Articles you may be interested inAmbipolar pentacene/ C 60 -based field-effect transistors with high hole and electron mobilities in ambient atmosphere Appl. Phys. Lett. 94, 023305 (2009); 10.1063/1.3072608 High-performance and electrically stable C 60 organic field-effect transistors Appl. Phys. Lett. 91, 092114 (2007); 10.1063/1.2778472 Ambipolar organic field-effect transistors fabricated using a composite of semiconducting polymer and soluble fullerene Appl. Phys. Lett. 89, 153505 (2006); 10.1063/1.2361269 Ambipolar operation of fullerene field-effect transistors by semiconductor/metal interface modificationMixed layers of copper-phthalocyanine ͑p-conductive͒ and fullerene ͑n-conductive͒ are used for the fabrication of organic field-effect transistors ͑OFET͒ and inverters. Regarding the electrical characteristics of these donor-acceptor blends they show ambipolar charge carrier transport, whereas devices made from only one of the materials show unipolar behavior. Such mixed films are model systems for ambipolar transport with adjustable field-effect mobilities for electrons and holes. By variation of the mixing ratio it is possible to balance the transport of both charge-carrier types. In this paper we discuss the variation of mobility and threshold voltage with the mixing ratio and demonstrate ambipolar inverters as a leadoff application. The gained results were analyzed by simulations using an analytical model for ambipolar transistors and subsequently compared to complementary inverters.

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 elec‐ tron 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. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of the alkyl-chains length on the electronic structure and interface properties of 1,4-octasubstituted zinc phthalocyanines on gold

Metal-insulator-semiconductor (MIS) diodes are the two-terminal pendants of thin film transistors sharing the same basic layer structure. However, instead of the current-voltage characteristics one has to study the capacitance-frequency and capacitance-voltage behavior, which can give information about doping, mobile charges and trapping processes in these devices. We have investigated MIS structures based on poly(alkyl-thiophene) as semiconductor which were fabricated on glass substrates with polymeric insulator layers and compared their response to devices fabricated on Si/SiO 2 substrates. From capacitance-voltage measurements the acceptor dopant concentration is determined for different preparation conditions. Typically these measurements show hysteresis between forward and reverse bias sweeps. We have investigated this behavior as a function of external parameters like sweep speed and temperature and discuss their possible origin. The analysis of the frequency response using appropriate equivalent circuits allows the extraction of material parameters, like conductivity and charge carrier mobility, which are compared to data obtained on thin film transistor structures.

One material system of interest for photovoltaic cells is the combination of the p-conducting copper-phthalocyanine (CuPc) and the n-conducting fullerene (C 60 ) as donor and acceptor materials, respectively. Therefore the transport properties for diodes containing neat and blended organic films are analysed in the space charge limited current regime. The charge carrier mobilities are found to decrease upon dilution of the respective conducting phase by the other species. Photovoltaic cells can be realised with bilayered or blended organic donor/acceptor films. The influence of both photo-active layer types on the electronic structure and the open circuit voltage is investigated. From photoelectron spectroscopy a higher open circuit voltage is predicted for bilayered solar cells. Due to mixing of the organic materials the intermolecular gap between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor is reduced. This prediction is proven true by photocurrent measurements.