Tuning Work Function of Noble Metals As Promising Cathodes in Organic Electronic Devices

Work function (WF) modification of metal electrodes by adsorbing electron-rich or electron-deficient molecules on metal surfaces has become a field of significant interest. The barrier for charge carrier injection in organic semiconductor devices can be reduced by molecular adsorption, leading to an interfacial dipole. Here, we demonstrate that the WF of noble metals such as Au can be decreased significantly by adsorbing air stable n-type dopant acridine orange base (AOB) thin film. When a (sub)monolayer AOB is deposited on sputter-cleaned Au, the WF of the substrate changes from 5.2 to 3.5 eV. At complete coverage of the Au surface, the WF is further reduced to 3.3 eV. When a (sub)monolayer of AOB is inserted between Au and C 60 thin film, the barrier of electron injection is decreased by 0.4 ± 0.1 eV as compared to an Au−C 60 interface without AOB. Polymer solar cells with AOB/Au as a cathode have a similar open circuit voltage and comparable power conversion efficiency with devices using LiF/Al as a cathode, demonstrating that the AOB-modified gold electrode is an efficient low-work-function contact. Given the low positive pinning energy of 3.3 eV for AOB, we expect that other conventional high-work-function materials (Ag, ITO, La 0.7 Sr 0.3 MnO 3 and even PEDOT:PSS) can be modified by AOB as effectively as Au.