UVM Theses and Dissertations
Format:
Print
Author:
Wo, Songtao
Dept./Program:
Materials Science Program
Year:
2011
Degree:
Ph. D.
Abstract:
Organic materials have some advantages to conventional inorganic semiconductor, such as the structural variability and the potential of low cost, large area fabrication of solar cells, organic lightemitting diode (OLED), organic thin film transistor (OTFT), display and so on. Anthony et al. have investigated some functionalized pentacenes, among those, TIPS-pentacene is an ideal material because it can be solution processed and it increases the [Pi - Pi] stacking so that reasonable mobility can be achieved.
In this thesis, three main areas will be addressed. The previous three years of my work were focused on (1) study of the interfacial structure between the pentacene monolayer and substrate by both model-dependent and model independent methods. My recent four years work was focused on using TIPS-pentacene as the organic material. We have made solution processed thin film by hollow rectangular capillary to fabricate the thin film transistor to study (2) characterization of thin film growth process by hollow capillary method. (3) grain size and orientation dependent mobility.
The thickness of the thin film is well controlled by the substrate speed and weight percentage of the solution under room temperature in the range of 1O-200nm. We found two distinct regimes of the thin film deposition depending on the substrate speed. In the slow speed regime, the evaporation effect dominates, while in fast regime, the viscous force effect dominates. The experimental data is explained by a simple film thickness model. At fast speed regime, the results are in close agreement with the Landau-Levich-Derjaguin (LLD) theory. Large grain size can be achieved at slow speed regime which can be 1 mm wide and more than 10 mm long along with the writing direction routinely. The grain structure has mosaic feature from LLD regime.
Grain orientation is found to be related to a preferred crystallographic orientation with respect to the crystallization direction, where the grain size in the direction transverse to the crystallization direction depends inversely on the writing speed, hence forming a regular array of oriented grain boundaries with controllable spacing. We utilize these controllable arrays to systematically study the role of large-angle grain boundaries in carrier transport and charge trapping in thin film transistors. The results show that the effective mobility scales inversely with the grain size, leading to an estimate of the potential drop at individual large-angle grain boundaries of more than one volt. Transient measurements after switching from positive to negative gate bias or between large negative and small negative gate bias reveal reversible charge trapping with time constants of 15 - 45 s that are correlated with grain boundary density.
These results indicate that the structure of grain boundary is not abrupt but extended, and probably associated with other defects full of traps. Similar effects and time constants are observed when switching between large negative and small negative gate bias, where the transients are possibly due to reversible hole trapping. We propose that diffusion along grain boundaries and other defects is the rate determining mechanism of the reversible trapping/detrapping for both electrons and holes. This model of a spatially correlated networks of trapped charge localized near grain boundaries explain the anisotropic mobility results, since trapped charge forms potential barrier that modulate the local density of mobile carriers thus impeding transport perpendicular to the grain boundaries.
Insitu synchrotron x-ray reflectivity is used to probe the early stages of pentacene growth in real time, under conditions relevant to the fabrication of organic thin film transistors. The results reveal that there is an interfacial water layer initially present on the SiO₂ substrate and that this water layer is still present at the interface after the deposition of a pentacene thin film. The thickness of the trapped interfacial water layer does not significantly change subsequent to film deposition, even after exposure to atmospheric pressure or during vacuum annealing at 70°C. However, a water layer is observed to form on the free surface of pentacene after sufficient exposure to water vapor, and the thickness of this layer can be reduced by subsequent vacuum annealing. These observations are correlated with organic thin film transistor mobilities measured at atmospheric pressure versus under vacuum.
In this thesis, three main areas will be addressed. The previous three years of my work were focused on (1) study of the interfacial structure between the pentacene monolayer and substrate by both model-dependent and model independent methods. My recent four years work was focused on using TIPS-pentacene as the organic material. We have made solution processed thin film by hollow rectangular capillary to fabricate the thin film transistor to study (2) characterization of thin film growth process by hollow capillary method. (3) grain size and orientation dependent mobility.
The thickness of the thin film is well controlled by the substrate speed and weight percentage of the solution under room temperature in the range of 1O-200nm. We found two distinct regimes of the thin film deposition depending on the substrate speed. In the slow speed regime, the evaporation effect dominates, while in fast regime, the viscous force effect dominates. The experimental data is explained by a simple film thickness model. At fast speed regime, the results are in close agreement with the Landau-Levich-Derjaguin (LLD) theory. Large grain size can be achieved at slow speed regime which can be 1 mm wide and more than 10 mm long along with the writing direction routinely. The grain structure has mosaic feature from LLD regime.
Grain orientation is found to be related to a preferred crystallographic orientation with respect to the crystallization direction, where the grain size in the direction transverse to the crystallization direction depends inversely on the writing speed, hence forming a regular array of oriented grain boundaries with controllable spacing. We utilize these controllable arrays to systematically study the role of large-angle grain boundaries in carrier transport and charge trapping in thin film transistors. The results show that the effective mobility scales inversely with the grain size, leading to an estimate of the potential drop at individual large-angle grain boundaries of more than one volt. Transient measurements after switching from positive to negative gate bias or between large negative and small negative gate bias reveal reversible charge trapping with time constants of 15 - 45 s that are correlated with grain boundary density.
These results indicate that the structure of grain boundary is not abrupt but extended, and probably associated with other defects full of traps. Similar effects and time constants are observed when switching between large negative and small negative gate bias, where the transients are possibly due to reversible hole trapping. We propose that diffusion along grain boundaries and other defects is the rate determining mechanism of the reversible trapping/detrapping for both electrons and holes. This model of a spatially correlated networks of trapped charge localized near grain boundaries explain the anisotropic mobility results, since trapped charge forms potential barrier that modulate the local density of mobile carriers thus impeding transport perpendicular to the grain boundaries.
Insitu synchrotron x-ray reflectivity is used to probe the early stages of pentacene growth in real time, under conditions relevant to the fabrication of organic thin film transistors. The results reveal that there is an interfacial water layer initially present on the SiO₂ substrate and that this water layer is still present at the interface after the deposition of a pentacene thin film. The thickness of the trapped interfacial water layer does not significantly change subsequent to film deposition, even after exposure to atmospheric pressure or during vacuum annealing at 70°C. However, a water layer is observed to form on the free surface of pentacene after sufficient exposure to water vapor, and the thickness of this layer can be reduced by subsequent vacuum annealing. These observations are correlated with organic thin film transistor mobilities measured at atmospheric pressure versus under vacuum.