4 research outputs found
Nanoscale Domain Imaging of All-Polymer Organic Solar Cells by Photo-Induced Force Microscopy
Rapid nanoscale imaging
of the bulk heterojunction layer in organic
solar cells is essential to the continued development of high-performance
devices. Unfortunately, commonly used imaging techniques such as tunneling
electron microscopy (TEM) and atomic force microscopy (AFM) suffer
from significant drawbacks. For instance, assuming domain identity
from phase contrast or topographical features can lead to inaccurate
morphological conclusions. Here we demonstrate a technique known as
photo-induced force microscopy (PiFM) for imaging organic solar cell
bulk heterojunctions with nanoscale chemical specificity. PiFM is
a relatively recent scanning probe microscopy technique that combines
an AFM tip with a tunable infrared laser to induce a dipole for chemical
imaging. Coupling the nanometer resolution of AFM with the chemical
specificity of a tuned IR laser, we are able to spatially map the
donor and acceptor domains in a model all-polymer bulk heterojunction
with resolution approaching 10 nm. Domain size from PiFM images is
compared to bulk-averaged results from resonant soft X-ray scattering,
indicating excellent quantitative agreement. Further, we demonstrate
that in our all-polymer system, the AFM topography, AFM phase, and
PiFM show poor correlation, highlighting the need to move beyond standard
AFM for morphology characterization of bulk heterojunctions
Domed Silica Microcylinders Coated with Oleophilic Polypeptides and Their Behavior in Lyotropic Cholesteric Liquid Crystals of the Same Polypeptide
Liquid crystals can organize dispersed
particles into useful and
exotic structures. In the case of lyotropic cholesteric polypeptide
liquid crystals, polypeptide-coated particles are appealing because
the surface chemistry matches that of the polymeric mesogen, which
permits a tighter focus on factors such as extended particle shape.
The colloidal particles developed here consist of a magnetic and fluorescent
cylindrically symmetric silica core with one rounded, almost hemispherical
end. Functionalized with helical polyÂ(Îł-stearyl-l-glutamate)
(PSLG), the particles were dispersed at different concentrations in
cholesteric liquid crystals (ChLC) of the same polymer in tetrahydrofuran
(THF). Defects introduced by the particles to the director field of
the bulk PSLG/THF host led to a variety of phases. In fresh mixtures,
the cholesteric mesophase of the PSLG matrix was distorted, as reflected
in the absence of the characteristic fingerprint pattern. Over time,
the fingerprint pattern returned, more quickly when the concentration
of the PSLG-coated particles was low. At low particle concentration
the particles were “guided” by the PSLG liquid crystal
to organize into patterns similar to that of the re-formed bulk chiral
nematic phase. When their concentration increased, the well-dispersed
PSLG-coated particles seemed to map onto the distortions in the bulk
host’s local director field. The particles located near the
glass vial–ChLC interfaces were stacked lengthwise into architectures
with apparent two-dimensional hexagonal symmetry. The size of these
“crystalline” structures increased with particle concentration.
They displayed remarkable stability toward an external magnetic field;
hydrophobic interactions between the PSLG polymers in the shell and
those in the bulk LC matrix may be responsible. The results show that
bio-inspired LCs can assemble suitable colloidal particles into soft
crystalline structures
Polypeptide Composite Particle-Assisted Organization of π‑Conjugated Polymers into Highly Crystalline “Coffee Stains”
We demonstrate that homopolypeptides
covalently tethered to anisotropically shaped silica particles induce
crystalline ordering of representative semiconducting polymers. Films
drop-cast from chloroform dispersions of polyÂ(Îł-stearyl-l-glutamate) (PSLG) composite particles and polyÂ(3-hexythiophene)
(P3HT) led to highly ordered crystalline structures of P3HT. Hydrophobic–hydrophobic
interactions between the alkyl side chains of P3HT and PSLG were the
main driving force for P3HT chain ordering into the crystalline assemblies.
It was found that the orientation of rigid P3HT fibrils on the substrate
adopted the directionality of the evaporating front. Regardless of
the PSLG-coated particle dimensions used, the drop-cast films displayed
patterns that were shaped by the coffee ring and Marangoni effects.
PSLG-coated particles of high axial ratio (4.2) were more efficient
in enhancing the electronic performance of P3HT than low axial ratio
(2.6) homologues. Devices fabricated from the ordered assemblies displayed
improved charge-carrier transport performance when compared to devices
fabricated from P3HT alone. These results suggest that PSLG can favorably
mediate the organization of semiconducting polymers
All-Small-Molecule Nonfullerene Organic Solar Cells with High Fill Factor and High Efficiency over 10%
We synthesized two
wide bandgap A–D–A structured <i>p</i>-type
organic semiconductor (<i>p</i>-OS) small
molecules with weak electron-withdrawing ester end groups: <b>SM1</b> with cyano group (CN) on the ester group and <b>SM2</b> without
the CN group. <b>SM1</b> showed stronger absorption, lower-lying
HOMO energy level, and higher hole mobility in comparison with that
of <b>SM2</b> without the CN groups. The all-small-molecule
organic solar cell (SM-OSC) with <b>SM1</b> as donor and a narrow
bandgap <i>n</i>-OS IDIC as acceptor demonstrated a high
power conversion efficiency (PCE) of 10.11% and a high fill factor
(FF) of 73.55%, while the PCE of the device based on <b>SM2</b>:IDIC is only 5.32% under the same device fabrication condition.
The PCE of 10.11% and FF of 73.55% for the <b>SM1</b>-based
device are the highest values for the nonfullerene SM-OSCs reported
in the literature so far. The results indicate that the cyano substitution
in <b>SM1</b> plays an important role in improving the photovoltaic
performance of the <i>p</i>-OS donors in the nonfullerene
SM-OSC. In addition, the photoinduced force microscopy (PiFM) was
first used in OSCs to characterize the morphology of its donor/acceptor
blend active layer