Decoupling the Effects of Self‐Assembled Monolayers on Gold, Silver, and Copper Organic Transistor Contacts

Abstract

In bottom‐contact organic field‐effect transistors (OFETs), the functionalization of source/drain electrodes leads to a tailored surface chemistry for film growth and controlled interface energetics for charge injection. This report describes a comprehensive investigation into separating and correlating the energetic and morphological effects of a self‐assembled monolayers (SAMs) treatment on Au, Ag, and Cu electrodes. Fluorinated 5,11‐bis(triethylsilylethynyl) anthradithiophene (diF‐TES‐ADT) and pentafluorobenzenethiol (PFBT) are employed as a soluble small‐molecule semiconductor and a SAM material, respectively. Upon SAM modification, the Cu electrode devices benefit from a particularly dramatic performance improvement, closely approaching the performance of OFETs with PFBT‐Au and PFBT‐Ag. Ultraviolet photoemission spectroscopy, polarized optical microscopy, grazing‐incidence wide‐angle X‐ray scattering elucidate the metal work function change and templated crystal growth with high crystallinity resulting from SAMs. The transmission‐line method separates the channel and contact properties from the measured OFET current–voltage data, which conclusively describes the impact of the SAMs on charge injection and transport behavior.

Potentiometric Parameterization of Dinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene Field‐Effect Transistors with a Varying Degree of Nonidealities

“…As a model system for fluorinated aromatic SAMs, PFBT can exert morphological and energetic effects. [13] Figure 4b suggests that, in this particular group of transistors, the SAM effect more strongly manifests itself as a growth template (Figure 2b) and a boost for the mobility (Figure 3d), while its effect on injection energy (or β) was weaker. In other words, the reduced R c in the SAM-functionalized OFETs is mainly due to the increase in µ ch , which in the end drove the overall performance enhancement (Figure 1c).…”

“…The fact that the measured work function increase widely varied in the literature (10 meV to 1 eV range) is an evidence, even if the experimental methods were similar and the device improvement was obvious in all cases. [13,45,46] Second, when morphological and energetic effects are involved simultaneously, one can simply dominate over the other, eventually screening the manifestation of the less significant. In our case, the SAMs promoted the tight packing of DNTT, and the associated transport enhancement was likely to be more dramatic than the barrier reduction.…”

“…Until recently, many experimental studies employed techniques such as the transmission-line method (TLM) or gated fourpoint probe measurements (gFPP), which can in principle directly probe these phenomena by separating the channel and contact properties. [8][9][10][11][12][13][14] In our view, the reported data bring strong evidence for the pronounced variability in relative strengths, and voltage-or structure dependence of the contact and channel effects, which makes different theoretical frameworks often necessary to understand different devices. Nonetheless, the MOSFET current-voltage model has been employed quite universally for simple parameter extraction.…”

Direct investigations of the interface impedance of organic field-effect transistors with self-assembled-monolayer-modified electrodes

“…Thus, the use of SAMs having negative electronic dipoles increases the work function of the electrodes, which results in the reduction of the carrier injection barrier at the metaleorganic interfaces [8,9,12]. Another report, however, showed a greater reduction in the contact resistance of the OFETs with SAM-modified electrodes, despite the fact that the modified electrode showed almost the same work function as that of bare electrodes [10]. Usually SAMs not only modify the surface energy of the metal electrodes, but also affect the crystallinity and grain sizes of the organic thin films [11].…”

Fully transparent organic transistors with junction-free metallic network electrodes

“…The PFBT SAM treatment has been previously employed on different continuous film based metal contacts including Cu, Ag, and Au. 17 It is believed that the strong electron withdrawing character of the PFBT would enhance the work function of the metal electrodes, 18,19 and thus the hole injection barrier between the metal contact and the highest occupied molecular orbital (HOMO) of the organic semiconductor can be reduced. Here, instead of the continuous thin film metal electrode, we applied PFBT (thiol-SAMs) onto network based electrode, and there was a significant improvement in the charge injection of the bottom contact devices.…”