Browsing by Subject "Organic Semiconductor"
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Item Carrier transport study for organic semiconductors using hydrostatic pressure.(2008-12) Schroepfer, Dominic DavidOrganic semiconducting materials show tremendous potential for use in low cost and light weight devices. However, transistors based on these materials are plagued with inconsistencies in their mobility and threshold voltage. To further the understanding of these devices, hydrostatic pressure is used in this research project to modify the transport properties of the free charge carriers within the semiconductor layer. Thin film transistors made with P3HT are found to respond approximately linear with pressure in both the mobility and threshold voltage. A linear mobility increase of 300% over 1GPa is found for one sample and an increase of 130% is found for a second sample. The threshold voltages change by 40V (from 40V at atmospheric pressure to 0V at 1GPa) for the former device and by 15V (15V to 0V) for the latter. The mobility increase is attributed to a decrease in inter-molecular spacing, which is well approximated by a linear relationship due to the small change in inter-molecular spacing. The threshold voltage changes show evidence of a change in trap site energy relative to the zero-bias Fermi level. Preliminary temperature data indicates that the traps are donor like. Pentacene thin film devices are tested and compared to the P3HT results. With large source to drain voltages (Vds = -20V to -40V) the FETs made from pentacene thin films are unstable, but show initial increases in mobility of nearly 100% from atmospheric pressure to 100MPa. Lowering Vds allows the data to appear more like the P3HT results, showing an approximately linear mobility increase of 100% through the entire 1GPa pressure cycle. The dependence of the threshold voltage on pressure for low Vds has some curvature, but is still roughly linear from -25V at atmospheric pressure to iii -15V at 1GPa. The threshold voltage change is positive (as opposed to negative for P3HT), which indicates increasing negative fixed space charge, but both films (P3HT and pentacene) shift closer to thresholds voltages of 0V. An alternative pentacene thin film device, a capacitor made of a pentacene film, SiO2, and doped Si, is used to study the mobility in another manner. This device shows a nearly linear mobility increase of 500% for 1GPa of pressure. The threshold for this device is nearly constant, in contrast to the FET. Carbon nanotubes and single crystals of organic semiconducting material are also made into FETs and tested versus pressure in this project. Carbon nanotubes are able to return a 50% increase in mobility with 1GPa of pressure, with an approximately constant threshold voltage. Data taken for single crystal rubrene devices extrapolates to a 1400% increase in mobility with 1GPa of pressure, based on data taken from atmospheric pressure to 70MPa (an increase of 100%). The single crystal device is unable to withstand any additional pressure, and the damage that occurs with pressure makes the threshold voltage shift difficult to characterize.Item An Engineered Approach to Specialty Chemicals Purification(2016-08) Morgan, NathanHigh purity is a near-universal requirement throughout the specialty chemicals industry, essential for many of the applications we take for granted in our daily life. The purification process is often a significant portion of the manufacturing cost for many specialty chemicals, including organic semiconductors and pharmaceuticals. Reducing this manufacturing cost is a key step in the effort to efficiently produce the necessary materials for our modern world. This dissertation examines two key purification processes, thermal gradient sublimation and crystallization, in order to offer potential routes for process improvement. Thermal gradient sublimation is examined through the lens of organic semiconductors, which are often purified using this technique at the industrial scale. Interestingly, the sublimation process is limited by vapor phase transport and deposition, not solid phase mechanisms. A model for this process is developed, suggesting potential routes to efficient scale-up and separation improvements. This dissertation also proposes a new method for crystallization control, pressure-swing. In this approach, rapid changes in pressure are used to control solubility during the crystallization process. A model describing the changes in solubility due to these pressure changes is developed, and several process validation experiments are performed using pharmaceutical molecules as model systems. While these tests show an enhanced control of solubility, attempts to replicate experimental results obtained using traditional crystallization control are only partially successful when using the pressure-swing technique.Item In-situ optical spectroscopy of the organic semiconductor/electrolyte dielectric interface.(2009-08) Kaake, Loren G.The implementation of organic thin film transistors into microelectronic devices hinges upon the development of organic semiconductors and gate dielectric materials. In a working device, the place where the two materials meet is critical to device performance. This buried interface between the organic semiconductor and the gate dielectric is notoriously difficult to characterize. One way to probe this interface is through the use of attenuated total internal reflection Fourier transform infrared and near infrared spectroscopy (ATR-FTIR). This method allows one to do optical spectroscopy on a working device and gain insight into the physical processes which occur at the semiconductor/dielectric interface during the application of voltage. One example of such a process is the induction of charge in the organic semiconductor layer. Charging of an organic semiconductor gives rise to distinct spectroscopic signatures which can be used to characterize properties intrinsic to the semiconductor. For example, high charge carrier density can give rise to unique spectroscopic signatures which may be related to the Mott insulator to metal transition. Crystallinity affects the spectral signatures of charge carriers, and these effects can give insight into sources of energetic disorder in the solid. Examining semiconductor charging also gives insight into the operating mechanisms of the dielectric material responsible for inducing charge carriers. Dielectric materials using mobile ions have become attractive for use in organic thin film transistors because they allow low voltage transistor operation. The physical mechanisms for charge induction are distinguishable when in-situ optical spectroscopy is applied. For example, the mobile ions in a dielectric material can penetrate the bulk of the semiconductor film or stop at the semiconductor/dielectric interface depending on the size of the ion. The rate of charging can be analyzed and used to estimate a material specific maximum operating speed of an electrochemical transistor. Using optical spectroscopy to examine the organic semiconductor/electrolyte dielectric interface gives insight into many aspects of device operation, many of which are critical to making organic thin film transistors a viable technology.Item Vibrational Sum Frequency Generation Spectroscopy of Organic Semiconducting Thin Film Interfaces(2017-03) Kearns, PatrickThis Dissertation is a compilation of my work done at the University of Minnesota in pursuit of a Ph.D. in Chemistry. The main theme is the use of vibrational sum frequency generation, an interface specific spectroscopic technique, to answer fundamentally interesting questions in the field of organic electronics, specifically the dielectric/semiconductor interface. This interface is of major importance in the function of organic field effect transistors. Chapters 1 and 2 will provide the relevant background on organic electronics and vibrational sum frequency generation, respectively. Chapter 3 is an explanation of the laser setup used to make the measurements. Chapter 4 explores the use of new modeling techniques to answer fundamental questions pertaining to the dielectric/organic interface under gate bias. Chapter 5 is the development of a new technique to collect multiple VSFG experiments at once. Finally, Chapter 6 uses techniques in both chapters 4 and 5 to examine how the organic semiconductor arranges itself on different gate dielectrics.