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

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

Influence of Fluorination and Molecular Weight on the Morphology and Performance of PTB7:PC₇₁BM Solar Cells

The device performance and microstructure of a series of PTB7-based polymers with varied molecular weight and degree of fluorination are investigated. Although the energy level of the highest occupied molecular orbital is found to increase with degree of fluorination, a strong relative molecular weight dependence of device performance dominates any underlying fluorination-dependent trend on overall performance. Microstructural investigation using a combination of X-ray techniques reveals a striking effect of polymer molecular characteristics on film morphology, with the size of PC₇₁BM domains systematically decreasing with increasing polymer molecular weight. Furthermore, the relative purity of the mixed PTB7:PC₇₁BM domain is found to systematically decrease with increasing molecular weight. When domain sizes with and without the use of the solvent additive diiodooctane (DIO) are compared, the effectiveness of DIO in reducing PC₇₁BM domain sizes is also found to be strongly dependent on the molecular weight of the polymer. It is found that molecular weights of at least 150 kg mol^–1 are required for DIO to be effective in reducing the PC₇₁BM domain size in order to produce high short-circuit current densities. Finally, it is shown that relatively high device efficiencies can be achieved with low degrees of fluorination; an efficiency of 4.6% is achieved for a degree of fluorination of only 5.3%

Structure Influence on Charge Transport in Naphthalenediimide–Thiophene Copolymers

Reported here is a characterization of a series of NDI–thiophene copolymers with one, two, three, and four thiophene units synthesized using Stille polycondensation of dibromo-naphthalene diimide and the trimethylstannylthiophene monomers. The effect of extension of the thiophene donor group is studied in terms of structure-charge transport correlation. The influence of side chains located on the thiophene units of copolymers with two and four thiophene units per monomer is also investigated. Charge transport of both signs is studied experimentally in field-effect transistors. Microstructural data obtained by near-edge X-ray absorption fine structure (NEXAFS) and grazing incidence wide-angle X-ray scattering (GIWAXS) is supported by AFM topography scans. Ultraviolet photoelectron spectroscopy (UPS) and UV–vis spectroscopy data are employed in the measurement of energy levels, and changes with annealing temperature are also discussed. Most of the polymers reach excellent electron and hole mobility with one copolymer (NDI-T4) exhibiting an especially balanced ambipolar charge transport of 0.03 cm² V^–1 s^–1. An odd–even effect in hole mobility is observed with higher values for polymers with an even number of thiophene units. The reported findings indicate that the final charge transport properties are a result of the interplay of many factors, including crystallinity, planarity and linearity of chain, spacing between acceptor units and packing of solubilizing branched side chains

Morphological and Device Evaluation of an Amphiphilic Block Copolymer for Organic Photovoltaic Applications

We report the morphological and photovoltaic evaluation of a novel fully conjugated donor/acceptor block copolymer system based on the P3HT-<i>b</i>-PFTBT scaffold. The incorporation of hydrophilic tetraethylene glycol side-chains into the PFTBT acceptor block generates an amphiphilic species whose properties provide demonstrable benefits over traditional systems. This design strategy facilitates isolation of the block copolymer from homopolymer impurities present in the reaction mixture, and we show that this purification leads to better-defined morphologies. The chemical disparity introduced between donor and acceptor blocks causes spontaneous microphase separation into well-defined domains, which we demonstrate with a combination of spectroscopy, microscopy, and X-ray scattering. The morphological advantages of this system are significant; however, preliminary device characterization indicates a loss of electron mobility in the hydrophilic acceptor block

Critical Role of Molecular Symmetry for Charge Transport Properties: A Paradigm Learned from Quinoidal Bithieno[3,4‑<i>b</i>]thiophenes

High molecular symmetry is always observed in high-performance organic semiconductors. However, whether it is an essential factor for molecular design is unclear. In this work, we designed and synthesized three quinoidal isomers, <b>QBTT-</b><b><i>o</i></b>, <b>QBTT-</b><i><b>i</b></i>, and <b>QBTT-</b><i><b>s</b></i>, with different sulfur orientations and a stable <i>E</i> configuration to investigate the relationship between the structure symmetry and organic thin-film transistor performance. We found that <b>QBTT-</b><i><b>o</b></i> and <b>QBTT-</b><i><b>i</b></i> with high <i>C</i>_2<i>h</i> symmetry exhibit electron mobilities of 0.02 and 0.15 cm² V^–1 s^–1, respectively, while <b>QBTT-</b><i><b>s</b></i> exhibits an unexpectedly high electron mobility of 0.32 cm² V^–1 s^–1 with <i>I</i>_on/<i>I</i>_off ratios of ≤10⁶. The enhanced electron mobilities from <b>QBTT-</b><i><b>o</b></i> and <b>QBTT-</b><i><b>i</b></i> to <b>QBTT-</b><i><b>s</b></i> can be attributed to the different sulfur orientations, especially, molecular symmetry. The thin-film microstructures of three <b>QBTT</b>s were systematically investigated by grazing incidence wide-angle X-ray scattering, near-edge X-ray absorption fine structure spectroscopy, atomic force microscopy, and molecular dynamic simulations. The crystallinities are critically dependent on sulfur orientations and increase from <b>QBTT-</b><i><b>o</b></i> to <b>QBTT-</b><i><b>i</b></i> to <b>QBTT-</b><i><b>s</b></i>, which agrees well with the organic thin-film transistor (OTFT) performance. The poor OTFT performance of <b>QBTT-</b><i><b>o</b></i> compared to that of <b>QBTT-</b><i><b>i</b></i> with the same <i>C</i>_2<i>h</i> symmetry can be attributed to the different sulfur orientations; meanwhile, we speculate that the strongest crystallinity of <b>QBTT-</b><i><b>s</b></i> might be attributed to the weak dipole moment that originated from the asymmetric molecular structure. Therefore, molecular symmetry is an important issue that needs to be carefully considered for the design of high-performance organic semiconductors

Blade Coating Aligned, High-Performance, Semiconducting-Polymer Transistors

Recent demonstration of mobilities in excess of 10 cm² V^–1 s^–1 have energized research in solution deposition of polymers for thin film transistor applications. Due to the lamella motif of most soluble, semiconducting polymers, the local mobility is intrinsically anisotropic. Therefore, fabrication of aligned films is of interest for optimization of device performance. Many techniques have been developed to control film alignment, including solution deposition via directed flows and deposition on topologically structured substrates. We report device and detailed structural analysis (ultraviolet–visible absorption, IR absorption, near-edge X-ray absorption (NEXAFS), grazing incidence X-ray diffraction, and atomic force microscopy) results from blade coating two high performing semiconducting polymers on unpatterned and nanostructured substrates. Blade coating exhibits two distinct operational regimes: the Landau–Levich or horizontal dip coating regime and the evaporative regime. We find that in the evaporative deposition regime, aligned films are produced on unpatterned substrates with the polymer chain director perpendicular to the coating direction. Both NEXAFS and device measurements indicate the coating induced orientation is nucleated at the air interface. Nanostructured substrates produce anisotropic bottom contact devices with the polymer chain at the buried interface oriented along the direction of the substrate grooves, independent of coating regime and coating direction. Real time studies of film drying establish that alignment occurs at extremely high polymer volume-fraction conditions, suggesting mediation via a lyotropic phase. In all cases the final films appear to exhibit high degrees of crystalline order. The independent control of alignment at the air and substrate interfaces via coating conditions and substrate treatment, respectively, enable detailed assessment of structure–function relationships that suggest the improved performance of the nanostructure aligned films arise from alignment of the less ordered material in the crystallite interphase regions

Critical Role of Alkyl Chain Branching of Organic Semiconductors in Enabling Solution-Processed N‑Channel Organic Thin-Film Transistors with Mobility of up to 3.50 cm² V^–1 s^–1

Substituted side chains are fundamental units in solution processable organic semiconductors in order to achieve a balance of close intermolecular stacking, high crystallinity, and good compatibility with different wet techniques. Based on four air-stable solution-processed naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)­malononitrile groups (NDI-DTYM2) that bear branched alkyl chains with varied side-chain length and different branching position, we have carried out systematic studies on the relationship between film microstructure and charge transport in their organic thin-film transistors (OTFTs). In particular synchrotron measurements (grazing incidence X-ray diffraction and near-edge X-ray absorption fine structure) are combined with device optimization studies to probe the interplay between molecular structure, molecular packing, and OTFT mobility. It is found that the side-chain length has a moderate influence on thin-film microstructure but leads to only limited changes in OTFT performance. In contrast, the position of branching point results in subtle, yet critical changes in molecular packing and leads to dramatic differences in electron mobility ranging from ∼0.001 to >3.0 cm² V^–1 s^–1. Incorporating a NDI-DTYM2 core with three-branched <i>N</i>-alkyl substituents of C_11,6 results in a dense in-plane molecular packing with an unit cell area of 127 Ų, larger domain sizes of up to 1000 × 3000 nm², and an electron mobility of up to 3.50 cm² V^–1 s^–1, which is an unprecedented value for ambient stable n-channel solution-processed OTFTs reported to date. These results demonstrate that variation of the alkyl chain branching point is a powerful strategy for tuning of molecular packing to enable high charge transport mobilities

Manipulation of Transition Metal Migration via Cr-Doping for Better-Performance Li-Rich, Co-Free Cathodes

The irreversible migration of transition metals is a primary issue, resulting in detrimental structural changes and poor battery performance in Li-rich layered oxide (LLO) cathodes. Herein, we propose that manipulating the migration of transition metals between octahedral and tetrahedral sites effectively inhibits undesirable phase transitions by stabilizing the delithiated structure of LLOs at high potential. This is demonstrated by introducing Cr into the Co-free LLO, Li1.2Ni0.2Mn0.6O2. A new spinel-like phase, accompanied by significant lattice variation, was observed in the heavily cycled Co-free LLO at high potential by using operando synchrotron characterizations. Benefiting from a well-maintained solid-solution reaction after long-term cycling, Cr-doped Li1.2Ni0.2Mn0.6O2 delivers up to 99% of its initial discharge capacity after 200 cycles at 1C (∼200 mAh g–1), far surpassing the pristine material (∼74%). The work provides valuable insights into the structural degradation mechanisms of LLOs and underscores the importance of stabilizing the delithiated structure at high potential

Single Crystal X-ray, AFM, NEXAFS, and OFET Studies on Angular Polycyclic Aromatic Silyl-Capped 7,14-Bis(ethynyl)dibenzo[<i>b</i>,<i>def</i>]chrysenes

The impact of molecular packing and alignment of 7,14-bis­((triethylsilyl)­ethynyl)­dibenzo­[<i>b,def</i>]­chrysene (TES-DBC) and 7,14-bis­((triisopropylsilyl)­ethynyl)­dibenzo­[<i>b,def</i>]­chrysene (TIPS-DBC) on OFET performance was investigated. The bulk solid state packing of these angular polycyclic aromatic hydrocarbons (PAHs) was analyzed via single crystal X-ray analysis, and their molecular stacking arrangements on HMDS modified SiO₂ substrate were studied using near edge X-ray absorption fine structure spectroscopy (NEXAFS) at the carbon K-edge. Our studies found that TES- and TIPS-DBC have significantly different solid state packing arrangements and tilt angles, yet have OFET mobilities that are comparable at 1.6 × 10^–3 and 1.0 × 10^–3 cm²/Vs, respectively. The deposition method also has a dramatic impact on film morphology