12 research outputs found
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:PC<sub>71</sub>BM 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 PC<sub>71</sub>BM domains systematically decreasing with increasing polymer
molecular weight. Furthermore, the relative purity of the mixed PTB7:PC<sub>71</sub>BM 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 PC<sub>71</sub>BM 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<sup>–1</sup> are required for DIO to be effective in reducing the PC<sub>71</sub>BM 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 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>. 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><sub>2<i>h</i></sub> symmetry
exhibit electron mobilities of 0.02 and 0.15 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, respectively, while <b>QBTT-</b><i><b>s</b></i> exhibits an unexpectedly
high electron mobility of 0.32 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> with <i>I</i><sub>on</sub>/<i>I</i><sub>off</sub> ratios of ≤10<sup>6</sup>. 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><sub>2<i>h</i></sub> 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 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> 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 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>
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 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>. Incorporating a NDI-DTYM2 core with three-branched <i>N</i>-alkyl substituents of C<sub>11,6</sub> results in a dense
in-plane molecular packing with an unit cell area of 127 Å<sup>2</sup>, larger domain sizes of up to 1000 × 3000 nm<sup>2</sup>, and an electron mobility of up to 3.50 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, 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 SiO<sub>2</sub> 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<sup>–3</sup> and 1.0 ×
10<sup>–3</sup> cm<sup>2</sup>/Vs, respectively. The deposition
method also has a dramatic impact on film morphology