Simulating Charge Injection and Dynamics in Microscale Organic Field-Effect Transistors

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.