Nanometer-scale Phase Separation Research Articles

Recent Advance in the Development of Singlet-Fission-Capable Polymeric Materials.

Singlet fission (SF) is a spin-allowed process in which a higher-energy singlet exciton is converted into two lower-energy triplet excitons via a triplet pair intermediate state. Implementing SF in photovoltaic devices holds the potential to exceed the Shockley-Queisser limit of conventional single-junction solar cells. Although great progress has been made in exploiting the underlying mechanism of SF over the past decades, the scope of materials capable of SF, particularly polymeric materials, remains poor. SF-capable polymer is one of the most potential candidates in the implementation of SF into devices due to their distinct superiorities in flexibility, solution processability and self-assembly behavior. Notably, recent advancements have demonstrated high-performance SF in isolated donor-acceptor (D-A) copolymer chains. This review provides an overview of recent progress in the development of SF-capable polymeric materials, with a significant focus on elucidating the mechanisms of SF in polymers and optimizing the design strategies for SF-capable polymers. Additionally, the paper discusses the challenges encountered in this field and presents future perspectives. It is expected that this comprehensive review will offer valuable insights into the design of novel SF-capable polymeric materials, further advancing the potential for SF implementation in photovoltaic devices.

Development of High-Efficient Ion Selective Membrane Separator for Lithium Polysulfide Redox Flow Batteries

As our society demands greater utilization of intermittent renewable energies from solar and wind, the development of a low cost, reliable energy storage technology with high energy density is of great interest. In particular, highly efficient large-scale electrochemical energy storage has been an important issue to enhance the efficiency and quality of electrical grid. Among many energy storage systems, lithium-polysulfide (Li-PS) redox flow batteries (RFBs) have been considered one of the most promising electrochemical storage systems because they can offer the high energy density of the Li-S batteries and the general features of RFBs (e.g. flexible system design from decoupled energy storage, a power generation, safer operation, long cycle life) simultaneously. To successfully adopt Li-PS RFBs for a large scale electrochemical storage technology, it is imperative that the membrane separator should prevent the crossover of redox-active materials between the electrodes which would otherwise result in low columbic efficiency, rapid capacity fading and poor cycle life. Recently, Bae group demonstrated that aromatic polymers based on biphenyl backbones have excellent chemical and mechanical stability and prevent crossover of redox-active materials for RFBs. In this work, we will present new, multifunctional, highly ion selective, biphenyl polymer (BPSA) with high ionic transport properties (Figure 1). The unique design of this polymer involves (1) a lithiated sulfonate group, (2) tunable hydrophobic polymer backbone, and (3) a hydrocarbon linker between them. The covalent connection of these two distinctively different moieties would lead to nanometer-scale phase separation between hydrophilic ion conducting channels (via aggregation of sulfonated groups) and hydrophobic polymer backbone domains. The BPSA membranes demonstrated resistance in swelling in the commonly used Li-S RFB solvent solution dioxolane/dimethoxyethane (15% in x-y direction) and achieved good mechanical properties (tensile strength ≥20 MPa, percentage elongation ≥50). In an effort to decrease the areal resistance and increase lithium conductivity of the polymer membranes, a variety of reinforcement materials such as poly(ethylene) (PE), polytetrafluoroethylene (PTFE), and Celgard were coated with a thin layer of BPSA. The low-cost, chemically inert, reinforcing materials provide additional mechanical strength and reduce the swelling ratio of BPSA, lowering the permeation rate of the polysulfide anions. Performance of these hydrocarbon cation exchange membranes in Li-S RFB with respect to the separation of polysulfides and conductivity are compared to the state-of-the-art cation exchange membrane Nafion®, and will be discussed. A brief description of battery performance carried out by UIC will follow. Figure 1

Unraveling the Morphology in Solution-Processed Pseudo-Bilayer Planar Heterojunction Organic Solar Cells.

The conventional bulk heterojunction (BHJ) structure is widely used for fabricating high-performance organic solar cells (OSCs) due to the nanometer-scale phase separation of the donor/acceptor component. However, the elaborate control of the BHJ morphology is difficult to carry out because the morphology evolution is such a complicated process. The compatibility requirement of materials in the same solvent restricts the structural diversity of the molecules to some extent. Meanwhile, the nanoscopic interpenetrating donor/acceptor domains reduce their crystallinity. The bilayer planar heterojunction (PHJ), by contrast, possesses complementary advantages that can make it an alternative candidate to achieve device fabrication and produce different vertical stratification in heterojunction films. However, the flat contact area limits the charge separation and transmission efficiency. The sequential solution processed approach was used to facilitate material diffusion in layers. Also, solvent additives were employed to further enhance the diffusion and thus the device performance. Nevertheless, the morphology of the formed pseudo-bilayer planar heterojunction (PPHJ) has not been fully revealed yet. Here, we carefully study the morphology of the nonfullerene-based PPHJ device in three dimensions. High hole mobility of 2.09 × 10-4 cm2 V-1 s-1 and electron mobility of 7.91 × 10-5 cm2 V-1 s-1 were obtained in the solution-processed PPHJ device. Meanwhile, a distinct phase separation size with a vertical rearrangement of donor and acceptor was observed, which enable the pseudo-bilayer devices to be equipped with a comparable spectral response to the BHJ devices. We demonstrate that a unique device architecture (ITO/ZnO/PBDB-T/ITIC/MoO3/Ag) with a power conversion efficiency of 7% can be obtained from a larger molecular weight of PBDB-T without using extra additives. The solution-processed PPHJ films have much in common with the BHJ films. The results proposed that with appropriate molecular design and vertical phase separation optimization, the performance of the solution-processed PPHJ-based OSCs can be further improved.

Nanometer-scale phase separation in Al60Ge30Mn10 amorphous alloy

Nanometer-scale phase separation has been explored in an Al60Ge30Mn10 amorphous alloy, where all atomic pairs have negative heats of mixing. Here we use a combination of experimental (X-ray diffraction, X-ray absorption fine structure, small angle X-ray scattering and transmission electron microscopy) and simulation (Ab initio molecular dynamics and reverse Monte Carlo) techniques to resolve the atomic- and micrometer-level structures in the phase-separated Al60Ge30Mn10 amorphous alloy. Three characteristic peaks appear in the structure factor, linking with three kinds of composition fluctuation regions with different heterogeneous structures. The origin of three characteristic peaks is investigated in details and attributed to different atomic pairs. By analyzing the local chemical ordering between main polyhedra, we find that various phases exhibit differences in atomic structure, composition and bond-orientational order. The results obtained here provide atomic-scale understanding of the nanometer-scale phase separation phenomenon in Al60Ge30Mn10 amorphous alloy.

Nanometer-scale phase separation and formation of delta ZrH2 in Cu-Zr binary amorphous alloys

Different Cu-Zr alloys were hydrogenated under 100 bars of hydrogen pressure at different temperatures. The hydrogenation induced transformation of the initially amorphous phase into a polycrystalline structure characterized by its nanoscale (<5 nm), mostly composed of metallic Cu and ZrH2. Calorimetry measurements after hydrogenation showed a low temperature exothermic transformation occurring in the alloys hydrogenated at temperatures below 473 K, followed by multiple endothermic peaks at higher temperature attributed to dehydrogenation of different hydride phases. Activation barrier energies of the phase transformation were derived from Kissinger's method, and further characterization involving transmission electron microscopy revealed the existence of delta ZrH2 with cubic structure contrasting with the typical epsilon ZrH2 with tetragonal structure. This study evidences the effect of hydrogen pressure, temperature, and the alloy chemistry on the nature of the hydride formation in Cu-Zr binary amorphous alloys during the hydrogenation procedure.

Role of Curvature to Produce Modulated Patterns of Lo + Ld Phases on GUV Surfaces

GUV studies of the three component lipid system DSPC/DOPC/CHOL show macroscopic phase separation of Ld + Lo phase domains. In contrast, GUVs of DSPC/POPC/CHOL appear uniform in the Ld + Lo region, but show nanometer-scale phase separation in FRET and ESR studies. Examination of the four component system DSPC/DOPC/POPC/CHOL enables study of macroscopic-to-nanoscopic transition as composition (DOPC/POPC ratio) is changed. The transition is observed to be rather abrupt, and reveals “modulated phase” patterning on the GUV in a small window of compositions. The patterns show stripe, bubble, and honeycomb structures. Following Helfrich, we attempt to explain these patterns through the energetics of curvature. We perform calculations and simulations to determine whether a heterogeneous membrane can stabilize modulated phases, subject only to a line tension and a bending rigidity. We also explore the energetic stability, shape, and size scales of these patterns.

Conjugated polymers based on quinoxaline for polymer solar cells

Quinoxaline-based π-conjugated donor polymers with different substituent groups (phenyl, p-fluorophenyl, p-methoxyphenyl, methyl, or hydrogen) on the quinoxaline unit were synthesized and characterized, focusing on the effects of these substituent groups on the performance of bulk heterojunction (BHJ)-based polymer solar cells fabricated with phenyl C71 butyric acid methyl ester ([70]PCBM) as an acceptor. Polymers with phenyl groups or p-methoxyphenyl groups showed nanometer-scale phase separation or uniform morphology in BHJ film, together with high field effect transistor (FET) hole mobility. On the other hand, polymers with methyl or hydrogen and a polymer with fluorine exhibited micrometer-scale domains in BHJ and low hole mobility. It is suggested that the mostly steric hindrance of the substituents regulate the polymer's aggregation properties and compatibility with [70]PCBM, which probably affects the hole transport properties and BHJ morphologies. We also investigated the effects of substituent groups on highest occupied molecular orbital energy level and optical band gap of the polymers.

Transmission Electron Microscopy Evidence of Spontaneous B-Cation Layered Distribution in NaNb1−xTaxO3

A transmission electron microscopy (TEM) study of the complex NaNb(1-x)Ta(x)O(3) (0.4 < or = x < or = 0.6) perovskites, combining high-resolution TEM and high-angle annular dark-field scanning TEM, has revealed the formation of extended areas on the crystals where niobium and tantalum order into layers in a 1:1 ratio. NaNb(1-x)Ta(x)O(3) oxides are stoichiometric, and there is neither charge difference nor significant ionic size discrepancy between Nb(V) and Ta(V) cations. As d(0) octahedrally coordinated cations, they show a propensity to second-order Jahn-Teller distortion. This distortion, however, manifests itself to different extents for the two cations and is considered the driving force for the layered ordered distribution observed. The niobium-tantalum segregation we have found can also be interpreted as a naturally occurring nanometer-scale phase separation. Albeit occurring in wide regions of the crystals and not in the entire grains, it shows a clear trend toward a long-range ordered disposition. This is reminiscent of the more general behavior of a recently documented class of perovskites that suffer spontaneous nanoscale phase separation to form a superlattice.

Synthesis and photophysical property of well-defined donor–acceptor diblock copolymer based on regioregular poly(3-hexylthiophene) and fullerene

A new, well-defined diblock copolymer (P3HT-b-C60) based on regioregular poly(3-hexylthiophene) (P3HT) and fullerene was synthesized. First, regioregular P3HT was synthesized through Grignard metathesis polymerization, and then methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) were copolymerized by using an end-functionalized P3HT as a macroinitiator for the atom transfer radical polymerization to yield a diblock copolymer (P3HT-b-P(MMA-r-HEMA)). A fullerene derivative functionalized with carboxylic acid, [6,6]-phenyl-C61-butyric acid (PCBA), was then chemically linked to the HEMA unit in the second block (P(MMA-r-HEMA)) to produce a diblock copolymer with the second block containing fullerenes. Annealing thin films of the copolymer revealed nanometer-scale phase separation, a more suitable morphology for enabling excitons generated in the P3HT domain to more efficiently reach the donor–acceptor interface, relative to simple blends of P3HT and C60. As a result, photoluminescence of the P3HT-b-C60diblock copolymer in the films showed a complete quenching of photoluminescence of P3HT, which is indicative of charge transfer between P3HT and fullerene.

Photoinduced Electron Transfer and Photovoltaic Response of a MDMO‐PPV:TiO2 Bulk‐Heterojunction

Combining conjugated poylmers and TiO2 in hybrid bulk‐heterojunctions is a promising method for producing novel solar cells. Here TiO2 is introduced into a poly(p‐phenylene vinylene) layer. Nanometer‐scale phase separation allows efficient photoinduced charge transfer between the two components. The films can be used as the active layer in a photovoltaic cell and provide external quantum efficiencies of up to 11 % (see Figure).

Photovoltaic Performance and Morphology of Polyfluorene Blends: A Combined Microscopic and Photovoltaic Investigation

The interplay between phase separation in polyfluorene blends which show photoinduced charge transfer and photovoltaic performance in photodiodes has been investigated. Phase separation length scales have been varied from several microns to tens of nanometers by limiting the time allowed for solvent-enhanced self-organization through several different processing routes. Concurrent with the decrease in feature size, an increase in maximum photovoltaic efficiency of nearly 1 order of magnitude was observed in photodiodes incorporating the phase-separated blends as the active layer. The structure of the blend films was investigated using fluorescence microscopy, fluorescence scanning near-field optical microscopy, and atomic force microscopy. In some cases, a hierarchy of micron- and nanometer-scale phase separation was observed which may explain the unexpectedly high photoresponse in devices with up to micron-scale phase separation structure. This result along with in situ fluorescence microscopy studies of...

Self-organized Cu–Ag nanocomposites synthesized by intermediate temperature ion-beam mixing

We perform an investigation of ion-beam mixing in the immiscible system Ag–Cu based on cross-sectional transmission electron microscopy. Multilayered samples consisting of ten periods of (6.7 nm Cu/11.2 nm Ag) are irradiated at temperatures ranging from 25 to 225 °C with normally incident 1 MeV Kr ions to doses in the range 1–2×1016 ion/cm2, enough to reach a nonequilibrium dynamical steady state. Regardless of the irradiation temperature, extensive grain growth takes place. At intermediate temperatures, competition between thermal decomposition and irradiation mixing results in a nanometer-scale phase separation. This spontaneous decomposition demonstrates that ion-beam mixing can be used as a processing tool to synthesize nanocomposite materials.

High photosensitivity and nanometer-scale phase separation in GeO_2–SiO_2 glass thin films

Amorphous xGeO(2)-(1-x)SiO(2) thin films exhibit large negative index changes (4-8%) in the high GeO(2) region (x>~0.25) on irradiation with ArF laser pulses. The sign of the index change is opposite the low GeO(2) region X<0.25, and the magnitude of the index change is larger by an order of magnitude than that reported so far. Cross-sectional transmission electron microscope observation has revealed that nanometer-scale phase separation is induced in these highly photosensitive glasses by irradiation with ArF excimer laser light pulses or electron beams. This is a first finding of microphase separation in SiO(2)-GeO(2) glasses by irradiation and provides an essential constraint on the modeling of photonic effects induced by irradiation in these glasses.

Effect of Nanometer-Scale Phase Separation on Wetting of Binary Self-Assembled Thiol Monolayers on Au(111)

The advancing contact angle of water has been measured on three types of the binary self-assembled monolayers (SAMs) on Au(111) with known surface structure at nanometer scale: a homogeneously mixed SAM of undecanethiol (UDT) and 11-mercaptoundecanoic acid (MUDA) (1), a SAM of hexadecanethiol (HDT) and 3-mercaptopropionic acid (MPA) phase-separated into nanometer-scale domains with different heights (2), and a SAM of UDT and MUDA artificially phase-separated into nanometer-scale domains having approximately equal heights (3). The homogeneously mixed SAM, 1, shows a linear variation of the cosine of the contact angle, cos θ, with the surface mole fraction of COOH-terminated thiol, , while in the case of 2 the cos θ versus plot is strongly concave; the cos θ depends little on for 0.5. In contrast, the phase-separated SAM, 3, shows a linear variation of cos θ with , as is the case of 1, indicating that the contact angle does not reflect the phase separation in nanomet...

Nanometer-scale phase separation in mixed composition self-assembled monolayers

Mixed composition monolayers of similar n-alkanethiols on are formed by self-assembly. While the average surface composition of these films accurately reflects the composition of the deposition solution, scanning tunneling microscopy and secondary ion mass spectroscopy measurements show that the films phase separate on the nanometer scale. Scanning tunneling microscopy has been used to follow molecular motions within these films. We discuss our observations in terms of the formation and stability of the phase-segregated domains, and their potential importance in nanoscale applications.

Anisotropic phase separation through the metal-insulator transition in amorphous alloys.

The characterization of nanometer-scale phase separation in sputtered, amorphous, metal-Ge and Fe-Si films has led to the observation of a new microstructure that extends through the metal-insulator transition. The phase separated regions, which are dependent on deposition conditions, are well correlated and of the order of 1 nm in the growth plane but poorly correlated and 1.5-2.0 nm in the growth direction. The results suggest that fluctuations during film growth play a pivotal role in preventing anticipated columnar structures, probably leading to unusual percolation properties.