Interpretation of the modulus spectra of organic field-effect transistors with electrode overlap and peripheral regions: determination of the electronic properties of the gate insulator and organic semiconductor

Abstract

The modulus spectra of organic field-effect transistors (OFETs) with electrode overlap and peripheral regions have been experimentally and theoretically investigated. The complex impedance of regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) OFETs with electrode overlap and peripheral regions was measured with a frequency response analyzer. The complex modulus was derived from an equivalent circuit of OFETs with overlap and peripheral regions using a four-terminal matrix approach. The modulus spectra of the P3HT OFETs were successfully fitted by those calculated using the expression derived from the equivalent circuit. Three structures were found in the modulus spectra of the P3HT OFETs owing to the dielectric properties of the gate insulator, transport properties of the organic semiconductor, and contact resistance from the low to high frequency ranges. The resistivity of the gate insulators and the field-effect mobility of working OFETs were determined using the values of the circuit components of the equivalent circuit obtained by fitting. I. INTRODUCTIONOrganic field-effect transistors (OFETs) are currently attracting a great deal of attention as next-generation electronic devices because they can be fabricated by a printing process on flexible large-area substrates with low cost. [1][2][3] The device performance of OFETs, which has been usually investigated by analysis of the transfer and output characteristics of OFETs, 4,5 is closely related to the device structures. For instance, a staggered configuration of OFETs has often been used. [6][7][8][9][10] In this configuration, a certain overlap region between the source and the gate electrodes is essential to reduce the contact resistance, which is caused by the strong electric field formed between the source and the gate electrodes. 5 However, excessive electrode overlap regions can act as parasitic capacitance, and thereby the cut-off frequency of OFETs decreases with increasing area of the electrode overlap region.

Theory of carrier accumulation in organic heterojunctions

“…The thickness of the accumulated region, the height of the relaxation peak of ImM was nearly constant and its relaxation frequency increased monotonically as the gate voltage up to 30V [23]. These behaviors are consistent with the above discussion.…”

“…If we accept it, ζ is at least larger than 10 in the OLEDs used for the evaluation of carrier density. In the case of OFET with a 500nm insulator layer and a 40nm organic layer, the height of the ImM of the relaxation peak of the organic layer which represents the effective thickness of the conductive region does not show obvious change up to the gate voltage of 30V [23]. From it, ζ is estimated at over 100.…”