Controlling Crystallization Kinetics Research Articles

Stabilizing precursor solution and controlling crystallization kinetics simultaneously for high-performance perovskite solar cells.

The efficiency of metal halide perovskite solar cells has skyrocketed, however defects created by aging precursor solutions and during crystallization pose a significant barrier to reproducibility and efficiency of solar cells. Herein, we report an additive method using Fluoro-N,N,N″,N″-tetramethylformamidinium hexafluorophosphate (F-(CH3 )4 CN2 PF6 , abbreviated as TFFH) to stabilize precursor solution and improve crysallization dynamics simutaneously for high-performance FAPbI3 (FA = formamidinium)-based perovskite indoor photovoltaics. The F-(CH3 )4 CN2 cations stabilize precursor solution by inhibiting oxidation of I- and reducing newly generated I0 to I- simultaneously. The PF6 - anions interact strongly with the Pb-I framework to passivate undercoordinated Pb2+ . Time-resolved optical diagnostics show a prolonged perovskite crystallization dynamics during which in situ defect passivation is achieved in the precesence of strong FA+ ···TFFH···Pb-I interaction. Simultaneous regulation of precursor solution and crystallizaiton dynamics guarantee larger perovskite grain sizes, better crystal orientation, and less defects of perovskite films as well as more efficient charge extraction in complete solar cells. The optimized solar cells achieve improved reproducibility, better stability and reach effciency of 42.43% at illumination of 1002 lux, which is the highest efficiency among all indoor photovoltaics. It is anticipated that the concurrent stabilization of solutions and regulation of crystallization dynamics will emerge as a prevalent approach for enhancing the reproducibility and efficiency of perovskite. This article is protected by copyright. All rights reserved.

Interface optimization and growth control for high efficiency wide bandgap perovskite solar cells

High-performance wide bandgap perovskite single-junction solar cells (PSCs) are the key elements for the fabrication of high-efficiency tandem solar cells. However, the development of wide bandgap PSCs is currently limited by pernicious interface contact and uncontrolled crystal evolution. In this work, the potassium iodide (KI) additive and solvent annealing method simultaneously are employed to control crystallization kinetics evolution of triple-cation hybrid halide wide bandgap perovskites. The interface is modified with phenethylammonium iodide (PEAI) to form 2D PEAI2PbI4 layer to further enhance the electron collection efficiency and reduce non-radiative recombination caused by the defects trap between the perovskite and electron transport layer. As a result, the photovoltaic performance parameters of the PSCs devices for open-circuit voltage, short-circuit current, and fill factor could be increased from 1.14 V, 14.98 mA cm−2, and 53% to 1.22 V, 17.65 mA cm−2, and 73%, respectively, obtaining the impressive solar cell device with 15.7% power conversion efficiency. The improvement in photovoltaic performance should be attributed to the optimization of crystal evolution and management of interface contact. Therefore, this study provides a promising strategy for fabricating the high-performance wide bandgap PSCs.

Effect of Carbon Nanofibers (CNF) on The Crystallization Kinetics of Polyphenylene Sulfide (PPS)

The presence of a reinforcement in polymeric materials can cause alterations in their physical properties, and increase the crystallization rate of their polymeric matrix. Therefore, this research work was oriented toward the incorporation of different percentages of carbon nanofibers (0.6-1-3 and 5%) in a PPS thermoplastic matrix, with the purpose to establish and understand each one of the variables that control crystallization kinetics. The results showed a gradual decrease in crystallization times for contents of up to 5% of CNF, where carbon nanofibers act as nucleation sites, accelerating the crystallization process. According to the values obtained in the Avrami exponent, these fell within a range that oscillates from 2.5 to 3 for pure PPS and PPS/1%CNF which indicates that the presence of carbon nanofibers does not have an effect on the nucleation mechanism of polyphenylene sulphide crystalline phase, thus suggesting crystallization with heterogeneous nucleation and two-dimensional growth with circular geometry

Managing Phase Orientation and Crystallinity of Printed Dion–Jacobson 2D Perovskite Layers via Controlling Crystallization Kinetics

AbstractTwo‐dimensional perovskites have attracted substantial attention for solar cell applications because of their higher stability as compared to their 3D analogs. To achieve efficient charge transport in thin‐film devices, obtaining high crystalline perovskite crystals perpendicularly aligned to the substrate is of great importance. This article reports the scalable printing of high‐quality Dion–Jacobson (DJ) perovskite thin films via tailoring crystallization kinetics. Introducing a small amount of 1‐methyl‐2‐pyrrolidinone to the conventional N,N‐dimethylformamide:dimethyl sulfoxide‐based precursor, the strong coordination with ammonium spacers enables a notably retarded crystallization, which results in perovskite films with distinctly enhanced crystallinity, highly vertical orientation, and graded phase distribution. Accordingly, efficient charge generation and ultrafast interphase charge transfer are realized. The champion DJ perovskite device delivers a high current density of 17.10 mA cm–2, an impressive open‐circuit voltage of 1.21 V, leading to a stabilized efficiency of 16.19%. In addition, the devices processed from the ternary solvent exhibit remarkably improved stability under stimuli with light, heat, and humidity, benefiting from their superb phase stability. This work demonstrates an important advancement in scalable deposition of DJ perovskite thin films for efficient and stable photovoltaic devices.

Switching between Crystallization from the Glassy and the Undercooled Liquid Phase in Phase Change Material Ge2Sb2Te5

Controlling crystallization kinetics is key to overcome the temperature-time dilemma in phase change materials employed for data storage. While the amorphous phase must be preserved for more than 10 years at slightly above room temperature to ensure data integrity, it has to crystallize on a timescale of several nanoseconds following a moderate temperature increase to near 2/3 Tm to compete with other memory devices such as dynamic random access memory (DRAM). Here, a calorimetric demonstration that this striking variation in kinetics involves crystallization occurring either from the glassy or from the undercooled liquid state is provided. Measurements of crystallization kinetics of Ge2 Sb2 Te5 with heating rates spanning over six orders of magnitude reveal a fourfold decrease in Kissinger activation energy for crystallization upon the glass transition. This enables rapid crystallization above the glass transition temperature Tg . Moreover, highly unusual for glass-forming systems, crystallization at conventional heating rates is observed more than 50 °C below Tg , where the atomic mobility should be vanishingly small.

Phase stability of rare earth sesquioxides with grain size controlled in the nanoscale

AbstractRare earth oxides are important materials due to their chemical and physical properties. The rare earth sesquioxides (RESOXs) possess a cubic, monoclinic, or hexagonal structure, depending on pressure, temperature and ionic radius. When RESOXs take the form of single crystalline grains with diameter in the nanoscale range (nanocrystals), it is suspected that the grain size determines the phase stability as well. RESOXs with medium ionic radius such as Gd2O3, Eu2O3 and Sm2O3 in the low temperature range up to 900°C may take the monoclinic structure or the cubic structure, depending on the synthesis process. The structure determining factor can be, besides temperature, also the grain size. In order to determine the stable structure taking into account grain size as a parameter, we produced Gd2O3 and Sm2O3 nanocrystals embedded in a neutral MgO matrix. This way, we prevented grain growth during annealing. For comparison, we annealed pure Gd2O3 and Sm2O3 nanocrystals in which temperature increase caused grain growth, as in traditional experiments for phase stability determination. We have shown that for coarse grained materials at low temperature monoclinic is the stable structure. For nano‐sized crystallites, cubic is the stable structure. This explains the controversial results as far as which phase is the stable one below 900°C and permits to program the needed structure by controlling crystallization kinetics and grain growth.

Electron‐Transport‐Layer‐Assisted Crystallization of Perovskite Films for High‐Efficiency Planar Heterojunction Solar Cells

AbstractCrystal engineering of CH3NH3PbI3 perovskite materials through template‐directed nucleation and growth on PbI2 nuclei dispersed in a polar fullerene (C60 pyrrolidine tris‐acid, CPTA) electron transport layer (ETL) (CPTA:PbI2) is proposed as a route for controlling crystallization kinetics and grain sizes. Chemical analysis of the CPTA:PbI2 template confirms that CPTA carboxylic acid groups can form a monodentate or bidentate chelate with Pb(II), resulting in a lower nucleation barrier that promotes rapid formation of the tetragonal perovskite phase. Moreover, it is demonstrated that a uniform CH3NH3PbI3 film with highly crystalline and large domain sizes can be realized by increasing the spacing between nuclei to retard perovskite crystal growth via careful control of the preferred nucleation site distribution in the CPTA:PbI2 layer. The improved perovskite morphology possesses a long photoluminescence lifetime and efficient photocarrier transport/separation properties to eliminate the hysteresis effect. The corresponding planar heterojunction photovoltaic yields a high power conversion efficiency (PCE) of 20.20%, with a high fill factor (FF) of 81.13%. The average PCE and FF values for 30 devices are 19.03% ± 0.57% and 78.67% ± 2.13%, respectively. The results indicate that this ETL template‐assisted crystallization strategy can be applied to other organometal halide perovskite‐based systems.

Solvent-aided layer crystallization—Case study glycerol–water

Solid-layer melt crystallization with an additional assisting liquid medium, just like an extraction solvent, is introduced. A case study of the purification of glycerol from mixtures containing 1–10wt% water and the use of 1-butanol as additional solvent assisting to control crystallization kinetics is presented. Cold finger experiments were carried out. It was found that in regular melt crystallization without the use of 1-butanol, water concentration could be reduced to 30–80wt% in comparison to the residual melt at growth rates between 4×10−8 and 1.4×10−7m/s, with lower growth rates resulting in lower water concentrations. No post-treatment like sweating or washing was applied. The dependence between purification success and growth rate could not be changed by initial water content or duration of the experiment and only little by agitation of the melt. The use of 1-butanol allowed an improvement of the process in terms of higher product purities and higher growth rates. Distribution coefficients improved by a factor of 2–4 without a loss in the growth rate or growth rates could be increased by a factor 5 without harming the separation quality. The influence of 1-butanol on the crystallization in comparison to that of water is further investigated. An understanding of how to use additional assisting solvents to control crystallization kinetics and to improve the separation is interesting for the purification of other components, especially high-viscous ones such as tocopherols or phytosterols as well.

Multi-scale thermal simulation of polymer crystallization

Crystallization from polymer melt is one of the most fundamental phenomena of material phase transformations. The possibility of controlling crystallization kinetics is essential to achieve the proper polymer microstructure and consequently obtain the desired material properties. The main objective of the presented work is to compute crystallization kinetics of semicrystalline thermoplastics in an injection molding process with multi-scale modeling. This is done by identifying the analytical parameters needed to connect crystallization kinetics with molecular material properties and applying the analytical scheme to the numerical simulation during cooling. The numerical method, crystallization kinetics and their implementation into numerical software are described as well as the experimental data.

Thermal Simulation of Polymer Crystallization during Post-Filling

Crystallization from polymer melt is one of the most fundamental phenomena of material phase transformations. The possibility of controlling crystallization kinetics is essential to achieve the proper polymer microstructure and consequently obtain desired material properties, reducing undesired effects such excessive anisotropy of shrinkage, warping and insufficient dimensional stability. Due to the high transformation rate, the simulation of crystallization is fundamental to mimic this important physical phenomenon under several testing and processing conditions by using commercial software. User subroutines were developed and implemented into finite element-based model to simulate crystal growth in semicrystalline polymers with various crystal morphologies. These subroutines allowed the commercial program Abaqus to be customized for solving the Kolmogoroff-Avrami-Evans equation with Hoffmann-Lauritzen model in order to simulate the variation of the polymer crystallization degree. The micro-structural evolution in non-isothermal conditions and with different cooling rates was considered. The study was performed on isotactic PP (SABIC PP 505) for its simplicity to the measure polymer crystals. A tensile test specimen, produced by injection molding, was chosen as case study to evaluate the crystallization evolution. The paper reports the numerical and experimental results.

Molecular mechanisms of crystallization impacting calcium phosphate cements.

The biomineral calcium hydrogen phosphate dihydrate (CaHPO(4).2H(2)O), known as brushite, is a malleable material that both grows and dissolves faster than most other calcium minerals, including other calcium phosphate phases, calcium carbonates and calcium oxalates. Within the body, this ready formation and dissolution can play a role in certain diseases, such as kidney stone and plaque formation. However, these same properties, along with brushite's excellent biocompatibility, can be used to great benefit in making resorbable biomedical cements. To optimize cements, additives are commonly used to control crystallization kinetics and phase transformation. This paper describes the use of in situ scanning probe microscopy to investigate the role of several solution parameters and additives in brushite atomic step motion. Surprisingly, this work demonstrates that the activation barrier for phosphate (rather than calcium) incorporation limits growth kinetics and that additives such as magnesium, citrate and bisphosphonates each influence step motion in distinctly different ways. Our findings provide details of how, and where, molecules inhibit or accelerate kinetics. These insights have the potential to aid in designing molecules to target specific steps and to guide synergistic combinations of additives.

Principles of controlling crystallization kinetics for a multicomponent coating on steel

Features of the crystallization of a multicomponent galvanizing whose main alloying element is nickel are studied. Kinetics of the process are described on the basis of x-ray microanalysis data and solution of the diffusion problem. A mechanism is discovered of the retarding effect of nickel on coating thickness growth. Data are provided for determining the diffusion coefficient for nickel in a zinc coating.

Applying Hot-Stage Microscopy to Co-Crystal Screening: A Study of Nicotinamide with Seven Active Pharmaceutical Ingredients

Co-crystal screening is routinely undertaken using high-throughput solution growth. We report a low- to medium-throughput approach, encompassing both a melt and solution crystallization step as a route to the identification of co-crystals. Prior to solution studies, a melt growth step was included utilizing the Kofler mixed fusion method. This method allowed elucidation of the thermodynamic landscape within the binary phase diagram and was found to increase overall screening efficiency. The pharmaceutically acceptable adduct nicotinamide was selected and screened against a small set of active pharmaceutical ingredients (APIs) (ibuprofen (both the racemic compound (R/S) and S-enantiomer), fenbufen, flurbiprofen (R/S), ketoprofen (R/S), paracetamol, piracetam, and salicylic acid) as part of a larger systematic study of synthon stability. From the screen, three new co-crystal systems have been identified (ibuprofen (R/S and S) and salicylic acid) and their crystal structures determined. Because of poor crystal growth synchrotron radiation was required for structure solution of the S-ibuprofen nicotinamide co-crystal. Two further potential systems have also been discovered (fenbufen and flurbiprofen), but crystals suitable for structure determination have yet to be obtained. A greater ability to control crystallization kinetics is required to yield phase-pure single-crystalline material for full verification of this crystal engineering strategy.

19aD04 Controlling crystallization kinetics of multicomponent fats by addition of high-melting point surfactants(NCCG-35)

19aD04 Controlling crystallization kinetics of multicomponent fats by addition of high-melting point surfactants(NCCG-35)

Kinetics of lactose crystallization and crystal size as monitored by refractometry and laser light scattering: effect of proteins

Lactose crystallization is a key step in the manufacture of whey powders. Both the crys- tal size and mass of crystals produced affect the subsequent step of spray drying and whey powder quality. Consequently, it is essential to control crystallization kinetics, crystal quality and how these properties are affected by whey components. For this purpose, two suitable and simple methods were developed. Refractometry was used to follow the kinetics of lactose crystallization in super- saturated solution of lactose. The extent of crystallization and the overall crystallization rate con- stant were then deduced. Laser light scattering was used to follow the crystal growth in the solid phase by measuring crystal size over time. The two methods were used to determine the impact of whey proteins on lactose crystallization from supersaturated solution. Our first results showed that the presence of whey proteins (5g·100 g-1 water) did not significantly modify the overall crystalli- zation rate constant but had a large lowering effect on the final size of crystals during the non-seeded crystallization process (~30 µm against ~105 µm in the absence of protein). lactose crystallization / whey proteins / refractometry / laser light scattering