Embedded Dipole Self‐Assembled Monolayers for Contact Resistance Tuning in p‐Type and n‐Type Organic Thin Film Transistors and Flexible Electronic Circuits

Abstract

Based on the powerful concept of embedded dipole self‐assembled monolayers (SAMs), highly conductive interfacial layers are designed, which allow tuning the contact resistance of organic thin‐film transistors over three orders of magnitude with minimum values well below 1 kΩ cm. This not only permits the realization of highly competitive p‐type (pentacene‐based) devices on rigid as well as flexible substrates, but also enables the realization of n‐type (C60‐based) transistors with comparable characteristics utilizing the same electrode material (Au). As prototypical examples for the high potential of the presented SAMs in more complex device structures, flexible organic inverters with static gains of 220 V/V and a 5‐stage ring‐oscillator operated below 4 V with a stage frequency in the range of the theoretically achievable maximum are fabricated. Employing a variety of complementary experimental and modeling techniques, it is shown that contact resistances are reduced by i) eliminating the injection barrier through a suitable dipole orientation, and by ii) boosting the transmission of charge carriers through a deliberate reduction of the SAM thickness. Notably, the embedding of the dipolar group into the backbones of the SAM‐forming molecules allows exploiting their beneficial effects without modifying the growth of the active layer.

Understanding, Optimizing, and Utilizing Nonideal Transistors Based on Organic or Organic Hybrid Semiconductors

“…o) Output characteristics of n‐type C 60 ‐OFETs with pyrimidine‐SAM‐treated Au. Reproduced with permission . Copyright 2018, Wiley‐VCH.…”

Critical Evaluation of Organic Thin-Film Transistor Models

“…The remaining sets of TFTs, that were analyzed for comparison, are a series of bottom-gate, top-contact TFTs [29] and series of bottom-gate, bottom-contact TFT with either pentacene or C60 as the semiconductor and Au contacts decorated with biphenyl-based SAMs containing embedded dipoles (one phenyl ring exchanged by pyrimidine) to adjust the work function of the contacts. [21] 3. Results…”

“…These other technologies include a similar TFT set as above, only the Au contacts were changed from bottom-contact to top-contact while the thickness of the DNTT layer was kept at 30 nm (called DNTT -TC in the following). [34] In addition, three other bottom-gate, bottom-contact TFT series were examined: pentacene on Au contacts coated with a SAM of 2-phenylpyrimidine-5-thiol (Pentacene -BP0-down), C60 with 4-(2-mercaptophenyl)pyrimidine (C60 -BP0-up) and C60 with biphenyl-4-thiol (C60 -BP0) (for detail, see [21]). For DNTT -TC and C60 -BP0, non-linearities in the linear regime of the output characteristics were modeled with a gate-voltage-dependent Schottky diode at the source side to get a reasonable agreement of measured and fitted characteristics (for details about the Schottky diode, see [21]).…”

“…[34] In addition, three other bottom-gate, bottom-contact TFT series were examined: pentacene on Au contacts coated with a SAM of 2-phenylpyrimidine-5-thiol (Pentacene -BP0-down), C60 with 4-(2-mercaptophenyl)pyrimidine (C60 -BP0-up) and C60 with biphenyl-4-thiol (C60 -BP0) (for detail, see [21]). For DNTT -TC and C60 -BP0, non-linearities in the linear regime of the output characteristics were modeled with a gate-voltage-dependent Schottky diode at the source side to get a reasonable agreement of measured and fitted characteristics (for details about the Schottky diode, see [21]). Figure 6 shows the ohmic part of the contact resistance r C,0 as a function of the channel length L for all of the additional four TFT series.…”

A Solvent‐Free Solution: Vacuum‐Deposited Organic Monolayers Modify Work Functions of Noble Metal Electrodes

“…[2][3][4][5][6][7][8][9] Braun et al have argued that different coupling mechanisms-ranging from charge transfer, chemical injection via tunneling as well as thermionic injection may be improved, thus reducing contact resistance on the injection side. [32] Since planar π-conjugated molecules are considerably thinner, vacuum-processed contact primers of these molecules have been proposed as an alternative to SAMs. [17][18][19][20][21][22][23][24][25][26][27] These works, however, have also highlighted some shortcomings of this particular type of monolayer, namely the poor conductivity of aliphatic SAMs and the poor solubility and therefore difficult processing of long aromatic SAMs.…”