Electric field confinement effect on charge transport in polycrystalline organic field-effect transistors

It is known that that the charge-carrier mobility in organic semiconductors is only weakly dependent on the lateral (source-drain) electric field at low fields [1,2]. However, in our study, the experimental charge-carrier mobility in organic field-effect transistors using 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-PEN) is found to be surprisingly field-dependent at low source-drain fields [3]. Corroborated by scanning Kelvin probe measurements, we explain this experimental observation by the severe difference between local conductivities within grains and at grain boundaries. The lower charge-carrier mobility in the disordered grain boundaries causes the major part of the source-drain voltage to drop over the boundaries, i.e. the local electric field in these boundaries is high. By rigorously accounting for the field-dependence of the mobility at the grain boundaries and inside the grains separately, we have shown that the overall field-dependence is strong because it is dominated by the disordered grain boundaries where the lateral field is high. We further confirm this picture by verifying that the charge-carrier mobility in channels without grain boundaries, made from the same organic semiconductor, is not significantly field-dependent. Finally, we show that our model allows us to quantitatively model the source-drain field dependence of the mobility in polycrystalline organic transistors. References: [1] H. Bässler, Phys. Stat. Sol. (b) 1993, 175, 15-56. [2] M. Bouhassoune, S.L.M. van Mensfoort, P.A. Bobbert, R. Coehoorn, Org. Electron. 2009, 10, 437-445. [3] X. Li, A. Kadashchuk, I.I. Fishchuk, W.T.T. Smaal, G. Gelinck, D.J. Broer, J. Genoe, P. Heremans, H. Bässler, Phys. Rev. Lett. 2012, 108, 066601.