Ambipolar charge carrier transport in organic semiconductor blends of phthalocyanine and fullerene

Ambipolar charge carrier transport is realised in organic field-effect transistors using mixtures of p-conductive copper-phthalocyanine and n-conductive buckminster fullerene as active layer. These blends are model systems for ambipolar transport with adjustable field-effect mobilities for electrons and holes by variation of the mixing ratio. Thereby balanced mobilities for both charge-carrier types are possible. In this paper we discuss the variation of mobility, threshold voltage and electronic energy levels with the mixing ratio. The charge carrier mobilities are strongly reduced upon dilution of the respective conducting phase by the other species. This shows that transport of each carrier species occurs by percolation through the respective phase in the blend. A strong correlation between contact resistance and mobility indicates that carrier injection is diffusion limited. A charge redistribution in the copper-phthalocyanine causes a hole accumulation at the organic/organic interface and affects thereby the threshold voltage for holes. By analysing the electronic structure using photoelectron spectroscopy no charge transfer was found in the ground state. The common work function of these blends changes linearly between the work functions of the neat materials. Thus a constant ionisation potential for the highest occupied molecular orbitals of the two materials is obtained. Furthermore we have fabricated ambipolar inverters using mixed organic semiconductor layers and compare their behaviour to complementary inverters consisting of discrete p-and n-channel transistors. The experimental findings and concomitant simulations demonstrate the need for balanced electron and hole mobilities in order to achieve symmetric inverter characteristics. However, they also reveal the superior performance of true complementary logic inverters towards their ambipolar pendants. [GRAPHICS] Sketch for the unipolar and ambipolar operation regimes of an organic field-effect transistor. V-D denotes the drain voltage, V-G and V-T are gate and threshold voltage, respectively. 0 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.