Study of contact resistance of high-mobility organic transistors through comparisons

Abstract

Realization of high-frequency low-cost organic electronics requires high-mobility organic field-effect transistors (OFETs) with short channels, where influence of contact resistance becomes more serious than either lower mobility or longer channel devices. To reduce the contact resistance, we systematically and quantitatively investigate the influence of the lowest unoccupied molecular orbital (LUMO) level of an electron acceptor layer, the active layer thickness, and the side chain of active layer itself on contact resistance of top-contact high-mobility OFETs through a series of comparative analysis. We find that the acceptor of 1,3,4,5,7,8-hexafluoro tetracyano naphtha quinodimethane (F 6 TNAP) with a deeper LUMO level is efficient for carrier injection and that the bulk resistance plays an important role in such devices. By optimizing the parameters, we get the lowest contact resistance of only 110 •cm, and thus recorded effective mobility of 8.0 cm 2 /Vs is attained for polycrystalline thin film transistors and still kept as high as 6 cm 2 /Vs at shorter channel lengths.

High‐Speed Organic Single‐Crystal Transistor Responding to Very High Frequency Band

“…Although the statistical errors from each fitting are not small, R C · W for OFETs with an F 4 ‐TCNQ layer is found to be lower than that for F 6 ‐TNAP‐inserted OFETs, whereas R sheet is almost identical for both OFETs, indicating that the dopant materials exert a significant influence on the contacts. The contact resistance evaluation shows an opposite trend compared to previous studies, where F 6 ‐TNAP gives a greater reduction in contact resistance than F 4 ‐TCNQ due to a slightly deeper lowest unoccupied molecular orbital (LUMO) level of F 6 ‐TNAP . The trend observed in this study can be explained by the bulk resistance of the dopant material itself.…”

Low‐Voltage, High‐Frequency Organic Transistors and Unipolar and Complementary Ring Oscillators on Paper

“…For the bending experiment, the banknote was rolled around a cylindrical bar with a radius of about 2.5 mm, which induces a tensile strain of about 3% in the TFTs. [38][39][40][41] Using the transmission line method (TLM), [25,27,[39][40][41][42][43] we have determined a width-normalized contact resistance of 1.4 kΩ cm and an intrinsic channel mobility of 0.8 cm 2 (Vs) −1 at an overdrive voltage (V GS -V th ) of −1.3 V for the DNTT TFTs (see also Table S2 in the Supporting Information). There appears to be no systematic effect of the bending on the threshold voltage and the gate current.…”

Materials Meets Concepts in Molecule‐Based Electronics

“…[ 169,170 ] As can be seen from Equation 4 , 5 , the MOSFET formalism further suggests an increase of output current I D with decreasing channel length L , as is also true for the OFET's cutoff frequency, which scales as [ 156 ] Additionally, the poor charge transport in organic semiconductors, especially across grain boundaries of amorphous or polycrystalline materials, favors short conduction paths. Furthermore, the contacts in such devices are often doped in order to enhance charge carrier injection and thus reduce the infl uence of contact resistance, [ 168,[178][179][180][181][182][183] a method which becomes increasingly diffi cult at short channel lengths due to diffusion of the dopant into the conductive channel. [ 156,167,171 ] Reduction of the channel length beyond a critical point (which depends on the device in question) always leads to so-called short channel behavior [ 156,[171][172][173][174] and an increasing limitation of the OFET performance by contact resistance.…”