Tuning of Metal Work Functions with Self-Assembled Monolayers
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Tuning of Metal Work Function with Self-assembled Monolayers B. de Boer, A. Hadipour, M. M. Mandoc, P. W. M. Blom Molecular Electronics, Materials Science Centreplus , University of Groningen, Nijenborgh 4, NL-9747AG, Groningen, The Netherlands ABSTRACT We demonstrate the tuning of metal work functions by chemically modifying the metal surface through the formation of chemisorbed self-assembled monolayers (SAMs) derived from 1H,1H,2H,2H-perfluorinated alkanethiols and hexadecanethiol. The ordering inherent in the SAMs creates an effective, molecular dipole at the metal/SAM interface, which increased the work function of Ag (ΦAg~4.4 eV) to 5.5 eV (∆Φ~1.1 eV) for 1H,1H,2H,2H-perfluorinated alkanethiols. Hexadecanethiol on the other hand shifted ΦAg to 3.8 eV (∆Φ~0.6 eV). On Au, the SAM of 1H,1H,2H,2H-perfluorodecanethiol raised ΦAu (4.9 eV) with 0.6 eV to 5.5 eV, whereas hexadecanethiol decreased ΦAu by 0.8 eV. These chemically modified electrodes were applied in the fabrication of polymer LEDs and the hole injection into poly[2-methoxy-5(2’-ethyl-hexyloxy)-1,4-phenylene vinylene (MEH-PPV) was investigated. An Ohmic contact for hole injection between a silver electrode functionalized with the perfluorinated SAMs, and MEH-PPV with a HOMO of 5.3 eV was established. Conversely, a silver electrode modified with a SAM of hexadecanethiol lowered ΦAg to 3.8 eV blocked the hole injection into PPV, which enables studying the electron transport in composite devices. The electron-only current was measured in a polymer/polymer blend photovoltaic cell based on MDMO-PPV (as donor) and poly[oxa-1,4-phenylene-(1-cyano-1,2-vinylene)-(2-methoxy-5-(3’,7’-dimethyloctyloxy)-1,4phenylene)-1,2-(2-cyanovinylene)-1,4-phenylene] (PCNEPV, acceptor). This method demonstrates a simple and attractive approach to modify and improve metal/organic contacts in organic electronic devices like LEDs, photovoltaic cells, and FETs. INTRODUCTION Metallic contacts in organic, opto-electronic devices are determinative for the ultimate device performance. Preparation methods, diffusion of metal atoms, reactivity toward air and organics,1 and roughness of the metallic contact can have a detrimental influence on the stability and performance of organic thin-film devices like light-emitting diodes (LEDs), photovoltaic (PV) cells, and (ambipolar) field-effect transistors (FETs). Although these influences have to be addressed before a reliable and reproducible device can be measured, one also has to take into account the intrinsic properties of the metal like transparency, reactivity, and especially its work function. In polymer LEDs, one has to choose an electron injecting contact with a work function that matches (within a few tenths of an eV) the energy level of the lowest unoccupied molecular orbital (LUMO) of the polymer layer to prevent the formation of an electron injection barrier. Analogous to the electron injection barrier, one can foresee a hole injection barrier if the metal work function of the hole injecting contact doesn’t line up with the h
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