Nonlinear conductivity of organic conductors is explained quantitatively starting from a phenomenological energy-balance equation. Experimental results of ͑TMET-TTP͒ 4 PF 6 are presented, where TMET-TTP is 2-͓4, 5-bis͑methylthio͒-1,3-dithiol-2-ylidene͔-5-͓4,5-ethylenedithio-1,3-dithiol-2-ylidene͔-1,3,4,6-tetrathiapentalene, and the simulation reproduces the nonlinear properties very well, including the voltage-current characteristics, power-law excess conductivity, and temperature dependence of the threshold field and current. The obtained heat capacity is much smaller than the lattice heat capacity, indicating unimportance of the Joule heating. Since the organic conductors show very little hysteretic switching, the time dependence of the energy balance is traced explicitly, defining nonlinear dynamics of hypothetical electron temperature, which reproduces oscillating states similar to that of the organic thyristor and even chaotic states under certain circumstances. It is expected that transient phenomena such as oscillatory and random outputs in some sort of organic and inorganic nonlinear conductors are quantitatively explained by this method, which in turn affords a playground of nonlinear dynamics.
N-channel organic field-effect transistors with stable performance at ambient conditions are fabricated on the basis of an electron-accepting molecule, dimethyldicyanoquinone diimine (DMDCNQI). The transistors are investigated by varying source and drain electrode materials: Au, Ag, Cu, and a highly conducting organic charge-transfer salt, (tetrathiafulvalene)(tetracyanoquinodimethane) [(TTF)(TCNQ)]. The devices with the Au electrode show lowest contact resistance and highest electron mobility (0.011 cm 2 V À1 s À1 for bottom-contact configuration), and the performance decreases in the order of Au > (TTF)(TCNQ) > Ag > Cu. This order does not seem related to the metal work functions, but is attributed to the organic-metal interfacial potentials. DMDCNQI forms highly conducting charge-transfer complexes with Ag and Cu, but the complex layer increases the interfacial potential as well as the electron-injection barrier and also increases the off-current for short channel devices. The air stability is not determined solely by the organic semiconductor but is considerably influenced by the electrode materials.
Remarkable nonlinear conductivity is observed in the organic conductor (MDT-TS)(I2Br)_{0.420} (MDT-TS: 5H-2-(1,3-diselenol-2-ylidene)-1,3,4,6-tetrathiapentalene) below the metal-insulator transition at 30 K. This compound is characterized by the incommensurate donor and anion columns, and the non-Ohmic behavior is associated with the collective excitation at the antiferromagnetic insulating state. We have observed spontaneous current oscillation analogous to the organic thyristor found in the theta-phase organic conductor.
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