Simultaneous control over both molecular order and long-range alignment in films of the donor-acceptor copolymer.

Control over both molecular order and long-range alignment order in films of the donor-acceptor copolymer of 3,6-bis(thiophen-2-yl)-N,N'-bis(2-octyl-1-dodecyl)-1,4-dioxo-pyrrolo[3,4-c]pyrrole and thieno[3,2-b]thiophene (PDBT-TT) was demonstrated via off-center spin-coating (OCSC) from its blend solution with polystyrene (PS). It was found that the dichroic ratio (DR) of OCSC blend films was dependent on both the physical process of spin-coating and the effect of PS chains. The highest DR of 2.75 was obtained via OCSC from the blend solution in oDCB at 1500 rpm. Meanwhile, both the intrachain and interchain molecular order were improved in blend films compared with neat ones, which were indicated by the red-shift of the max absorption, enhanced J-aggregation absorption, and smaller π-π stacking distance (from 3.77 to 3.70 Å). According to the results of the investigation into the macro anisotropy, micro morphology, solution rheology properties, and photophysics features of films, an overall mechanism of simultaneous control over molecular and long-range order of D-A copolymer films was proposed. On the one hand, a larger viscosity and the pseudoplastic nature of the solution tuned by choosing good solvents with high boiling points and adding PS resulted in a better chain disentanglement, better shear transfer, and a slower contact line receding velocity to induce an enhanced alignment of chains and thus fibrillar aggregates. Also, the critical contact line receding velocity for alignment dominated by the solvent evaporation rate accounted for the variation of DR with OCSC rates. A vertical phase separation accompanying the formation of aligned fibrils during OCSC was also confirmed due to the friction shear between air and solution surface. On the other hand, the negligible dependence of the blend OCSC film's photophysical and morphological features on the solvent suggested the critical role of PS in determining the better intrachain conjugation in blend films, which was attributed to multiple reasons, like limited phase separation room, a coil-toward conformation promotion, and a high surface energy. Furthermore, the enhanced π-attraction and smaller steric hindrance induced by improved intrachain conjugation accounted for the smaller π-π stacking distance in the blend films than that in the neat ones.

Ordered fibrillar morphology of donor-acceptor conjugated copolymers at multiple scales via blending with flexible polymers and solvent vapor annealing: insight into photophysics and mechanism.

The ordered, aligned fibrillar morphology at multiple scales of a donor-acceptor (D-A) conjugated copolymer of 3,6-bis-(thiophen-2-yl)-N,N'-bis(2-octyl-1-dodecyl)-1,4-dioxo-pyrrolo[3,4-c]pyrrole and thieno[3,2-b]thiophene (PDBT-TT) was prepared via blending with flexible polymers (PS13.7k, PDBT-TT/PS = 1/10 w/w) followed by chloroform (CF) solvent vapor annealing (SVA) for 24 h. The aligned fibrillar bundles were of about 500 nm width, consisting of parallel aligned nanofibrils of ab. 10 nm width. It was found that the direction of backbones in nanofibrils was parallel to the long axis of nanofibrils, which implied an intense intra-chain conjugation associated with extended backbones and J-aggregation of PDBT-TT. This ordered morphology corresponded to the characteristic photophysical features of (i) red-shifted absorption arising from J-aggregation, (ii) larger Davydov splitting, (iii) the prevailing absorbance of J-aggregation over H-aggregation in its UV-Vis spectrum and (iv) more red-shifted max photoluminescence emission, compared with the films prepared via the other methods. By investigating the Raman spectra and XRD profiles, it is proposed that the origin of the best morphological and photophysical order is the combination of blending and SVA. The limited and "flexible" space formed due to phase separation between PDBT-TT and PS facilitated the motion of rigid PDBT-TT chains and promoted their stacking order as templates, and CF vapor assisted the conformational transition of chains to more "coil-like" to help them reorganize in a thermodynamic stable way.

Achieving balanced intermixed and pure crystalline phases in PDI-based non-fullerene organic solar cells via selective solvent additives.

Herein, balanced intermixed and pure crystalline phases in N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (EP-PDI)-based non-fullerene organic solar cells (OSCs) were achieved via selective solvent additives (SAs). Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(6-fluoro-4-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (F-DTS) possessing different compatibilities with EP-PDI were selected as model systems to investigate the guideline of SAs selection for different non-fullerene-based systems. According to the solubility parameter difference (Δδ) between EP-PDI and SAs, five different SAs were divided into two types: (I) strong intermolecular interactions with EP-PDI molecules (with Δδ values less than 5 MPa(1/2)), (II) weak intermolecular interactions with EP-PDI molecules (with large Δδ values). For PTB7:EP-PDI system with large and obvious phase separation, the introduction of type (II) SAs provided extra interactions with EP-PDI molecules, thus effectively reducing EP-PDI aggregate domains and increasing intermixed fractions. The incorporation of type (II) SAs resulted in a greater yield of dissociated polarons, and the final device efficiency increased from 0.02% to 1.65%. On the contrary, for finely mixed F-DTS:EP-PDI systems, type (I) SAs were considerably more effective because of the fact that the required pure crystalline phases were readily induced by the unfavorable interactions. The charge transport pathways optimized by type (I) SAs improved device efficiency from 0.18% to 2.82%. Hence, by processing selective SAs, the fraction of intermixed and pure crystalline phases for PDI-based non-fullerene OSCs can be well regulated; therefore, the final performance for both systems can be significantly improved.

Cooperative effects of solvent and polymer acceptor co-additives in P3HT:PDI solar cells: simultaneous optimization in lateral and vertical phase separation.

In this work, solvent chloronaphthalene (CN) and polymer acceptor an alternating copolymer of perylene diimide and carbazole (PCPDI) were utilized as co-additives to optimize the nanoscale phase-separated morphology and photovoltaic properties of bulk-heterojunction (BHJ) polymer solar cells based on the poly(3-hexyl thiophene) (P3HT)/N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (EP-PDI) system. The domain size of EP-PDI molecules together with that of P3HT distinctly decreased by adding a 0.75 vol% CN additive. The optimized lateral phase separation increased the donor-acceptor interfacial area and facilitated the exciton dissociation process, leading to 5-fold enhancement of short-circuit current (JSC). Furthermore, when PCPDI was employed as a co-additive, acceptor materials (including PCPDI and EP-PDI) were prone to aggregation towards the top surface of blend films, improving vertical phase separation of active layers. PCPDI incorporation, which improved the percolation pathways for electron carriers, suppressed the crystallinity of P3HT distinctly. Thus, much more balanced charge transport was achieved by PCPDI addition, which resulted in almost 1-fold enhancement of open-circuit voltage (VOC) by reducing nongeminate recombination. As a consequence, cooperative effects of CN and PCPDI additives improved the nanoscale phase-separated morphology in lateral and vertical directions simultaneously, achieving the enhancement in both VOC and JSC.

Uniaxial alignment of triisopropylsilylethynyl pentacene via zone-casting technique.

Uniaxially aligned triisopropylsilylethynyl pentacene (TIPS-pentacene) crystals over a large area were fabricated using zone-casting technique. The array of TIPS-pentacene displayed a high orientation degree with a dichroic ratio (DR) of 0.80. The crystals were arranged with c axis perpendicular to the substrate and the long axis of the ribbon corresponded to the a axis of TIPS-pentacene. The properties of the solutions and the processing parameters were shown to influence the formation of the oriented TIPS-pentacene crystalline array. Solvent with a low boiling point (such as chloroform) favoured the orientation of the ribbon-like crystals. The concentration of the solution should be appropriate, ensuring the crystallization velocity of TIPS-pentacene matching with the receding of the meniscus. Besides, we proved that the casting speed should be large enough to induce a sufficient concentration gradient. The orientation mechanism of TIPS-pentacene was attributed to a synergy of the ordered nuclei and a match between the crystallization velocity and the casting speed. Field effect transistors (FETs) based on the oriented TIPS-pentacene crystalline array showed a mobility of 0.67 cm(2) V(-1) s(-1).

Oriented poly(3-hexylthiophene) nanofibril with the π-π stacking growth direction by solvent directional evaporation.

In this article, the uniaxial alignment of poly(3-hexylthiophene) (P3HT) nanofibrils with a π-π stacking growth direction in which P3HT chains adopt a flat-on conformation was obtained by solvent directional evaporation using a glass cover slide and a poly(dimethylsiloxane) (PDMS) sheet to press the P3HT film in a carbon disulfide (CS(2)) atmosphere. By controlling the CS(2) vapor pressure during the film-forming process, we got a well-oriented P3HT film whose order parameter reached as high as 0.97. The orientation of the film was induced by the crystallization nucleation of P3HT and the directional evaporation of the solvent. Under a CS(2) vapor atmosphere, P3HT crystals preferred to adopt the form II modification, which started by nucleation. Owing to the solvent directional evaporation from the center to the margin, P3HT at the center of the sample would precipitate first to induce nucleation. Then the peripheral P3HT would directly diffuse, precipitate, and then adhere to the nucleus to form the uniaxial alignment of P3HT nanofibrils along the direction of solvent evaporation. Furthermore, in the P3HT nanofibrils, the π-π stacking direction of P3HT lamellae was parallel to the crystal growth direction, which would provide an effective path for charge transport.

Single cell migration dynamics mediated by geometric confinement.

The migration dynamics of cells plays a key role in tissue engineering and regenerative medicine. Previous studies mostly focus on regulating stem cell fate and phenotype by biophysical cues. In contrast, less is known about how the geometric cues mediate the migration dynamics of cells. Here, we fabricate graphene oxide (GO) microstripes on cell non-adhesive PEG substrate by using micromolding in capillary (MIMIC) method. Such micropatterns with alternating cell adhesion and cell resistance enable an effective control of selective adhesion and migration of single cells. The sharp contrast in cell adhesion minimizes the invasion of cells into the PEG patterns, and thereby strongly confines the cells on GO microstripes. As a result, the cells are forced to adapt highly polarized, elongated, and oriented geometry to fit the patterns. A series of pattern widths have been fabricated to modulate the extent of cell deformation and polarization. Under strong confinement, the cytoskeleton contractility, intracellular traction, and actin filament elongation are highly promoted, which result in enhanced cell migration along the patterns. This work provides an important insight into developing combinatorial graphene-based patterns for the control of cell migration dynamics, which is of great significance for tissue engineering and regenerative medicine.

Donor-acceptor cocrystal based on hexakis(alkoxy)triphenylene and perylenediimide derivatives with an ambipolar transporting property.

An organic donor-acceptor cocrystal with an ambipolar transporting property was constructed based on N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (EP-PDI) and 2,3,6,7,10,11-hexakis-(hexyloxy)-triphenylene (H6TP). The cocrystal with an alternating stacking of H6TP and EP-PDI molecules was formed through both drop-casting and spin-coating processes, especially at the optimized ratios of H6TP/EP-PDI (2/1, 1/1). The formation of the cocrystal was driven by the strong π-π interaction and the weaker steric hindrance, resulting from the smaller side groups, between the donor and acceptor molecules. Field effect transistors (FETs) based on the H6TP/EP-PDI cocrystal exhibited relatively balanced hole/electron transport, with a hole mobility of 1.14 × 10(-3) cm(2) V(-1) s(-1) and an electron mobility of 1.40 × 10(-3) cm(2) V(-1) s(-1).

Donor/Acceptor Molecular Orientation-Dependent Photovoltaic Performance in All-Polymer Solar Cells.

The correlated donor/acceptor (D/A) molecular orientation plays a crucial role in solution-processed all-polymer solar cells in term of photovoltaic performance. For the conjugated polymers PTB7-th and P(NDI2OD-T2), the preferential molecular orientation of neat PTB7-th films kept face-on regardless of the properties of processing solvents. However, an increasing content of face-on molecular orientation in the neat P(NDI2OD-T2) films could be found by changing processing solvents from chloronaphthalene (CN) and o-dichlorobenzene (oDCB) to chlorobenzene (CB). Besides, the neat P(NDI2OD-T2) films also exhibited a transformation of preferential molecular orientation from face-on to edge-on when extending film drying time by casting in the same solution. Consequently, a distribution diagram of molecular orientation for P(NDI2OD-T2) films was depicted and the same trend could be observed for the PTB7-th/P(NDI2OD-T2) blend films. By manufacture of photovoltaic devices with blend films, the relationship between the correlated D/A molecular orientation and device performance was established. The short-circuit current (Jsc) of devices processed by CN, oDCB, and CB enhanced gradually from 1.24 to 8.86 mA/cm(2) with the correlated D/A molecular orientation changing from face-on/edge-on to face-on/face-on, which could be attributed to facile exciton dissociation at D/A interface with the same molecular orientation. Therefore, the power conversion efficiency (PCE) of devices processed by CN, oDCB, and CB improved from 0.53% to 3.52% ultimately.

Development of large-scale size-controlled adult pancreatic progenitor cell clusters by an inkjet-printing technique.

The generation of transplantable ß-cells from pancreatic progenitor cells (PPCs) could serve as an ideal cell-based therapy for diabetes. Because the transplant efficiency depends on the size of islet-like clusters, it becomes one of the key research topics to produce PPCs with controlled cluster sizes in a scalable manner. In this study, we used inkjet printing to pattern biogenic nanoparticles, i.e., mutant tobacco mosaic virus (TMV), with different spot sizes to support the formation of multicellular clusters by PPCs. We successfully achieved TMV particle patterns with variable features and sizes by adjusting the surface wettability and printing speed. The spot sizes of cell-adhesive TMV mutant arrays were in the range of 50-150 µm diameter. Mouse PPCs were seeded on the TMV-RGD (arginine-glycine-aspartate)-patterned polystyrene (PS) substrate, which consists of areas that either favor (TMV-RGD) or prohibit (bare PS) cell adhesion. The PPCs stably attached, proliferated on top of the TMV-RGD support, thus resulting in the formation of uniform and confluent PPC clusters. Furthermore, the aggregated PPCs also maintained their multipotency and were positive for E-cadherin, indicating that the formation of cell-cell junctions is critical for enhanced cell-cell contact.

Ordered droplet formation by thin polymer film dewetting on a stripe-patterned substrate.

The dewetting process of thin polystyrene (PS) film on flat and stripe-patterned substrates is presented. Different dewetting processes were observed when the thin PS films annealed at above the glass transition temperature on these different kinds of substrates. The final dewetting on the flat substrate led to formation of polygonal liquid droplets, while on the stripe-patterned substrate, the droplets were observed to align at the centers of the stripes. A possible explanation for the dewetting process on the stripe-patterned substrate is proposed.

Micro-contact printing of graphene oxide nanosheets for fabricating patterned polymer brushes.

A robust strategy is developed to fabricate micro-patterned graphene oxide (GO) films on a hydroxylated surface via micro-contact printing induced supramolecular self-assembly. Existing photoactive sites on the surface of GO allow further amplification by growing polymer brushes via self-initiated photografting and photopolymerization (SIPGP).

A microcontact printing induced supramolecular self-assembled photoactive surface for patterning polymer brushes.

A facile and robust strategy for creating micropatterned polymer brushes via the combination of a micro-contact printing (µCP) induced supramolecular self-assembled photoactive surface with subsequent self-initiated photografting and photopolymerization (SIPGP) is reported. The results contribute to polymeric functionalization on a wide range of hydroxylated surfaces or graphene based materials.

Adhesive lithography for fabricating organic electronic and optoelectronics devices.

Improvements in organic electronic materials have led to novel device applications, ranging from large-area flexible displays to lightweight plastic electronics. Progress on these applications would benefit from development of low-cost fabrication techniques for organic semiconductors. In this review, several fabrication processes based on adhesion force (i.e. van der Waals forces, thiol-metal reactions, and cold welding) are introduced. These patterning techniques are dry patterning techniques, i.e., the electronic materials are patterned from the raised regions of molds onto a substrate directly by additive or subtractive patterning methods. Patterning of organic small molecule, polymer thin films and metal electrodes by adhesive lithography is demonstrated. The operating properties of patterned organic light-emitting diodes (OLEDs) and organic thin film transistors (OTFTs) are comparable with the performance of devices fabricated by conventional evaporation deposition methods.

Self-assembly of carbazole-based dendrimers by solvent vapor annealing: from fibers to spherulites.

The self-assembly behavior of H1-BCz, a first-generation dendrimer with phenyl-carbazole as core and carbazole as dendrons, was systemically studied by solvent-vapor annealing. The morphologies of spin-coated H1-BCz film on various substrates changed from giant crystalline fibers of several millimeters to spherulites with a decrease in tetrahydrofuran vapor pressure. At high vapor pressure, dewetting easily took place in the amorphous films, and single fibers or fiber clusters formed. The fibers grew from nuclei formed at the place where it does not dewet. The fibers grew larger by continuous nucleation and growth at the growth front of the fiber until all the substance on the substrate was consumed. As the vapor pressure decreased, dewetting did not so easily happen, and fibers packed more tightly and spherulites were obtained. Through characterization of the crystals and films, we speculate that 3D H1-BCz crystal formed by overlapping 2D lamellar structure. The π-π interactions and van der Waals interactions among H1-BCz molecules and solvent may be the main force driving the self-assembly of H1-BCz.

Highly oriented tunable wrinkling in polymer bilayer films confined with a soft mold induced by water vapor.

The authors report the formation of highly oriented wrinkling on the surface of the bilayer [polystyrene (PS)/poly(vinyl pyrrolidone) (PVP)] confined by a polydimethylsiloxane (PDMS) mold in a water vapor environment. When PVP is subjected to water vapor, the polymer loses its mechanical rigidity and changes to a viscous state, which leads to a dramatic change in Young's modulus. This change generates the amount of strain in the bilayer to induce the wrinkling. With a shape-controlled mold, they can get the ordered wrinkles perfectly perpendicular or leaned 45 degrees to the channel orientation of the mold because the orientation of the resultant force changes with the process of water diffusion which drives the surface to form the wrinkling. Additionally, they can get much smaller wrinkles than the stripe spacing of PDMS mold about one order. The wrinkle period changes with the power index of about 0.5 for various values of the multiplication product of the film thicknesses of the two layers, namely, lambda approximately (h(PS)h(PVP))(1/2).

Dewetting of thin polystyrene films under confinement.

The dewetting behavior of thin polystyrene (PS) film has been investigated by placing an upper plate with a ca. 140 nm gap from the underlying substrate with the spin-coated thin polymer films. Three different kinds of dewetting behaviors of thin PS film have been observed after annealing according to the relative position of the PS film to the upper plate. Since the upper plate is smaller than the underlying substrate, a part of the polymer film is not covered by the plate. In this region (I), thin PS film dewetting occurs in a conventional manner, as previously reported. While in the region covered by the upper plate (III), the PS film exhibits unusual dewetted patterns. Meanwhile, in the area right under the edge of the plate (II) (i.e., the area between region I and region III), highly ordered arrays of PS droplets are formed. Formation mechanisms of different dewetted patterns are discussed in detail. This study may offer an effective way to improve the understanding of various dewetting behaviors and facilitate the ongoing exploration of utilizing dewetting as a patterning technique.

Micropatterning of organic semiconductor microcrystalline materials and OFET fabrication by "hot lift off".

Considerable progress toward the development of electronic devices that rely on organic semiconductors as the active material component has been made in recent years. The key step for realization of the advanced organic electronic, or optical, device is the ability to micropattern different kinds of electronic materials, such as organic semiconductor/conducting materials, over large areas with micrometer-sized resolution. Here we demonstrate a simple and direct method for micropatterning small-molecule microcrystalline films using an epoxy stamp. The "hot lift off" method is highly selective, creating patterns with high resolution and relies on straightforwardly tailoring the adhesive properties between the epoxy and the film. This process is well suited for patterning many types of materials.