Tuning the work functions of metals was demonstrated 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 toward 3.8 eV (ΔΦ ~ 0.6 eV) and raised the energy barrier for hole injection. These SAMs on Au were less efficient. 1H,1H,2H,2H-perfluorodecanethiol raised ΦAu (4.9 eV) by 0.5 eV to 5.4 eV, whereas hexadecanethiol decreased ΦAu by only 0.1 eV. These chemically modified electrodes were applied in the fabrication of pLEDs and the hole conduction of 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 5.2 eV was established. Conversely, a silver electrode modified with a SAM of hexadecanethiol lowered ΦAg to 3.8 eV, creating an efficient energy barrier for hole injection. This method demonstrates a simple and attractive approach to modify and improve metal/organic contacts in organic electronic devices like LEDs and photovoltaic cells.
The contact energy barrier of poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT:PSS) on poly(9,9-dioctylfluorene) (PFO) amounts to 1 eV, resulting in a hole injection current that is reduced by 6-7 orders of magnitude. However, upon electrical addressing it is demonstrated that the hole contact barrier completely vanishes, resulting in an Ohmic contact. Furthermore, a change of the built-in electric field has been observed with two different techniques. The shift of the built-in electric field is in agreement with a charge dipole layer that reduces the injection energy barrier for the PEDOT:PSS/PFO contact.