N-channel materials

Soluble derivatives of fused aromatic systems are not limited to p-channel materials however, they are far fewer in number. A soluble derivative of NTCDA has recently been developed that can be cast from a,a,a-trifluorotoluene (Figure 5.3.lid) [66]. Devices prepared from this derivative showed electron mobilities of 0.01 cmWs [67]. This is currently the only nonfullerene solution-cast n-channel material. 

L6pez Navarrete et al. reported that the introduction of cyano groups in an oligothiophene as presented in 2.83 enhances the electron affinity of the oligomers, which stabilizes the radical anions or dianions and promotes them to prospective n-channel materials [176]. The application of some of these dicyano derivatives in OFETs has been reviewed [177]. 

The design question is given the heat rate Q, the length L, and width W, select a working fluid with mass flow rate m, channel dimension d, channel spacing Sxd (for 5" is a number >1), number of channels n, and material and thickness of the block H. We will be particularly interested in the pumping power P. 

To produce a very thick n-channel device, the resistivity of the silicon must be made relatively high, about 5,000 to 10,000 H-cm, as opposed to the 20-100 H-cm material used in standard n-channel CCDs. Higher resistivity is required for greater penetration depth of the fields produced by the frontside polysilicon wires (penetration depth is proportional to the square root of the resistivity). These thick high resistivity CCDs have been developed for detection of soft x-rays with space satellites and can be procured from E2V and MIT/LL. 

HOMO of DBTTF and the LUMO of TCNQ, respectively (Fig. 4b). Source and drain electrodes are several organic metals of the TTF TCNQ type having different chemical potentials predicted using Fig. 4c which is the same as Fig. 2a. For the electrodes whose chemical potentials are set within the conduction band of the channel material, FET exhibited n-type behavior (A in Fig. 4d). When the chemical potentials of organic metals are allocated within or near the valence band of the channel, p-type behaviors were observed (E, F in Fig. 4d). When the chemical potentials of the electrodes are within the gap of the channel, FET exhibited ambipolar-type behavior (B-D in Fig. 4d). Since the channel material is the alternating CT solid, the drain current is not excellent and a Mott type insulator of DA type or almost neutral CT solid having segregated stacks is much preferable in this context. 

The usual TFT structure is shown in Fig. 10.7 and comprises the a-Si H channel, a gate dielectric, and source, drain, and gate contacts. N-channel accumulation mode operation using an undoped a-Si H channel is the only structure widely used. Depletion mode devices are prevented by the high defect density of doped material, which makes it difficult to deplete the channel. The much lower mobility of holes compared to electrons gives p channel devices a lower current by about a factor 100, which is undesirable. 

It is remarkable, however, that despite a roughness which is larger than the film thickness in some of our films (see below), all of them show n-channel as well as p-channel transport. Obviously, there still exists a percolation path for conduction in both materials independent of phase separation and order formation. We also note that the bulk morphology does not necessarily provide precise information for organic ficld-cffcct transistors since in these devices the active channel is restricted to the first few molecular layers at the interface to the gate dielectric [48-50]. 

As already mentioned, there was the suggestion of using ambipolar FETs to realise complementary-like organic integrated circuits [3, 10, 17, 66]. Here we investigate ambipolar inverters consisting of mixed-layer ambipolar FETs and compare their characteristics to a complementary inverter made of discrete p- and n-channel transistors from neat materials. 

Laquindanum, J.G. et al., n-channel organic transistor materials based on naphthalene... 

C. R. Newman, C. D. Flisbie, D. A. da Silva Filho, J. L. Bredas, P. C. Ewbank, and K. R. Mann, Introduction to organic thin film transistors and design of n-channel organic semiconductors, Chemistry of Materials, 16, 4436, 2004. 

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