Large and continuous tuning of the work function of indium tin oxide using simple mixing of self-assembled monolayers

Self-assembled monolayers (SAMs) have been extensively investigated in opto-electronic applications, such as organic light emitting diodes (OLEDs). SAMs are used to tune the energy level alignment by allowing Ohmic contact at the interface between electrodes and organic semiconductors. To achieve the required energy level alignment and modify the electrode work function, molecules carrying a permanent dipole are chemically grafted at the electrode surface. Typically, the electrodes are modified by choosing one specific molecule carrying the appropriate dipole to achieve the desired (discrete) work function value. In this contribution, we propose a simple way to continuously tune the work function over almost 1 eV and demonstrate this on the most commonly used transparent electrode, namely, indium tin oxide (ITO). The continuous tuning is achieved by selecting two molecules able to form SAMs, each carrying a different permanent dipole. Solutions comprising the molecules are mixed at different relative concentrations and deposited on the ITO surface. The composition of the resulting densely packed mixed SAM is directly related to the composition of the initial mixing in solution. The effect of the SAM on the ITO electronic landscape was analyzed by various surface sensitive measurements. Furthermore, the differently functionalized transparent electrodes have been integrated in prototypical OLEDs. Through electrical characterization, we confirm the ability to continuously tune the carrier injection and thereby improve the luminescence.