11 research outputs found
INFLUENCE OF MOLECULAR STRUCTURES ON THE PROPERTIES OF COMPLEMENTARY SEMICONDUCTING POLYMER BLENDS
Semiconducting polymers are considered as the materials for the next generation large-area and flexible electronics. In the past several decades, extensive effort has been to understand and improve the charge transport properties of semiconducting polymers. Chapter one is an introduction to organic semiconductors and complementary semiconducting polymer blends. Complementary semiconducting polymer blends is recently proposed and established by our research group. The complementary semiconducting polymer blends consist of a matrix polymer (main component, with intentionally introduced flexible conjugation break spacer along the polymer backbone) and a tie chain polymer (fully-conjugated polymer). Previous studies on these blends have shown that with 1 wt% of the tie chain polymer in the matrix polymer, charge carrier mobility becomes two orders of magnitude higher than the matrix polymer and lies in the same order of the tie chain polymer. The subsequent chapters describe the synthesis and characterizations of semiconducting polymers with different structural features. Chapter two details the influence of the length in the matrix polymer to the physical properties and charge transport of the matrix polymer and the complementary semiconducting polymer blends. The results reveal that with longer side chains, matrix polymers exhibit lower melting points, higher crystallinity and lower charge carrier mobilities. Profound odd-even effects are also observed in these properties. As for the complementary semiconducting polymer blends, charge carrier mobilities is less sensitive to the conjugation-break spacers. Chapter three presents a series of matrix polymers with different side chains and the physical properties and charge transport of these polymers and the complementary semiconducting polymer blends. The chapter describes that side chain has a significant influence on the physical properties of the polymers. Specifically, polymers with asymmetric side chains exhibit much lower melting points than the symmetric analogues. As for charge carrier mobilities, polymers with alkyl and siloxane-terminated side chains show similar properties, whereas the presence of oligoether side chain leads to a decrease for orders of magnitudes. Chapter four focus on the influence of the molecular weight of the tie chain polymer on the charge carrier mobilities of the complementary semiconducting polymer blends. The result shows that although for the tie chain polymer itself, molecular weights higher than a certain region leads to decrease in charge carrier mobilities, for the complementary semiconducting polymer blends tie chain polymer with higher molecular weight is in favor of charge transport
Development and Evaluation of Low-Cost CO2 Sensors for Buildings
There is a significant opportunity to improve building energy efficiency and indoor environmental quality by accurately monitoring CO2 levels. However, current CO2 sensors tend to be expensive or require regular recalibration. This work presents research related to the initial development and evaluation of two novel CO2 sensors based on chemiresistive and resonant mass sensing techniques. Prototype sensors were assessed in a bench-top test chamber at temperatures, humidity levels, and CO2 concentrations, typical of indoor environments. Under these conditions, prototype sensors required only 60 mW of power, or less. Further, each sensor was developed to have a footprint of less than 25 mm2 and a cost of less than $50. Given the relative low cost, small size, and potential for low power consumption, these sensors may serve as an attractive alternative to the commercial CO2 sensors that are currently available
Attaining Melt Processing of Complementary Semiconducting Polymer Blends at 130 °C via Side-Chain Engineering
Complementary
semiconducting polymer blends (c-SPBs) have been proposed and tested
to achieve melt-processed high-performance organic field-effect transistors
(OFETs). Prior to this study, melt processing requires temperatures
as high as 180 °C. To implement this technique into low-cost
and large-area thin-film manufacturing for flexible organic electronics,
semiconducting materials meltable at temperatures tolerable by ubiquitous
plastic substrates are still needed. We report here the design and
melt processing of a c-SPB consisting of a matrix polymer (DPP-C5)
and its fully conjugated analogue. By utilizing a siloxane-terminated
alkyl chain and a branched alkyl chain as solubilizing groups, the
matrix polymer DPP-C5 presents a melting temperature of 115 °C.
The resulting c-SPB containing as low as 5% of the fully conjugated
polymer could be melt-processed at 130 °C. The obtained OFET
devices exhibit hole mobility approaching 1.0 cm<sup>2</sup>/(V s),
threshold voltages below 5 V, and <i>I</i><sub>ON</sub>/<i>I</i><sub>OFF</sub> around 10<sup>5</sup>. This combination
of efficient charge-carrier transport and considerably low processing
temperatures bode well for melt processing of semiconducting polymer-based
organic electronics
Conjugation-Break Spacers in Semiconducting Polymers: Impact on Polymer Processability and Charge Transport Properties
Conjugation-break
spacers (CBSs) are intentionally introduced into
the diketopyrrolopyrrole (DPP)-based polymer backbones. We reveal
that the solution processability progressively increases with the
percentage of CBSs, while charge mobility inversely varies to the
CBS ratio. For instance, the polymer DPP-30 with solubility of ∼10
mg/mL in dichlorobenzene provides an average mobility over 1.4 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, while DPP-0
exhibits an average mobility of 4.3 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> with solubility of ∼3 mg/mL. This correlation
provides a general guidance to design polymers with desired electronic
performance and solution processability for large-scale roll-to-roll
processing. Most encouraging, DPP-70 can be melt processed in air
and provide hole mobilities up to 0.30 cm<sup>2</sup> V<sup>–1 </sup>s<sup>–1</sup>, substantially higher value than their solution-processed
counterparts about 0.1 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>. The mobility boost in melt-processed devices, together
with completely eliminating the need to use toxic solvent in the processing,
encourages to design melt-processable polymers for electronic devices
Complementary Semiconducting Polymer Blends for Efficient Charge Transport
Charge
transport in polymeric thin films is a complicated process,
which involves a multitude of coupled electronic events. Because of
the growing appeal of semiconducting polymers in organic electronics,
it makes the fundamental understanding of charge transport increasingly
important. On the other hand, it urges the solution of the processability
problem, frequently associated with high-performance polymers. In
this study, we introduce complementary semiconducting polymer blends
(<i>c</i>-SPBs), aiming to provide solutions for both the
fundamental understanding of charge transport and the processability
problem. The <i>c</i>-SPBs contain a highly crystalline
matrix polymer with intentionally placed conjugation-break spacers
(CBSs) along the polymer backbone, thus eliminating intrachain transport,
and a tie chain polymer that is a fully conjugated polymer, restoring
intrachain transport by connecting π-crystalline aggregates
in the matrix polymer. The results show that the addition of as little
as 1 wt % tie chain polymer into the matrix polymer induces a nearly
2 order of magnitude improvement in charge carrier mobility from ∼0.015
to 1.14 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, accompanied by substantial lowering of activation energies from
100.1 to 64.6 meV. The morphological characterizations and electrical
measurements confirm that tie chains are able to build the connectivity
between crystalline aggregates, leading to efficient charge transport
in the polymer blend films. Furthermore, this study suggests that <i>c</i>-SPBs can be a new platform for designing high-mobility
electronic materials with enhanced solution processability for future
organic electronics
Complementary Semiconducting Polymer Blends: The Influence of Conjugation-Break Spacer Length in Matrix Polymers
The
concept of complementary semiconducting polymer blends (<i>c</i>-SPBs) for efficient charge transport was recently proposed
and established by our group. In this study, we aim to reveal the
influence of the length of conjugation-break spacers (CBSs) on charge
transport properties of the matrix polymers and their corresponding
complementary polymer blends. A series of 11 DPP-based semiconducting
polymers DPP-C<i>m</i> (<i>m</i> = 2–12)
that incorporate CBSs of 2–12 methylene units along the polymer
backbones were prepared and characterized. The UV–vis spectra
and the ultraviolet photoelectron spectroscopy (UPS) measurements
show that the CBS length has marginal influence on the polymer absorption
spectra, energy levels, and band gaps. It also has little impact on
polymer decomposition temperatures. However, the CBS length has a
profound influence on polymer phase transition and the heat of fusion.
As for the melt transitions, an odd–even effect is observed
from DPP-C2 to DPP-C7, in which polymers with even-numbered CBSs show
higher melting points than their adjacent odd-numbered derivatives.
The trend is opposite for heat of fusion. The polymers with odd-numbered
CBSs exhibit larger heat of fusion, indicating higher ordering and
crystallinity. The odd–even effect is also found in surface
morphologies of the polymers by atomic force microscopy (AFM). The
polymers with the even CBSs have a more interconnected feature that
appear more fibrillar than the polymers with the odd linkages. As
far as charge carrier mobility is concerned, the average number drops
from 0.023 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> to 7.9 × 10<sup>–6</sup> cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> as the CBS moves from C2 to C12. It is intriguing
to observe that even-numbered polymers outperform the adjacent odd-numbered
polymers, despite the fact that the latter show higher ordering and
crystallinity in thin films. When these polymers are mixed with fully
conjugated DPP-C0 (2 wt %, designated as tie chain polymer), the obtained <i>c</i>-SPBs witness a dramatic increase (2–4 orders of
magnitude) in charge carrier mobility. Interestingly, the odd–even
effect is not found for charge transport in the <i>c</i>-SPBs. This work reveals that the length of CBSs plays a significant
role in charge transport properties of the matrix polymers and reconfirms
that efficient charge transport properties of the <i>c</i>-SPB result from the interactions between matrix polymers and tie
chain polymers. This begins to provide guidelines as to what spacer
lengths may be utilized to offer the best balance between processing
and charge transport properties
Symmetry Breaking in Side Chains Leading to Mixed Orientations and Improved Charge Transport in Isoindigo-<i>alt</i>-Bithiophene Based Polymer Thin Films
The selection of
side chains is important in design of conjugated polymers. It not
only affects their intrinsic physical properties, but also has an
impact on thin film morphologies. Recent reports suggested that a
face-on/edge-on bimodal orientation observed in polymer thin films
may be responsible for a three-dimensional (3D) charge transport and
leads to dramatically improved mobility in donor–acceptor based
conjugated polymers. To achieve a bimodal orientation in thin films
has been seldom explored from the aspect of molecular design. Here,
we demonstrate a design strategy involving the use of asymmetric side
chains that enables an isoindigo-based polymer to adopt a distinct
bimodal orientation, confirmed by the grazing incidence X-ray diffraction.
As a result, the polymer presents an average high mobility of 3.8 ±
0.7 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> with
a maximum value of 5.1 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, in comparison with 0.47 and 0.51 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> obtained from the two reference
polymers. This study exemplifies a new strategy to develop the next
generation polymers through understanding the property-structure relationship
Complementary Semiconducting Polymer Blends: Influence of Side Chains of Matrix Polymers
The concept of complementary semiconducting
polymer blends (<i>c</i>-SPBs) has been recently proposed
to achieve enhanced solution
processability and/or melt-processing capability for organic electronics.
In the previous study, we demonstrated the impact of conjugation-break
spacers of matrix polymers. In the current work, we explore the influence
of the side chains of the matrix polymer on the physical properties
of the pure polymers and their corresponding <i>c</i>-SPBs,
including electrical properties and phase transition behaviors. Six
diketoÂpyrrolopyrrole (DPP)-based polymers with pentamethylene
conjugation-break spacers (CBSs) and various side chains, including
branched-alkyl, triethylene glycol (TEG), and siloxane-terminated
side chains, were synthesized and characterized. The UV–vis
spectra show that the side chains have a noticeable impact on the
intermolecular interactions in the solid states. In addition, side
chains also have a significant influence on the thermal behaviors
of the polymers. Polymers with asymmetric side chains attached to
the same DPP unit exhibit lower melting points compared to the congeners
with symmetric side chains. The polymer with both branched-alkyl and
TEG side chain exhibits the lowest melting point of 104 °C. As
for charge transport properties, polymers with branched-alkyl and/or
siloxane-terminated side chains give hole mobilities on the same order
of magnitude, whereas the polymers with TEG side chains exhibits much
lower mobilities. When <i>c</i>-SPBs with a fully conjugated
polymer with branched-alkyl side chains are concerned, the <i>c</i>-SPBs of all polymers, except for the polymer with only
TEG side chains (TEG-DPP-C5), show hole mobilities 2 orders of magnitude
higher than the corresponding pure matrix polymers. In contrast, TEG-DPP-C5
merely presents an improvement of 20 times, which resulted from the
incompatibility of TEG side chains from the matrix polymer and the
alkyl side chains from the tie chain polymer. These results provide
new insights into structural design for semiconducting materials with
both high performance and better processability
Understanding Interfacial Alignment in Solution Coated Conjugated Polymer Thin Films
Domain
alignment in conjugated polymer thin films can significantly enhance
charge carrier mobility. However, the alignment mechanism during meniscus-guided
solution coating remains unclear. Furthermore, interfacial alignment
has been rarely studied despite its direct relevance and critical
importance to charge transport. In this study, we uncover a significantly
higher degree of alignment at the top interface of solution coated
thin films, using a donor–acceptor conjugated polymer, polyÂ(diketopyrrolopyrrole-<i>co</i>-thiophene-<i>co</i>-thienoÂ[3,2-<i>b</i>]Âthiophene-<i>co</i>-thiophene) (DPP2T-TT), as the model
system. At the molecular level, we observe in-plane π–π
stacking anisotropy of up to 4.8 near the top interface with the polymer
backbone aligned parallel to the coating direction. The bulk of the
film is only weakly aligned with the backbone oriented transverse
to coating. At the mesoscale, we observe a well-defined fibril-like
morphology at the top interface with the fibril long axis pointing
toward the coating direction. Significantly smaller fibrils with poor
orientational order are found on the bottom interface, weakly aligned
orthogonal to the fibrils on the top interface. The high degree of
alignment at the top interface leads to a charge transport anisotropy
of up to 5.4 compared to an anisotropy close to 1 on the bottom interface.
We attribute the formation of distinct interfacial morphology to the
skin-layer formation associated with high Peclet number, which promotes
crystallization on the top interface while suppressing it in the bulk.
We further infer that the interfacial fibril alignment is driven by
the extensional flow on the top interface arisen from increasing solvent
evaporation rate closer to the meniscus front