Tuning of Metal Work Functions with Self-Assembled Monolayers

Abstract

AbstractWe 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 (Δφ∼1.1 eV) for 1H,1H,2H,2H-perfluorinated alkanethiols. Hexadecanethiol on the other hand shifted φAg to 3.8 eV (Δφ ∼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)-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.