14 research outputs found
Direct Calorimetric Observation of the Rigid Amorphous Fraction in a Semiconducting Polymer
The performance of
polymeric semiconductors is profoundly affected
by the thermodynamic state of its crystalline and amorphous fractions
and how they affect the optoelectronic properties. While intense research
has been conducted on the crystalline features, fundamental understanding
of the amorphous fraction(s) is still lacking. Here, we employ fast
scanning calorimetry to provide insights on the glass transition of
the archetypal conjugated polymer polyĀ(3-hexylthiophene) (P3HT). According
to the conceptual definition of the glass transition temperature (<i>T</i><sub>g</sub>), that is, the temperature marking the crossover
from the melt in metastable equilibrium to the nonequilibrium glass,
an enthalpy relaxation should be observed by calorimetry when the
glass is aged below <i>T</i><sub>g</sub>. Thus, we are able
to identify the enthalpy relaxations of mobile and rigid amorphous
fractions (MAF and RAF, respectively) of P3HT and to determine their
respective <i>T</i><sub>g</sub>. Our work moreover highlights
that the RAF should be included in structural models when establishing
structure/property interrelationships of polymer semiconductors
Reversibly Slowing Dewetting of Conjugated Polymers by Light
Dewetting, i.e., the retraction of
a fluid from a surface it ādislikesā,
is a macroscopic phenomenon controlled through parameters like viscosity
and surface tension on length-scales much larger than the size of
the molecules. So far, dewetting was known to proceed in the same
manner, independent of the dewetting film being illuminated by light
or not, e.g., through an optical microscope. Here, we demonstrate
that the velocity of dewetting of conjugated polymers can be reversibly
tuned through appropriate exposure to light. We relate this observation
to the absorption of photons of suitable energy resulting in the generation
of excitons which may partially delocalize along and across polymer
chains and so induce changes in polymer chain conformation. Such changes,
in turn, may cause stiffening or overlap of polymer chains and thus
lead to macroscopically detectable differences in behavior of an ensemble
of conjugated molecules expressed via material properties like viscosity
Coexisting Glassy Phases with Different Compositions in NFA-Based Bulk Heterojunctions
Organic solar cell (OSC) bulk heterojunctions (BHJ) typically
feature
a rich phase morphology with the phase composition and distribution
significantly affecting processes such as charge generation, recombination
and extraction, and in turn, device performance. While fullerene-based
BHJs are relatively well understood structurally, especially when
blends with a flexible-chain donor are employed, donor: non-fullerene
acceptor (NFA) blends are more challenging to elucidate. The reason
is that NFAs often display different polymorphs; moreover, their glassy
states can be complex. Focusing on blends of the widely investigated
donor polymer, poly(3-hexylthiophene-2,5-diyl) (P3HT), and the prototype
NFA, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2ā²,3ā²-dā²]-s-indaceno[1,2-b:5,6-bā²]dithiophene
(ITIC), we reveal here the coexistence of two glassy phases: a molecularly
intermixed and an ITIC-rich one. In P3HT-rich blends, both glassy
phases are present as nanosized domains, evenly distributed in the
BHJ, as visualized via vapor phase infiltration (VPI) āstainingā.
In contrast, the 1:1 (by weight) and NFA-rich blends show clear, lateral
phase separation between large (>500 nm) domains of the glassy
phases
and thinner polymer-rich domains that are unaffected by annealing.
Our observations help to explain earlier P3HT: ITIC device studies;
and also highlight the complexity of NFA-based BHJs, emphasizing the
need for a deeper understanding of the phase behavior of such systems
Designing Small Molecules as Ternary Energy-Cascade Additives for Polymer:Fullerene Solar Cell Blends
Designing Small
Molecules as Ternary Energy-Cascade
Additives for Polymer:Fullerene Solar Cell Blend
Effects of Side-Chain Length and Shape on Polytellurophene Molecular Order and Blend Morphology
We investigate the
molecular order and thin film morphology of
the conjugated polymer polytellurophene, in order to understand how
the tellurium atom and the choice of side-chain influence the conjugated
polymerās backbone planarity and performance in organic transistors.
We find that polyĀ(3hexyltellurophene) (P3HTe) continues the trend
from polythiophene (P3HT) to polyselenophene (P3HS): substitution
with Tellurium leads to a more planar backbone, evident from the shifts
of the Cī»C vibrational peak to lower wavenumbers (ā¼1389
cm<sup>ā1</sup>) and a smaller optical band gap (ā¼1.4
eV). Resonant Raman spectroscopy revealed that molecular order was
highly dependent on the structure of the P3ATe alkyl side-chain: a
longer chains introduces kinetic hindrance, reducing the fraction
of ordered phase obtained at room temperature, while a branched side-chain
introduces steric hindrance, with intrinsic disorder present even
when deposited at higher temperatures. When blended with the insulator
HDPE, all three polymers exhibit little additional disorder and instead
form phase-separated networks of high molecular order that are beneficial
to percolated charge transport in transistors. We find that molecular
order, as measured by Raman, correlates well with reported transistor
mobilities and provides a greater understanding of the structureāproperty
relationships that determine the performance of these novel organometallic
polymers in electronic devices
Bis(triisopropylsilylethynyl)pentacene/Au(111) Interface: Coupling, Molecular Orientation, and Thermal Stability
The
assembly and the orientation of functionalized pentacene at the interface
with inorganics strongly influence both the electric contact and the
charge transport in organic electronic devices. In this study electronic
spectroscopies and theoretical modeling are combined to investigate
the properties of the bisĀ(triisopropylsilylethynyl)Āpentacene (TIPS-Pc)/Au(111)
interface as a function of the molecular coverage to compare the molecular
state in the gas phase and in the adsorbed phase and to determine
the thermal stability of TIPS-Pc in contact with gold. Our results
show that in the free molecule only the acene atoms directly bonded
to the ligands are affected by the functionalization. Adsorption on
Au(111) leads to a weak coupling which causes only modest binding
energy shifts in the TIPS-Pc and substrate core level spectra. In
the first monolayer the acene plane form an angle of 33 Ā± 2Ā°
with the Au(111) surface at variance with the vertical geometry reported
for thicker solution-processed or evaporated films, whereas the presence
of configurational disorder was observed in the multilayer. The thermal
annealing of the TIPS-Pc/Au(111) interface reveals the ligand desorption
at ā¼470 K, which leaves the backbone of the decomposed molecule
flat-lying on the metal surface as in the case of the unmodified pentacene.
The weak interaction with the metal substrate causes the molecular
dissociation to occur 60 K below the thermal decomposition taking
place in thick drop-cast films
Conjugated Copolymers of Vinylene Flanked Naphthalene Diimide
We report the synthesis of a novel
naphthalene diimide (NDI) monomer
containing two (tributylĀstannyl)Āvinyl groups. The utility of
this building block is demonstrated by its copolymerization with five
different electron-rich comonomers under Stille conditions. The resulting
high molecular weight polymers show red-shifted optical absorptions
in comparison to the analogous polymers without the vinylene spacer
and a significant increase in the intensity of the low-energy intramolecular
charge transfer band. The polymers all exhibit ambipolar behavior
in bottom-gate, top-contact organic thin-film transistors. The insertion
of a solution-processed barium hydroxide layer between the polymer
and the gold electrode led to unipolar behavior with improved electron
mobilities
Comparative Optoelectronic Study between Copolymers of Peripherally Alkylated Dithienosilole and Dithienogermole
Here we report a simple methodology for the synthesis
of dithienosilole
and dithienogermole monomers in which the necessary solubilizing long
chain alkyl groups are incorporated into the peripheral 3,5-positions
of the fused ring. We report four novel monomers in which methyl or
butyl groups are attached to the bridging Si and Ge atom. Copolymers
with bithiophene were synthesized by a Stille polymerization in high
molecular weight. We report the optical, electrical, electrochemical
and morphological properties of the resulting polymers. We find that
the nature of the bridging heteroatom (Si or Ge) has only a minor
influence on these properties, whereas the nature of the alkyl chain
attached to the bridging atom is found to have a much larger effect
Temperature-Dependence of Persistence Length Affects Phenomenological Descriptions of Aligning Interactions in Nematic Semiconducting Polymers
Electronic and optical
properties of conjugated polymers are strongly
affected by their solid-state microstructure. In nematic polymers,
mesoscopic order and structure can be theoretically understood using
MaierāSaupe (MS) models, motivating us to apply them to conjugated
macromolecular systems and consider the problem of their material-specific
parametrization. MS models represent polymers by worm-like chains
(WLC) and can describe collective polymer alignment through anisotropic
MS interactions. Their strength is controlled by a phenomenological
temperature-dependent parameter, Ļ
Ā(<i>T</i>). We undertake
the challenging task of estimating material-specific Ļ
Ā(<i>T</i>), combining experiments and Self-Consistent Field theory
(SCFT). Considering three different materials and a spectrum of molecular
weights, we cover the cases of rod-like, semiflexible, and flexible
conjugated polymers. The temperature of the isotropicānematic
transition, <i>T</i><sub>IN</sub>, is identified via polarized
optical microscopy and spectroscopy. The polymers are mapped on WLC
with temperature-dependent persistence length. Fixed persistence lengths
are also considered, reproducing situations addressed in earlier studies.
We estimate Ļ
Ā(<i>T</i>) by matching <i>T</i><sub>IN</sub> in experiments and SCFT treatment of the MS model.
An important conclusion is that accounting explicitly for changes
of persistence length with temperature has significant qualitative
effects on Ļ
Ā(<i>T</i>). We moreover correlate our
findings with earlier discussions on the thermodynamic nature of phenomenological
MS interactions
Effects of a Heavy Atom on Molecular Order and Morphology in Conjugated Polymer:Fullerene Photovoltaic Blend Thin Films and Devices
We study the molecular order and morphology in poly(3-hexylthiophene) (P3HT) and poly(3-hexylselenophene) (P3HS) thin films and their blends with [6,6]-phenyl-C<sub>61</sub>-butyric acid methyl ester (PCBM). We find that substitution of the sulfur atoms in the thiophene rings of P3HT by heavy selenium atoms increases the tendency of the molecules to form better ordered phase; interestingly, their overall fraction of ordered phase is much lower than that of P3HT-based films. The higher tendency of P3HS molecules to order (aggregate) is consistent with more planar chain conformation simulated. The lower fraction of ordered phase (or the higher fraction of disordered phase) in P3HS-based films is clearly identified by in-plane skeleton Raman modes under resonant excitation conditions, such as a smaller ratio of the Cī»C modes associated with the ordered (ā¼1422 cm<sup>ā1</sup>) and disordered (ā¼1446 cm<sup>ā1</sup>) phases (<i>I</i><sub>1422cm<sup>ā1</sup></sub>/<i>I</i><sub>1446cm<sup>ā1</sup></sub> = 1.4 for P3HS and 0.6 for P3HS:PCBM), compared with P3HT-based films (<i>I</i><sub>1449cm<sup>ā1</sup></sub>/<i>I</i><sub>1470cm<sup>ā1</sup></sub> = 2.5 for P3HT and 1.0 for P3HT:PCBM) and a larger Raman dispersion of the Cī»C mode: P3HS (17 cm<sup>ā1</sup>) <i>versus</i> P3HT (6 cm<sup>ā1</sup>) and P3HS:PCBM (36 cm<sup>ā1</sup>) <i>versus</i> P3HT:PCBM films (23 cm<sup>ā1</sup>). The higher fraction of disordered phase in P3HS prevents the formation of micrometer-sized PCBM aggregates in blend films during thermal annealing. Importantly, this lower fraction but better quality of ordered phase in P3HS molecules strongly influences P3HS:PCBM photovoltaic performance, producing smaller short-circuit current (<i>J</i><sub>sc</sub>) in pristine devices, but significantly larger increase in <i>J</i><sub>sc</sub> after annealing compared to P3HT:PCBM devices. Our results clarify the effects of heavy atom substitution in low band gap polymers and their impact on blend morphology and device performance. Furthermore, our study clearly demonstrates resonant Raman spectroscopy as a simple, but powerful, structural probe which provides important information about āfraction/quantity of ordered phaseā of molecules, not easily accessible using traditional X-ray-based techniques