Harding University Course Catalog 1988-1989

Catalog of Harding University 1988-1989https://scholarworks.harding.edu/catalogs/1050/thumbnail.jp

Radiation-induced conductivity in poly(phenylene vinylene) derivatives

\u3cp\u3eUsing the pulse-radiolysis time-resolved microwave conductivity technique the mobility and decay kinetics of radiation-induced charge carriers is studied in a series of poly(2,5-dialkoxy-phenylene vinylene) derivatives. The lower limit to the sum of the mobilities of the positive and negative charge carriers. Σμ(min), depends strongly on the alkoxy functionalization and ranges from 1.2·10\u3csup\u3e-7\u3c/sup\u3e to 1.4·10\u3csup\u3e-6\u3c/sup\u3e m\u3csup\u3e2\u3c/sup\u3eN/V·s at room temperature. Σμ(min) increases with the degree of order in the material. The after-pulse conductivity decay kinetics are disperse and are controlled by a combination of charge recombination and trapping.\u3c/p\u3

Dual-gate organic thin-film transistors

\u3cp\u3eA dual-gate organic thin-film transistor is realized using solution-processed organic semiconductor and insulator layers. Electrodes are made from gold. Compared to conventional single-gate transistors, this device type has a higher on current and steeper subthreshold slope. We show that the improved performance is the result of a nonconstant threshold voltage rather than formation of a second accumulation channel. Formation of a second accumulation channel does occur but the field-effect mobility associated with this channel is a factor 104 lower than the primary channel due to the relatively rough insulator-semiconductor interface.\u3c/p\u3

Polaron pair formation, migration, and decay on photoexcited poly(phenylenevinylene) chains

\u3cp\u3ePoly(2-(3,7-dimethyloctoxy)-5-methoxy-1,4-phenylenevinylene), (dMOM-PPV), undergoes a thermochromic transition on heating in benzene at approximately 35°C from a red gel to a yellow solution. The latter is metastable on cooling to room temperature and eventually reverts to the gel form after several hours. The absorption and emission spectrum of the gel can be resolved into two components, one which is identical with that for the solution and which is therefore associated with single-strand segments, and a second of similar spectral shape but lower energy which is ascribed to aggregated segments. Photoexcitation of dMOM-PPV in both the gel and solution results in a transient change in the microwave conductivity as monitored using the time-resolved microwave conductivity (TRMC) technique. The change in microwave conductivity for the gel is approximately an order of magnitude larger than for the solution. No TRMC transient is found on flash photolysis of a benzene solution of a dMOM-PPV derivative with 13% nonconjugated units in the polymer backbone. The effects are attributed to formation of mobile charge carriers (polaron pairs) on a single polymer chain in the case of the solution, whereas in the gel interchain charge transfer results in the predominant formation of charge carriers located on separated chains. The product of the quantum efficiency for polaron pair formation, φ\u3csub\u3ep\u3c/sub\u3e, and the sum of the mobilities, Σ\u3csub\u3eμ\u3c/sub\u3e, is φ\u3csub\u3ep\u3c/sub\u3eΣ\u3csub\u3eμ\u3c/sub\u3e = (2.0 ± 0.3) × 10\u3csup\u3e-8\u3c/sup\u3e m\u3csup\u3e2\u3c/sup\u3e/Vs and (2.0 ± 0.5) × 10\u3csup\u3e-9\u3c/sup\u3e m\u3csup\u3e2\u3c/sup\u3e/Vs for the gel and solution respectively. The decay in microwave conductivity is nonexponential and extends to hundreds of nanoseconds.\u3c/p\u3

High-performance all-polymer integrated circuits

\u3cp\u3eIn this letter, we demonstrate the integration of all-polymer field-effect transistors in fully functional integrated circuits with operating frequencies of several kHz. One of the key items is an approach to incorporate low-Ohmic vertical interconnects compatible with an all-polymer approach. Inverters, NAND gates, and ring oscillators with transistor channel lengths down to 1 μm have been constructed. Inverters show voltage amplification at moderate biases and pentacene seven-stage ring oscillators show switching frequencies of a few kHz. The potential to realize large integrated circuits is demonstrated by a 15 bit code generator circuit using several hundreds of devices. The proposed concept was evaluated for three solution-processable organic semiconductors.\u3c/p\u3

Scalable electronic ratchet with over 10% rectification efficiency

\u3cp\u3eElectronic ratchets use a periodic potential with broken inversion symmetry to rectify undirected (electromagnetic, EM) forces and can in principle be a complement to conventional diode-based designs. Unfortunately, ratchet devices reported to date have low or undetermined power conversion efficiencies, hampering applicability. Combining experiments and numerical modeling, field-effect transistor-based ratchets are investigated in which the driving signal is coupled into the accumulation layer via interdigitated finger electrodes that are capacitively coupled to the field effect transistor channel region. The output current–voltage curves of these ratchets can have a fill factor >> 0.25 which is highly favorable for the power output. Experimentally, a maximum power conversion efficiency well over 10% at 5 MHz, which is the highest reported value for an electronic ratchet, is determined. Device simulations indicate this number can be increased further by increasing the device asymmetry. A scaling analysis shows that the frequency range of optimal performance can be scaled to the THz regime, and possibly beyond, while adhering to technologically realistic parameters. Concomitantly, the power output density increases from ≈4 W m\u3csup\u3e−2\u3c/sup\u3e to ≈1 MW m\u3csup\u3e−2\u3c/sup\u3e. Hence, this type of ratchet device can rectify high-frequency EM fields at reasonable efficiencies, potentially paving the way for actual use as energy harvester.\u3c/p\u3

Organic transistors in optical displays and microelectronic applications

\u3cp\u3eOrganic thin-film transistors (OTFTs) offer unprecedented opportunities for implementation in a broad range of technological applications spanning from large-volume microelectronics and optical displays to chemical and biological sensors. In this Progress Report, we review the application of organic transistors in the fields of flexible optical displays and microelectronics. The advantages associated with the use of OTFT technology are discussed with primary emphasis on the latest developments in the area of active-matrix electrophoretic and organic light-emitting diode displays based on OTFT backplanes and on the application of organic transistors in microelectronics including digital and analog circuits.\u3c/p\u3