Mixed Lead Halide Research Articles

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.

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.

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.

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.

Improving Stability and Performance of Cesium Mixed Lead Halides for Photovoltaic Applications

We have focused on mixed halide perovskite thin films of the formula CsPbX3 where (X3 = Br3, Cl3, I3, Br2Cl, Br2I, and I2Cl) prepared by spin-coating in order to study the effects of partial and total Br substitution. For this purpose, we performed a series of characterizations, including x-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM), and UV–visible spectroscopy. All the films except CsPbI2Br showed two characteristic peaks at 2θ angles of 26.80° and 51.80° corresponding to the (111) and (220) crystallographic planes. In the case of CsPbI2Br, we have the same peaks, but the main ones are located at 15° and 30° for the (100) and (200) planes, respectively. SEM examined the surface morphology of the different mixed lead halide films; the best surface was that of the CsPbBr2I sample, which is well-coated, dense, with no pinholes and no cracks, and has the largest grain size. In addition, all the mixed halide films showed good absorbance, especially between 600 nm and 900 nm, with band gap values between 1.94 and 2.92 eV.

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.

Mixed halide head perovskites thin films: Stability and growth investigation

Herein, we investigate the mixed halide perovskite films with the formula MAPbX3 where (X3 = Br3, Cl3, I3, Br2Cl, Br2I, and I2Cl) prepared by Spin-Coating in order to study the effects of partial and total substitution of Br. For this purpose, we have carried out a series of characterization techniques, including x-ray diffraction (XRD), Scanning electron microscopy (SEM), UV–visible spectroscopic; we find that all films content with partial Bromide exhibit two peaks located at 2θ angles of 14,94° and 30,2° matching to crystallographic planes (110) and (220) respectively correspond to perovskite structure, in the case of MAPbCl3, MAPbI3 or mixed MAPbI2Cl the characteristics peaks (110) and (220) were identified at 2θ angles of 14,16° and 28,53° respectively. The surface morphology of the different mixed lead halides films was examined by SEM, MAPbBr2I has the highest crystallinity and grain size. Furthermore, All the mixed halide films exhibited strong absorbance with bandgap values ranging between 1.5 eV and 2.5 eV. Moreover, we investigated the crystal structure, morphology, and color change of MAPbX3 after four weeks under dark with 22% humidity; the Bromide-doped samples demonstrate a higher degree of stability based on the study of their degradation mechanisms.

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.

Effects of the PbBr2:PbI2 Molar Ratio on the Formation of Lead Halide Thin Films, and the Ratio's Application for High Performance and Wide Bandgap Solar Cells.

We investigate the effects of the molar ratio (x) of PbBr2 on the phases, microstructure, surface morphology, optical properties, and structural defects of mixed lead halides PbI2(1−x)Br2x for use in solar cell devices. Results indicate that as x increased to 0.3, the surface morphology continued to improve, accompanied by the growth of PbI2 grains. This resulted in lead halide films with a very smooth and continuous morphology, including large grains when the film was formed at x = 0.3. In addition, the microstructure changed from (001)-oriented pure PbI2 to a highly (001)-oriented β (PbI2-rich) phase. The plausible mechanism for the enhanced morphology of the lead halide films by the addition of PbBr2 is proposed based on the growth of a Br-saturated lead iodide solid solution. Furthermore, iodine vacancies, identified by X-ray photoelectron spectroscopy, decreased as the ratio of PbBr2 increased. Finally, an electrical analysis of the solar cells was performed by using a PN heterojunction model, revealing that structural defects, such as iodine vacancies and grain boundaries, are the main contributors to the degradation of the performance of pure PbI2-based solar cells (including high leakage, low stability, and high hysteresis), which was significantly improved by the addition of PbBr2. The solar cell fabricated at x = 0.3 in air showed excellent stability and performance. The device lost merely 20% of the initial efficiency of 4.11% after 1500 h without encapsulation. This may be due to the dense microstructure and the reduced structural defects of lead halides formed at x = 0.3.

Tin Oxide Modified Titanium Dioxide as Electron Transport Layer in Formamidinium-Rich Perovskite Solar Cells

The design of electron transport layers (ETLs) with good optoelectronic properties is one of the keys to the improvement of the power conversion efficiencies (PCEs) and stability of perovskite solar cells (PSCs). Titanium dioxide (TiO2), one of the most widely used ETL in PSCs, is characterized by low electrical conductivity that increases the series resistance of PSCs, thus limiting their PCEs. In this work, we incorporated tin oxide (SnO2) into titanium dioxide (TiO2) and studied the evolution of its microstructural and optoelectronic properties with SnO2 loading. The thin films were then integrated as ETLs in a regular planar Formamidinium (FA)-rich mixed lead halide PSCs so as to assess the overall effect of SnO2 incorporation on their charge transport and Photovoltaic (PV) characteristics. Analysis of the fabricated PSCs devices revealed that the best performing devices; based on the ETL modified with 0.2 proportion of SnO2; had an average PCE of 17.35 ± 1.39%, which was about 7.16% higher than those with pristine TiO2 as ETL. The improvement in the PCE of the PSC devices with 0.2 SnO2 content in the ETL was attributed to the improved electron extraction and transport ability as revealed by the Time Resolved Photoluminescence (TRPL) and Electrochemical Impedance Spectroscopy (EIS) studies.

A Lead Mixed Halide with Three Different Coordinated Anions and Strong Second-Harmonic Generation Response.

Nonlinear optical (NLO) crystals are widely applied in information technology, micro-manufacturing and medical treatment. Herein, a new lead mixed halide with strong second-harmonic generation (SHG) response, Cs3 Pb2 (CH3 COO)2 Br3 I2 , has been designed and rationally synthesized. Cs3 Pb2 (CH3 COO)2 Br3 I2 represents the rare NLO crystal featuring that three different anions (I- , Br- and O2- ) simultaneously coordinate the Pb(II) atom to form a severely distorted [PbBr2 I2 O2 ] polyhedron with a large polarizability. Remarkably, Cs3 Pb2 (CH3 COO)2 Br3 I2 not only exhibits a very strong phase-matching SHG response of 9×KH2 PO4 (KDP), but also possesses a large birefringence (0.27@1064 nm) and high laser damage threshold (LDT). The strong SHG effect of Cs3 Pb2 (CH3 COO)2 Br3 I2 mainly originates from the oriented arrangement of [Pb2 Br3 I2 ] chains. This study points out an effective strategy to develop new NLO crystals with strong SHG response.

Impact of stress testing and passivation strategies on low-cost carbon-based perovskite solar cell under ambient conditions

The use of organolead trihalide perovskite material has been an important milestone in the field of photovoltaics with power conversion efficiencies reaching up to 25.6% in 2021. The light absorbing layer of the perovskite solar cell (PSC) plays a pivotal role in establishing long-term efficiency and stability. Stabilization of these perovskite phases is crucial for the efficient charge transfer dynamics, leading to a better optoelectronic performance. However, the perovskite layer may undergo undesirable phase transitions when exposed to environmental stressors. This impediment can be allayed by having a better understanding of the stress testing regimes and proper passivation strategies leading to their commercial scalability. In the present study, three types of perovskite materials are used for stress testing so as to observe the influence of atmospheric stressors on the type of perovskite materials used. An accelerated ageing test was performed on PSCs with simultaneous combination of heat and humidity. Our results show that FA-MA-Cs based mixed cationic lead halide perovskite are better suited for endurance against environmental stressors retaining almost 50% of PCE post stress test. The champion PSC samples were then subjected to two different modes of low-cost passivation techniques for perovskite absorber layer using acetic acid and CaCl2 as passivating agents. Acetic acid treated samples managed to have lower hysteresis values, lower yellow phase-based degradation and a PCE of up to 15% as compared to control samples. CaCl2 treated samples on the other hand had improved open-circuit voltage values and had more homogenous distribution with better resistance against recombined electrons.

Effect of CH3NH3I/CH3NH3Br precursors on the structural and surface morphology properties of the electrodeposited methylammonium lead–mixed halide perovskite films

Organometallic halide perovskites have been arisen as a class of multi-purpose materials with exciting applications in optoelectronic devices such as solar cells, light-emitting diodes, photo detectors, and laser diodes. In spite of a significant hope in performance, still production process in ambient air and its stability must be addressed as the main issues for this type of material. In this work, a versatile and scalable fabricating process of halide mixed perovskite layers through electrochemical deposition of PbO2 on FTO (fluorine-doped tin dioxide–coated glass) substrates and double-vapor transformation with HX and CH3NH3X (X = I & Br) vapors without using a glove box or a high vacuum system is demonstrated. Highly crystalline hollow-free electrodeposited perovskite layers on FTO substrate were achieved. Treatments with CH3NH3Br/CH3NH3I precursors were also investigated to check the possible impact on the quality of the final perovskite layer. It was illustrated that the use of CH3NH3I or CH3NH3Br precursors in the last step of the electrochemical synthesis can determine the properties of the perovskite film. When PbI2 and PbBr2 films exposed to CH3NH3Br vapor, bromine-treated perovskites were achieved with uniform grain size of ~ 1.5 to 2.3 μm. Similar treatment with CH3NH3I vapor led to the smaller grain size perovskite layers (~ 0.7 μm). Final properties of all fabricated layers, including optical absorption, bandgaps the crystalline phases, and chemical composition, were characterized and compared.

Inhibition of Phase Segregation in Cesium Lead Mixed-Halide Perovskites by B-Site Doping.

SummaryThe emergence of all-inorganic halide perovskites has shown great potential in photovoltaic and optoelectronic devices. However, the photo-induced phase segregation in lead mixed-halide perovskites has severely limited their application. Herein, by real-time monitoring the photoluminescence (PL) spectra of metal mixed-halide perovskites under light irradiation, we found that the photo-induced phase transition can be significantly inhibited by B-site doping. For pristine mixed-halide perovskites, an intermediate phase of CsPbBrxI3-x can only be stabilized under low excitation power. After introducing Sn2+ ions, such intermediate phase can be stabilized in nitrogen atmosphere under high excitation power and phase segregation can be started after the exposure in oxygen due to oxidization of Sn2+. Replacing Sn2+ by Mn2+ can further improve the intermediate phase's tolerance to oxygen proving that B-site doping in perovskites structure by Sn2+ or Mn2+ could effectively minimize the light-induced phase segregation and promote them to serve as promising candidates in photovoltaic and light-emitting devices.

Connecting Femtosecond Transient Absorption Microscopy with Spatially Coregistered Time Averaged Optical Imaging Modalities.

Multimodal all-optical imaging involving coregistered femtosecond transient absorption microscopy (TAM), time-integrated photoluminescence (PL), and steady-state modalities such as confocal reflectance and transmission offers an appealing approach to gain a comprehensive understanding of complex electronic excited-state phenomena in spatially heterogeneous systems. A unique combination of these modalities allows us to unravel not only the competing electronic excited-state dynamical processes but also the underlying morphological information with simultaneous high temporal and spatial resolution. However, correlating the various images obtained from time-resolved and time-independent modalities is generally nontrivial and particularly challenging when the electronic dynamics under study evolve in both time and space. Here, we demonstrate a new approach for rationally correlating time-resolved microscopy with coregistered time-integrated or steady-state modalities. Specifically, our approach involves an extended global lifetime analysis of the time-resolved microscopic data set to separate distinct dynamical processes taking place on commensurate time scales, and the resulting decay-associated amplitude maps (DAAMs) were applied to explore correlations with the images acquired using time-independent modalities. The feasibility of our approach was validated through analyzing a multimodal data set acquired from a thin film of chloride-containing mixed lead halide perovskites (CH3NH3PbI3-xClx) using femtosecond transient absorption, time-integrated PL, and confocal reflectance microscopies. Analysis of the results obtained enable us to gain new insight into the complex ultrafast relaxation dynamics in this highly heterogeneous system.