Mixed Lead Halide Perovskite Research Articles

Stabilization of mixed-halide lead perovskites under light by photothermal effects

Mixed-halide lead perovskites (MHLPs) are semiconductor materials with bandgaps that are tunable across the visible spectrum and have seen promising applications in photovoltaics and optoelectronics. However, their segregation into phases with enriched halide components, under resonant light illumination and/or electric field, have hindered their practical applications. Herein, we demonstrate the stabilization of the MHLP photoluminescence (PL) peak as a function of their excitation intensities. This effect is associated with the phase segregation of MHLPs and their subsequent remixing by photothermal heating. We conclude that the balance between these opposing processes dictates the equilibrium PL peak of the MHLPs. The findings in this work could serve as a potential approach to obtain MHLP with stable emission peaks under operating conditions.

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.

Stabilizing Mixed Halide Lead Perovskites against Photoinduced Phase Segregation by A-Site Cation Alloying

Mixed halide perovskites as top absorbers with relatively high bandgaps (Eg > 1.70 eV) are essential for tandem and multijunction solar cells. Light-induced phase segregation of mixed halide perovs...

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.

Defect Energetics in Pseudo-Cubic Mixed Halide Lead Perovskites from First-Principles

Owing to the increasing popularity of lead-based hybrid perovskites for photovoltaic (PV) applications, it is crucial to understand their defect energetics and its influence on their optoelectronic...

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.

Azetidinium as cation in lead mixed halide perovskite nanocrystals of optoelectronic quality

Previous theoretical calculations show that azetidinium has the right radial size to form a 3D perovskite with lead halides [G. Kieslich et al., Chem. Sci. 5, 4712 (2014)] and has been shown to impart, as the A-site cation of the ABX3 unit, beneficial properties to ferroelectric perovskites [B. Zhou et al., Angew. Chem., Int. Ed. 50, 11441 (2011)]. However, there has been very limited research into its use as the cation in lead halide perovskites to date. In this communication, we report the synthesis and characterization of azetidinium-based lead mixed halide perovskite colloidal nanocrystals. The mixed halide system is iodine and chlorine unlike other reported nanocrystals in the literature, where the halide systems are either iodine/bromine or bromine/chlorine. UV-visible absorbance data, complemented with photoluminescence spectroscopy, reveal an indirect-bandgap of about 2.018 eV for our nanocrystals. Structural characterization using transmission electron microscopy shows two distinct interatomic distances (2.98 Å ± 0.15 Å and 3.43 Å ± 0.16 Å) and non-orthogonal lattice angles (≈112°) intrinsic to the nanocrystals with a probable triclinic structure revealed by X-ray diffraction. The presence of chlorine and iodine within the nanocrystals is confirmed by energy dispersive X-ray spectroscopy. Finally, light-induced electron paramagnetic resonance spectroscopy with PCBM confirms the photoinduced charge transfer capabilities of the nanocrystals. The formation of such semiconducting lead mixed halide perovskites using azetidinium as the cation suggests a promising subclass of hybrid perovskites holding potential for optoelectronic applications such as in solar cells and photodetectors.

Phase Segregation and Photothermal Remixing of Mixed-Halide Lead Perovskites.

Mixed-halide lead perovskites (MHPs) are promising materials for photovoltaics and optoelectronics due to their highly tunable band gaps. However, they phase segregate under continuous illumination or an electric field, the mechanism of which is still under debate. Herein we systematically measure the phase segregation behavior of polymer-encapsulated CH3NH3Pb(BrxI1-x)3 MHPs as a function of excitation intensity and the nominal halide ratio by in situ photoluminescence microspectroscopy and observe surprising phase dynamics at the beginning of the illumination. The initial phase segregation to I-rich and Br-rich phases is observed followed by the formation of a new mixed-halide phase within several seconds that has not been reported before. We propose that the photothermal effect is amplified at the small-size I-rich domains, which significantly changes the local phase segregation in the otherwise uniform film within milliseconds after illumination.

Long-Distance Ionic Diffusion in Cesium Lead Mixed Halide Perovskite Induced by Focused Illumination

Understanding the nature of mobile ions is crucial for revealing their energy landscape in halide perovskites. Here we visualize the diffusion of light-activated mobile ions in mixed halide perovsk...

Cesium Lead Mixed-Halide Perovskites for Low-Energy Loss Solar Cells with Efficiency Beyond 17%

In spite of rapid progress in cesium lead halide (CsPbX3) perovskites solar cells (PSCs), their power conversion efficiencies (PCEs) achieved to date leave much space for further improvement, princ...

Compositional Engineering of Mixed-Cation Lead Mixed-Halide Perovskites for High-Performance Photodetectors.

The mixed-cation lead mixed-halide perovskites can combine the advantages of the constituents while avoiding their drawbacks, and they have been widely explored in solar cells. However, there are only few research studies on the mixed-cation lead mixed-halide perovskites for photodetectors. In this work, we fabricate photodetectors based on FA(1-x)CsxPb(BryI(1-y))3 perovskite and reveal the effect of the chemical composition on the crystal phase stability and device performance of mixed-cation mixed-halide perovskite photodetectors. The FA0.7Cs0.3Pb(I0.8Br0.2)3 photodetectors exhibit high specific detectivity, high responsivity, and excellent stability in ambient conditions. Especially, the flexible perovskite photodetectors fabricated on poly(ethylene terephthalate) substrates exhibit extremely high specific detectivity of 2.8 × 1013 Jones, which is the highest value to date for flexible perovskite photodetectors, as well as excellent stability and outstanding flexibility. These results indicate that mixed-cation mixed-halide perovskites are promising to be applied in high-performance photodetectors and other flexible optoelectronic devices.

Mixed-halide perovskite synthesis by chemical reaction and crystal nucleation under an optical potential

The development of new methods to engineer lead halide perovskite crystals with a controlled band gap and emission properties is an active subject in materials science and chemistry. We present the preparation of mixed-halide lead perovskites by spatially- and temporally- controlled chemical reactions and crystal growth under an optical potential in unsaturated precursor solutions. The crystals are characterized by transmission and photoluminescence spectral measurements and X-ray diffraction analysis. When compared with the spontaneous formation of multiple crystals in saturated precursor solutions, the optical potential creates large single crystals with a high chloride composition, providing distinct blue and green fluorescent crystals of chloride–bromide lead perovskites. We discuss the formation of mixed-halide perovskites from the viewpoints of an increased rate of chemical reaction via the formation and desolvation of precursor complexes and a decreased free energy potential.

Multi-cation Synergy Suppresses Phase Segregation in Mixed-Halide Perovskites

Summary Mixed lead halide perovskite solar cells have been demonstrated to benefit tremendously from the addition of Cs+ and Rb+, but its root cause is yet to be understood. This hinders further improvement, and processing approaches remain largely empirical. We address the challenge by tracking the solidification of precursors in situ and linking the evolutions of different crystalline phases to the presence of Cs+ and Rb+. In their absence, the perovskite film is inherently unstable, segregating into MA-I- and FA-Br-rich phases. Adding either Cs+ or Rb+ is shown to alter the solidification process of the perovskite films. The optimal addition of both Cs+ and Rb+ drastically suppress phase segregation and promotes the spontaneous formation of the desired α phase. We propose that the synergistic effect is due to the collective benefits of Cs+ and Rb+ on the formation kinetics of the α phase and on the halide distribution throughout the film.

Air-Stable and Oriented Mixed Lead Halide Perovskite (FA/MA) by the One-Step Deposition Method Using Zinc Iodide and an Alkylammonium Additive.

We present a one-step method to produce air-stable, large-grain mixed cationic lead perovskite films and powders under ambient conditions. The introduction of 2.5 % of Zn(II), confirmed by X-ray diffraction (XRD), results in stable thin films which show the same absorption and crystal structure after 2 weeks of storage under ambient conditions. Next to prolonged stability, the introduction of Zn(II) affects photophysical properties, reducing the bulk defect density, enhancing the photoluminescence (PL), and extending the charge carrier lifetime. Furthermore, 3-chloropropylamine hydrochloride is applied as the film-forming agent. The presence of this amine hydrochloride additive results in highly oriented and large crystal domains showing an ulterior improvement of PL intensity and lifetime. The material can also be prepared as black precursor powder by a solid–solid reaction under ambient conditions and can be pressed into a perovskite pellet. The prolonged stability and the easy fabrication in air makes this material suitable for large-scale, low-cost processing for optoelectronic applications.

Hydration of mixed halide perovskites investigated by Fourier transform infrared spectroscopy

The mixed lead halide perovskites are promising candidates for solar cells, but the intrinsic instabilities due to phase segregation, halide migration, and hydration hinder the practical applications. Currently, less attention is paid to the humidity induced degradation. This important issue of mixed halide perovskites is now investigated by Fourier transform infrared (FTIR) spectroscopy. After hydration of CH3NH3Pb(I1−xBrx)3, a broad band at around 3200 cm−1 of –OH vibration appears in the FTIR spectra and the bands in the range of 1200–1700 cm−1 become much stronger. The dominant FTIR mode at 1660 cm−1 due to the intermediate complex is sensitive on hydration, implying it could be used as a characteristic peak to evaluate the hydration degree of mixed halide perovskites. These information are of interest to optimize the crystal structure of this material to improve the stability so that to promote the photovoltaic applications.

On the origin of spatially dependent electronic excited-state dynamics in mixed hybrid perovskite thin films

The fundamental photophysics underlying the remarkable performance of organic-inorganic hybrid perovskites in optoelectronic device applications has been increasingly studied using complementary spectroscopic techniques. However, the spatially heterogeneous polycrystalline morphology of the solution-processed thin films is often overlooked in conventional ensemble measurements and therefore the reported results are averaged over hundreds or even thousands of nano- and micro-crystalline grains. Here, we apply femtosecond transient absorption microscopy to spatially and temporally probe ultrafast electronic excited-state dynamics in chloride containing mixed lead halide perovskite (CH3NH3PbI3–xClx) thin films. We found that the electronic excited-state relaxation kinetics are extremely sensitive to the spatial location probed, which was manifested by position-dependent transient absorption signal amplitude and decay behaviour, along with an obvious rise component at some positions. The analysis of transient absorption kinetics acquired at several distinct spatial positions enabled us to identify Auger recombination as the dominant mechanism underlying the initial portions of the spatially dependent dynamics with variable rate constants. The different rates observed suggest occurrence of distinct local electronic structures and variable contributions from impurities/defects and phonons in this nonlinear dynamical process.

Electronic Traps and Phase Segregation in Lead Mixed-Halide Perovskite

An understanding of the factors driving halide segregation in lead mixed-halide perovskites is required for their implementation in tandem solar cells with existing silicon technology. Here we report that the halide segregation dynamics observed in the photoluminescence from CH3NH3Pb(Br0.5I0.5)3 is strongly influenced by the atmospheric environment, and that encapsulation of films with a layer of poly(methyl methacrylate) allows for halide segregation dynamics to be fully reversible and repeatable. We further establish an empirical model directly linking the amount of halide segregation observed in the photoluminescence to the fraction of charge carriers recombining through trap-mediated channels, and the photon flux absorbed. From such quantitative analysis we show that under pulsed illumination, the frequency of the modulation alone has no influence on the segregation dynamics. Additionally, we extrapolate that working CH3NH3Pb(Br0.5I0.5)3 perovskite cells would require a reduction of the trap-related c...

Growth of single crystals of methylammonium lead mixedhalide perovskites

We report the growth and characterization of different bulk single crystals of organo lead mixed halide perovskites CH3NH3PbI3−xBrx by two different crystal growth approaches: (i)anti-solvent diffusion, and (ii) inverse temperature crystallization. In order to control the size and the shape of crystals, we have investigated different experimental growth parameters such as temperature and precursor concentration. The morphology of obtained crystals was observed by optical microscope, whereas their intrinsic crystalline properties were characterized by single crystal as well as powder X-ray diffraction. The results illustrated that the growth and crystalline structure of mixed halide perovskites CH3NH3PbI3−xBrx could be easily tuned.

Cesium lead mixed-halide perovskites in polymer matrix

Inorganic perovskites of CsPbX3 (X = Cl, Br, I) type are excitonic semiconductors for optoelectronics and nanophotonics showing outstanding optical properties that can be controlled via their chemical composition tuning. Pure chloride, bromide and iodide phases of such materials exhibit intense photoluminescence with FWHF = 12-30 nm (FWHM – full width at half maximum) peaked at 420, 525 and 710 nm, respectively. In the meanwhile, mixed-halide perovskites are aimed to fill these gaps and demonstrate emission in the whole visible spectrum. The conventional method of formation of mixed-halide perovskites is mixing of different metal halides in organic solvents affording a purpose solution for further casting and annealing. This method has a serious disadvantage that stems from the inequivalent solubility of the precursors. Herein, we report an easy way of CsPbCl3-xBrx (0<x<3) formation by means of Chemical Vapor Anion Exchange (CVAE) method. The thin films consisting of CsPbBr3 grains embedded into polymer matrix were treated by HCl vapor to give mixed-halide phases emitting light in the 420-525 nm range.