SYMPOSIUM ON SURFACE SCIENCE 2011 Baqueira Beret, Lleida ...

Charge Transport Through and Within Self-Assembled Monolayers:
New Insights from Nanofabricated Model Devices
Christof Wöll
Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology, KIT, 76344 Karlsruhe
E-mail: Christof.Woell@KIT.edu, www.ifg.kit.edu
The use of organic molecules as active
semiconductors in electronic devices is reaching
the stage where commercial products are arriving
at the market. There are, however, still a number
of fundamental issues, e.g. a proper description of Fig. 1: Schematic diagram of an “ideal” organic
diode with two tunnel contacts [3].
phenomena (e.g. charge injection) occurring at the
electrode/OSC interface. Unfortunately, there are pronounced problems with growing highly
ordered OSC-films on (strictly clean) metal substrate [1]. An attractive alternative to produce
well-defined interfaces also suited for a theoretical analysis is to modify the metal electrode by
adsorbing a well-defined thiolate-based self-assembled monolayer (SAM) [2]. This approach not
only allows adjusting the work-function of the metal, but also provides structurally perfect
substrates well suited for the OMBD-process.
In this talk we will demonstrate that on an Au substrate covered with a decanethiolate SAM,
pentacene, one of the most interesting molecules to be used as organic semiconductor in OFETs, can
be grown in a rather perfect, bulk-like structure. The high structural quality and the absence of any
contaminations make it possible to produce a two-terminal organoelectronic model device as shown
in Fig. 1 [3]. This “ideal” device is essentially free of imperfections and well suited for a theoretical
analysis. The results of numeric simulations [3] reveal that, at positive bias, n-transport dominates
p-transport, an unusual behavior for pentacene-based devices.
The n-conduction within the OSC pentacene can be suppressed by introducing OH-groups at the
SAM/OSC interface, a finding which is in accord with related experiments on OFETs [4]. Recent
experiments indicate that loading of the OH-traps with electrons is a reversible process [5].
Fabrication of functioning OFET devices confirms the positive effect of SAM surface modification
on device properties [6].
A recent investigation of nanographene-based SAMs has demonstrated that using the
SAM-approach relative molecular orientations can be achieved which are different from that present
in the corresponding bulk phases, with very positive effects on the charge carrier mobilities [7].
References
[1] G.Witte and Ch.Wöll, J.Mater.Res. 19, 1889 (2004)
[2] M. Kind and C. Wöll, Prog. Surf. Sci. 84, 230 (2009)
[3] L.Ruppel, A.Birkner, G.Witte, C.Busse, T.Lindner, G.Paasch, C.Wöll, J.Appl.Phys. 102,
033708 (2007)
[4] L.Chua, J.Zaumseil, J.Chang, E.Ou, P.Ho, H.Sirringhaus, R.Friend, Nature 434, 194 (2005)
[5] Z.-H. Wang, D. Käfer, A. Bashir, J. Götzen, A. Birkner, G. Witte, Ch. Wöll,
PhysChemChemPhys, 12, 4317-4323 (2010)
[6] C.Bock, D.Pham, U.Kunze, D.Käfer, G.Witte, Ch.Wöll, J.Appl.Phys. 100, 114517 (2006)
[7] D. Käfer, A. Bashir, X. Dou, G. Witte, K. Müllen, Ch. Wöll, Adv. Mater, 22, 384-388 (2010)
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