Organic semiconductors have attracted a great deal of attention in recent years, due to their low cost, the possibilities for depositing them on large flat, flexible substrates, and the enormous choice of suitable molecules. Organic semiconductors can be used in switchable components such as transistors and in optoelectronic components, such as organic light-emitting diodes (OLEDs) and photovoltaic cells.
Organic transistors and, in particular, organic field-effect transistors (OTFTs) have been the subject of investigation and development for many years. It is expected that OTFTs will be usable on a large scale, for example, in low-cost integrated circuits for identification tags (RFID) and for screen control (backplane) applications. In order to enable low-cost applications, thin film technology is generally required for producing the transistors. Over the last few years, performance characteristics have improved to such an extent that the commercialization of organic transistors is foreseeable. For example, high field-effect mobility values of up to 6 cm2/Vs have been found in OTFTs for electrons based on fullerene-C60 and up to 5.5 cm2/Vs for holes based on pentacene.
Charge-carrier transport in thin organic films is generally described in terms of temperature-activated charge carrier hopping, which leads to relatively low charge carrier mobility and a strong influence of disorder. Therefore the field-effect mobility in OTFTs generally depends on the charge carrier density. For this reason, a relatively high gate voltage is usually necessary in order to fill localized states and to achieve high charge carrier mobility in the organic layer.
One possibility for increasing charge carrier density and thus charge carrier mobility in organic semiconductors is electrical doping by means of donors or acceptors. This brings about a change in the Fermi level of the semiconductor and, depending on the type of donor used, an increase in the initially very low conductivity by generating charge carriers in a matrix material. U.S. Pat. No. 5,093,698 describes the general requirements placed on a combination of organic materials for electrical doping.
In the last few years, electrical doping of organic semiconductors by means of molecular dopants has been investigated in detail. These investigations have shown that the charge carrier mobility of mixed layers increases depending on the doping concentration. This phenomenon is thereby explained that additional charge carriers gradually fill states of the matrix material from the lower end of the state density distribution, that is, states with lower charge carrier mobility. Equally, the Fermi level of the semiconductor is gradually changed, depending on the type of dopant used. It is increased for n-type doping and reduced for p-type doping, so that an increase in the initially very low conductivity is achieved.
In OTFTs with an electrically doped active layer, as the charge carrier mobility increases, the threshold voltage falls and with it, quite generally, the operating voltage. For most areas of application of OTFTs, it is desirable to achieve very low OFF currents. A high doping concentration leads to a high background charge density which, in turn, leads to an undesirable ohmic charge carrier transport which, due to the field effect, cannot be effectively controlled.
The OFF state of a transistor is understood to be an applied gate voltage of less than the threshold voltage of the component for the n-type conductor and of greater than the threshold voltage for the p-type conductor. For the widely investigated OTFT of the enhancement type, the OFF state exists at a gate voltage Vg=0 V for p-type and n-type.
However, it has also been found that in semiconductor layers with excellent charge carrier mobility, the mixing in of dopants leads to an increase in impurity scattering and thereby also restricts the maximum charge carrier mobility in OTFTs. (Harada et al., Appl. Phys. Lett. 91 092118 (2007)). An alternative arrangement wherein the background charge carrier concentration is enhanced without dopants being mixed into the semiconductor layer is therefore desirable. An arrangement of this type makes it possible, in principle, to increase the charge carrier mobility beyond the usual extent.
Methods for determining charge carrier mobility in a field-effect transistor are per se known in various embodiments. One example is described by US 2004/191952 A by Shtein et al. From the saturation region of a current-voltage graph between the source and drain electrodes, the charge carrier mobility is calculated for a particular gate voltage.
OTFTs with additional layers arranged on the active semiconductor layer, also designated encapsulation layers or cover layers, have been described. Examples are double layers of pentacenes and fullerene C60, in order to achieve ambipolar component functionality (Wang et al., Org. Electron. 7, 457 (2006)). In this special case, it can be deduced from the energy level that no technically relevant change in the charge carrier density takes place in the active layer. In US 2007/034860 A1 Nakamura et al. describe a structure of this type which even has a greater charge carrier mobility in the active layer compared with the encapsulation layer.
U.S. Pat. No. 5,500,537 by Tsumura et al. describes, inter alia, an OTFT structure wherein a further layer, similar to an encapsulation layer, is applied to the active layer. The requirement placed on the active layer is that it should be a polymer layer. The requirement placed on the further layer is that it controls the conductivity of the active layer. This requirement is actually too general for a switchable component. The proposed arrangement can only function in geometries wherein the source/drain contacts are not in direct contact with the other layer of higher conductivity, since otherwise large OFF currents would be inevitable.
US 2006/0202196 by Kawakami et al. describes structures with an encapsulation layer, which is configured as an electrically homogeneously doped layer, wherein the matrix material of the encapsulation layer is the same as or similar to the material of the active layer. This means that the charge carrier mobilities of the active layer and the encapsulation layer are the same or at least similar and that the electrical conductivity of the encapsulation layer is even greater than the electrical conductivity of the active layer in the OFF state, due to the electrical doping. The doped layer also acts as a parallel resistor and very severely impairs the ON/OFF ratio.