P3HT Nanopillars for Organic Photovoltaic Devices Nanoimprinted by AAO Templates

Free-standing nanorod arrays of poly(3-hexylthiophene) (P3HT) were fabricated on indium tin oxide/glass substrates using anodic aluminum oxide (AAO) templates. The AAO templates were treated with a low molecular weight polydimethylsiloxane mold-release agent to reduce their surface energy of the template and interactions with the P3HT. Using a thermal nanoimprinting process, the templates were easily removed, generating nanorods on the surfaces of P3HT thin films. These unique structures were investigated for application in organic photovoltaic devices

Measuring the Degree of Crystallinity in Semicrystalline Regioregular Poly(3-hexylthiophene)

By using a combination of wide-angle X-ray diffraction (WAXD), mass density, and ¹³C solid-state nuclear magnetic resonance (NMR) measurements, a quantification of the absolute degree of crystallinity in regioregular poly­(3-hexylthiophene) (rr-P3HT) is presented. A regiorandom P3HT (rra-P3HT), lacking long-range order, was used to separate the crystalline contribution from the total scattering in WAXD, thus yielding degrees of crystallinity in the range of 47–56% at room temperature for three different rr-P3HTs. For the same rr-P3HT with identical processing history, NMR yields degrees of crystallinity that are consistently ∼10% greater than that obtained by WAXD, which can only be explained by ordered chain segments in the amorphous phase. NMR results also suggest that rra-P3HT contains weakly ordered chain segments, which likely contribute to an underestimation of degree of crystallinity when determined from mass density measurements, if rra-P3HT is used to approximate a fully amorphous P3HT. The results shown in this study provide direct proof of three different types of P3HT chain segments: crystallites (i.e., long-range ordered chain packing), amorphous phase (i.e., disordered chain packing), and short-range ordered chain packing embedded in the amorphous phase. The presence of the short-range ordered chain packing is particularly important when correlating the morphology to macroscopic charge transport properties in P3HT-based devices. In general, those locally ordered chain segments, though not constituting a distinct phase, are believed to be of critical importance in determining the transport characteristics of conjugated semiconducting polymers with or without a distinct crystalline phase present

Assembly of Graphene Oxide at Water/Oil Interfaces: Tessellated Nanotiles

The interfacial assembly of graphene oxide (GO) at the water/oil interface and its kinetics were systematically studied. GO nanosheets were found to segregate to the water/oil interface and interact with quaternized block copolymer chains by the peripheral carboxyl groups on the GO. If the interfacial area is decreased, then GO, assembled at and confined to the interface, jams and then buckles. An analysis of the kinetics of the assembly processes leads to the conclusion that the diffusion of GO to the interface is the rate-determining step. The morphology of the jammed GO film was investigated, and TEM images show that GO sheets form a mosaic or tile across the whole oil/water interface

Assembly of Graphene Oxide at Water/Oil Interfaces: Tessellated Nanotiles

The interfacial assembly of graphene oxide (GO) at the water/oil interface and its kinetics were systematically studied. GO nanosheets were found to segregate to the water/oil interface and interact with quaternized block copolymer chains by the peripheral carboxyl groups on the GO. If the interfacial area is decreased, then GO, assembled at and confined to the interface, jams and then buckles. An analysis of the kinetics of the assembly processes leads to the conclusion that the diffusion of GO to the interface is the rate-determining step. The morphology of the jammed GO film was investigated, and TEM images show that GO sheets form a mosaic or tile across the whole oil/water interface

A Small Molecule Composed of Dithienopyran and Diketopyrrolopyrrole as Versatile Electron Donor Compatible with Both Fullerene and Nonfullerene Electron Acceptors for High Performance Organic Solar Cells

A new conjugated small molecule composed of dithienopyran and diketopyrrolopyrrole (DTP-DPP) was synthesized. Since DTP-DPP has advantageous optoelectronic properties for photovoltaic applications such as intense and broad light absorption, proper frontier energy levels, high crystallinity, and high hole mobility, the devices fabricated from the blend with PC71BM and P­(NDI2OD-T2) exhibit high power conversion efficiencies of 6.88% and 4.82%, respectively. This work demonstrates that DTP-DPP is a versatile electron donor compatible with both fullerene and nonfullerene acceptors for high performance SM-OSCs

Highly Crystalline Low Band Gap Polymer Based on Thieno[3,4‑<i>c</i>]pyrrole-4,6-dione for High-Performance Polymer Solar Cells with a >400 nm Thick Active Layer

Two thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione (TPD)-based copolymers combined with 2,2′-bithiophene (BT) or (<i>E</i>)-2-(2-(thiophen-2-yl)­vinyl)­thiophene (TV) have been designed and synthesized to investigate the effect of the introduction of a vinylene group in the polymer backbone on the optical, electrochemical, and photovoltaic properties of the polymers. Although both polymers have shown similar optical band gaps and frontier energy levels, regardless of the introduction of vinylene bridge, the introduction of a π-extended vinylene group in the polymer backbone substantially enhances the charge transport characteristics of the resulting polymer due to its strong tendency to self-assemble and thus to enhance the crystallinity. An analysis on charge recombination in the active layer of a solar cell device indicates that the outstanding charge transport (μ = 1.90 cm²·V^–1·s^–1) of PTVTPD with a vinylene group effectively suppresses the bimolecular recombination, leading to a high power conversion efficiency (PCE) up to 7.16%, which is 20% higher than that (5.98%) of the counterpart polymer without a vinylene group (PBTTPD). More importantly, PTVTPD-based devices do not show a large variation of photovoltaic performance with the active layer thickness; that is, the PCE remains at 6% as the active layer thickness increases up to 450 nm, demonstrating that the PTVTPD-based solar cell is very compatible with industrial processing

Assembly of Acid-Functionalized Single-Walled Carbon Nanotubes at Oil/Water Interfaces

The efficient segregation of water-soluble, acid-functionalized, single-walled carbon nanotubes (SWCNTs) at the oil/water interface was induced by dissolving low-molecular-weight amine-terminated polystyrene (PS-NH₂) in the oil phase. Salt-bridge interactions between carboxylic acid groups of SWCNTs and amine groups of PS drove the assembly of SWCNTs at the interface, monitored by pendant drop tensiometry and laser scanning confocal microscopy. The impact of PS end-group functionality, PS and SWCNT concentrations, and the degree of SWCNT acid modification on the interfacial activity was assessed, and a sharp drop in interfacial tension was observed above a critical SWCNT concentration. Interfacial tensions were low enough to support stable oil/water emulsions. Further experiments, including potentiometric titrations and the replacement of SWCNTs by other carboxyl-containing species, demonstrated that the interfacial tension drop reflects the loss of SWCNT charge as the pH falls near/below the intrinsic carboxyl dissociation constant; species lacking multivalent carboxylic acid groups are inactive. The trapped SWCNTs appear to be neither ordered nor oriented

Hydrolysis-Induced Morphology Evolution of Linear and Bottlebrush Block Copolymers in Thin Films with Acid Vapor or Photoacid Generators

The self-assembly of high-χ low-N block copolymers (BCPs) can give patterns with sub-10 nm full pitch, serving as a promising alternative to photolithographic methods. In this work, we synthesized poly(solketal methacrylate)-block-polystyrene copolymers, PSM-b-PS, with various volume ratios of the two blocks. After hydrolysis of the PSM block into poly(glycerol monomethacrylate), PGM, the BCPs had both lamellar and cylindrical microdomain morphologies in the bulk phase and in thin films. In addition to our previously developed solid-state hydrolysis strategy involving trifluoroacetic acid vapor, we developed a new photoinduced solid-state hydrolysis using photoacid generators, PAGs, embedded within the polymer films. After exposure to UV followed by a postexposure baking or solvent vapor annealing, the BCPs transitioned from the disordered, phase-mixed state into laterally ordered cylindrical patterns. In comparison to linear BCPs that rely on a random copolymer layer to modify interfacial interactions with the substrate to promote an orientation of the microdomains normal to the interface, we found that the microdomains in bottlebrush multiblock copolymers oriented normal to the interface absent substrate modification due to the chain architecture

Nanomechanical Mapping of a Deformed Elastomer: Visualizing a Self-Reinforcement Mechanism

Mapping the structure evolution and mechanical properties of elastic polymers or biomaterials during bulk deformation has been difficult, yet this information has long been thought to be key for understanding the structure–mechanical property relationship necessary to guide the design of new materials. Here we use a nanomechanical mapping to assess the structural evolution and mechanical properties of a deformed isoprene rubber (IR) to elucidate a self-reinforcement mechanism in this material. A hierarchical nanofibrillar structure, ranging from several to a hundred nanometers in size, comprised of fibers oriented parallel to the stretching direction was found. The nanofibers, connected by oriented amorphous tie chains, form a network structure that is responsible for significantly enhanced stress, a key factor giving rise to the self-reinforcement of IR and, more than likely, most elastomers that undergo strained-induced crystallization