Field effect in organic devices with solution-doped arylamino-poly-(phenylene-vinylene)

MOS capacitors and field effect transistors with arylamino-PPV as an active layer have been prepared and characterized. For this material we developed a doping method to increase the conductivity. The field effect has been demonstrated by the measurement of capacitance voltage (CV)-curves of the MOS capacitor. In accumulation the oxide capacitance is only achieved for low frequencies. At positive gate voltages inversion has not been observed. The p-doping concentrations are in the range of 10(16),..., 10(18) cm(-3). With the realized thin film transistor structure a typical transistor behaviour has been demonstrated. The estimated value for the mobility is in the order of 10(-4) cm(2)/Vs. This low value causes a high relaxation time. The measured characteristics of both devices show a large hysteresis for different sweep directions and a shift of the curves for repeated measurements. Mobile ions, the kinetics of incomplete ionization, chemical reactions of iodine or the polaron-bipolaron conversion might influence this behaviour. Conditions to achieve inversion were determined by two-dimensional (2-D) simulation of both devices. It turns out that, due to the low intrinsic density of organic materials, one cannot demonstrate an inversion layer in an MOS capacitor. But in the transistor structure the minority carriers are injected from source and drain into the channel generating an inversion layer. Further experimental investigations are necessary to prove this prediction and to clarify the hysteresis effect. (C) 2000 Elsevier Science Ltd. All rights reserved.