Effect of mixing hopping sites for carrier transport in organic semiconductor nanodevices

In order to investigate the carrier transport in organic semiconductor nanodevices, the hopping model is a powerful method to describe the electrical conduction in the disorder material. The doping or mixing with other material is essential for the conduction mechanism in these organic semiconductor materials. Thus the influence of hopping site mixing was evaluated by Monte Carlo simulation. This technique simulates the carrier hopping by using hopping rate given by Miller-Abrahams expression. The energy of each site follows the Gaussian distribution and rectangular lattice site with the dimension of 41times41times41 was employed to calculate the ensemble and time averages of diffusion coefficient. For mobility evaluation, the electric field with the magnitude of 4 kV/cm was applied to modify the energy on each lattice site. In order to investigate the effect of mixing hopping site, the lattice sites were randomly selected according to the mixing ratio (Xc). The selected site energy was modified by the factor (Xf) varied in the range between 0.80E₀ and 1.10 E₀, where Eo refers to site energy of host material. The tunneling rate to the selected mixing site was also modified with the value varied from 0.0 to 1.0 times of the tunneling value of the host material. The variation of the calculated mobility was investigated when these hopping parameters were changed systematically. The results indicate that the calculated mobility is very sensitive to the Xf value in the range of 0.97Eo to 1.03Eo. It found that for the same tunneling rate case, the mobility in mix hopping site with relative high energy in minority sites gained higher value than that in non-mix material while the mobility in material with low energy minority site obtained lower value. The result also showed that with the mixing hopping site at different value of tunneling rate, the mobility can be much higher value depending on the mixing ratio.