Abstract
As a potential low-cost alternative to traditional amorphous-silicon based
devices, organic field-effect transistors (OFETs) are expected to be incorporated into
all-plastic integrated circuits and flexible display backplanes. More recently,
breakthroughs have been made in the performance of OFETs based on pi-
conjugated small molecules, among which, tri-isopropylsilylethynyl pentacene (TIPS-PEN)
and its derivatives are currently under extensive investigations due to their
good charge-transport properties combined with decent air-stability, as well as the
possibility of inexpensive solution-processing. Fundamental understanding of the
charge transport is not only important to deepen the understanding of structure-property
relationships of organic functional layers, but also to optimize the
performance of various organic electronic devices. The charge-carrier mobility is a
critical parameter for the operating speed of a device, notably, in an OFET.
Structural inhomogeneity within a single component or between phase-separated
blends has a significant impact on the local charge-transport properties. Thus,
controlling the morphology and molecular order of organic semiconductors is the key
to achieve optimal performance for OFETs.
This thesis is aiming at highly reproducible solution-processed organic
transistors, with device parameters relevant to practical applications (e.g. low
operating-voltages, steep sub-threshold slopes and uniform performance in large
areas), through controlling the morphology and molecular order of small-molecule
organic semiconductors. More specifically, this thesis intends to achieve a balanced
combination of (i) a solvent-based processing method that can manipulate the
morphology of organic semiconductors; (ii) a composite semiconductor formulation
consisting of TIPS-PEN and a binder material such as a polymer; (iii) use of
patterning methods in line with the requirements of large-area electronics, such as
ink-jet printing; and (iv) an improved understanding of charge-transport mechanisms
in (realistic) high-performance transistor devices based on these single-component
or composite semiconductors. This combination results in highly reproducible
solution-processed OFETs exhibiting high mobility as well as decent uniformity in
large areas, as demonstrated throughout the thesis.
Aiming at the first objective (i) of this thesis, in Chapter 2, a new approach
was developed to prepare large single crystals of organic semiconductors, by using
azeotropic binary solvent mixtures. The two solvents form a positive azeotrope and
have significantly different solubilities for TIPS-PEN. At solvent compositions close
to the azeotropic point, an abrupt transition of morphology from polycrystalline thin-films
to large single crystals was observed. We found that the solvent composition at
the late-stage of evaporation determines the final morphology, which can be facilely
controlled by adjusting the initial volume ratio of the binary solvents. The charge-carrier
mobilities were substantially enhanced by a factor of 4, from the morphology
of thin-films to large single crystals used as active layer in OFETs. Additionally, this
approach was extended to other pi-conjugated organic molecules to elucidate its
broad applicability.
To achieve a balanced combination of the objectives (ii) & (iii), i.e. large-area
patterning of composite semiconductors, next, we set out to study the effects of
blending an organic semiconductor with an insulating polymer on the morphology
and transistor performance. In Chapter 3 we presented a systematic study of the
influence of material composition and ink-jet processing conditions on the charge
transport in bottom-gate/bottom-contact OFETs based on single droplets of TIPS-PEN/
polystyrene blends. After careful process optimization of blending ratio and
printing temperature we routinely make transistors with an average mobility of 1
cm2/Vs (maximum 1.5 cm2/Vs), on/off ratio exceeding 107, sharp turn-on in current
(sub-threshold slopes approaching 60 mV/decade, the second steepest value for
OFETs reported so far), and decent uniformity in large areas. These characteristics
are superior to the neat TIPS-PEN devices. Using channel scaling measurements
and scanning Kelvin probe microscopy, the sharp turn-on in current in the blends
was attributed to a contact (tunneling) barrier that originates from a thin insulating
polystyrene layer between the injecting contacts and the semiconductor channel.
These new insights on device operations of our blend transistors provide valuable
guidelines towards next-generation organic transistors based on small-molecule
semiconductor and insulating polymer blends.
Following the knowledge gained in Chapter 3, and in line with the objective
(iv) of this thesis on the fundamental understanding of device operation, a so-called
‘electric field confinement effect’ on charge transport in polycrystalline OFETs was
presented in Chapter 4. It is known that the charge-carrier mobility in organic
semiconductors is only weakly dependent on the electric field at low fields; our
experimental charge-carrier mobility in OFETs using TIPS-PEN was found to be
surprisingly field-dependent at low source-drain fields. Corroborated by scanning
Kelvin probe measurements, we explained this experimental observation by the
severe difference between the local lateral-field dependences within grains and at
grain boundaries. Redistribution of accumulated charges creates very strong local
lateral fields in the latter regions. These strong local fields in the grain boundaries
result in the carrier mobility in grain boundaries to become field-dependent, and as
the mobility in grain-boundaries limits the overall mobility its field-dependence
translates to a field-dependence of the average mobility. We further confirmed this
picture by verifying that the charge-carrier mobility in channels having no grain
boundaries, made from the same type of organic semiconductor, is not significantly
field-dependent. Finally, we showed that our model allows us to "quantitatively"
describe the source-drain field dependence of mobility in polycrystalline OFETs.
Then, we moved to using molecular design to control the morphology and
molecular order of organic semiconductors. In Chapter 5 we presented a new TIPS-PEN
derivative, namely BTE-TIPS-PEN, with ethyl substituents at the 2,3,9,10
backbone positions to modulate the solubility and film-forming properties. High-performance
OFETs were readily fabricated using a single-step process without the
need to form blends or the use of top-gate architecture. Average mobilities above 1
cm2/Vs were measured at low-operating voltages for specific crystal orientations,
with the highest saturation mobility reaching as high as 3.92 cm2/Vs, confirming that
an improved molecular design can indeed result in a controlled macro- and micro-structure
of BTE-TIPS-PEN thin films that positively influences the electronic
properties. The high device reproducibility obtained for BTE-TIPS-PEN is also
promising for the technological exploitation of such discrete devices in large-area
organic electronics.
Next, we demonstrated in Chapter 6, that a careful selection of the casting temperature alone can allow a rapid production of OFETs with uniform and reproducible device performance over large areas. Based on a systematic investigation on the thermal behaviour of 5,11-bis(triethyl silylethynyl) anthradithiophene (TES ADT), we presented four distinctive solid-state phases of TES ADT exhibiting drastically different charge-transport properties, deduced from OFET device characteristics corroborated by Lateral Time-of-Flight (L-ToF) photoconductivity measurements. The best-performing crystal polymorph of TES ADT was identified: when casting solutions of TES ADT dissolved in chloroform at a substrate temperature of more than 20 °C below its glass transition temperature, highly-crystalline and homogeneous TES ADT thin films can be facilely produced in a single-step, without the need for any post-depositions as previously reported, opening pathways towards high-throughput and reliable fabrication of high-performance OFETs.
In Chapter 7 we presented the first highly-reproducible n-type SAMFET, based on a perylene derivative (namely PBI-PA) with a phosphonic acid anchoring group which enables an efficient fixation to aluminum oxide. Simple device fabrication under ambient conditions leads to a complete surface coverage of the aluminum oxide with a monolayer of PBI-PA, and to transistors with electron mobilities up to 10-3 cm2/Vs for channel length as long as 100 µm. By implementing p- and n-type SAMFETs in one circuit, a complementary inverter based solely on SAMFETs, with a large noise margin of 7 volts and a gain up to 17, was demonstrated for the first time, paving the way to robust and low-power self-assembled monolayer based complementary circuits.
As a side topic of this thesis, in the last Chapter, we introduced an
unconventional use of the molecular (polymer chain/dipole) alignment, in the
dielectric layer of organic field-effect transistors. Chapter 8 presented a voltage-programmable
light-emitting field-effect transistor (LEFET) using a ferroelectric
polymer as the gate dielectric. We showed by both experimental observations and
numerical modeling that, when the ferroelectric gate dielectric is polarized in
opposite directions at the drain and source sides of the channel, respectively, both
electron and hole currents are enhanced, resulting in more charge recombination
and ~ 10 times higher light emission in a ferroelectric LEFET, compared to the
device with non-ferroelectric gate. As a result of the ferroelectric poling (dipole
alignment), our ferroelectric LEFETs exhibit repeated programmability in light
emission, and an external quantum efficiency (EQE) of up to 1.06 %. Numerical
modeling revealed that the remnant polarization charge of the ferroelectric layer
tends to ‘pin’ the position of the recombination zone, paving the way to integrate
specific optical out-coupling structures in the channel of these devices to further
increase the brightness.
The results and new insights obtained in this thesis will serve as important
guidelines for the development of new generation solution-processed organic
transistors towards large-area organic (opto-) electronics.
Original language | English |
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Qualification | Doctor of Philosophy |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 24 Sept 2012 |
Place of Publication | Eindhoven |
Publisher | |
Print ISBNs | 978-90-386-3191-2 |
DOIs | |
Publication status | Published - 2012 |