Electric Field Confinement Effect on Charge Transport in Organic Field-Effect Transistors

While it is known that the charge-carrier mobility in organic semiconductors is only weakly dependent on the electric field at low fields, the experimental mobility in organic field-effect transistors using silylethynyl-substituted pentacene is found to be surprisingly field dependent at low source-drain fields. Corroborated by scanning Kelvin probe measurements, we explain this observation by the severe difference between local conductivities within grains and at grain boundaries. Redistribution of accumulated charges creates very strong local lateral fields in the latter regions. We further confirm this picture by verifying that the charge mobility in channels having no grain boundaries, made from the same organic semiconductor, is not significantly field dependent. We show that our model allows us to quantitatively model the source-drain field dependence of the mobility in polycrystalline organic transistors.

Figure 1

Lateral-field dependences of the OFET mobility measured in TIPS-PEN channel with GB (circles) and without GB (squares) at $T=300\mathrm{K}$. Charge mobilities calculated assuming homogenous electric field ($q=1$) for $\sigma =65\mathrm{meV}$ and $\sigma =73\mathrm{meV}$ within Pasveer-Coehoorn model (curves 2 and 1, respectively, in blue) and by Fishchuk model (curves 2′ and 1′, respectively, in red). Other parameters used are $a=0.7\mathrm{nm}$, $c={10}^{-3}$, and the ratio $a/b=10$ and 5 for Pasveer-Coehoorn and Fishchuk models, respectively. The best-fit curve for the channel with GB calculated assuming strong local electric fields accounted for by the field magnification parameter $q=115$ within Pasveer-Coehoorn model (curve 3, in blue) and by $q=64$ within Fishchuk model (curve 3’, in red). Note that curves 1 and 1′ coincide.

Figure 2

Surface potential profiles of pure TIPS-PEN (a) and the TIPS-PEN PS blend (b) measured by SKPM in the active layer of OFET devices at $V_D=-5\mathrm{V}$ and for different $V_G$ voltages. Positions of GB are indicated by red arrows in (a).

Figure 3

Lower panel: Distribution of lateral electric field calculated from the surface potential distribution from Fig. 2a at $V_D=-5\mathrm{V}$ and $V_G=-15\mathrm{V}$ in pure TIPS-PEN channel. Positions of GB are indicated by arrows. Upper panel: Corresponding AFM topography image of the studied channel. The red horizontal line in the upper panel depicts the position where the cross section of the SKPM scan in Fig. 2a was taken.