Invited Paper EM-MoM8
Electronic Transport Processes in Polymer Transistors

Monday, October 15, 2007, 10:20 am, Room 612

Solution deposited polymer thin film transistors have mobility reaching of order 1 cm2/Vsec making them interesting for applications such as flat panel displays. The increase in mobility is a result of new materials synthesis and an understanding of how the surface energy of the deposition surface controls the structural order of the film. A polymer such as polythiophene is highly anisotropic and conduction occurs essentially in a single 2-dimensional sheet of polymer next to the dielectric interface. Band conduction, coupled with a simple density of states model to account for disorder effects, are able to explain the main features of the electronic transport, based on theoretical calculations of the electronic structure. The upper limits on the mobility, of about 10 cm2/Vs can also be estimated from calculations. Although polymer semiconductors cannot be doped by conventional atomic substitution, doping is possible by the incorporation of molecular impurities, and allows further test of transport models. We describe a particular case of acceptor doping with ozone and show that the transport models are able to explain the results and that acceptor binding energies can be deduced. In common with other disordered semiconductors, polymer TFTs exhibit electrical bias stress effects, which induce a change in the threshold voltage due to trapping in states that communicate weakly with the conduction holes. Electrical stress measurements in polythiophene TFTs show properties with a wide range of time constants. The threshold voltage shift increases as a power law in time and in gate voltage. However, after a few days of stress, the threshold voltage shift stabilizes because there is a thermally activated recovery mechanism. The physical mechanisms of the bias stress effects, and whether they are intrinsic to the polymer semiconductor, will be discussed.