Dipolar SAMs Reduce Charge Carrier Injection Barriers in n-Channel Organic Field Effect Transistors

Abstract

In this work we examine small conjugated molecules bearing a thiol headgroup as self assembled monolayers (SAM). Functional groups in the SAM-active molecule shift the work function of gold to n-channel semiconductor regimes and improve the wettability of the surface. We examine the effect of the presence of methylene linkers on the orientation of the molecule within the SAM. 3,4,5-Trimethoxythiophenol (TMP-SH) and 3,4,5-trimethoxybenzylthiol (TMP-CH2-SH) were first subjected to computational analysis, predicting work function shifts of -430 and -310 meV. Contact angle measurements show an increase in the wetting envelope compared to that of pristine gold. Infrared (IR) measurements show tilt angles of 22 and 63°, with the methylene-linked molecule (TMP-CH2-SH) attaining a flatter orientation. The actual work function shift as measured with photoemission spectroscopy (XPS/UPS) is even larger, -600 and -430 meV, respectively. The contact resistance between gold electrodes and poly[N,N'-bis(2-octyldodecyl)-naphthalene-1,4:5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene) (Polyera Aktive Ink, N2200) in n-type OFETs is demonstrated to decrease by 3 orders of magnitude due to the use of TMP-SH and TMP-CH2-SH. The effective mobility was enhanced by two orders of magnitude, significantly decreasing the contact resistance to match the mobilities reported for N2200 with optimized electrodes.

Indium‐Tin‐Oxide (ITO) Work Function Tailoring by Covalently Bound Carboxylic Acid Self‐Assembled Monolayers

“…The resulting data confirm that it is possible to improve the device performance by aligning the energy levels in organic optoelectronic devices. This results in a lower contact resistance mediated by the insertion of the SAM between the metal‐oxide contact and the organic layer …”

“…These barriers can be attributed to energetic misalignments of the electrode work function and the molecular conduction orbital . Recently it was shown that a barrier reduction leads to improved charge transport and hence more efficient devices . To reach a minimal injection barrier, the electrode work function has to match the conduction orbital of the adjacent semiconductor.…”

Bithiazole: An Intriguing Electron‐Deficient Building for Plastic Electronic Applications

“…More recently, independent of the type of electrode used, it has been crucial to adjust the work functions of the electrodes to the energy levels of the semiconductor in order to avoid injection and extraction barriers. One popular approach is to functionalize the electrodes with self‐assembling monolayers (SAMs), which allows the introduction of dipoles at the interface, thus shifting the effective work function of the metal . In OFETs, both molecular and polymeric semiconductors have been successfully employed and, in some cases, the measured mobilities are comparable with that of amorphous silicon.…”

Controlled Growth of Ultrathin Film of Organic Semiconductors by Balancing the Competitive Processes in Dip-Coating for Organic Transistors

“…[1][2][3][4][5][6][7][8][9] Generally, the thickness of organic semiconductor layer in organic electronic devices is in the range of several tens of to hundreds of nanometer (nm), which consists of tens of or more molecular layers. [10][11][12][13][14][15][16] On the other hand, ultrathin film (< 15 nm, Figure 1a) of organic semiconductors represents a kind of nano-scale film consisting of monolayer to few molecular layers (Figure 1b). [17][18][19][20][21][22][23] 3 According to the surface morphology, ultrathin film includes continuous and microstructured film (Figure 1a), both of which provide excellent platform for fundamental researches and practical applications.…”