Tuning of Metal Work Functions with Self-Assembled Monolayers

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 (φ Ag ∼4.4 eV) to 5.5 eV (Δφ∼1.1 eV) for 1H,1H,2H,2H-perfluorinated alkanethiols. Hexadecanethiol on the other hand shifted φ Ag to 3.8 eV (Δφ ∼0.6 eV). On Au, the SAM of 1H,1H,2H,2H-perfluorodecanethiol raised φ Au (4.9 eV) with 0.6 eV to 5.5 eV, whereas hexadecanethiol decreased φ Au 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 φ Ag 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)-phenylene)-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.