Analyzes of field effect devices based on poly(3-octylthiophene)

Abstract

Poly(3-octylthiophene) (P3OT) based MOS capacitors and transistors have been prepared. The spin-coated P3OT-layers have been investigated by ellipsometry. The measured index of refraction and the gap energy are n=1.8 and E G=2 eV, respectively. The layer thickness has been varied by the technological parameters of the deposition process from 65 nm down to about 10 nm. Quasi-static and dynamic capacitance voltage measurements on the capacitors prove an unintentional p-doping and indicate that layers up to about 30 nm are fully depleted. An unintentional doping concentration of approximately 5×10 17,…,2×10 18 cm −3 has been estimated by analytical approximations and comparison with numerical two-dimensional (2D) simulations. Both types of devices show hysteresis effects which have been already observed and analyzed for devices based on arylamino-poly-( p-phenylene-vinylene) and also reported for poly(3-hexylthiophene). The hysteresis is formally described by different flat band voltages for the sweep from positive to negative bias and the reverse direction. The dependence of the hysteresis on measuring and geometrical parameters allows for a qualitative discussion of the possible origin of the hysteresis. The analysis of the impedance measurements on the capacitors by estimations from the dielectric relaxation and comparison with 2D simulations is complemented by proposing an appropriate equivalent circuit. The extracted mobility perpendicular to the layer is as low as ≈5×10 −8 cm 2/V s. The thin film transistors have a subthreshold slope of S≈200 mV/dec which is rather good for an organic device. This value is in accordance with simulations and analytical calculations provided there are effectively no rechargeable trap states neither at the interface nor in the bulk. The mobilities estimated for the parallel transport are with ≈2×10 −5 cm 2/V s almost three orders of magnitude larger than for the perpendicular transport. The influence of ageing and subsequent annealing has been investigated. The results support the assumption that the high unintentional doping is at least to some extent caused by oxidation.