Metastable Morphology Research Articles

High Performance As-Cast Organic Solar Cells Enabled by a Refined Double-Fibril Network Morphology and Improved Dielectric Constant of Active Layer.

High performance organic solar cells (OSCs) are usually realized by using post-treatment and/or additive, which can induce the formation of metastable morphology, leading to unfavorable device stability. In terms of the industrial production, the development of high efficiency as-cast OSCs is crucially important, but it remains a great challenge to obtain appropriate active layer morphology and high power conversion efficiency (PCE). Here, efficient as-cast OSCs are constructed via introducing a new polymer acceptor PY-TPT with a high dielectric constant into the D18:L8-BO blend to form a double-fibril network morphology. Besides, the incorporation of PY-TPT enables an enhanced dielectric constant and lower exciton binding energy of active layer. Therefore, efficient exciton dissociation and charge transport are realized in D18:L8-BO:PY-TPT-based device, affording a record-high PCE of 18.60% and excellent photostability in absence of post-treatment. Moreover, green solvent-processed devices, thick-film (300nm) devices, and module (16.60 cm2) are fabricated, which show PCEs of 17.45%, 17.54%, and 13.84%, respectively. This work brings new insight into the construction of efficient as-cast devices, pushing forward the practical application of OSCs.

How can we improve the stability of organic solar cells from materials design to device engineering?

AbstractAmong a promising photovoltaic technology for solar energy conversion, organic solar cells (OSCs) have been paid much attention, of which the power conversion efficiencies (PCEs) have rapidly surpassed over 20%, approaching the threshold for potential applications. However, the device stability of OSCs including storage stability, photostability and thermal stability, remains to be an enormous challenge when faced with practical applications. The major causes of device instability are rooted in the poor inherent properties of light‐harvesting materials, metastable morphology, interfacial reactions and highly sensitive to external stresses. To get rid of these flaws, a comprehensive review is provided about recent strategies and methods for improving the device stability from active layers, interfacial layers, device engineering and encapsulation techniques for high‐performance OSC devices. In the end, prospectives for the next stage development of high‐performance devices with satisfactory long‐term stability are afforded for the solar community.

Powering the Future: A Critical Review of Research Progress in Enhancing Stability of High‐Efficiency Organic Solar Cells

AbstractOrganic solar cells (OSCs) are a promising photovoltaic technology that employs organic semiconductor material as the photoactive layer, which has the unique advantages of light weight, large‐area flexible fabrication, low‐cost, and semitransparent. In recent years, the performance of OSCs has been significantly improved, and the highest power conversion efficiency has exceeded 19%. Despite the tremendous progress in OSCs, the major bottleneck in realizing the commercialization of OSCs is the device stability. Therefore, reviewing the recent research progress on the stability of high‐performance OSCs is urgent and necessary. This review discusses the factors limiting device lifetime, such as metastable morphology, air, irradiation, heat, and mechanical stresses. Additionally, this review presents the research progress over the last 5 years, focusing on enhancing device stability from the perspective of photoactive layers and other functional layers, which includes material design and device engineering, such as solid additives, device fabrication, optimizing buffer layers, using stable electrodes, and encapsulation. Lastly, this review explores current commercialization challenges and prospects, including using advanced machine learning techniques to assist experimental research.

Ultrasound-assisted solution crystallization of fotagliptin benzoate: Process intensification and crystal product optimization

The ultrasound-assisted crystallization process has promising potentials for improving process efficiency and modifying crystalline product properties. In this work, the crystallization process of fotagliptin benzoate methanol solvate (FBMS) was investigated to improve powder properties and downstream desolvation/drying performance. The direct cooling/antisolvent crystallization process was conducted and then optimized with the assistance of ultrasonic irradiation and seeding strategy. Direct cooling/antisolvent crystallization and seeding crystallization processes resulted in needle-like crystals which are undesirable for downstream processing. In contrast, the ultrasound-assisted crystallization process produced rod-like crystals and reduced the crystal size to facilitate the desolvation of FBMS. The metastable zone width (MSZW), induction time, crystal size, morphology, and process yield were studied comprehensively. The results showed that both the seeding and ultrasound-assisted crystallization process (without seeds) can improve the process yield and the ultrasound could effectively reduce the crystal size, narrow the MSZW, and shorten the induction time. Through comparing the drying dynamics of the FBMS, the small rod-shaped crystals with a mean size of 9.6 μm produced by ultrasonic irradiation can be completely desolvated within 20 h, while the desolvation time of long needle crystals with an average size of about 157 μm obtained by direct cooling/antisolvent crystallization and seeding crystallization processes is more than 80 h. Thus the crystal size and morphology were found to be the key factors affecting the desolvation kinetics and the smaller size produced by using ultrasound can benefit the intensification of the drying process. Overall, the ultrasound-assisted crystallization showed a full improvement including crystal properties and process efficiency during the preparation of fotagliptin benzoate desolvated crystals.

Quasicrystals and Their Approximants in a Crystalline–Amorphous Diblock Copolymer

Discoveries of Frank–Kasper phases and closely related dodecagonal quasicrystals (DDQCs) in soft, mesoscopic systems have galvanized efforts to unveil the fundamental mechanisms that drive the formation of these remarkably complex micellar packings. Toward this end, we report temperature-dependent small-angle X-ray scattering analyses of a crystalline–amorphous poly(ethylene oxide)-block-poly(2-ethyl hexylacrylate) (OA) diblock copolymer with Mₙ = 8300 g/mol, Đ = Mw/Mₙ = 1.10, and volume composition fO = 0.21. On heating at ambient temperature, this polymer assembles sequentially into five distinct morphologies prior to melt disordering at TODT = 69 °C: semicrystalline lamellae (Lc), a liquid-like packing (LLP) of particles lacking translational order, an aperiodically ordered DDQC, a periodic FK σ phase, and a body-centered cubic (BCC) packing of particles. Detailed investigations of thermal processing conditions that foster DDQC formation reveal that this metastable morphology only forms in melts exhibiting LLP characteristics arising from either melting the Lc phase at low temperature or quenching a high-temperature disordered state, and that this DDQC eventually evolves into a σ approximant phase. Cooling a well-ordered BCC phase induces direct formation of a σ phase with no evidence of DDQC formation, suggesting the critical importance of particle size distribution of the disorganized yet segregated LLP state in triggering the emergence of a DDQC.

Suppressing Kinetic Aggregation of Non‐Fullerene Acceptor via Versatile Alloy States Enables High‐Efficiency and Stable Ternary Polymer Solar Cells

AbstractDespite considerable advances devoted to improving the operational stability of organic solar cells (OSCs), the metastable morphology degradation remains a challenging obstacle for their practical application. Herein, the stabilizing function of the alloy states in the photoactive layer from the perspective of controlling the aggregation characteristics of non‐fullerene acceptors (NFAs), is revealed. The alloy‐like model is adopted separately into host donor and acceptor materials of the state‐of‐the‐art binary PM6:BTP‐4Cl blend with the self‐stable polymer acceptor PDI‐2T and small molecule donor DRCN5T as the third components, delivering the simultaneously enhanced photovoltaic efficiency and storage stability. In such ternary systems, two separate arguments can rationalize their operating principles: (1) the acceptor alloys strengthen the conformational rigidity of BTP‐4Cl molecules to restrain the intramolecular vibrations for rapid relaxation of high‐energy excited states to stabilize BTP‐4Cl acceptor. (2) The donor alloys optimize the fibril network microstructure of PM6 polymer to restrict the kinetic diffusion and aggregation of BTP‐4Cl molecules. According to the superior morphological stability, non‐radiative defect trapping coefficients can be drastically reduced without forming the long‐lived, trapped charge species in ternary blends. The results highlight the novel protective mechanisms of engineering the alloy‐like composites for reinforcing the long‐term stability of NFA‐based ternary OSCs.

An Optimized Fibril Network Morphology Enables High-Efficiency and Ambient-Stable Polymer Solar Cells.

Morphological stability is crucially important for the long‐term stability of polymer solar cells (PSCs). Many high‐efficiency PSCs suffer from metastable morphology, resulting in severe device degradation. Here, a series of copolymers is developed by manipulating the content of chlorinated benzodithiophene‐4,8‐dione (T1‐Cl) via a random copolymerization approach. It is found that all the copolymers can self‐assemble into a fibril nanostructure in films. By altering the T1‐Cl content, the polymer crystallinity and fibril width can be effectively controlled. When blended with several nonfullerene acceptors, such as TTPTT‐4F, O‐INIC3, EH‐INIC3, and Y6, the optimized fibril interpenetrating morphology can not only favor charge transport, but also inhibit the unfavorable molecular diffusion and aggregation in active layers, leading to excellent morphological stability. The work demonstrates the importance of optimization of fibril network morphology in realizing high‐efficiency and ambient‐stable PSCs, and also provides new insights into the effect of chemical structure on the fibril network morphology and photovoltaic performance of PSCs.

Progress in Stability of Organic Solar Cells.

The organic solar cell (OSC) is a promising emerging low‐cost thin film photovoltaics technology. The power conversion efficiency (PCE) of OSCs has overpassed 16% for single junction and 17% for organic–organic tandem solar cells with the development of low bandgap organic materials synthesis and device processing technology. The main barrier of commercial use of OSCs is the poor stability of devices. Herein, the factors limiting the stability of OSCs are summarized. The limiting stability factors are oxygen, water, irradiation, heating, metastable morphology, diffusion of electrodes and buffer layers materials, and mechanical stress. The recent progress in strategies to increase the stability of OSCs is surveyed, such as material design, device engineering of active layers, employing inverted geometry, optimizing buffer layers, using stable electrodes and encapsulation materials. The International Summit on Organic Photovoltaic Stability guidelines are also discussed. The potential research strategies to achieve the required device stability and efficiency are highlighted, rendering possible pathways to facilitate the viable commercialization of OSCs.

Phase- and shape-controlled synthesis of VO2 by a hydrothermal-calcination method

VO2(M) structures with various shapes were synthesized by vacuum annealed VO2 metastable phase, and this metastable phase (VO2(B/D)) were formed by hydrothermal method used H2C2O4 and V2O5 powders as starting reactants. The phase structure, morphology, surface composition and phase transition characteristics of VO2 were characterized by XRD, SEM, XPS and DSC. Obtained results show that the VO2's metastable phase structure and morphology strongly depended on the n-butanol/water ratio, molar ratio of V2O5 and H2C2O4 and molar concentration of V2O5, which leads a different synthesis pathway. The influence mechanism has also been studied. Low n-butanol/water ratio (2:45) and low molar ratio (1:1, 1:2) favor the formation of VO2(B) nanosheets/nanorods while high n-butanol/water ratio (45:2) and high molar concentration (0.22 mol/L) favor the formation of VO2(D) micro/nanospheres. Ultimately, the VO2(M) nanoparticles can be obtained at a relatively low vacuum annealing temperature of 300 °C, and with a lowest TC was as low as 55.43 °C. In addition, the effect of vacuum annealing temperature and morphology on the phase transition characteristic of VO2(M) has also been studied.

Enhancement of lysozyme crystallization under ultrasound field.

With the increasing demand for biopharmaceuticals, a method to crystallize biomolecule products with high quality, high yield and uniform size distribution as well as regular crystal habit is needed. In this work, ultrasound was used as a nucleation accelerator to decrease the energy barrier for lysozyme crystal formation. Crystallization experiments on egg-white lysozyme were carried out with and without ultrasound. The effect of ultrasound on induction time, metastable zone width, crystal size and morphology and process yield was investigated in detail. The nucleation-promoting effect produced by ultrasound is illustrated by the reduction of metastable zone width and induction time. By inducing faster nucleation, ultrasound leads to protein crystals grow at lower supersaturation levels with shorter induction time. It was found that ultrasound could result in uniform size distribution of the product due to the preventing of aggregation. However, long time continuous application of ultrasound could result in smaller particle size. Hence, ultrasonic-stop method was found to be a more appropriate strategy to enhance the crystallization process of proteins such as lysozyme.

Crystallization process and nucleation kinetics of Mg2+//SO4 2-, NO3 --H2O system.

In order to achieve efficient recovery of ions from the simultaneous desulfurization and denitrification wastewater, the effects of various factors (i.e. the saturation temperature, the cooling termination temperature, the stirring rate and the cooling rate) on crystallization yield, metastable zone width and crystal morphology were investigated to determine the optimal crystallization conditions of Mg2+//SO4 2-, NO3 --H2O system. According to the results of experiments, the nucleation kinetics were also speculated by Nývlt self-consistent equation and classical 3D nucleation theory. Also, the crystallization products were characterized by X-ray diffraction and scanning electron microscopy. Under the determined optimal conditions, the yield of the crystal can reach 78%, and the crystal products were verified as pure MgSO4· 7H2O, whose morphology is complete without defects.

Engineering the Kinetics of Directed Self-Assembly of Block Copolymers toward Fast and Defect-Free Assembly.

Directed self-assembly (DSA) of block copolymers (BCPs) can achieve perfectly aligned structures at thermodynamic equilibrium, but the self-assembling morphology can become kinetically trapped in defective states. Understanding and optimizing the kinetic pathway toward domain alignment is crucial for enhancing process throughput and lowering defectivity to levels required for semiconductor manufacturing, but there is a dearth of experimental, three-dimensional studies of the kinetic pathways in DSA. Here, we combined arrested annealing and TEM tomography to probe the kinetics and structural evolution in the chemoepitaxy DSA of PS- b-PMMA with density multiplication. During the initial stages of annealing, BCP domains developed independently at first, with aligned structures at the template interface and randomly oriented domains at the top surface. As the grains coarsened, the assembly became cooperative throughout the film thickness, and a metastable stitch morphology was formed, representing a kinetic barrier. The stitch morphology had a three-dimensional structure consisting of both perpendicular and parallel lamellae. On the basis of the mechanistic information, we studied the effect of key design parameters on the kinetics and evolution of structures in DSA. Three types of structural evolutions were observed at different film thicknesses: (1) immediate alignment and fast assembly when thickness < L0 ( L0 = BCP natural periodicity); (2) formation of stitch morphology for 1.25-1.45 L0; (3) fingerprint formation when thickness >1.64 L0. We found that the DSA kinetics can be significantly improved by avoiding the formation of the metastable stitch morphology. Increasing template topography also enhanced the kinetics by increasing the PMMA guiding surface area. A combination of 0.75 L0 BCP thickness and 0.50 L0 template topography achieved perfect alignment over 100 times faster than the baseline process. This research demonstrates that an improved understanding of the evolution of structures during DSA can significantly improve the DSA process.

Real-Time Atomic Force Microscopy Imaging of Block Copolymer Directed Self Assembly.

The kinetics of directed self-assembly of symmetric PS-b-PMMA diblock copolymer on chemically patterned templates were measured during in situ thermal annealing. Although these chemical guide patterns lead to well-aligned, defect-free lamellar patterns at thermodynamic equilibrium, in practice, challenges remain in understanding and optimizing the kinetic evolution for technological applications. High-speed, environmentally controlled atomic force microscopy imaging was used to track pattern evolution on the time scale of individual microdomain connections in real space and time, allowing the direct visualization of defect healing mechanisms. When we apply this highly general technique to films on chemically patterned substrates, we find that pattern alignment is mediated by a metastable nonbulk morphology unique to these samples, referred to as the "stitch" morphology. We observe diverse and anisotropic mechanisms for the conversion from this morphology to equilibrium lamellar stripes. Directed self-assembly on chemical templates is observed to follow exponential kinetics with an apparent energetic barrier of 360 ± 80 kJ/mol from 210-230 °C, a significant enhancement when compared with ordering rates on unpatterned substrates. Ultimately, from local imaging, we find that the presence of a chemical guiding field causes morphological ordering and lamellar alignment to occur irreversibly.

Recovery and carbonation of 100% of calcium in waste concrete fines: Experimental results

Mineral sequestration stabilizes carbon dioxide in carbonate form, its thermodynamic ground state. In the case of waste concrete, it produces precipitated calcium carbonate, a commodity. We propose an indirect, aqueous process consisting of six steps: (i) recovery of waste cement (primarily calcium silicate hydrate) from waste concrete, (ii) leaching of calcium from waste cement using hydrochloric acid, (iii) purification of the leachate through alkalinity addition, (iv) precipitation of calcium carbonate via aqueous reaction with sodium carbonate, four of which were investigated here. The omissions include regeneration of hydrochloric acid and sodium hydroxide solutions from sodium chloride solution through bipolar membrane electrodialysis and absorption of CO2 by sodium hydroxide solution. Hydrochloric acid was capable of leaching all the calcium ions present in the fines when the stoichiometric amount was added. Results showed up to 70% of the total calcium ions recovered were associated with waste cement with the remainder from limestone in the aggregates. A bright, high-purity PCC was produced after purification to pH ∼11. The high-purity PCC particles were larger than what is typically used in paper manufacturing (∼2 μm) with metastable morphology. The particle size was reduced to ∼2 μm when the purification stopped at pH 9.0 and a magnesium containing PCC was produced; the morphology remained stable in de-ionized water for 24 h.

Role of copper on Laves phase morphology in 9-12%Cr steels

In this work the Laves phase was found to appear in two different morphologies, namely granular shapes and in an elongated shape. No difference in crystallography could be detected between these morphologies. The Laves phase was only observed in its elongated form in Cu-containing steels, where it was the primary morphology present after short term ageing. After long term ageing, the elongated Laves phase was replaced by the granular morphology. It is speculated that Cu precipitates act as nucleation sites for the elongated Laves phase, resulting in an unstable orientation relationship with the matrix, an in the meta-stable elongated morphology of Laves phase precipitates.

Diverse morphologies in thin-film bijels by varying film thickness and composition.

Thin-film bijels have many potential energy and environmental applications. In this work, the metastable bijel morphology space in thin-film confinement is explored with a Cahn-Hilliard/Brownian-Dynamics computational model. The key parameters varied are the bijel liquid phase blend ratio and the bijel film thickness. Simulations reveal a broad spectrum of structurally unique morphologies that have yet to be observed in experiments and which could have interesting applications in membrane science and other domains. Extensive analyses of surface-to-volume ratios, interfacial particle attachment statistics, and topological interfacial curvatures within the bijels are presented for a complete characterization of the morphological structure.

Simultaneous recovery and crystallization control of saline organic wastewater by membrane distillation crystallization

Membrane distillation crystallization (MDC) was introduced and investigated for the simultaneous treatment of saline organic wastewater generated from oil extraction, in this article. The developed process model of MDC for the investigated ternary system was validated by the experiments with good agreement. Under the tested feed ethylene glycol (EG) composition and operation conditions, the highly concentrated EG (residual side, recovery ratio &gt; 98.7%), pure water (permeate side, purity &gt; 99%), pure crystals with narrow crystal‐size distribution, and desired morphology were simultaneously manufactured. By this simultaneous recovery of EG and H2O in the feed flow, the overall separation efficiency was enhanced. The impact factors of the crystal properties during MDC were also revealed by comprehensive analysis. Moreover, the diverse metastable zone width and crystal morphology obtained in different feed EG composition indicated the change of the nucleation barrier and the kinetic crystallization control mechanism. © 2016 American Institute of Chemical EngineersAIChE J, 63: 2187–2197, 2017

Process-directed self-assembly of block copolymers: a computer simulation study

The free-energy landscape of self-assembling block copolymer systems is characterized by a multitude of metastable minima and concomitant protracted relaxation times of the morphology. Tailoring rapid changes (quench) of thermodynamic conditions, one can reproducibly trap the ensuing kinetics of self-assembly in a specific metastable state. To this end, it is necessary to (1) control the generation of well-defined, highly unstable states and (2) design the unstable state such that the ensuing spontaneous kinetics of structure formation reaches the desired metastable morphology. This process-directed self-assembly provides an alternative to fine-tuning molecular architecture by synthesis or blending, for instance, in order to fabricate complex network structures. Comparing our simulation results to recently developed free-energy techniques, we highlight the importance of non-equilibrium molecular conformations in the starting state and motivate the significance of the local conservation of density.

Effect of Reflux Time on Nanoparticle Shape

In the present work, Pt nanoparticles were produced from a reaction mixture containing a trace amount of cobalt carbonyl salt acting as a shape inducer. Nanoparticle shape evolution during reaction mixture reflux was monitored by characterizing particles extracted from the reaction mixture at different times. It was observed that 5 min of reflux produced spherical nanoparticles, 30 min of reflux produced cube shaped nanoparticles, and 60 min of reflux produced truncated octahedron morphology nanoparticles. It is illustrated that during nanoparticle synthesis the reflux process can provide energy needed for shape transformation from a metastable cube morphology to a truncated octahedron morphology which is thermodynamically the most stable geometry for fcc crystals. An optimization of the reaction reflux is thus needed for isolating metastable shapes.

β-phase formation in a crosslinkable poly(9,9-dihexylfluorene)

The synthesis of a crosslinkable poly(9,9-dihexylfluorene) derivative is reported. Vinyl ether moieties allow for photolithographic patterning, resulting in an immiscible polyfluorene network, which enables subsequent wet thin-film processing. The metastable β-phase morphology, inherent to most polyfluorene homopolymers, can be induced in this crosslinkable polymer. This is verified by optical spectroscopy, X-ray diffraction and by thermal analysis. The β-phase in polyfluorenes is of interest due to its superior optical and electrical characteristics compared to the amorphous phase. Here, amplified stimulated emission is studied. Surprisingly, the β-phase polymer exhibits higher emission thresholds than the amorphous analogue.