Observation of a Distinct Surface Molecular Orientation in Films of a High Mobility Conjugated Polymer

The molecular orientation and microstructure of films of the high-mobility semiconducting polymer poly­(<i>N</i>,<i>N</i>-bis-2-octyldodecylnaphthalene-1,4,5,8-bis-dicarboximide-2,6-diyl-alt-5,5-2,2-bithiophene) (P­(NDI2OD-T2)) are probed using a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS) and near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy. In particular a novel approach is used whereby the bulk molecular orientation and surface molecular orientation are simultaneously measured on the same sample using NEXAFS spectroscopy in an angle-resolved transmission experiment. Furthermore, the acquisition of bulk-sensitive NEXAFS data enables a direct comparison of the information provided by GIWAXS and NEXAFS. By comparison of the bulk-sensitive and surface-sensitive NEXAFS data, a distinctly different molecular orientation is observed at the surface of the film compared to the bulk. While a more “face-on” orientation of the conjugated backbone is observed in the bulk of the film, consistent with the lamella orientation observed by GIWAXS, a more “edge-on” orientation is observed at the surface of the film with surface-sensitive NEXAFS spectroscopy. This distinct edge-on surface orientation explains the high in-plane mobility that is achieved in top-gate P­(NDI2OD-T2) field-effect transistors (FETs), while the bulk face-on texture explains the high out-of-plane mobilities that are observed in time-of-flight and diode measurements. These results also stress that GIWAXS lacks the surface sensitivity required to probe the microstructure of the accumulation layer that supports charge transport in organic FETs and hence may not necessarily be appropriate for correlating film microstructure and FET charge transport

Highly Exfoliated MWNT–rGO Ink-Wrapped Polyurethane Foam for Piezoresistive Pressure Sensor Applications

The fabrication of pressure sensors based on reduced graphene oxide (rGO) as the sensing material is challenging due to the intrinsic hydrophobic behavior of graphene oxide inks as well as the agglomeration of graphene oxide flakes after reduction. Hydrazine (a reducing agent) and a dual-component additive comprising benzisothiazolinone and methylisothiazolinone in appropriate proportion were used to synthesize a rGO ink with a hydrophilic nature. Utilizing this hydrophilic rGO ink mixed with multiwalled carbon nanotubes (MWNTs), a very simple, low-cost approach is demonstrated for the fabrication of a pressure sensor based on polyurethane (PU) foam coated with the MWNT–rGO ink (MWNT–rGO@PU foam). The MWNT–rGO@PU foam-based devices are shown to be versatile pressure sensors with the potential to detect both small-scale and large-scale movements. At low pressure (below 2.7 kPa, 50% strain), the formation of microcracks that scatter electrical charges results in a detectable increase in resistance suitable for detecting small-scale motion. At a higher pressure, the compressive contact of the coated faces of the PU foam results in a sharp decrease in resistance suitable for monitoring of large-scale motion. Moreover, these sensors exhibit good flexibility and reproducibility over 5000 cycles. The versatility of this sensor has been demonstrated in a wide range of applications, such as speech recognition, health monitoring, and body motion detection. The significant advantages of this sensor are that its cost is low, it is easy to fabricate, and it has a versatility that renders it favorable to health-monitoring applications

In-Depth Understanding of the Morphology–Performance Relationship in Polymer Solar Cells

It is well-established that thermal annealing optimizes the morphology and improves the efficiency of P3HT-based organic solar cells, but the effects of different cooling rates after annealing are not well understood. In this paper, we use a model system based on poly­(3-hexylthiophene) (P3HT) and phenyl-C₆₁-butyric acid methyl ester (PCBM) to examine the relationship between morphology and device performance for annealing before (preannealing) and after (postannealing) the application of the electrode, with different cooling rates and in different device architectures. In the conventional structure, postannealing is confirmed to significantly enhance efficiency. The device prepared with a slow cooling rate (3.6%) shows a higher average power conversion efficiency than that prepared with a fast cooling rate (3.3%). The microstructural changes underlying this 10% increase in device performance and further effects of cooling rate, pre- and postannealing, and device architecture are comprehensively examined with a combination of synchrotron-based techniques, including grazing incidence wide-angle X-ray scattering, near-edge X-ray absorption fine structure spectroscopy, and X-ray photoelectron spectroscopy. The best device in the conventional architecture (postannealed with slow cooling rate) shows a more face-on orientation and narrower orientational distribution of P3HT crystallites. In addition, postannealing leads to PCBM diffusion toward the blend/top electrode interface. The enrichment of PCBM at the blend/top electrode interface plays a positive role in aiding electron collection at the electrode in the conventional structure, but it has a negative effect on the performance of the inverted structure, where hole collection at the top electrode instead is required. For this reason, in an inverted structure, preannealed films with slow cooling exhibit the best photovoltaic performance

Highly Exfoliated MWNT–rGO Ink-Wrapped Polyurethane Foam for Piezoresistive Pressure Sensor Applications

The fabrication of pressure sensors based on reduced graphene oxide (rGO) as the sensing material is challenging due to the intrinsic hydrophobic behavior of graphene oxide inks as well as the agglomeration of graphene oxide flakes after reduction. Hydrazine (a reducing agent) and a dual-component additive comprising benzisothiazolinone and methylisothiazolinone in appropriate proportion were used to synthesize a rGO ink with a hydrophilic nature. Utilizing this hydrophilic rGO ink mixed with multiwalled carbon nanotubes (MWNTs), a very simple, low-cost approach is demonstrated for the fabrication of a pressure sensor based on polyurethane (PU) foam coated with the MWNT–rGO ink (MWNT–rGO@PU foam). The MWNT–rGO@PU foam-based devices are shown to be versatile pressure sensors with the potential to detect both small-scale and large-scale movements. At low pressure (below 2.7 kPa, 50% strain), the formation of microcracks that scatter electrical charges results in a detectable increase in resistance suitable for detecting small-scale motion. At a higher pressure, the compressive contact of the coated faces of the PU foam results in a sharp decrease in resistance suitable for monitoring of large-scale motion. Moreover, these sensors exhibit good flexibility and reproducibility over 5000 cycles. The versatility of this sensor has been demonstrated in a wide range of applications, such as speech recognition, health monitoring, and body motion detection. The significant advantages of this sensor are that its cost is low, it is easy to fabricate, and it has a versatility that renders it favorable to health-monitoring applications

Highly Efficient and Balanced Charge Transport in Thieno[3,4‑<i>c</i>]pyrrole-4,6-dione Copolymers: Dramatic Influence of Thieno[3,2‑<i>b</i>]thiophene Comonomer on Alignment and Charge Transport

The design, synthesis, characterization, and application of a novel series of copolymers based on the electron deficient thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione building block, copolymerized with either thieno­[3,2-<i>b</i>]­thiophene (PTPDTT) or thiophene (PTPDT), are reported. High molecular weights were obtained for PTPDTT via Stille polycondensation. For the PTPDTs, different molecular weights were achieved by varying the polymerization conditions. The increase in molecular weight (PTPDT-2) favors face-on alignment and increases the charge carrier mobility. Grazing-incidence wide-angle X-ray scattering measurements reveal higher crystallinity for PTPDTT with up to 5 orders of lamellar stacking compared to PTPDTs. All polymers show ambipolar charge transport with highly balanced hole and electron mobilities in organic field effect transistors (OFETs), which improve considerably upon thermal annealing. A shift of comonomer from simple thiophene in PTPDT-2 to planar and electron-dense thienothiophene in PTPDTT drastically changes the alignment from face-on to edge-on fashion. Consequently, the charge carrier mobility increases considerably by 1 order of magnitude in PTPDTT, reaching excellent charge carrier mobilities for both holes (0.11 cm² V^–1 s^–1) and electrons (0.17 cm² V^–1 s^–1). PTPDTT was tested as a donor material in combination with PC₇₁BM as well as an acceptor material along with a donor polymer. As a donor material, a power conversion efficiency of 4.3% was reached in combination with PC₇₁BM

Azido-Functionalized Thiophene as a Versatile Building Block To Cross-Link Low-Bandgap Polymers

We unveil a concept for the design of cross-linkable semiconducting polymers that is based on a modular tercopolymerization which stands out by its low synthetic effort, easy accessibility, and its broad range of applications. 3-(6-Azidohexyl)­thiophene was used as a comonomer in the synthesis of a variety of low-bandgap copolymers using different polymerization techniques such as Suzuki–Miyaura cross-coupling and Stille cross-coupling. We show that when only a small amount (5–10 mol %) of azide groups is introduced into the polymers, the impact on absorption and electrochemical properties (HOMO/LUMO values) is negligible. The small amount of azide functionality is however enough to obtain polymers that can easily be cross-linked by UV illumination. Thermal stability of the solid state packing and alignment is studied in neat polymer thin films as well as in blends with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) as a relevant model blend system. Solvent resistivity of these polymer films is investigated by absorption and photoluminescence measurements. It is finally shown in organic field effect transistors that the introduction of 10% azide-functionalized monomer does not considerably influence hole transport mobility (0.20–0.45 cm² V^–1 s^–1)

Diffractive X‑ray Waveguiding Reveals Orthogonal Crystalline Stratification in Conjugated Polymer Thin Films

The depth dependence of crystalline structure within thin films is critical for many technological applications but has been impossible to measure directly using common techniques. In this work, by monitoring diffraction peak intensity and location and utilizing the highly angle-dependent waveguiding effects of X-rays near grazing incidence, we quantitatively measure the thickness, roughness, and orientation of stratified crystalline layers within thin films of a high-performance semiconducting polymer. In particular, this diffractive X-ray waveguiding reveals a self-organized 5 nm thick crystalline surface layer with crystalline orientation orthogonal to the underlying 65 nm thick layer. While demonstrated for an organic semiconductor film, this approach is applicable to any thin film material system with stratified crystalline structure where orientation can influence important interfacial processes such as charge injection and field-effect transport

Microstructure of Polycrystalline PBTTT Films: Domain Mapping and Structure Formation

We utilize near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning transmission X-ray microscopy (STXM) to study the microstructure and domain structure of polycrystalline films of the semiconducting polymer poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-<i>b</i>]thiophene) (PBTTT). Total electron yield NEXAFS spectroscopy is used to examine the surface structure of the first 1–2 molecular layers, while bulk-sensitive STXM is used to produce maps of domain orientation and order sampled through the entire film thickness. We study different phases of PBTTT including as-cast, terraced and nanoribbon morphologies produced <i>via</i> spin-coating as well as aligned films of as-cast and nanoribbon morphologies produced by zone-casting. For the terraced morphology, domains are observed that are larger than the size of the terraced surface features, and the calculated degree of order is reduced compared to the nanoribbon morphology. For zone-cast films, we find that, although little optical anisotropy is observed in the bulk of as-cast films, a high degree of surface structural anisotropy is observed with NEXAFS spectroscopy, similar to what is observed in annealed nanoribbon films. This observation indicates that the aligned surface structure in unannealed zone-cast films templates the bulk ordering of the aligned nanoribbon phase. STXM domain mapping of aligned nanoribbon films reveals elongated, micrometer-wide domains with each domain misoriented with respect to its neighbor by up to 45°, but with broad domain boundaries. Within each nanoribbon domain, a high degree of X-ray dichroism is observed, indicating correlated ordering throughout the bulk of the film

Tuning Dipole Orientation of 2,6-Azulene Units in Conjugated Copolymers by C–H Activation Strategy toward High-Performance Organic Semiconductor

Azulene has aroused widespread interest for constructing optoelectronic materials. However, controlling the dipole orientation of 2,6-azulene units in the conjugated polymer backbone is a significant challenge so far. Herein, by C–H activation strategy, three 2,6-azulene-TPD-based conjugated copolymers with different dipole arrangements were synthesized, where TPD = thieno[3,4-c]pyrrole-4,6-dione. The dipole arrangements of 2,6-azulene units were random for P(AzTPD-1), head-to-head/tail-to-tail for P(AzTPD-2), and head-to-tail for P(AzTPD-3). These polymers exhibited unipolar n-type semiconductor characteristics in organic field effect transistors. Moreover, regioregular polymer P(AzTPD-3) displayed the best device performance with an electron mobility of up to 0.33 cm2 V–1 s–1, which makes P(AzTPD-3) a high-performance n-type polymeric semiconductor. These results demonstrate that incorporation of 2,6-azulene units into the polymeric backbone together with the regulation of the dipole orientation of 2,6-azulene units is an effective strategy for obtaining high-performance organic optoelectronic materials

Correlating the Efficiency and Nanomorphology of Polymer Blend Solar Cells Utilizing Resonant Soft X-ray Scattering

Enhanced scattering contrast afforded by resonant soft X-ray scattering (R-SoXS) is used to probe the nanomorphology of all-polymer solar cells based on blends of the donor polymer poly(3-hexylthiophene) (P3HT) with either the acceptor polymer poly((9,9-dioctylfluorene)-2,7-diyl-<i>alt</i>-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (F8TBT) or poly([<i>N</i>,<i>N</i>′-bis(2-octyldodecyl)-11-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-12-bithiophene)) (P(NDI2OD-T2)). Both P3HT:F8TBT and P3HT:P(NDI2OD-T2) blends processed from chloroform with subsequent annealing exhibit complicated morphologies with a hierarchy of phase separation. A bimodal distribution of domain sizes is observed for P3HT:P(NDI2OD-T2) blends with small domains of size ∼5–10 nm that evolve with annealing and larger domains of size ∼100 nm that are insensitive to annealing. P3HT:F8TBT blends in contrast show a broader distribution of domain size but with the majority of this blend structured on the 10 nm length scale. For both P3HT:P(NDI2OD-T2) and P3HT:F8TBT blends, an evolution in device performance is observed that is correlated with a coarsening and purification of domains on the 5–10 nm length scale. Grazing-incidence wide-angle X-ray scattering (GI-WAXS) is also employed to probe material crystallinity, revealing P(NDI2OD-T2) crystallites 25–40 nm in thickness that are embedded in the larger domains observed by R-SoXS. A higher degree of P3HT crystallinity is also observed in blends with P(NDI2OD-T2) compared to F8TBT with the propensity of the polymers to crystallize in P3HT:P(NDI2OD-T2) blends hindering the structuring of morphology on the sub-10 nm length scale. This work also underscores the complementarity of R-SoXS and GI-WAXS, with R-SoXS measuring the size of compositionally distinguishable domains and GI-WAXS providing information regarding crystallinity and crystallite thickness