Length Scale Of Phase Separation Research Articles

Kinetics and mechanism of light-induced phase separation in a mixed-halide perovskite

•Utilized in situ cryoelectron-microscopy-cathodoluminescence spectroscopy •Observed the morphological and band-gap evolution of iodide-rich clusters in situ •Performed quantitative studies of the characteristic length scale of phase separation •Phase field modeling prediction is consistent with spinodal decomposition Halide ion phase separation is a barrier to the application of mixed-halide perovskites whereby the presence of large populations of photogenerated or injected carriers causes undesirable changes in the local band gap. We investigate the mechanism of phase separation in CsPbIxBr3-x perovskite single crystals driven by light. The phase separation process and its dynamics are visualized at the nanometer scale at cryogenic temperatures using in situ scanning transmission electron microscopy and cathodoluminescence. Combined with phase field modeling, which accounts for the coupling between electronic and chemical driving forces, our observations point to a spinodal decomposition mechanism in which both the amplitude of composition fluctuation and the characteristic length scale grow non-linearly with time. Our findings provide microscopic insights that can assist in further engineering mixed-halide perovskites either for stability or for intentional programming of the local halide ion composition, opening pathways to a wide range of applications. Halide ion phase separation is a barrier to the application of mixed-halide perovskites whereby the presence of large populations of photogenerated or injected carriers causes undesirable changes in the local band gap. We investigate the mechanism of phase separation in CsPbIxBr3-x perovskite single crystals driven by light. The phase separation process and its dynamics are visualized at the nanometer scale at cryogenic temperatures using in situ scanning transmission electron microscopy and cathodoluminescence. Combined with phase field modeling, which accounts for the coupling between electronic and chemical driving forces, our observations point to a spinodal decomposition mechanism in which both the amplitude of composition fluctuation and the characteristic length scale grow non-linearly with time. Our findings provide microscopic insights that can assist in further engineering mixed-halide perovskites either for stability or for intentional programming of the local halide ion composition, opening pathways to a wide range of applications.

Oligoester-Derivatized (Semi-)Interpenetrating Polymer Networks as Nanostructured Precursors to Porous Materials with Tunable Porosity

Porous polymeric materials with tunable porosity can be engineered from oligoester-derivatized semi-interpenetrating polymer networks (semi-IPNs) or IPNs, respectively composed of either uncrosslinked or crosslinked aliphatic oligoesters entangled in a stiff subnetwork. In this paper, miscellaneous polyester/poly(methyl methacrylate)-based semi-IPN and IPN systems are first prepared as precursors with varying structural parameters, especially the nature [i.e., poly(d,l-lactide), poly(e-caprolactone)] and the molar mass (i.e., from 560 to 3700 g mol−1) of the oligoester precursor. (Nano)porous networks with defined porosity are then generated through two complementary routes. This original paper discusses the scope and limitations of both approaches and investigates the correlation between the structure and morphology of the generated networks and the porosity of the resulting porous materials. We demonstrate that the choice of the precursors with defined compatibility is of paramount significance in the length scale of phase separation associated with nanostructured networks as well as in the porosity scale of (nano)porous materials derived therefrom. Indeed, we find that the quantitative extraction of uncrosslinked oligoesters from semi-IPNs allows for the elaboration of nanoporous networks with pore diameters lower than 150 nm, provided that a high miscibility between both partners in semi-IPN precursors is attained, i.e. when using the lower molar mass oligoester. Alternately, the total hydrolysis of the polyester subnetwork associated with IPNs offers more versatility, since nanoporous networks can be obtained with a pore size range of 20–150 nm, regardless of the oligoester nature and molar mass in IPN precursors.

Triptycene as a Supramolecular Additive in PTB7:PCBM Blends and Its Influence on Photovoltaic Properties.

Additives play an important role in modifying the morphology and phase separation of donor and acceptor molecules in bulk heterojunction (BHJ) solar cells. Here, we report triptycene (TPC) as a small-molecule additive for supramolecular control of phase separation and concomitant improvement of the power conversion efficiency (PCE) of PTB7 donor and fullerene acceptor-based BHJ polymer solar cells. An overall 60% improvement in PCE is observed for both PTB7:PC61BM and PTB7:PC71BM blends. The improved photovoltaic (PV) performance can be attributed to three factors: (a) TPC-induced supramolecular interactions with donor:acceptor components in the blends to realize a nanoscale phase-separated morphology; (b) an increase in the charge transfer state energy that lowers the driving force for electron transfer from donor to acceptor molecules; and (c) an increase in the charge carrier mobility. An improvement in efficiency using TPC as a supramolecular additive has also been demonstrated for other BHJ blends such as PBDB-T:PC71BM and P3HT:PCBM, implying the wide applicability of this new additive molecule. A comparison of the photostability of TPC as an additive for PTB7:PCBM solar cells to that of the widely used 1,8-diiodooctane additive shows ∼30% higher retention of PV performance for the TPC-added solar cells after 34 h of AM 1.5G illumination. The results obtained suggest that the approach of using additives that can promote supramolecular interactions to modify the length scale of phase separation between donor and acceptor is very promising and can lead to the development of highly efficient and stable organic photovoltaics.

Influence of Molecular Excluded Volume and Connectivity on the Nanoscale Morphology of Conjugated Polymer/Small Molecule Blends

Over the past couple of decades, organic photovoltaics (OPV) based on conjugated polymer/fullerene derivative bulk heterojunctions have been extensively studied, resulting in single-junction efficiencies of order 10%. The need to push the efficiency toward 15% has resulted in the synthesis of a large number of non-fullerene electron acceptors with ever-increasing absorption coefficients in the red. Though some new acceptors have recently begun to be competitive with the fullerene, there is very little systematic understanding of which molecular geometries and spatial frontier orbital extent correlate with improved performance. One of the most important factors that determines the OPV efficiency is the nanoscale, phase-separated morphology of the blend. In this article, using a combination of resonant elastic X-ray scattering and elemental mapping, we investigate the influence of relatively small chemical changes to a nonplanar conjugated small molecule on the nanoscale morphology of the resulting polymer/molecule blend. We find that subtle modifications of the number and placement of peripheral functional groups can have an enormous influence on the length scale of phase separation. We then quantify the extent of phase separation by using the generalized indirect Fourier transform to convert resonant scattering intensities to pair-distance distribution functions. Our results point toward the large influence of the molecular excluded volume as a major morphology determinant. This work has implications for synthetic efforts to create non-fullerene electron acceptors that can substantially outcompete fullerenes in OPV devices.

Morphology Characterization of Bulk Heterojunction Solar Cells

AbstractRevealing the nanostructure of bulk‐heterojunction (BHJ) photovoltaic blends is a critical task in the field of organic photovoltaics. The complicated morphology, ranging from binary blends to ternary mixtures, shows quite varied structural details that need to be fully characterized in terms of correlating them with device performance. The nonequilibrium nature of the BHJ thin‐film morphology makes the structure optimization difficult, and fundamental principles that relate to chemical structure, processing methods, and device performances have not yet been fully understood. On the other hand, various high‐powered techniques ranging from in‐house instrumentation to synchrotron techniques have been applied to this tremendously difficult research topic. More structural details, including length scale of phase separation, structural ordering, chain/molecular orientation, surface topology, etc., can be obtained in a quantitative manner, which finally allows the structure–property relationships to be determined. Here, the most important characterization techniques in understanding the morphology of BHJ solar cells are summarized, targeting chemists and materials scientists.

Toward a better understanding of conjugated polymer blends with non-spherical small molecules: coupling of molecular structure to polymer chain microstructure

Abstract

Influence of C60 co-deposition on the growth kinetics of diindenoperylene-From rapid roughening to layer-by-layer growth in blended organic films.

We investigated the growth of the two phase-separating materials diindenoperylene (DIP) and buckminsterfullerene C60 with different mixing ratio in real-time and in situ by X-ray scattering experiments. We found that at room temperature, mixtures with an excess of DIP show a growth mode which is very close to the perfect layer-by-layer limit with DIP crystallites forming over the entire film thickness. An unexpected increase in the island size is observed for these mixtures as a function of film thickness. On the other hand, equimolar and C60 dominated mixtures grow with poor crystallinity but form very smooth films. Additionally, it is observed that higher substrate temperatures lead to an increase in the length scale of phase separation with film thickness.

New insight of molecular interaction, crystallization and phase separation in higher performance small molecular solar cells via solvent vapor annealing

Solvent vapor annealing (SVA) studies on the morphology and performance of a porphyrin-based deep-absorption organic solar cells consisting of a strongly segregated bulk heterojunction (BHJ) blend, are presented. It is seen that the solvent vapor annealing of a well-mixed BHJ blends induces molecular motion, leading to a phase separated morphology governed by a spinodal decomposition mechanism. The earlier stage of solvent vapor swelling (<10 s) led to an obvious phase separation but not device performance. The device performance showed a dramatic increase in short circuit current and fill factor between 15 and 20 s of SVA. Thus, phase purity is a critical parameter in determining the performance of this binary blend. SVA on a thermally annealed BHJ thin film showed two distinctive processes, a crystal dissolution and a recrystallization, accompanied by phase mixing and then phase separation. The final morphology of SVA films that were initially thermally annealed showed a reduced length scale of phase separation, in comparison to SVA on as-cast films. Thus preformed donor crystallites appear to lock-in the morphology, even in a small molecule blend setting. The best performing device was obtained by a slight SVA (10 s) of films that were initially thermally annealed, reaching a power conversion efficiency of 8.48%. This suggests that the localized morphological optimization and domain size reduction are most important factors in dictating organic photovoltaic device efficiencies.

Influence of nano-scale morphology on impact toughness of epoxy blends

Abstract The microstructure and ballistic impact toughness was investigated for a series of dynamically heterogeneous epoxy blends composed of diglycidyl ether of bisphenol A (DGEBA) that is cross-linked with stoichiometric mixtures of a rigid cycloaliphatic diamine and flexible polypropylene oxide based diamines. The ballistic impact resistance of the blends correlates with the presence of nano-scale structure, where blends that contain 2–5 nm domains exhibit nearly a doubling of impact toughness when compared to blends that exhibit large scale phase separation. The length scale of phase separation was manipulated by controlling the ratio of short chain to long chain propylene oxide diamines. At low long chain content, the materials nanoscale phase separate into 2–5 nm domains, resulting in a transparent epoxy that exhibits relatively good impact toughness. At high long chain volume fractions, the mixtures undergo macroscale phase separation and lose both optical clarity and impact performance. Interestingly, some of the macro-phase separated blends with intermediate long chain polypropylene oxide diamine content still exhibit high impact resistance if a nanoscale structure is still present in the rigid continuous phase. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to characterize the phase separated structure in these blends. When the SAXS peak associated with nanoscale phase separation shifts to lower scattering vector and its intensity is reduced, the impact performance of the mixtures is likewise reduced. We hypothesize that the small scale structure in the blends facilitates rapid deformation during high rate ballistic impact events, which allows the material to absorb more energy before failure.

Subtle Balance Between Length Scale of Phase Separation and Domain Purification in Small-Molecule Bulk-Heterojunction Blends under Solvent Vapor Treatment.

A series of solvents with different solubilities for DR3TBDTT and PC71 BM, and different boiling points, is used for solvent vapor annealing (SVA) treatment to systematically investigate the solvent-morphology-performance relationship. The presence of solvent molecules inside bulk-heterojunction (BHJ) thin films promotes the mobility of both donor and acceptor molecules, leading to crystallization and aggregation, which are important in modulating morphology.

Controlling phase separation in La5/8−yPryCa3/8MnO3 (y = 0.45) epitaxial thin films by strain disorder

Present study reveals that the length-scale of phase separation in La5/8−yPryCa3/8MnO3 thin films can be controlled by strain disorder invoked during the growth and relaxation process of film. Strain disorder provides an additional degree of freedom to tune colossal magnetoresistance. Magneto-transport measurements following cooling and heating in unequal fields protocol demonstrate that coherent strain stabilizes antiferromagnetic insulating phase, while strain disorder favors ferromagnetic metallic phase. Compared to bulk, antiferromagnetic-insulating phase freezes at lower temperatures in strain disordered films. Raman spectroscopy confirms the coexistence of charge-ordered-insulating and ferromagnetic-metallic phases which are structurally dissimilar and possess P21/m and R-3C like symmetries, respectively.

Controlling the Dominant Length Scale of Liquid–Liquid Phase Separation in Spin‐coated Organic Semiconductor Films

Organic electronic devices are often made by solution processing a multi‐component ink. During solution processing, for example, via spin coating, the solvent evaporates and the solid components deposit on the substrate. The morphology of this layer can range from well‐mixed to extensively phase separated. To optimize device performance, it is essential to control the degree and dominant length scale of phase separation. Currently, the mechanism of phase separation induced by solvent evaporation is poorly understood. It has been shown that length scales are influenced by spin speed, drying time, final layer thickness and the ratio between the solid components, but a complete experimental dataset and consistent theoretical understanding are lacking. In this contribution, in situ measurements during spin coating and a simple numerical model are used to understand the drying process. In addition, an advanced image analysis of transmission electron micrographs of films processed under a wide range of processing conditions is carried out. A normalized drying rate is proposed as the key parameter that controls the dominant length scale of phase separation.

Directed Polystyrene/Poly(methyl methacrylate) Phase Separation and Nanoparticle Ordering on Transparent Chemically Patterned Substrates

We investigate the surface-directed phase separation of spin-coated polystyrene/poly(methyl methacrylate) (PS/PMMA) blends on prepatterned octadecyltrichlorosilane (OTS)-glass substrates under various experimental conditions. As a result of tandem processes of spinodal decomposition and selective wetting of polymer components during spin-coating, low-energy OTS stripes and high-energy glass surfaces laterally arrange the phase-separated polymers according to the chemical pattern on the substrate. Optimal pattern replication was achieved when the length scale of phase separation, controlled via the polymer concentration of the spin-coating solution, matched the smallest feature dimension in a striped chemical pattern possessing two alternating distances between stripes. It was also shown that polymer blend patterns were most closely registered with the underlying substrate when the PS/PMMA composition ratio (30/70, w/w) matched the areal fraction of OTS on the glass surface (∼30%). The influence of solvents demonstrated that a solvent with a relatively low volatility, such toluene, was required for patterning so that domain feature sizes were able to coarsen to the size of the patterned features before film vitrification. As well, we showed that the technique and optimized conditions developed in this study could be applied to pattern photoluminescent CdS quantum dots into microscale arrays of parallel lines via spin-coating onto transparent OTS-glass substrates.

Hierarchical self-assembly in diblock copolypeptides of poly(γ-benzyl-l-glutamate) with poly poly(l-leucine) and poly(O-benzyl-l-tyrosine)

Block copolypeptides with their inherent nanometer length scale of phase separation, provide means of manipulating the type (α-helices, β-strands) and persistence of peptide secondary structures. Two such examples are employed based on the α-helical poly(γ-benzyl-l-glutamate) (PBLG) polypeptide as one block and poly(l-leucine) (α-helical) or poly(O-benzyl-l-tyrosine) (POBT) (β-strands) as the second block. Although both secondary structures are present in the copolypeptides the effect of nano-scale confinement is to induce folding in the POBT β-sheets and to maintain the defected α-helices of PBLG and PLEU with a limited lateral coherence.

The effect of the resultant microphase-separated structures of polymer matrices on the electro-optical properties of polymer dispersed liquid crystal films by Iniferter polymerization

Iniferter polymerization was employed to prepare polymer dispersed liquid crystal (PDLC) films and an additional photoinitiator was introduced to induce the phase separation of polymer matrices themselves on the process of preparing the PDLC. The effect of the polymerization kinetics and the resultant microphase-separated structures of polymer matrices on the electro-optical properties of PDLC films were studied. It was found that the bigger length scale of phase separation of polymer matrices induced strong light-scattering resulting in low ON-state transmittance. And faster polymerization kinetic induced higher threshold and saturation voltages.

Time-resolved electrostatic force microscopy of polymer solar cells

Blends of conjugated polymers with fullerenes, polymers, or nanocrystals make promising materials for low-cost photovoltaic applications. Different processing conditions affect the efficiencies of these solar cells by creating a variety of nanostructured morphologies, however, the relationship between film structure and device efficiency is not fully understood. We introduce time-resolved electrostatic force microscopy (EFM) as a means to measure photoexcited charge in polymer films with a resolution of 100 nm and 100 micros. These EFM measurements correlate well with the external quantum efficiencies measured for a series of polymer photodiodes, providing a direct link between local morphology, local optoelectronic properties and device performance. The data show that the domain centres account for the majority of the photoinduced charge collected in polyfluorene blend devices. These results underscore the importance of controlling not only the length scale of phase separation, but also the composition of the domains when optimizing nanostructured solar cells.