Simulating Charge Injection
and Dynamics in Microscale Organic Field-Effect Transistors
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Abstract
Monte Carlo simulations were used to investigate the
carrier dynamics in realistic, finite-sized, small-molecule, organic
field-effect transistors (OFETs) within the first few nanoseconds
of device turn-on as well as when the system equilibrates. The results
show that the device current exhibits large magnitude oscillations
(64 ± 27 nA) during device turn-on if the initial configuration
assumed no carriers in the device (i.e., carriers only arrive through
injection from the source electrode). After equilibration (125 ns),
the current continues to oscillate, however, at lower magnitude (64 ±
2 nA), even if the initial configuration assumed randomly placed charges.
Fourier transforms of device current as a function of simulation time
show that these oscillations occur at well-defined device geometry-dependent
frequencies, independent of initial configuration of the system. Examination
of the carrier lifetimes and path lengths, which were found to vary
nonlinearly with device length, are used to argue that the oscillations
are the result of the charge injection procedure, which assumed a
constant probability event. The results suggest that carriers travel
in waves in realistically finite-sized devices and that carrier lifetime
and path length vary nonlinearly by device geometry. Alternating current
studies of OFETs may be useful in confirming these findings