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Biocompatibility and Conductivity Assessment of PANI-PAAMPSA for Neural Recording Applications
No full textEffects of Solution and Film Annealing on Morphology and Performance in Polythiophene-Fullerene Solar Cells
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Developing non-invasive processing methodologies and understanding the materials properties of solution-processable organic semiconductors for organic electronics
Get PDFtextEssential to the success of organic electronics, and in particular organic thin-film
transistors, is the realization of stable, high-mobility, electrically-active organic materials
that can enable low-cost solution-based processing methods. The development of viable
solution-processable organic semiconductors helps make this possible. Consequently,
understanding the materials properties of solution-processable organic semiconductors
and how the processing conditions associated with device fabrication affect device
performance are key to realizing low-cost organic electronics. In this work, we focused
on understanding the processing-structure-property relationships of a solutionprocessable
organic semiconductor, triethylsilylethylnyl anthradithiophene (TES ADT).
Specifically, we demonstrated how a solvent-vapor annealing process can induce the
crystallization of TES ADT post device processing. Bottom-contact thin-film transistors
with annealed TES ADT routinely exhibit an average charge-carrier mobility of 0.1
cm
2
/V-s, which is sufficient to drive backplane circuitry in flexible display applications.
Additionally, we demonstrated that the manner in which source and drain
electrodes are defined significantly affects the performance of the resulting TES ADT
thin-film transistors. Specifically, the yield of functioning top-contact TES ADT thinfilm
transistors with electrodes defined by evaporation through a shadow mask directly
on the organic semiconductor is low, and of the functioning devices, the charge-carrier
mobility varies significantly (0.01 – 0.1 cm2
/V-s). In comparison, top-contact TES ADT
thin-film transistors with electrodes defined separately and then laminated against the
organic semiconductors have high yield and high charge-carrier mobility (0.2 ± 0.06
cm
2
/V-s). This result emphasizes the importance of adapting existing or developing new
thin-film transistor fabrication techniques to overcome the materials limitations of
organic semiconductors. Along the same vein, we also demonstrated an elastomeric
stamp-based, solventless printing process, nanotransfer printing (nTP), for the additive
patterning of copper electrodes and interconnects of feature sizes 1 – 500 μm. These
printed copper patterns differ from similarly printed gold patterns in that they are not
electrically conductive. Leaching the elastomeric stamps in hot toluene prior to printing,
however, allowed us to routinely print conductive copper features with an average
resistivity of 31 μΩ-cm.
Another aspect of thin-film transistor fabrication that is crucial for optimal device
performance (i.e., low off currents and low leakage currents) is the patterning and
isolation of the organic semiconductor between neighboring devices. We demonstrated
two novel techniques for patterning TES ADT. The first technique utilizes UV light in
the presence of dichloroethane vapors to simultaneously pattern and crystallize TES
ADT. TES ADT thin-film transistors patterned with this technique exhibit high chargecarrier
mobility (0.1 cm2
/V-s) and low off currents (10-11 A). The second patterning
technique uses a PDMS stamp to selectively remove TES ADT from the non-channel
regions of the thin-film transistor. This technique can be used to pattern both as-spun and
crystalline TES ADT thin films. Crystalline TES ADT thin-film transistors patterned
with this technique exhibit an average charge-carrier mobility of 0.2 cm2
/V-s and low off
currents on the order of 10-11 A, while amorphous TES ADT thin films that are first
patterned and then crystallized exhibit an average charge-carrier mobility of 0.1 cm2
/V-s
and off currents on the order of 10-10 A.Chemical Engineerin
Structure-function Relationships of Organic Semiconductors in Electronic Devices
No full textOrganic field-effect transistors (OFETs) are essential components of organic semiconductor-based electronics. In order to engineer the best performance from OFETs, it is desirable to understand the processing-structure-function relationships present in the organic active layer of these devices. To elucidate the processing-structure-function relationships present in thin films of contorted hexabenzocoronene (HBC), I examined the effect of changing the active layer film thickness of HBC OFETs on the device’s performance. Fabricating OFETs from thin films with thicknesses ranging from 15 – 250 nm, I first determined device mobilities, then determined the orientational and growth kinetics characteristics of these films using grazing incidence x-ray diffraction, near edge x-ray absorption fine structure spectroscopy, and in situ microscopy during isothermal crystallization experiments. Although mobility magnitudes were not reproducible across multiple data sets, HBC gradient thickness OFETs showed an increase in mobility with increasing film thickness between 30 – 70 nm within the same data set, corresponding with an increase in the proportion of crystals with “edge-on” oriented pi planes perpendicular to the substrate. Crystal growth and nucleation modes do not appear to be primary factors in determining the mobility increase, although differences in spherulite characteristics could be responsible for the variation across data sets
Organic photovoltaics: tracking the open-circuit voltage of bulk-heterojunction solar cells comprising non-fullerene ternary blends
No full textOrganic solar cells, with attractive qualities, such as mechanical flexibility and
solution processability, have the potential to be useful alternatives to their silicon
counterparts in unique form factor applications if their power conversion
efficiencies can be improved. One processing method shown to improve OPV
efficiency is ternary blend active layers, comprised of two electron acceptors and a
donor, or two electron donors and an acceptor. It has been proven that the open
circuit voltage (VOC) can be tuned in ternary blends comprised of two polymer
donors and a small molecule acceptor or of one donor and two fullerene acceptors;
the tunability of the VOC is dependent on the chemical and compatibility of the two
polymer donors or the two fullerene acceptors. In this thesis, the behavior of binary
and ternary blends comprised of a single donor (P3HT, a common polymer donor)
and three structurally similar non-fullerene acceptors was characterized. Two
ternary systems were demonstrated to exhibit tunable VOCs across the acceptor
composition range explored, while holding P3HT fraction constant. The binary and
ternary systems were characterized from an electronic as well as morphological
perspective through methods including ionization potential (IP) measurements, UV-visible spectroscopy (UV-vis), differential scanning calorimetry (DSC), grazing-incidence
X-ray diffraction (GIXD), and Flory-Huggins interaction (χ) parameters. In
agreement with the literature, both acceptor pairs were found to be highly miscible
and to have small energetic offsets, meeting the requirements for VOC tunability
Synthesis and Study of Supersized Heteroaromatic Small Molecules for Organic Photovoltaics
No full textOrganic photovoltaics (OPVs) are promising alternatives to conventional inorganic solar cells because of their potential for low cost (low-energy) fabrication, and suitability for lightweight, flexible devices. Polycyclic aromatic hydrocarbons (PAHs) are utilized as active ingredients in OPVs because they can be strongly absorbing, chemically stable, and have properties that are highly tunable through organic synthesis. In particular, contorted
hexabenzocoronene (cHBC) is shape-complementary with commonly used fullerene
electron acceptors, but is limited by poor absorption overlap with the solar spectrum.
Replacing benzene rings on cHBC with heterocyclic moieties, such as benzofuran, results in a bathochromic shift that enables improved visible light absorption. Despite these improvements, heteroatom-containing cHBC derivatives do not absorb broadly between 400 - 500 nm, instead exhibiting more local absorption around 400 nm and 500 nm. This thesis reports the synthesis and characterization of contorted tetrabenzofuranyltetrabenzocircumbiphenyl
(cTBFTBCB), a novel heteroaromatic derivative of contorted octabenzocircumbiphenyl (cOBCB) that incorporates benzofuran moieties, and the device properties of solar cells comprising this material. The parent compound, cOBCB, shows strong visible absorption between 400 - 500 nm and has been used in solar cells that exhibit power conversion efficiencies of 2.9%. cTBFTBCB exhibits strong absorption from 400 - 500 nm, as well as at 550 nm. This material has the narrowest optical band gap of any contorted PAH reported thus far and shows promise for applications in OPVs
Biomass Pyrolysis Pathways to Produce Low-Carbon Transportation Fuel
No full textSustainable biofuels can serve as an alternative to petroleum-derived
transportation fuels. These synfuels can be integrated with conventional gasoline and
diesel without altering current infrastructure in the transportation sector. Depending on
the conversion process, biofuels can have a low or net negative carbon footprint,
providing a means of reducing atmospheric carbon dioxide levels.
Pyrolysis is a thermochemical conversion pathway involving the degradation of
cellulose, hemicellulose, and lignin components of biomass. Biomass pyrolysis products
include non-condensable gases, condensable vapors (both hydrocarbon and aqueous), and
solid char. The condensed hydrocarbon liquid, called pyrolysis-oil, is the desired product
for transportation fuel production, but high oxygen content (~ 40 wt%) prevents direct
use in motor vehicles. Upgrading processes can reduce oxygen levels, but in general
cannot produce the high quality fuels required to replace gasoline and diesel. Advanced
pyrolysis techniques, which directly integrate traditional upgrading processes into the
pyrolysis reactor, on the other hand, can produce high quality transportation fuels. KiOR
has recently commercialized one such pathway.
Researchers at Gas Technology Institute (GTI) have developed a particularly
interesting pilot scale catalytic pyrolysis pathway, which integrates catalytic
hydrotreatment with thermal pyrolysis. Process designs based around GTI¿s Integrated
Hydropyrolysis and Hydroconversion (IH) pathway were evaluated to determine
technical and economic feasibility of producing liquid transportation fuels. The technoeconomic
analyses of configurations that co-process natural gas with and without
electricity generation, both yield a levelized cost of fuel of 28-59/tCO avoided. Ultimately, processes modeled on GTI¿s IH system can provide a
renewable, low carbon liquid transportation fuel
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