Polyfluorene-based organic field-effect transistors

The electrical performance and device stability of a patterned-gate, gate-planarized, inverted, coplanar thin-film transistor with an organic polymer F8T2 active layer semiconductor was studied. The validity of electrical performance parameter extraction methods was studied and the inclusion of a field-dependent mobility provided the most reliable results. A gate-bias dependent activation energy near 0.2eV for the field-effect mobility of holes in F8T2 was found. The source and drain contact and channel resistances were characterized and are similar with values near 109O. Additionally, these devices showed promise for use in high-voltage applications. The effects of broadband and monochromatic illumination were characterized and strongly absorbed illumination reduced the threshold voltage. Application of a photo-field effect theory provided an estimation of the density and slope of the gap states. This method also provided an estimate of the flat-band voltage of -10V. The performance of the device as a photodetector showed a responsivity of 1A/W, a photosensitivity greater than 100, an external quantum efficiency greater than 100%, a noise equivalent power of 10-14 WHz-0.5 and a specific detectivity of approximately 2x1011 cmHz0.5W-1. Hysteresis in the transfer characteristics was characterized by the dependence on the applied gate-bias, temperature and illumination. It was observed that the hysteresis is a charge trapping effect that is dependent on the charge density within the channel. The hysteresis was eliminated by incorporating an organic insulator layer between the inorganic insulator and the organic semiconductor. Bias temperature stress effects were characterized and the major effect was an increase in threshold voltage. Analysis using a stretched exponential behavior for DC bias stress effects showed that there exists a distribution of trap states centered at 0.25eV above the valence band. Negative AC BTS resulted in a dependence on the total stress time and demonstrated the combination of several physical phenomena, including slow carrier transport and the existence of few reversible and many irreversible trap states. A relatively low (65°C) optimal operating temperature of organic-based devices was observed. The trap states were further characterized using the photodischarge method to investigate the kinetics and distribution of trap states. A narrow distribution of trap states at 0.3eV above the valence band was found, which is consistent with field-effect mobility and bias temperature stress results.