Controlling morphology and molecular order of solution-processed organic semiconductors for transistors

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.