Tailoring the work function of surfaces: Effects of charge transfer in surface modifiers on metals and transparent conducting oxides

Abstract

There has been much research into modifying the work function of metals and transparent conducting oxides for use in organic optoelectronic device applications, particularly as electrode materials in organic photovoltaic devices (OPVs) and organic light-emitting diodes (OLEDs). Materials’ work functions may need to be raised or lowered depending on the desired application. OPVs and OLEDs require a low work function electrode to serve as the electron-selective and electron-injecting electrode, respectively. Traditional low work function materials are highly reactive to air and water and thus devices that use these materials require encapsulation for the device to operate as intended. Methods for making low work function materials are being examined, such as taking an air-stable, high work function material and using a modifier to lower the effective surface work function. This method would give a low work function electrode when interfacing with the active materials and it could still behave as a high work function material when interacting with the environment. Increasing the work function of an electrode material is also desirable in order to more efficiently extract holes from OPVs and inject holes into OLEDs. This is generally done with surface modifiers that have an internal dipole moment in which the negative end points away from the surface, in order to increase the electrostatic potential and thus raise the work function, but systems with large charge transfer, presented here, have now shown that this is not the only method. In systems to both raise and lower the work function, charge transfer has been shown to affect the method by which the work function is modified. Charge transfer can be so important in the system, that if it is not present, the work function modification ability will be reduced or the work function may possibly remain unchanged from the unmodified surface.