Sustainable Thermoplastic Elastomers Derived from Fatty Acids

Vegetable oils are an attractive source for polymers due to their low cost, abundance, annual renewability, and ease of functionalization. Stearyl and lauryl acrylate, derived from vegetable oils such as soybean, coconut, and palm kernel oil, have been polymerized through reversible addition–fragmentation chain transfer polymerization, resulting in poly­(styrene-<i>b</i>-(lauryl acrylate-<i>co</i>-stearyl acrylate)-<i>b</i>-styrene) (SAS) triblock copolymers. Varying the length of the side chain on the polyacrylate midblock (C18 and C12 in stearyl and lauryl acrylate repeat units, respectively) is a convenient tool for tuning the physical properties of the triblock copolymers. The SAS triblock copolymers exhibit properties appropriate for thermoplastic elastomer (TPE) applications. Small-angle X-ray scattering and transmission electron microscopy experiments have elucidated the microphase-separated morphology of the SAS triblock copolymers, consistent with a spherical morphology lacking long-range order. The physical properties of the polymers can be readily tuned by varying the acrylate midblock composition, including the melting temperature, viscosity, and triblock copolymer tensile properties. Tensile testing reveals elastomeric behavior with high elongation at break. Surprisingly, the order–disorder transition temperature of the triblock copolymer is not dependent on the acrylate composition in the midblock. This indicates that the acrylate composition can be used as a tool to manipulate the physical properties of the triblock copolymers without affecting the order–disorder transition temperature, or processing temperature, of the TPEs

Impact of Low Molecular Weight Poly(3-hexylthiophene)s as Additives in Organic Photovoltaic Devices

Despite tremendous progress in using additives to enhance the power conversion efficiency of organic photovoltaic devices, significant challenges remain in controlling the microstructure of the active layer, such as at internal donor–acceptor interfaces. Here, we demonstrate that the addition of low molecular weight poly­(3-hexylthiophene)­s (low-MW P3HT) to the P3HT/fullerene active layer increases device performance up to 36% over an unmodified control device. Low MW P3HT chains ranging in size from 1.6 to 8.0 kg/mol are blended with 77.5 kg/mol P3HT chains and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) fullerenes while keeping P3HT/PCBM ratio constant. Optimal photovoltaic device performance increases are obtained for each additive when incorporated into the bulk heterojunction blend at loading levels that are dependent upon additive MW. Small-angle X-ray scattering and energy-filtered transmission electron microscopy imaging reveal that domain sizes are approximately invariant at low loading levels of the low-MW P3HT additive, and wide-angle X-ray scattering suggests that P3HT crystallinity is unaffected by these additives. These results suggest that oligomeric P3HTs compatibilize donor–acceptor interfaces at low loading levels but coarsen domain structures at higher loading levels and they are consistent with recent simulations results. Although results are specific to the P3HT/PCBM system, the notion that low molecular weight additives can enhance photovoltaic device performance generally provides a new opportunity for improving device performance and operating lifetimes

Passive Parity-Time Symmetry in Organic Thin Film Waveguides

Periodic media are fundamentally important for controlling the flow of light in photonics. Recently, the emerging field of non-Hermitian optics has generalized the notion of periodic media to include a new class of materials that obey parity-time (PT) symmetry, with real and imaginary refractive index variations that transform into one another upon spatial inversion, leading to a variety of unusual optical phenomena. Here, we introduce a simple approach based on interference lithography and oblique angle deposition to achieve PT-symmetric modulation in the effective index of large area organic thin film waveguides with the functional form Δ<i>ñ</i>_eff(<i>z</i>) ∼ <i>e</i>^<i>iqz</i>. Passive PT symmetry breaking is observed through asymmetry in the forward and backward diffraction of waveguided light that maximizes at the exceptional point, resulting in unidirectional reflectionless behavior that is visualized directly via leakage radiation microscopy. These results establish the basis for organic PT waveguide media that can be tuned for operation throughout the visible to near-infrared spectrum and provide a direct pathway to incorporate gain sufficient to achieve active PT symmetric lattices and gratings

Backbone and Side Group Interchain Correlations Govern Wide-Angle X‑ray Scattering of Poly(3-hexylthiophene)

Identifying the origin of scattering from polymer materials is crucial to infer structural features that can relate to functional properties. Here, we use our recently developed virtual-site coarse graining to accelerate atomistic simulations and show how various molecular features govern wide-angle X-ray scattering from a conjugated polymer, poly(3-hexylthiophene) (P3HT). The efficient molecular dynamics simulations can represent the structure and capture the emergence of crystalline order from amorphous melts upon cooling while retaining atomistic details of chain configurations. The scattering extracted from simulations shows good agreement with wide-angle X-ray scattering experiments. Amorphous P3HT exhibits broad scattering peaks: a high-q peak from interchain side-group correlations and a low-q peak from interchain backbone–backbone correlations. During amorphous to crystalline phase transitions, the distance between backbones along the side-group direction increases because of lack of interdigitation in the crystalline phase. Scattering from π–π stacking emerges only after crystallization takes place. Intrachain correlations contribute negligibly to the scattering from the amorphous and crystalline phases

Elemental Mapping of Interfacial Layers at the Cathode of Organic Solar Cells

One of the limitations in understanding the performance of organic solar cells has been the unclear picture of morphology and interfacial layers developed at the active layer/cathode interface. Here, by utilizing the shadow-Focused Ion Beam technique to enable energy-filtered transmission electron microscopy imaging in conjunction with X-ray photoelectron spectroscopy (XPS) experiments, we examine the cross-section of polythiophene/fullerene solar cells to characterize interfacial layers near the semiconductor-cathode interface. Elemental mapping reveals that localization of fullerene to the anode interface leads to low fill factors and S-shaped current–voltage characteristics. Furthermore, the combination of elemental mapping and XPS depth profiles of devices demonstrate oxidation of the aluminum cathode at the active layer interface for devices without S-shaped characteristics and fill factors of 0.6. The presence of a thin dielectric at the semiconductor-cathode interface could minimize electronic barriers for charge extraction by preventing interfacial charge reorganization and band-bending

Signatures of Multiphase Formation in the Active Layer of Organic Solar Cells from Resonant Soft X‑ray Scattering

Resonant soft X-ray scattering (RSOXS) is a complementary tool to existing reciprocal space methods, such as grazing-incidence small-angle X-ray scattering, for studying order formation in polymer thin films. In particular, RSOXS can exploit differences in absorption between multiple phases by tuning the X-ray energy to one or more resonance peaks of organic materials containing carbon, oxygen, nitrogen, or other atoms. Here, we have examined the structural evolution in poly­(3-hexylthiophene-2,5-diyl)/[6,6]-phenyl-C₆₁-butyric acid methyl ester mixtures by tuning X-rays to resonant absorption energies of carbon and oxygen. Our studies reveal that the energy dependence of RSOXS profiles marks the formation of multiple phases in the active layer of organic solar cells, which is consistent with elemental maps obtained through energy-filtered transmission electron microscopy

Close-Packed Spherical Morphology in an ABA Triblock Copolymer Aligned with Large-Amplitude Oscillatory Shear

A microphase-separated poly­(styrene-<i>b</i>-(lauryl-<i>co</i>-stearyl acrylate)-<i>b</i>-styrene) (SAS) triblock copolymer exhibiting a disordered spherical microstructure with randomly oriented grains was aligned through the application of large-amplitude oscillatory shear (LAOS) at a temperature below the order–disorder transition temperature of the triblock copolymer, yet above the glass transition temperature of the polystyrene spherical domains. The thermoplastic elastomeric behavior of the SAS triblock copolymer provided a convenient means to observe the aligned morphology. Following application of LAOS, the specimen was quenched to room temperature (below the glass transition temperature of polystyrene), and small-angle X-ray scattering data were obtained in the three principal shear directions: shear gradient, velocity, and vorticity directions. The analysis revealed that the SAS triblock copolymer formed coexisting face-centered cubic and hexagonally close-packed spherical microstructures. The presence of a close-packed microstructure is in stark contrast to an extensive body of literature on sphere-forming bulk block copolymers that favor body-centered cubic systems under quiescent conditions and under shear. The aligned microstructure observed in this bulk block copolymer was reminiscent of that observed in various spherical soft material systems such as colloidal spheres, sphere-forming block copolymer solutions, and star polymer solutions. The highly unanticipated observation of close-packed spherical microstructures in a neat block copolymer under shear is hypothesized to originate from the dispersity of the block copolymer

Glass Transition Temperature of Conjugated Polymers by Oscillatory Shear Rheometry

The stiff backbones of conjugated polymers can lead to a rich phase behavior that includes both crystalline and liquid crystalline phases, making measurements of the glass transition challenging. In this work, the glass transitions of regioregular poly­(3-hexyl­thiophene-2,5-diyl) (RR P3HT), regiorandom (RRa) P3HT, and poly­((9,9-bis­(2-octyl)-fluorene-2,7-diyl)-<i>alt</i>-(4,7-di­(thiophene-2-yl)-2,1,3-benzo­thiadiazole)-5′,5″-diyl) (PFTBT) are probed by linear viscoelastic measurements as a function of molecular weight. We find two glass transition temperatures (<i>T</i>_g’s) for both RR and RRa P3HT and one for PFTBT. The higher <i>T</i>_g, <i>T</i>_α, is associated with the backbone segmental motion and depends on the molecular weight, such that the Flory–Fox model yields <i>T</i>_α = 22 and 6 °C in the long chain limit for RR and RRa P3HT, respectively. For RR P3HT, a different molecular weight dependence of <i>T</i>_α is seen below <i>M</i>_n = 14 kg/mol, suggesting this is the typical molecular weight of intercrystal tie chains. The lower <i>T</i>_g (<i>T</i>_αPE ≈ −100 °C) is associated with the side chains and is independent of molecular weight. RRa P3HT exhibits a lower <i>T</i>_α and higher <i>T</i>_αPE than RR P3HT, possibly due to a different degree of nanophase separation between the side chains and the backbones. In contrast, PFTBT only exhibits one <i>T</i>_g above −120 °C, at 144 °C in the long chain limit

Synthesis of Perfluoroalkyl End-Functionalized Poly(3-hexylthiophene) and the Effect of Fluorinated End Groups on Solar Cell Performance

A series of well-defined perfluoroalkyl end-functionalized poly­(3-hexylthiophenes) (P3HT) were synthesized by Stille coupling of stannylated 2-perfluoralkylthiophene with the bromine end of P3HT. The length of the perfluoroalkyl end group was varied from −C₄F₁₃ to −C₈F₁₇. These polymers were fully characterized and tested in bulk heterojunction solar cells with phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor. Performance of the solar cells was highest for the unmodified P3HT and decreased as the length of the perfluoroalkyl end increased. The most affected device parameters were the short-circuit current density (<i>J</i>_sc) and series resistance, pointing to lower charge carrier mobility and poor morphology as the cause for the lower performance. While the morphology of blends did not significantly change with perfluoroalkyl end modification, analysis of blended films by energy-filtered transmission electron microscopy (EF-TEM) revealed wider P3HT domains, consistent with the perfluorinated end groups segregating to the edge or exterior of P3HT domains, causing two domains to join. This study demonstrates that the perfluoroalkyl end group can be detrimental to polymer solar cell device performance, and further work toward understanding the interface between the donor and acceptor phases is required to fully understand this effect

Incorporating Fluorine Substitution into Conjugated Polymers for Solar Cells: Three Different Means, Same Results

Fluorinating conjugated polymers is a proven strategy for creating high performance materials in polymer solar cells, yet few studies have investigated the importance of the fluorination method. We compare the performance of three fluorinated systems: a poly­(benzodithieno-dithienyltriazole) (PBnDT-XTAZ) random copolymer where 50% of the acceptor units are difluorinated, PBnDT-mFTAZ where every acceptor unit is monofluorinated, and a 1:1 physical blend of the difluorinated and nonfluorinated polymer. All systems have the same degree of fluorination (50%) yet via different methods (chemically vs physically, random vs regular). We show that these three systems have equivalent photovoltaic behavior: ∼5.2% efficiency with a short-circuit current (<i>J</i>_sc) at ∼11 mA cm^–2, an open-circuit voltage (<i>V</i>_oc) at 0.77 V, and a fill factor (FF) of ∼60%. Further investigation of these three systems demonstrates that the charge generation, charge extraction, and charge transfer state are essentially identical for the three studied systems. Transmission electron microscopy shows no significant differences in the morphologies. All these data illustrate that it is possible to improve performance not only via regular or random fluorination but also by physical addition via a ternary blend. Thus, our results demonstrate the versatility of incorporating fluorine in the active layer of polymer solar cells to enhance device performance