Organic semiconductors have the advantage over traditional inorganic semiconductors, such as Si or GaAs, in that they do not require perfect single crystal films to operate in real devices. Complicated multi-layer structures with nanometer scale thicknesses can thus be easily fabricated from organic materials using low-cost roll-to-roll manufacturing techniques. However, the discrete nature of organic semiconductors also implies that they typically contain almost no intrinsic charge carriers (i.e., electrons or holes), and thus act as insulators until electrical charges are injected into them. In electrical device applications this means that all of the holes and electrons within a device must be injected from the anode and cathode respectively. As a result, device stability, performance, and lifetime are greatly influenced by the interface between the organic materials and the electrode contacts. Despite the fundamental importance of the electrode/organic contacts, much of the basic physical understanding of these interfaces remains unclear. As a result, the current design of state-of-the-art organic optoelectronic devices tends to be based on trial and error experimentation, resulting in overly complicated structures that are less than optimal. In the present thesis, various electrode/organic interfaces relevant to device applications are studied using a variety of different techniques, including photoelectron spectroscopy and the iii temperature dependent current-voltage characteristics of single carrier devices. The fundamental understanding gleaned from these studies has been used to develop new strategies for controlling the energy-level alignment at electrode/organic interfaces. A universal method for tuning the work function of electrode materials using a halogenated organic solvent and UV light has been developed. Application of this technique in organic light emitting diodes enabled the first highly simplified two-layer device with a state-of-the-art record breaking efficiency.

In this diploma work, organic electrochemical transistors based on PEDOT:PSS have been studied, focusing on the influence of the geometry and physical dimensions on the transistor characteristics. The geometrical parameters studied are the area ratio between the gate and channel, the channel width and the channel length. Each parameter has been varied in five steps with each step containing three identical transistors. Results concerning the geometrical influence of the linear region resistance, the saturation current (density) and the on/off ratio are presented and discussed. Also, empirical curve-fits of the geometrical influence on the linear region resistance and the saturation current have been performed. In addition, tentative results of the locus of the saturation current versus saturation voltage for specific transistors have been obtained.

Dithienopyrrole-based conjugated materials, including oligomers and polymers, for potential organic electronic applications, were designed, synthesized and characterized. The optical and electrochemical properties of these materials were investigated, and their structure-property relationships were studied. Some of the materials can be oxidized (or reduced) chemically or electrochemically. Furthermore, the utility of these materials in organic electronic devices, such as OFETs and OPVs, were assessed. In OFETs, they can function as hole-transport materials with mobilities up to 4.8 × 10-2 cm2/(Vs), and one example serves as an ambipolar material with comparable hole and electron mobilities of 1.2 × 10-3 and 5.8 ×10-4 cm2/(Vs), respectively. Some of the materials can also be used as electron donors in OPVs in conjunction with PCBM, and exhibited power conversion efficiencies up to 1.4% after optimizations. They may also be used in other applications such as electrochromic devices, photodetectors, and optical limiting.

The synthesis and characterization of some phosphonic acids as well as the modification of indium tin oxide (ITO) substrates using these phosphonic acids are presented in this thesis. Phosphonic acids have been known to bind strongly to the surface of a number of metal oxides. ITO substrates were reported to be modified with a variety of surface modifiers. Herein the ITO substrates were modified with the chosen phosphonic acids with different functional groups in order to tune the work function and compare the work function changes with the functional group properties.

<p>In this diploma work, organic electrochemical transistors based on PEDOT:PSS have been studied, focusing on the influence of the geometry and physical dimensions on the transistor characteristics. The geometrical parameters studied are the area ratio between the gate and channel, the channel width and the channel length. Each parameter has been varied in five steps with each step containing three identical transistors. Results concerning the geometrical influence of the linear region resistance, the saturation current (density) and the on/off ratio are presented and discussed. Also, empirical curve-fits of the geometrical influence on the linear region resistance and the saturation current have been performed. In addition, tentative results of the locus of the saturation current versus saturation voltage for specific transistors have been obtained.</p>

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Graham, Samuel; Committee Member: Garimella, Srinivas; Committee Member: Kippelen, Bernard; Committee Member: Melkote, Shreyes; Committee Member: Ready, Jud. Part of the SMARTech Electronic Thesis and Dissertation Collection.

This thesis reports on the design, fabrication, and testing of state-of-the-art, high-performance inverted top-emitting organic light-emitting diodes (OLEDs). The vast majority of research reports focuses on a device architecture referred to as a conventional OLED which has its anode on the bottom of the device and its cathode on the top. Moreover, most conventional OLEDs are bottom-emitting such that light exits the structure through both a semitransparent bottom electrode of indium-tin oxide and a glass substrate. The particular device architecture developed in this thesis is one in which the devices are inverted (i.e. their cathode is on the bottom as opposed to on top) and top-emitting. Despite the advantages that inverted top-emitting OLEDs possess over conventional bottom-emitting OLEDs, their development has been relatively slow. This is because inverted OLEDs have traditionally been hampered by the difficulty of injecting electrons effectively into the device. In this work, a novel method of injecting electrons from bottom cathodes into inverted OLEDs is discovered. In several previous reports, bottom Al/LiF cathodes had been used with the electron-transport material Alq3 to produce inverted OLEDs, but the resulting inverted OLEDs exhibited inferior performance to conventional OLEDs with top cathodes of Al/LiF. A new route for the development of highly efficient inverted OLEDs is shown through the use of electron-transport materials with high electron mobility values and large electron affinities. After systematic device optimization, inverted top-emitting OLEDs are demonstrated that currently define the state-of-the-art in terms of device efficiency. Optimized green and blue inverted top-emitting OLEDs are demonstrated that have a current efficacies of 92.5 cd/A and 32.0 cd/A, respectively, at luminance values exceeding 1,000 cd/m2. Finally, this discovery has enabled the development of the first stacked inverted top-emitting OLEDs ever made, combining all of the advantages offered by an inverted architecture, a top-emissive design, and a stacked structure. These OLEDs have a current efficacy of 200 cd/A at a luminance of 1011 cd/m2, attaining a maximum current efficacy of 205 cd/A at luminance of 103 cd/m2.

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Organic thin film transistors (TFTs) based on the field effect transistor architecture provide a methodology for sensing by exhibiting a change in the transport properties such as shifts in mobility, threshold voltage and conductivity. Chemical recognition is achievable by various methods including the two processes, which we are studying, direct analyte interactions with the semiconductor and specific receptor molecules on the semiconducting surface. Previous work demonstrates the effects of carrier concentration, grain size (surface morphology), and channel length on the sensing response to analytes such as alcohols, which exhibit a moderate dipole moment. When the alcohol interacts with the organic channel the addition of a trap and a positive charge occurs at the grain boundaries. At low carrier concentrations the added charge has the effect of producing an increase in current for the sensing response. At higher carrier concentrations the occurrence of trapping overwhelms the effect of the positive charge and you see and reduction in current. Typically the mobility shifts, which occur during sensing are correlated with trapping for polar analytes. The magnitude of the mobility decreases are dependent on the dipole moment of the polar analyte. Another aspect of organic materials is the fine-tuning of the chemical sensitivity by modifying the surface with receptor sites to increase the partition coefficient. In our study we pull the polarization, molecular dipole moment, transport and trapping, and partition coefficient concepts together to produce a model, which describes how an OFET based sensor interacts with an analyte with and without receptor molecules and under aqueous conditions. / text