Electroluminescence in ion gel gated organic polymer semiconductor transistors

This thesis reports the light emission in ion gel gated, thin film organic semiconductor transistors and investigates the light emission mechanism behind these devices. We report that ion gel gated organic polymer semiconductor transistors emit light when the drain source voltage is swept slightly beyond the energy gap of the polymer divided by the elementary charge (Vds > Eg/e). In particular, the light emission in poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) polymer semiconductor, with 1-ethyl-3-methylimidazoliumbis (trifluoromethylsulfonyl) imide/ poly(styrene-block-ethylene oxide-block-styrene (EMIM TFSI/ SOS) ion gel as dielectric material is reported. The current-voltage characteristics corresponding to the light emission, where the systematic increase of the drain current, correlated with light emission is reported. In low voltage regime, (Vds < Eg/e), well saturated transistor characteristics are observed. By charge modulation spectroscopy (CMS) study we show that there is a prominent electrochemical doping occurring with gate voltages. Further, owing to the movement of ions with voltages, irrespective of the location of electrodes, we show that the ion gel, bilayer planar devices emit light in Vds > Eg/e regime (without any gate voltages), at room temperature. Based on the location of the recombination zone in the proximity of electron injecting electrode and CMS results showing prominent di ffusion of negative ions into the polymer layer, we conclude that the light emitting mechanism is akin to light emitting electrochemical cells (LECs). Even in the the transistor regime, where Vds << Eg/e, with the signatures of increasing drain current for fixed Vg and Vds values, we show that the transistor can not be of purely electrostatic operation alone. We study the fluorescence quenching of an operating bilayer device under a constant bias over a period of time and compare the results with the electroluminescence of the device and show that the formation of the p-n junction within the polymer layer due to the penetrated ions from the gel dielectric into the polymer semiconductor layer on the application of the voltage is the cause behind the light emission. We show that diffusivity of the cation (EMIM) is very low compared to the anion (TFSI). This is consistent with the fact that the recombination zone is near the electron injecting electrode in these devices. We have developed a theoretical model for the ions movement within the semiconductor polymer matrix governed by both diffusion and drift independently, for the bilayer, polymer ion gel planar, light emitting electrochemical cells. We have further developed a 2- dimensional numerical model based on the theoretical model and have compared the results of the numerical model with the results of a fluorescence probing of the bilayer device with time, at constant potential across the bilayer LEC and report that the drift coefficient of 1x10^-13 cm^2/V.s and a diffusion coefficient of 1 x 10^-15cm2/V.s for TFSI ions in F8BT matrix.Gates Cambridge Trust and Cambridge Overseas Trus

VISUALIZING AND CONTROLLING CHARGE TRANSPORT IN CONJUGATED POLYMER NETWORKS AND FILMS

VISUALIZAING AND CONTROLLING CHARGE TRANSPORT IN CONJUGATED POLYMER NETWORKS AND FILMS MAY 2014 ANDREW R. DAVIS, B.S., UNIVERSITY OF VIRGINIA M.S., UNIVERSITY OF MASSACHUSETTS AMHERST Ph.D., UNIVERSITY OF MASSACHUSETTS AMHERST Directed by: Professor Kenneth R. Carter The desire for more commercially feasible flexible electronic plastics has led to the development of increasingly complex conjugated polymer architectures and device geometries. Through these efforts, tremendous advances have been made in the design and performance of electronic devices fabricated with solution-processable semiconducting polymers. However, none of these materials have yet reached commercial maturity, so the opportunity for their further exploration from both a fundamental science and an application-driven point of view motivates this dissertation. Chapter 1 presents a background introduction to many of the concepts, ideas, and existing research necessary to set the context of this dissertation’s work. The first component of this work (Chapters 2-6) investigates thiol-ene cross-linked conjugated polymer networks. By installing vinyl functionalities in poly(fluorene) molecules at chain ends (Chapters 2 and 3) and along the polymer backbone (Chapters 4 and 5), the polymers can be rapidly and efficiently cross-linked by photo-reaction with thiol cross-linkers into highly tunable semiconducting polymer networks. It is shown that the thiol-ene cross-linking reaction allows for a new molecular handle on modifying interchain electronic communication via network density, which is visualized using characteristic and unambiguous photoemission from low-energy fluorenone species. Light emitting diodes fabricated using these networks as an emissive layer show enhanced color stability compared to as-spun counterparts, and the robustness of the networks allows for solution processing of multiple stratified emissive layers for controlling color emission. Furthermore, the highly efficient thiol-ene cross-linking reaction is shown to work as an effective means for grafting poly(fluorene)s onto functionalized surfaces. This work’s second component in Chapter 7 details the direct visualization of charge carrier density in polymeric thin film transistors using Modulation-Amplified Reflectance Spectroscopy (MARS) measurements. Owing to the unique changes in optical behavior following the formation of a charged state within the conjugated polymer film, optical spectroscopy coupled to a CCD camera offers a powerful visualization tool for observing and mapping charges across large areas as they interact with electrodes, defects, and film morphologies in active devices. The MARS technique illuminates the spatial distribution of carriers in electronic polymer films, allowing direct spatial visualization of charge density and film defects. Finally, a brief concluding comment on this work and an outlook for the field in general is presented in Chapter 8

Synthesis and characterization of novel organic semiconducting materials for organic field effect transistors (OFETs) and photovoltaics (OPVs)

No abstract available.No abstract available

Probing dynamic interfaces in organic electronics

Organic semiconductors are emerging in solar cells, photodetectors, light-emitting diodes and field-effect transistors. The main advantages are the electrical transport properties that can be tailored by chemical design, and their mechanical flexibility. Applications are foreseen in the field of large-area organic electronics, where numerous discrete devices are required on low-cost substrates such as glass or plastic. Widespread introduction however is hampered by two main bottlenecks, which are the limited operational stability and the complex formulations needed for the large-area processing. The basic building block of organic electronics is the field-effect transistor; a microelectronic device used to manipulate the magnitude of a current with an external electrical field. Transistors have to be reliable under operation i.e. when biases are applied to the electrodes, the resulting device current should be constant. In organic transistors however, the on-current slowly decreases with time, an effect of which the origin is unknown. This so-called bias-stress-effect has puzzled the scientific community for more than two decades. In chapter 2 it was shown that the decrease of current with time is caused by a shift of the threshold voltage, i.e. the bias needed to turn the transistor on. The temperature dependence of the threshold voltage shift is determined to be independent of the semiconductor used. This observation led to the conclusion that the threshold voltage shift is due to a common physical origin, but the exact nature remained elusive. Additional measurement techniques were required to understand the cause of the threshold voltage shift. In chapter 3 scanning Kelvin probe microscopy (SKPM) was used to monitor trapped charges in transistors under applied bias. SKPM measures the local surface potential and provides microscopic insight in the electrical performance of organic transistors. It was shown that the potential profiles changed upon gate bias stress, but only in the channel region of the transistor. The reliability issues were found to be not due to an increase in the contact resistance, but due to trapping of charges in the channel area of the transistor. The question remained where the charges were trapped exactly. The threshold voltage shift could be due to trapping in the semiconductor or in the gate dielectric. In chapter 4 SKPM proved that charges can be stored at the dielectric-semiconductor interface even without a semiconductor present. By exfoliating the semiconductor after stress, and subsequent probing of the surface potential of the exposed gate dielectric, it was shown that the threshold voltage shift was indeed due to charge trapping in the dielectric and not in the semiconductor. In chapter 5 a scenario is proposed to explain the observed threshold voltage shift quantitatively. The model consists of two ingredients: (i) hole-assisted electrolytic production of protons from water in the accumulation layer and (ii) the subsequent diffusion of these protons into the SiO2 gate dielectric. The proposed model captures the most important features of the gate bias stress effect, such as the time dependence of the threshold voltage shift and the semiconductor-independent temperature dependence of the threshold voltage shift. The model can also explain the recovery of the transistor upon grounding all electrodes and predicts an anomaly in the current transients, which was experimentally verified in chapter 5. By combining the experimental and theoretical evidence as presented in this thesis, it was concluded that a layer of water at the gate dielectric together with diffusion of protons into the gate dielectric can explain the most important features of the bias stress effect. At the moment large area processing, like evaporation and spin coating, is top-down. A promising process technology for organic electronics is bottom-up self-assembly, which is the autonomous organization of components into patterns and structures without human intervention. Self-assembly has the advantage over conventional processing techniques that it can be made substrate selective and can potentially cover large areas uniformly. Self-assembled monolayers (SAMs) gained a great scientific and industrial interest because of their ability to change the macroscopic properties of surfaces by a single sheet of molecules. Reported surface modifications are for instance changes in workfunction, wettability and adhesion. The advances in SAM-based electronics however have been slow. To investigate the possibility of self-assembly as a processing tool to fabricate organic electronics, first a well-studied system - thiols on gold - was considered in chapter 6. Thiols with opposite built-in dipole moment were chosen and in this way the workfunction of the injecting electrode could be increased as well as decreased. The change in workfunction resulted in a modification of the current injection. In this way patterned light-emitting diodes (LEDs) were fabricated. Only a single layer of molecules determined the light emission in an organic LED. The expertise gained on self-assembly in organic electronics was used to fabricate self-assembled monolayer electronics in chapter 7. Making integrated circuits using a bottom-up approach involving self-assembling molecules was already proposed in the 1970s. The basic building block of such an integrated circuit is the self-assembled monolayer field-effect transistor (SAMFET), where the semiconductor is a monolayer spontaneously formed on the gate dielectric. In SAMFETs fabricated so far, current modulation has only been observed in sub-micrometer channels. Low field-effect carrier mobility, low yield and poor reproducibility have prohibited the realization of bottom-up integrated circuits. We were able to identify and remove these bottlenecks by studying the charge injection and transport in monolayer semiconductors. By circumventing the main roadblocks, real logic functionality was demonstrated in integrated circuits by constructing a 15-bit code generator in which hundreds of SAMFETs were addressed simultaneously. Additionally, we investigated the cause of the absence of current in long channel length transistors. The extracted device mobility in SAMFETs was found to be determined by the monolayer coverage and the channel length. The dependence on coverage and channel length were quantitatively explained numerically and analytically. At partial coverage, SAMFETs form a unique model system to study charge percolation in two dimensions. The SAMFETs were made with silicon dioxide as the gate dielectric, which could be a possible disadvantage when they are used as building blocks in flexible electronics. As a final step the self-assembly process was transferred to organic substrates in chapter 8, which yielded fully functional 4-bit code generators based on organic dielectrics

CHARGE TRAPPING IN POLYMER DIELECTRICS AND POTENTIALS AT ORGANIC DONOR-ACCEPTOR JUNCTIONS—THE ROLE OF INTERFACE AND BULK CONTRIBUTIONS

Organic electronics have attracted increasing interest during the past decade due to their potential applications in transparent, large-area, printable, and stretchable devices. Solution based material deposition considerably reduces processing costs, and allows the use of non-standard substrates in device design. Many organic electronic device parameters are controlled by interfacial as well as bulk properties. Organic donor-acceptor junctions are relevant to organic photovoltaics (OPVs) as well as organic light emitting diodes (OLEDs). In an OPV, interfacial potentials between the hole transporting (donor) organic semiconductor (OSC) and electron transporting OSC (acceptor) lead to separation and recombination of electrons and holes. The mechanisms behind interfacial potential formation in organic donor-acceptor junctions are not fully understood and are an active area of study. In this thesis, the interfacial potential was measured, and interface and bulk contributions were separated by fabricating lateral organic donor-acceptor junctions both with and without a gap between the donor and acceptor materials. Contact between the donor and acceptor materials increases the interfacial potential beyond that calculated from bulk values. This can be explained through differences in electron affinity of the donor and acceptor, and also by differences in the delocalization of molecular orbitals (MOs) of the two OSC films. Greater delocalization of MOs allows for electron donation to adjacent molecules, a surprising result in organic electronics. In addition, the effect of the substrate on the potential was examined. The field is persistently negative on the acceptor side when the junction is made on a SiO2 substrate. When Al2O3, a substrate with higher dielectric constant, is used, the field decreases in one case, and reverses in the other. For organic field effect transistors (OFETs), the instability of switching voltages is an interface-dominated issue which causes the device left on to turn off over time, referred to as bias stress. Bias stress, caused by charges trapped at the dielectric/OSC interface, can be quantified by a shift in the threshold voltage (Vth) of the device. This thesis discusses localizing trapped charges in an OFET dielectric to control bias stress and operating voltages. By changing numbers and positions of trapped charges in the dielectric, the voltage at which the OFET turns on can be defined, and by pre-populating interfacial traps before running the device, bias stress may be reduced. In this thesis, charging of bilayer and trilayer dielectrics made from in-house synthesized ‘chargeable’ substituted polymers was studied. There was greater stabilization of trapped charges at the dielectric/OSC interface in chargeable polymers adjacent to the OSC, indicating charging occurs through an interface-driven mechanism. However, when they were encapsulated such that the chargeable polymer was situated between two layers of unsubstituted polymer, there was less response to charging than in the fully unsubstituted control. This reduction in bias stress susceptibility could stem from the bulk dielectric polarization of the chargeable layer, which counteracts the charge trapping mechanism at the dielectric/OSC interface

Novel solution processable dielectrics for organic and graphene transistors

In this thesis we report the development of a range of high-performance thin-film transistors utilising different solution processable organic dielectrics grown at temperatures compatible with inexpensive substrate materials such as plastic. Firstly, we study the dielectric properties and application of a novel low-k fluoropolymer dielectric, named Hyflon AD (Solvay). The orthogonal nature of the Hyflon formulation, to most conventional organic semiconductors, allows fabrication of top-gate transistors with optimised semiconductor/dielectric interface. When used as the gate dielectric in organic transistors, this transparent and highly water-repellent polymer yields high-performance devices with excellent operating stability. In the case of top-gate organic transistors, hole and electron mobility values close to or higher than 1 cm2/Vs, are obtained. These results suggest that Hyflon AD is a promising candidate for use as dielectric in organic and potentially hybrid electronics. By taking advantage of the non-reactive nature of the Hyflon AD dielectric, the p-doping process of an organic blend semiconductor using a molybdenum based organometallic complex as the molecular dopant, has also been investigated for the first time. Although the much promising properties of Hyflon AD were demonstrated, the resulting transistors need, however, to be operated at high voltages typically in the range of 50-100 V. The latter results to a high power consumption by the discrete transistors as well as the resulting integrated circuits. Therefore, reduction in the operating voltage of these devices is crucial for the implementation of the technology in portable battery-operated devices. Our approach towards the development of low-voltage organic transistors and circuits explored in this work focused on the use of self-assembled monolayer (SAM) organics as ultra-thin gate dielectrics. Only few nanometres thick (2-5 nm), these SAM dielectrics are highly insulating and yield high geometrical capacitances in the range 0.5 - 1 μF/cm2. The latter has enabled the design and development of organic transistors with operating voltages down to a few volts. Using these SAM nanodielectrics high performance transistors with ambipolar transport characteristics have also been realised and combined to form low-voltage integrated circuits for the first time. In the final part of this thesis the potential of Hyflon AD and SAM dielectrics for application in the emerging area of graphene electronics, has been explored. To this end we have employed chemical vapour deposited (CVD) graphene layers that can be processed from solution onto the surface of the organic dielectric (Hyflon AD, SAM). By careful engineering of the graphene/dielectric interface we were able to demonstrate transistors with improved operating characteristics that include; high charge carrier mobility (~1400 cm2/Vs), hysteresis free operation, negligible unintentional doping and improved reliability as compared to bare SiO2 based devices. Importantly, the use of SAM nanodielectrics has enabled the demonstration of low voltage (<|1.5| V) graphene transistors that have been processed from solution at low temperature onto flexible plastic substrates. Graphene transistors with tuneable doping characteristics were also demonstrated by taking advantage of the SAM’s flexible chemistry and more specifically the type of the chemical SAM end-group employed. Overall, the work described in this thesis represents a significant step towards flexible carbon-based electronics where large-volume and low-temperature processing are required