Organic 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
-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.
University of Minnesota Ph.D. dissertation. December 2008. Major: Electrical Engineering. Advisor: Prof. P. Paul Ruden. 1 computer file (PDF); xii,109 pages, appendices A-D.
Schroepfer, Dominic David.
Carrier transport study for organic semiconductors using hydrostatic pressure..
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