Engineering of Printable and Air-Stable Silver Electrodes with High Work Function using Contact Primer Layer: From Organometallic Interphases to Sharp Interfaces

Contact engineering is an important issue for organic electronics as it allows to reduce charge carrier injection barriers. While the use of molecular contact primer layers to control the energy level alignment is demonstrated in many concept studies, mainly using (single crystalline) model substrates, the processability of electrodes and their robustness must also be considered in real devices. Although silver electrodes can be printed using silver ink, their low work function and sensitivity to oxidation severely limits their use for printable organic electronics. The present study demonstrates that monolayers of F(4)TCNQ and F(6)TCNNQ provide a reliable approach to engineer high work function silver electrodes, which is examined for Ag(111) as well as polycrystalline and silver ink substrates. Notably, upon multilayer growth, a pronounced intercalation of silver into the molecular adlayer occurs, yielding thermally stabilized organometallic interphases extending over the entire adlayer. It is shown that heating allows their controlled desorption leaving behind a well-defined monolayer that is further stabilized by additional charge transfer. Especially F(6)TCNNQ contact primer layers can also be prepared on oxidized silver electrodes yielding work functions of 5.5-5.6( )eV, which can even withstand air exposure. Such contact primers show no interdiffusion into subsequently deposited layers of the prototypical p-type organic semiconductor pentacene, hence validating their use for organic electronic devices.