Stress effects on charge transport in organic semiconductors

Organic semiconductor devices have unique advantages over inorganic semiconductor devices. Most important are their suitability for low-temperature, low-cost, large-area fabrication, and their compatibility with flexible substrates. Compared with inorganic semiconductors, organic semiconductors have low charge mobilities and high threshold voltages, which limit their uses in microelectronics. In order to obtain organic electronic devices with high charge carrier mobilities, the charge transport in organic materials is studied through an examination of stress effects on charge mobility in conjugated molecular semiconductors. This thesis presents the results obtained for pentacene, tetracene, and rubrene based electronic devices.; Pentacene, tetracene, and rubrene show very low dark conductivity but readily observable photoconductivity when illuminated with light in the visible spectrum. We measured the change of photoconductivity with hydrostatic pressure in single-crystal samples of these materials. Possible mechanisms for the observed increase in photoconductivity with pressure are discussed. We conclude that a charge mobility increase under pressure is most likely the cause of the observed increase in photoconductivity in the case of pentacene. For tetracene and rubrene, changes in the absorption spectrum in the range of the excitation wavelengths may also be significant. We also observe a phase transition near 0.3 GPa in tetracene, in agreement with previous results.; We also report the results of measurements of electrical characteristics for top contact pentacene thin film transistors under hydrostatic pressure at room temperature. A drastic increase of drain current and hole mobility with pressure is observed. Possible reasons are discussed to account for the difference in current and mobility increase ratio from sample to sample. Preliminary results on uni-axial stress effects and bending effects on charge carrier transport in pentacene thin film transistors are depicted.; A steady increase of hole mobility with hydrostatic pressure is also observed in rubrene single crystal field effect transistors. The zero pressure hole mobility ranges from 3 to 6 cm2/Vs in different rubrene single crystal FETs and all rubrene FETs show similar hole mobility increase ratio with pressure. These results are very reversible. Threshold voltage shifts with pressure are typically in the range of 0.1 V.