Grazing Incidence Wide-angle X-ray Scattering Research Articles

Crystallization of Poly(ethylene oxide) on the Surface of Aqueous Salt Solutions Studied by Grazing Incidence Wide-Angle X-ray Scattering.

We report the thin layer crystallization of high-molar mass poly(ethylene oxide) (PEO) on a liquid support using a 4 M K2CO3 aqueous solution as a subphase. Because of the Hofmeister effect, PEO does not dissolve and remains at the surface during compression on a Langmuir trough. The transition from the flat pancake conformation upon compression of the spread polymer film to an entangled monolayer results in a plateau region of the Langmuir isotherm. Using grazing incidence wide-angle X-ray scattering, the final crystallization of PEO was observed, and the crystal orientation was determined. The fold surface was (209̅), that is, the helix axis has a tilt angle of 2.9° to the normal vector of the water surface.

Alkyl Branching Position in Diketopyrrolopyrrole Polymers: Interplay between Fibrillar Morphology and Crystallinity and Their Effect on Photogeneration and Recombination in Bulk-Heterojunction Solar Cells

Diketopyrrolopyrrole (DPP)-based donor–acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP-based donor–acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (ECT) of the blend films lies exceptionally c...

Control and Characterization of Organic Solar Cell Morphology Through Variable-Pressure Solvent Vapor Annealing

Solvent vapor annealing (SVA) of organic photovoltaics (OPVs) has become an important postdeposition treatment, but current OPV SVA methods are difficult to reproduce and neither tunable nor scalable. Herein, it is shown that a variable-pressure solvent vapor annealing (VP-SVA) system can be used to reproducibly and tunably anneal OPV active layers to produce highly controlled film morphologies. We show that VP-SVA is useful not only for the well-studied P3HT:PC61BM model system, but also for modern OPV active layers based on nonfullerene acceptors. Phase separation and material crystallinity are precisely tuned by controlling the solvent vapor concentration used during the annealing process, as evidenced by photoinduced force microscopy (PiFM) and grazing incidence wide-angle X-ray scattering (GIWAXS). The results show that overannealing occurs at saturated solvent vapor pressures, highlighting the importance of the VP-SVA technique.

Highly twisted ladder-type backbone bearing perylene diimides for non-fullerene acceptors in organic solar cells

A ladder-type π-system PDIPY that bearing perylene diimide and pyrene units have been developed for constructing non-fullerene acceptors. The vinyl bridged PDIPY dimer PDIPY-V and polymer PPDIPY-V have been synthesized via bromonation and Stille coupling recation. PDIPY-V and PPDIPY-V exhibit intense and broad absorption within 400–650 nm that complement the absorption spectra of the narrow bandgap polymeric donors. DFT calculation indicated that PDIPY-V and PPDIPY-V hold highly twisted structure, which prevent the formation of π−π stacking and crystallization in the aggregated states. The grazing-incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM) investigations confirmed the amorphous nature of the pure films and the PDIPY-V/PPDIPY-V:PTB7-Th blend films. The PDIPY-V:PTB7-Th based photovoltaic device showed a poor performance due to the small LUMO offset of donor and acceptor. While the PPDIPY-V:PTB7-Th based device showed a high PCE of 5.20%, indicating the PDIPY derivatives are promising non-fullerene acceptors for polymer solar cells.

Two-Dimensional Halide Perovskites Incorporating Straight Chain Symmetric Diammonium Ions, (NH3C mH2 mNH3)(CH3NH3) n-1Pb nI3 n+1 ( m = 4-9; n = 1-4).

Low-dimensional halide perovskites have recently attracted intense interest as alternatives to the three-dimensional (3D) perovskites because of their greater tunability and higher environmental stability. Herein, we present the new homologous 2D series (NH3C mH2 mNH3)(CH3NH3) n-1Pb nI3 n+1 ( m = 4-9; n = 1-4), where m represents the carbon-chain number and n equals layer-thickness number. Multilayer ( n > 1) 2D perovskites incorporating diammonium cations were successfully synthesized by the solid-state grinding method for m = 4 and 6 and by the solution method for m = 7-9. Structural characterization by single-crystal X-ray diffraction for the m = 8 and m = 9 series ( n = 1-4) reveals that these compounds adopt the Cc space group for even n members and Pc for odd n members. The optical bandgaps are 2.15 eV for two-layer ( n = 2), 2.01 eV for three-layer ( n = 3), and 1.90 eV for four-layer ( n = 4). The materials exhibit excellent solution processability, and casting thin-films of the n = 3 members was successfully accomplished. The films show a clear tendency for the higher- m members to have preferred orientation on the glass substrate, with m = 8 exhibiting almost perfect vertical layer orientation and m = 9 displaying both vertical and parallel layer orientation, as confirmed by grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. The vertical layer orientation for the (NH3C8H16NH3)(CH3NH3)2Pb3I10 member results in the best thermal, light, and air stability within this series, thus showing excellent potential for solar cell applications.

Importance of Nucleation during Morphology Evolution of the Blade-Cast PffBT4T-2OD-Based Organic Solar Cells

Different temperatures, solvents, and additives are used as influencing parameters to drive molecular packing and phase separation for the development of the controlled evolution of nanostructures for organic solar cells (OSCs). The temperature-dependent aggregation (TDA) features of polymers are explored by investigating aggregation in solution and solid thin-film states using solution small-angle neutron scattering (SANS), soft/hard X-ray scattering, and transmission electron microscopy (TEM) characterizations. In situ grazing incidence wide angle X-ray scattering (GIWAXS) reveals that the nucleation process is highly significant and responsible for the ultimate film morphology. Processing conditions such as temperatures, solvents, and additives were used to influence the nucleation and evolution of film morphology. The nucleation process may improve the polymer packing and phase separation. It may translate into optimized multilength scale domains and efficient charge percolation pathways, a strong imp...

Elucidating the Failure Mechanisms of Perovskite Solar Cells in Humid Environments Using In Situ Grazing-Incidence Wide-Angle X-ray Scattering

The main downfall of perovskite solar cells is that they degrade rapidly when exposed to heat, light, and moisture. Although previous work has elucidated the decomposition pathways of lead halide perovskites, there has been little direct insight into how perovskite decomposition affects device performance. Therefore, in order to better understand this correlation, we performed in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements on methylammonium lead iodide solar cells. We show that the formation of hydrate phases is not the most important device degradation pathway; rather, as water penetrates the cell, the mobility of iodide ions increases, leading to corrosion of the metallic top contact. Furthermore, the work reveals a temperature dependence to the perovskite decomposition pathway, with higher temperatures suppressing the formation of both intermediates and byproducts. The work suggests that the rigorous exclusion of moisture and the design of corrosion-resistant electrodes may...

Effects of solvent vapor annealing on organic photovoltaics with a new type of solution-processable oligothiophene-based electronic donor material

We synthesized a pair of A–D–A–D–A (A = acceptor, D = donor unit) oligomers containing a benzothiadiazole (BT) unit, namely, DRCN5BT-HH and DRCN5BT-TT, which differs in the positions of alkyl side chains. Solution-processed bulk heterojunction solar cells consisting of PC61BM and DRCN5BT-HH gave a high Voc of 1.10 V and markedly improved power conversion efficiencies up to 3.26% after solvent vapor annealing (SVA). The effects of SVA on the morphology and microstructure of the active layer were investigated by atomic force microscopy (AFM) and grazing-incidence wide angle X-ray scattering (GIWAXS). For DRCN5BT-HH, crystallization, orientation, and molecular ordering were improved, and a fibrillar structure was formed with acicular aggregates of the donor. For DRCN5BT-TT, no marked structural changes were induced by SVA. We found that these morphological and microstructural changes induced by SVA correlate with the differences in the photovoltaic properties of both of the isomers.

Synthesis of Polythiophene Derivatives with Oligoisobutylene Side Chains

In recent years, organic electronics including organic light emitting diode, organic photovoltaic cell and organic thin film transistor have received much attention due to their severals advantages based on organic materials, such as mechanical flexibility and solution-based low-cost processability. π-Conjugated polymers are known as mechanically soft and flexible material in organic semiconductors. However, their durability for stretched stress is not enough for practical application. Therefore, the molecular design is required for enabling stretchable organic semiconductor. The low strethcability of π-conjugated polymer is due to high crystallinity which induced their stiff main chain and strong intermolecular interactions. Here, we developed novel polythiophene derivatives, P3IBTs, which contain oligoisobutylene units in their side chains. We expect that several branched points on side chains weaken π-π stacking interaction and decrease crystallinity of the polymers. In addition, we tried to develop cross-linked P3IBT to realize the formation of an inherently stretchable π-conjugated polymer. We prepared the monomer of polythiophene derivatives with diisobutylene and triisobutylene side chains according to Scheme 1. The oligoisobutylene unit was synthesized based on a carbocationic addition reaction. Finally, polythiophene derivatives with diisobutylene sidechain (P3IB2T) (M n = 12,000, M w/M n = 1.17) and triisobutylene side chain (P3IB3T) (M n = 13,000, M w/M n = 1.21) could be synthesized by Kumada catalyst-transfer polycondensation. P3IB2T is dark red solid and P3IB3T is dark red chewing-gum like solid at room temperature. Subsequently, we characterized their thermal, optical properties and crystalline nature compared with poly(3-hexylthiophene)(P3HT) (M n = 10,000, M w/M n = 1.17). The thermal properties of the polymers were investigated by differential scanning calorimetry (DSC). The melting temperature of P3IB2T (T m = 130 ˚C) is lower than P3HT (T m = 242 ˚C). On the other hand, there are neigher endothermic nor exothermic peaks in the DSC curve of P3IB3T. Therefore, this polymer would be more amorphous-like than P3IB2T. These result suggests that oligoisobutylene side chains decrease crystallinity of polymers. The absorption spectrum of P3IB2T film showed lower absorption intensity around 620 nm than the P3HT film, indicating weaker π-π stacking interaction. Moreover, P3IB3T films showed the blue-shifted absorption spectrum compared to P3HT and P3IB2T ones probably due to larger torsion of main chains affected by triisobutylene side chain. Next, the crystalline nature was evaluated by Grazing incidence wide-angle x-ray scattering (GIWAXS). P3IB2T and P3IB3T films showed edge-on orientation similar to the P3HT ones, judged from the azimuth plots of (200) diffraction patterns. Moreover, (010) diffraction patterns were not observed for P3IB2T and P3IB3T films, probably due to less π-π stacking interaction by oligoisobutylene side chains. Finally, we cross-linked P3IB3T to realize the formation of an inherently stretchable p-conjugated polymer. 4,4’-Methylenebis(2,6-bis(methoxymethyl)phenol) and isopropyl p-toluenesulfonate were used as cross-linker and thermal acid generator, respectively. The cross-linking reaction was proceeded by heating at 200 ˚C for 3 h. The mechanical properties of the cross-linked P3IB3T was investigated by the tensile test using a TMA instrument. As a result, P3IB3T showed much lower tensile strength (0.68 MPa) and higher elongation at break (141%) than P3HT (984 MPa, 59 %). In conclusion, we have synthesized polythiophene derivatives with oligoisobutylene side chains. These polymer showed lower crystallinity and weaker π-π stacking interaction than P3HT. These properties might be caused by the effect of several branch points on oligoisobutylene side chains. In addition, the cross-linked P3IB3T film could be formed, possessing good mechanical durability. Figure 1

Directing the Aggregation of Native Polythiophene during in Situ Polymerization.

The performance of semiconducting polymers strongly depends on their intra- and intermolecular electronic interactions. Therefore, the morphology and particularly crystallinity and crystal structure play a crucial role in enabling a sufficient overlap between the orbitals of neighboring polymers. A new solution-based in situ polymerization for the fabrication of native polythiophene thin films is presented, which exploits the film formation process to influence the polymer crystal structure in the resulting thin films. The synthesis of the insoluble polythiophene is based on an oxidative reaction in which the oxidizing agent, iron(III) p-toluenesulfonate (FeTos), initially oxidizes the monomers to enable the polymer chain growth and secondly the final polymers, thereby chemically doping the polythiophene. To exploit the fact that the doped polythiophene has a different crystal packing structure compared to the undoped polythiophene, we investigate the structural effect of this inherent doping process by varying the amounts of FeTos in the reaction mixture, creating polythiophene thin films with different degrees of doping. The structural investigation performed by means of grazing incidence wide-angle X-ray scattering (GIWAXS) suggests that the strongly doped polymer chains aggregate in a π-stacked manner in the film formation process. Moreover, this π-stacking can be maintained after the removal of the dopant molecules. GIWAXS measurements, molecular dynamics simulations, and spectroscopic analysis suggest the presence of polythiophene in a novel and stable crystal structure with an enhanced intermolecular interaction.

Processing Methods for Obtaining a Face-On Crystalline Domain Orientation in Conjugated Polymer-Based Photovoltaics

The polymer chain orientation and degree of crystallinity within a polymer:fullerene bulk heterojunction (BHJ) photovoltaic can greatly impact device performance. In general, a face-on chain orientation is preferred for charge conduction through sandwich-structure photovoltaic devices, but for many conjugated polymers, an edge-on conformation is energetically favored. In this work, we examine the effects of different processing techniques on photovoltaics based on the poly[4,8-bis(2-ethylhexyloxy)-benzol[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxy-1-one)thieno [3,4-b]thiophene-2,6-diyl] (PBDTTT-C):[6,6]-phenyl-C71-butyric-acid-methylester (PC71BM) materials combination. We examine the extent of polymer crystallinity and crystalline domain orientation using both traditional blend-casting (BC), where the polymer and fullerene are cast from a single, codissolved solution, as well as sequential processing (SqP), where the polymer film is deposited first, and then the fullerene is infiltrated into the polymer film in a second solution processing step. We show using two-dimensional grazing-incidence wide-angle X-ray scattering (GIWAXS) that BC leads to a disordered, isotropic polymer network in the resulting BHJ film with a correspondingly poor device efficiency. By contrast, SqP preserves the preferred face-on chain orientation seen in pure polymer films, yielding higher short-circuit currents that are consistent with the increased hole mobility of face-on oriented polymer chains. We also study the effects of the widely used processing additive 1,8-diiodooctane (DIO) on polymer chain orientation and crystallinity in photovoltaic devices made by both processing techniques. We show that DIO results in increased polymer crystallinity, and in devices made by BC, DIO also causes a partial recovery of the face-on PBDTTT-C domain orientation, improving device performance. The face-on chain orientation in SqP devices produces efficiencies similar to those of optimized BC devices made with DIO but without the need for solvent additives or other postprocessing steps.

In Situ Analysis of Solvent and Additive Effects on Film Morphology Evolution in Spin‐Cast Small‐Molecule and Polymer Photovoltaic Materials

AbstractTo elucidate the details of film morphology/order evolution during spin‐coating, solvent and additive effects are systematically investigated for three representative organic solar cell (OSC) active layer materials using combined in situ grazing incidence wide angle x‐ray scattering (GIWAXS) and optical reflectance. Two archetypical semiconducting donor (p‐type) polymers, P3HT and PTB7, and semiconducting donor small‐molecule, p‐DTS(FBTTh2)2 are studied using three neat solvents (chloroform, chlorobenzene, 1,2‐dichlorobenzene) and four processing additives (1‐chloronaphthalene, diphenyl ether, 1,8‐diiodooctane, and 1,6‐diiodohexane). In situ GIWAXS identifies several trends: 1) for neat solvents, rapid crystallization occurs that risks kinetically locking the material into multiple crystal structures or crystalline orientations; and 2) for solvent + additive processed films, morphology evolution involves sequential transformations on timescales ranging from seconds to hours, with key divergences dependent on additive/semiconductor molecular interactions. When π‐planes dominate the additive/semiconductor interactions, both polymers and small molecule films follow similar evolutions, completing in 1–5 min. When side chains dominate the additive/semiconductor interactions, polymer film maturation times are up to 9 h, while initial crystallization times <10 s are observed for small‐molecule films. This study offers guiding information on OSC donor intermediate morphologies, evolution timescales, and divergent evolutions that can inform OSC manufacture.

Fabrication and characterization of Ba3Zn2Fe24O41 (Zn2Z) hexaferrites films on silicon substrates

We report the fabrication and characterization of Ba3Zn2Fe24O41 (Zn2Z) hexaferrite films on silicon substrates using a simple sol-gel drop casting method. A combination of X-ray diffraction (XRD), scanning electron microscope (SEM), grazing incidence wide angle X-ray scattering (GIWAXS) and vibrating sample magnetometer (VSM) was used to characterize the structural and magnetic properties of the films. The XRD results confirmed the formation of the Z-type hexaferrite with some ZnFe2O4 and α-Fe2O3 phases. SEM images showed several micron-size platelet-like hexagonal crystals with nanocrystals grown onto their surfaces. Interestingly, a few regions of the film exhibit self-assembly of the nanocrystals in ordered patterns. GIWAXS measurements indicated that the crystals are not randomly oriented within the film. The magnetic results revealed typical characteristics of a soft magnetic material consistent with the Z-type characteristics. The magnetization did not reach saturation up to the maximum applied field of 6 kOe. The saturation magnetization obtained with the field applied perpendicular to the film plane was found to be lower than that for a parallel geometry.

Efficient ternary polymer solar cells with a shelf-life stability for longer than 410 days

We have prepared an efficient ternary polymer solar cell incorporating a poly(indacenodithiophene)-based conjugated polymer (PIDTBT), [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2´,3´-d´]-s-indaceno[1,2-b:5,6-b´]-dithiophene (ITIC). Grazing-incidence wide-angle X-ray scattering (GIWAXS), atomic force microscopy (AFM), and photoluminescence (PL) spectroscopy revealed that intermolecular interactions between the PC71BM and ITIC units disrupted the formation of large ITIC crystals. The ITIC and PC71BM components formed compatible domains that dispersed well within the PIDTBT matrix, providing optimized ternary blends for efficient carrier transport and led to greater photon-to-electron conversion efficiency. Compared with the pre-optimized PC71BM binary device, the ternary device displayed an improvement in short circuit current density (Jsc) from 12.0 ± 0.3 to 14.2 ± 0.5 mA cm–2, due primarily to complementary light harvesting in the visible and near-infrared regions; as a result, the device performance improved by 20%—from 5.5 ± 0.2 to 6.6 ± 0.1% under AM 1.5G (100 mW cm–2) irradiation. Furthermore, the ternary cell exhibited outstanding long-term stability, with its performance remaining high (at 6.8%) after storage for 410 days in a glove box (ISOS-D-1 (shelf lifetime)).

Every Atom Counts: Elucidating the Fundamental Impact of Structural Change in Conjugated Polymers for Organic Photovoltaics

As many conjugated polymer-based organic photovoltaic (OPV) materials provide substantial solar power conversion efficiencies (as high as 13%), it is important to develop a deeper understanding of how the primary repeat unit structures impact device performance. In this work, we have varied the group 14 atom (C, Si, Ge) at the center of a bithiophene fused ring to elucidate the impact of a minimal repeat unit structure change on the optical, transport, and morphological properties, which ultimately control device performance. Careful polymerization and polymer purification produced three “one-atom change” donor–acceptor conjugated alternating copolymers with similar molecular weights and dispersities. DFT calculation, absorption spectroscopy, and high-temperature solution 1H nuclear magnetic resonance (NMR) results indicate that poly(dithienosilole-alt-thienopyrrolodione), P(DTS-TPD), and poly(dithienogermole-alt-thienopyrrolodione), P(DTG-TPD) exhibit different rotational conformations when compared to poly(cyclopentadithiophene-alt-thienopyrrolodione), P(DTC-TPD). Solid-state 1H MAS NMR experiments reveal that the greater probability of the anticonformation in P(DTS-TPD) and P(DTG-TPD) prevail in the solid phase. The conformational variation seen in solution and solid-state NMR in turn affects the polymer stacking and intermolecular interaction. Two-dimension 1H-1H DQ-SQ NMR correlation spectra shows aromatic–aromatic correlations for P(DTS-TPD) and P(DTG-TPD), which on the other hand is absent for P(DTC-TPD). In a thin-film interchain packing study using grazing incidence wide-angle X-ray scattering (GIWAXS), we observe the π-face of the conjugated backbones of P(DTC-TPD) aligned edge-on to the substrate, whereas in contrast the π-faces of P(DTS-TPD) and P(DTG-TPD) align parallel to the surface. These differences in polymer conformations and backbone orientations lead to variations in the OPV performance of blends with the fullerene PC71BM, with the device containing P(DTC-TPD):PCBM having a lower fill factor and a lower power conversion efficiency. Ultrafast transient absorption spectroscopy shows the P(DTC-TPD):PCBM blend to have a more pronounced triplet formation from bimolecular recombination of initially separated charges. With a combination of sub-bandgap external quantum efficiency measurements and DFT calculations, we present evidence that the greater charge recombination loss is the result of a lower lying triplet energy level for P(DTC-TPD), leading to a higher rate of recombination and lower OPV device performance. Importantly, this study ties ultimate photovoltaic performance to morphological features in the active films that are induced from the processing solution and are a result of minimal one-atom differences in polymer repeat unit structure.

Thin Film Ion Transport and Morphology of Poly(ethylene oxide) and Lithium Salt Mixtures

With the ability to tune ion transport and mechanical properties, polymer electrolytes have demonstrated promise as solid electrolyte materials for lithium-metal anode batteries. In particular, polyethylene oxide (PEO) and lithium salt mixtures have been the most widely studied polymer electrolyte system.1 Moreover, block copolymer electrolytes (e.g. block polystyrene-block-PEO) have yielded more advanced nanostructured architectures with increased mechanical robustness and efficient ion transport.2 Driven by their potential applications, enormous efforts have been devoted to elucidating mechanisms for ion transport and the formation of lithium dendrites. However, majority of reported studies have focused only on thick bulk samples (100’s of microns thick films). This is significant to note as interfaces3 play a critical role in the performance of batteries, but researchers have mostly inferred interfacial effects using bulk properties. As a consequence, we focus on investigating polymer electrolytes in the (ultra)-thin film regime (5-500 nm), which can be used as a platform to directly study fundamental interfacial effects. In addition, structural characteristics limiting charge transport in polymer electrolytes such as domain orientation and interconnectivity can be more easily probed using thin films.4 Ultimately, investigating polymer electrolyte in the context of thin films will lead to better fundamental understanding of charge transport in polymer electrolytes and the charge transfer limitations at the electrode/electrolyte interfaces. Here, we report on our initial study on charge transport and morphology of a model polymer electrolyte system (mixtures of PEO and LiTFSI, lithium bis(trifluoromethanesulfonyl)imide)) in the thin film regime (<200 nm). We specifically probe the structure-property relationships as function of salt concentration, temperature, and film thickness. We utilized synchrotron-based grazing-incidence wide angle X-ray scattering (GIWAXS) to probe the effects of salt concentration and film thickness on the semicrystalline morphology. At room temperature, the PEO crystallites become more ordered, but the relative degree of crystallinity (rDoC) of PEO gradually decreases with increasing salt concentration. The film becomes completely amorphous at the highest salt concentration r = 0.15, where r is the molar ratio of lithium ions to ethylene oxide repeat units. This trend is consistent with the established theories that adding Li salt disrupts the crystallite structure of PEO and reduces the overall degree of crystallinity. Ionic transport measurements were performed using a.c. impedance spectroscopy on PEO-LiTFSI thin films on custom-designed interdigitated electrode devices (IDE). Above the melting point of PEO (≈ º60 C), the temperature dependence of ionic conductivity of all samples can be well-described using Vogel-Tammann-Fulcher (VTF) model. This suggests that segmental motion of PEO chains facilitate ionic transport in PEO-LiTFSI thin films. Ionic conductivity is found to increase with increasing salt concentration at first but decreases at high salt concentration. This is likely due to the reduced dissociation rate and increased glass transition temperature at high concentration, similar to the behavior of bulk samples.1,2 Our results have demonstrated the first successful fabrication and characterization of ionic transport in PEO-Li salt thin films. This provides an important step toward exploiting thin film configuration to reveal many morphological and interfacial effects on ion conduction mechanism of polymer electrolytes and will be subject of investigations in our future studies. REFERENCES S. Lascaud et al., Macromolecules, 27, 7469–7477 (1994).D. T. Hallinan and N. P. Balsara, Annu. Rev. Mater. Res., 43, 503–525 (2013) http://www.annualreviews.org/doi/abs/10.1146/annurev-matsci-071312-121705.E. Peled and S. Menkin, J. Electrochem. Soc., 164, A1703–A1719 (2017).Y. Kambe, C. G. Arges, S. N. Patel, M. P. Stoykovich, and P. F. Nealey, ECS Interface (2017).

Molecular engineering of perylene-diimide-based polymer acceptors containing heteroacene units for all-polymer solar cells

Abstract Polymer acceptors based on perylene diimide (PDI) with three symmetrical S-heteroacene backbone units of different sizes were synthesized for use in all-polymer solar cells (all-PSCs). The effects of varying the size of the heteroacene unit on the backbone of the PDI polymer are evident in the absorption spectra and their energy level offsets, which are correlated with their morphological and photovoltaic properties. These newly synthesized polymers were employed as acceptors with the polymer poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)] (PPDT2FBT) as the donor in all-PSCs that were found to exhibit modest power conversion efficiencies. The variations in photovoltaic properties of all-PSCs are investigated by characterizing the charge generation, carrier mobilities and recombination. The morphological disorder at the polymer/PPDT2FBT interface and average composition variations are revealed by using grazing incidence wide angle X-ray scattering (GIWAXS) and resonance soft X-ray scattering (R-SoXS) characterizations.

Thermal transitions in semi-crystalline polymer thin films studied via spectral reflectance

Herein we report the finding that spectral reflectance (SR) can be used to identify thermal transitions in semi-crystalline polymer thin films. By studying film thickness as a function of temperature, we found that semi-crystalline polymers exhibit characteristic expansion and contraction profiles during the melting (Tm) and crystallization (Tc) transitions, respectively. Prior to this discovery, studies on the crystalline Tm in thin films have involved more expensive and complex techniques such as atomic force microscopy (AFM), ellipsometry, and grazing-incidence wide-angle X-ray scattering (GIWAXS). We correlate Tm and Tc as measured by SR with differential scanning calorimetry (DSC) in the bulk and temperature-controlled GIWAXS or AFM in thin films. We show that SR is accurate for measuring changes in films with thicknesses of 500 nm down to 21 nm and for detecting melting point depression due to thin film confinement. Thus, we demonstrate that SR is a powerful tool for measuring thermal transitions in semi-crystalline polymer films with single-degree resolution.

Main-chain/side-chain liquid crystalline polymer based on “jacketing” effect with different-length tails: Phase structures and spontaneous oriented behavior in film

Based on side-chain “Jacketing” effect, a series of main-chain/side-chain liquid crystalline polymers (MCSCLCPs) with different lengths of alkyl tail, poly (2,5-bis {[6-(4-alkoxy-4′-oxybiphenyl)-6-hexyl] oxycarbonyl} styrene) (denoted as Pm, m represents the number of carbons in the alkyl tails and m = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.) were synthesized through radical polymerization. Their phase structures were investigated using combined techniques, such as polarized optical microscope (POM), differential scanning calorimetry (DSC), one- and two-dimensional wide-angle X-ray diffraction (1D- and 2D-WAXD) and grazing-incidence wide angle X-ray scattering (GI-WAXS). It has been identified that the phase structure of Pms is strongly dependent on the length of alkyl tail. Pms (m ≤ 4) with short alkyl tail exhibit double ordered structure on the nanometer and subnanometer scales. Pms (m ≥ 5) with lengthened alkyl tail form typical smectic C phase. Furthermore, GI-WAXS result reveals that both smectic and hierarchically ordered structure can form preferred orientation on the naked silicon wafer because of the homeotropic property of the side-chain mesogens.

Structure, nanomechanics, and dynamics of dispersed surfactant‐free clay nanocomposite films

The current work presents a new approach to achieve high quality dispersion of surfactant‐free nanoclay tactoid particles in sub‐micron thin films despite the absence of organic modifier. Natural Montmorillonite particles, Cloisite, were dispersed in thin films of polycaprolactone (PCL) through a flow coating technique assisted by ultra‐sonication. Wide‐angle X‐ray scattering (WAXS), grazing‐incidence wide‐angle X‐ray scattering (GI‐WAXS), and transmission electron microscopy (TEM) were used to confirm the level of natural clay dispersion down to the level of tactoids (sub‐micron scale stacks of clay sheets). These characterization techniques were carried out in conjunction with an analysis of nanomechanical properties via strain‐induced buckling instability for modulus measurements (SIEBIMM), a high‐throughput technique to characterize thin film mechanical properties. The buckling patterns indicate that the natural clay tactoids separate buckling‐enhancing (high‐modulus) crystalline regions and interconnect buckling‐suppressing amorphous (low‐modulus) regions. Due to the tactoid length scale, the glass transition behavior of the composites as characterized by broadband dielectric relaxation spectroscopy was unmodified by the clay. Likewise, the glass transition temperature, Tg, and fragility (slope of relaxation time behavior approaching Tg), remain unaffected, indicating that these dispersed tactoids do not induce pronounced confinement effects on dynamics. POLYM. ENG. SCI., 58:1285–1295, 2018. © 2018 Society of Plastics Engineers