energy barrier, and their influence on photoinduced carrier lifetimes. Importantly, COC treatment increases the lifetime charge carriers in mixed halide thin films, improving efficient charge transport in perovskite solar cell applications.

How Photogenerated I2 Induces I-Rich Phase Formation in Lead Mixed Halide Perovskites.

Bandgap tunability of lead mixed halide perovskites (LMHPs) is a crucial characteristic for versatile optoelectronic applications. Nevertheless, LMHPs show the formation of iodide-rich (I-rich) phase under illumination, which destabilizes the semiconductor bandgap and impedes their exploitation. Here, it is shown that how I2 , photogenerated upon charge carrier trapping at iodine interstitials in LMHPs, can promote the formation of I-rich phase. I2 can react with bromide (Br- ) in the perovskite to form a trihalide ion I2 Br- (Iδ- -Iδ+ -Brδ- ), whose negatively charged iodide (Iδ- ) can further exchange with another lattice Br- to form the I-rich phase. Importantly, it is observed that the effectiveness of the process is dependent on the overall stability of the crystalline perovskite structure. Therefore, the bandgap instability in LMHPs is governed by two factors, i.e., the density of native defects leading to I2 production and the Br- binding strength within the crystalline unit. Eventually, this study provides rules for the design of chemical composition in LMHPs to reach their full potential for optoelectronic devices.

Raman Spectroscopy of Formamidinium-Based Lead Mixed-Halide Perovskite Bulk Crystals.

In recent years, there has been an impressively fast technological progress in the development of highly efficient lead halide perovskite solar cells. Nonetheless, the stability of perovskite films and associated solar cells remains a source of uncertainty and necessitates sophisticated characterization techniques. Here, we report low- to mid-frequency resonant Raman spectra of formamidinium-based lead mixed-halide perovskites. The assignment of the different Raman lines in the measured spectra is assisted by DFT simulations of the Raman spectra of suitable periodic model systems. An important result of this work is that both experiment and theory point to an increase of the stability of the perovskite structure with increasing chloride doping concentration. In the Raman spectra, this is reflected by the appearance of new lines due to the formation of hydrogen bonds. Thus, higher chloride doping results in less torsional motion and lower asymmetric bending contributing to higher stability. This study yields a solid basis for the interpretation of the Raman spectra of formamidinium-based mixed-halide perovskites, furthering the understanding of the properties of these materials, which is essential for their full exploitation in solar cells.

Effective Suppressing Phase Segregation of Mixed-Halide Perovskite by Glassy Metal-Organic Frameworks.

Lead mixed-halide perovskites offer tunable bandgaps for optoelectronic applications, but illumination-induced phase segregation can quickly lead to changes in their crystal structure, bandgaps, and optoelectronic properties, especially for the Br-I mixed system because CsPbI3 tends to form a non-perovskite phase under ambient conditions. These behaviors can impact their performance in practical applications. By embedding such mixed-halide perovskites in a glassy metal-organic framework, a family of stable nanocomposites with tunable emission is created. Combining cathodoluminescence with elemental mapping under a transmission electron microscope, this research identifies a direct relationship between the halide composition and emission energy at the nanoscale. The composite effectively inhibits halide ion migration, and consequently, phase segregation even under high-energy illumination. The detailed mechanism, studied using a combination of spectroscopic characterizations and theoretical modeling, shows that the interfacial binding, instead of the nanoconfinement effect, is the main contributor to the inhibition of phase segregation. These findings pave the way to suppress the phase segregation in mixed-halide perovskites toward stable and high-performance optoelectronics.

Towards Stable Free Lead Mixed Halide Perovskite Thin Films on FTO-Coated Glass Substrate

All-inorganic cesium (Cs) lead perovskites have better thermal and chemical stability than organic–inorganic hybrids. They therefore represent a hope for stability and increased performance of perovskites as absorber layers in photovoltaic solar cells. In the present work, we have deposited different layers on FTO-coated glass substrates using the one-step spin-coating method. The results of the lead substitution are presented and critically discussed. The X-ray diffraction (XRD) results show four peaks for all three samples. The main peaks of the different films are located at the 2θ angles of 26.45° and 51.50° for the Muller indices (220) and (242), respectively. These two main peaks indicate that the prepared thin films all have two preferred crystallographic orientations. Beyond these two main peaks, we have two other smaller peaks at 2θ of 33.67° and 37.70° corresponding to the Muller indices of (210) and (211), respectively. The smoother the surface of the thin films, the more light they reflect, resulting in poor light absorption by the films. It is therefore important to obtain a surface image of the prepared films, as the larger the surface image, the better the film. With this in mind, we carried out a scanning electron microscope (SEM) analysis, which gave us the surface images. Figure 3 shows the SEM images of thin CsPb1−xSnxI1.5Br1.5 layers (x = 0, 0.5, and 1) grown on FTO-coated glass substrate with different [Pb]/[Sn] ratios. As can be seen from the figure, the effect of the [Pb]/[Sn] ratio is visible in the surface images of the different thin films. The smallest grain size is that of the unsubstituted CsPbI1.5Br1.5, while the largest grain size corresponds to the partially Pb-substituted layer (CsPb0.5Sn0.5I1.5Br1.5). The fully Pb-substituted layer (CsSnI1.5Br1.5) has an intermediate grain size. The surface images of the films show that the surfaces are well coated with grain sizes that vary greatly depending on the layer. The best grain size is that of the thin film with partial lead substitution (CsPb0.5Sn0.5I1.5Br1.5). Regarding the UV-visible absorption of the different films, we can say that the films absorb the maximum amount of light in the wavelength range of 350–550 nm. Above 550 nm, the absorption coefficients drop significantly. The absorption coefficients of the tin-free (Sn) layer remain higher than the coefficients of the other layers throughout the UV-visible spectrum. The degradation study revealed that the Sn-free layer retains good light absorption compared to the other layers after 4 weeks of exposure to the ambient environment. The crystal structure of all the layers shows good resistance to the elements during the 4 weeks, as shown by the renewed XRD results after the 4 weeks of exposure.

Generation of Hybrid Lead Halide CH3NH3PbI3-xClx Perovskite Crystals via Convective Self-Assembly

In this study, we have employed a film deposition technique based on convective forces acting at the triple air–solution–substrate contact line to tune the rate of solvent evaporation and, thus, to exert control over the morphology of the resulting mixed lead halide perovskite CH3NH3PbI3-xClx films. By varying the speed of film deposition while processing at two different temperatures, perovskite crystals of various shapes and dimensions were generated upon the crystallization of the initial perovskite precursor ink. More precisely, longer than half a millimeter 3D needle-like perovskite crystals exhibiting sharp edges co-existing with large cross-like 3D perovskite crystals could only be obtained upon the slow deposition of films at a low sample processing temperature of 17 °C, i.e., under conditions of low solvent evaporation rate. On the contrary, the utilization of higher film deposition speeds and/or processing temperatures led to smaller 3D needle-like or quasi-2D rectangular perovskite crystals that often appeared interconnected and coalesced. Moreover, as it was revealed by the photoluminescence measurements, the emission intensity of perovskite crystals was larger and dominated by a shorter wavelength peak, as compared to the uncrystallized material that emitted much less light, but at a longer wavelength.

Defect Engineering to Achieve Photostable Wide Bandgap Metal Halide Perovskites.

Bandgap tuning is a crucial characteristic of metal-halide perovskites, with benchmark lead-iodide compounds having a bandgap of 1.6 eV. To increase the bandgap up to 2.0 eV, a straightforward strategy is to partially substitute iodide with bromide in so-called mixed-halide lead perovskites. Such compounds are prone, however, to light-induced halide segregation resulting in bandgap instability, which limits their application in tandem solar cells and a variety of optoelectronic devices. Crystallinity improvement and surface passivation strategies can effectively slow down, but not completely stop, such light-induced instability. Here we identify the defects and the intragap electronic states that trigger the material transformation and bandgap shift. Based on such knowledge, we engineer the perovskite band edge energetics by replacing lead with tin and radically deactivate the photoactivity of such defects. This leads to metal halide perovskites with a photostable bandgap over a wide spectral range and associated solar cells with photostable open circuit voltages.

Growth of Hybrid Perovskite Crystals from CH3NH3PbI3–xClx Solutions Subjected to Constant Solvent Evaporation Rates

In this work, we subjected hybrid lead-mixed halide perovskite (CH3NH3PbI3-xClx) precursor inks to different solvent evaporation rates in order to facilitate the nucleation and growth of perovskite crystals. By controlling the temperature of perovskite solutions placed within open-air rings in precise volumes, we established control over the rate of solvent evaporation and, thus, over both the growth rate and the shape of perovskite crystals. Direct utilization of diluted lead-mixed halide perovskites solutions allowed us to control the nucleation and to favor the growth of only a low number of perovskite crystals. Such crystals exhibited a clear sixfold symmetry. While crystals formed at a lower range of temperatures (40-60 °C) exhibited a more compact dendritic shape, the crystals grown at a higher temperature range (80-110 °C) displayed a fractal dendritic morphology.

Excitation Intensity- and Size-Dependent Halide Photosegregation in CsPb(I0.5Br0.5)3 Perovskite Nanocrystals.

Although broad consensus exists that photoirradiation of mixed-halide lead perovskites leads to anion segregation, no model today fully rationalizes all aspects of this near ubiquitous phenomenon. Here, we quantitatively compare experimental, CsPb(I0.5Br0.5)3 nanocrystal (NC) terminal anion photosegregation stoichiometries and excitation intensity thresholds to a band gap-based, thermodynamic model of mixed-halide perovskite photosegregation. Mixed-halide NCs offer strict tests of theory given physical sizes, which dictate local photogenerated carrier densities. We observe that mixed-anion perovskite NCs exhibit significant robustness to photosegregation, with photosegregation propensity decreasing with decreasing NC size. Observed size- and excitation intensity-dependent photosegregation data agree with model predicted size- and excitation intensity-dependent terminal halide stoichiometries. Established correspondence between experiment and theory, in turn, suggests that mixed-halide perovskite photostabilities can be predicted a priori using local gradients of (empirical) Vegard's law expressions of composition-dependent band gaps.

The Electronic Disorder Landscape of Mixed Halide Perovskites.

Band gap tunability of lead mixed halide perovskites makes them promising candidates for various applications in optoelectronics. Here we use the localization landscape theory to reveal that the static disorder due to iodide:bromide compositional alloying contributes at most 3 meV to the Urbach energy. Our modeling reveals that the reason for this small contribution is due to the small effective masses in perovskites, resulting in a natural length scale of around 20 nm for the "effective confining potential" for electrons and holes, with short-range potential fluctuations smoothed out. The increase in Urbach energy across the compositional range agrees well with our optical absorption measurements. We model systems of sizes up to 80 nm in three dimensions, allowing us to accurately reproduce the experimentally observed absorption spectra of perovskites with halide segregation. Our results suggest that we should look beyond static contribution and focus on the dynamic temperature dependent contribution to the Urbach energy.

In Situ Ellipsometry Measurements on the Halide Phase Segregation of Mixed Halide Lead Perovskites.

Methylammonium lead iodide bromides MAPb(BrxI1‐x)3 are a class of mixed halide lead perovskites, materials that offer high‐power conversion efficiencies and bandgap tunability. For these reasons, they are a promising absorber material for future solar cells, although their use is still limited due to several factors. The reversible phase segregation under even low light intensities is one of them, lowering the effective bandgap due to local segregation into iodide‐rich and bromide‐rich phases. While several studies have been done to illuminate the mechanism and suppression of phase segregation, challenges remain to understand its kinetics. We obtained dynamic ellipsometric measurements from x=0.5 mixed halide lead perovskite thin films protected by a polystyrene layer under green laser light with a power density of ∼11 W/cm2. Time constants between 1.7(±0.7)×10−3 s−1 for the segregation and 1.5(±0.6)×10−4 s−1 for recovery were calculated. The phase segregation rate constants are surprisingly two orders of magnitude slower than and the recovery rate is consistent with those measured using photoluminescence methods under similar conditions. These results confirm a concern in the literature about the complexity in the phase separation kinetics measured from photoluminescence. We expect ellipsometry to serve as a complementary technique to other spectroscopies in studying mixed‐halide lead perovskites phase segregation in the future.

Study the effect of defects on the quantum efficiency of perovskite solar cells

Perovskite solar cells (PSCs) have generated considerable interest in the field of solar energy. With the rapid development of fabrication technology, high efficiency, environmentally friendly perovskite solar cells are becoming more attractive as candidates for power generation. Defects in the absorber and interface layers of mixed lead halide perovskite solar cells reduce the efficiency and stability of the cells. The current study examined the impact of defects density, defect capturing cross-section, defect level, and defect type on the quantum efficiency (QE) of the absorber layer. Furthermore, the impact of defects in the hole transport layer/absorber and electron transport layer/absorber interface layers is also be extensively studied. SCAPS-1D will be used to simulate perovskite solar cells in this work. The perovskite solar cells structure proposed is FTO/ ETM/ absorber/ HTM structure based on TiO2 as ETM layer; Cu2O as HTM layer, and on CH3NH3PbI3 as absorber layer. According to the results, the quantum efficiency QE reduced sharply from 90.2% to 20% with varying the density of the defect Nt and the capture cross-section carriers σ in absorber layer from 1015 cm-3 to 10 18 cm-3 and 2×10-13 cm2 to 2×10-10 cm2, respectively. Additionally, it is discovered that the capture cross-section carriers exhibits behavior similar to that of the absorber layer's defect density.

Optical absorption and stability enhancement in mixed lead, tin, and germanium hybrid halide perovskites for photovoltaic applications

Hybrid halide perovskite (HHP) materials present flexible version of next-generation solar cells and among these, Methyl ammonia lead iodide, i.e., CH3NH3PbI3 (MAPbI3), dominated the research field owing to its excellent photovoltaic properties, but its commercial deployment was halted due to the presence of toxic Pb. Mixed cation and anion strategies can play a significant role in improving structural stability and reducing toxicity. Partial or full replacement of Pb with congener but non-toxic elements such as Sn and Ge can facilitate the large uptake of HHP solar cells. In this work by first-principles calculations, we explore the potential alternatives to MAPbI3 by investigating the structural, electronic and optical properties of MAPb1-x-ySnxGeyI3 [(x, y) = (0, 0.5), (0.25, 0.25) (0.5, 0)]. The present report indicates that via Ge and Sn-doping in MAPbI3, the bandgap can be tuned from 1.16 eV to 0.77 eV. Meanwhile, in the Sn/Ge/Sn–Ge doped MAPbI3, the optical absorption coefficients get enhanced in the visible region and in the regions as far as mid-infrared, making them better alternatives to toxic MAPbI3 for photovoltaic applications. Moreover, owing to the outstanding photovoltaic characteristics, MAPb0.50Sn0.25Ge0.25I3 is proposed as an absorber layer material for the efficient perovskite solar cell modules.

Enhancing the Electronic Properties and Stability of High-Efficiency Tin-Lead Mixed Halide Perovskite Solar Cells via Doping Engineering.

Overcoming Voc loss to increase the efficiency of perovskite solar cells (PSCs) has been aggressively studied. In this work, we introduce and compare rubidium iodide (RbI) and potassium iodide (KI) alkali metal halides (AMHs) as dopants in a tin-lead (SnPb)-based perovskite system to improve the performance of PSCs by enhancing their Voc. Improvement in terms of surface morphology, crystallinity, charge transfer, and carrier transport in the SnPb perovskites was observed with the addition of AMH dopants. Significant power conversion efficiency improvement has been achieved with the incorporation of either dopant, and the highest efficiency was 21.04% in SnPb mixed halide PSCs when the RbI dopant was employed. In conclusion, we can outline the enhancement strategy that yields a remarkable efficiency of >20% with a smaller Voc loss and improved storage, light, and thermal stability in SnPb PSCs via doping engineering.