Surface Energy and Work Function Control of AlOx/Al Surfaces by Fluorinated Benzylphosphonic Acids

Abstract

The performance of organic electronic devices can be significantly improved by modifying metal electrodes with organic monolayers, which alter the physical and chemical nature of the interface between conductor and semiconductor. In this paper we examine a series of twelve phosphonic acid compounds deposited on the native oxide layer of aluminum (AlOx/Al), an electrode material with widespread applications in organic electronics. This series includes dodecylphosphonic acid as a reference and eleven benzylphosphonic acids, seven of which are fluorinated, including five newly synthesized derivatives. The monolayers are experimentally characterized by contact angle goniometry and by X-ray photoemission spectroscopy (XPS), and work function data obtained by low-intensity XPS are correlated with molecular dipoles obtained from DFT calculations. We find that monolayers are formed with molecular areas ranging from 17.7 to 42.9 Å2/molecule, and, by the choice of appropriate terminal groups, the surface energy can be tuned from 23.5 mJ/m2 to 70.5 mJ/m2. Depending on the number and position of fluorine substituents on the aromatic rings, a variation in the work function of AlOx/Al substrates over a range of 0.91 eV is achieved, and a renormalization procedure based on molecular density results in a surprising agreement of the changes in surface potential with interface dipoles as expected from Helmholtz’ equation. The ability to adjust energetics and adhesion at organic semiconductor/AlOx interfaces should have immediate applications in devices such as OLEDs, OTFTs, organic solar cells, and printed organic circuits.