Tuning of metal work function with organic carboxylates and its application in top-emitting electroluminescent devices

By fine-tuning of work functions and thus the hole-injection properties of Ag and Al anodes, an electroluminescent device was achieved by using various self-assembled monolayers of organic carboxylate on the electrode surfaces. The IR spectra evidenced different binding behaviors of the carboxylates on Ag and Al. A correlation between the change in work function with the effective dipole moment along the surface normal and the currents in the hole-only devices was observed. These self-assembled-monolayer-modified metals were used as anodes in the fabrication of top-emitting organic light-emitting diodes (TOLEDs). The TOLED with the Ag anode modified by the perfluoroalkanoate exhibited a luminous efficiency as high as 18 cdA(-1), superior to that of the Ag2O-based device. With Al as the anode, the highest luminous efficiency was merely 6 cdA(-1) and decayed rapidly. The poorer EL property and performance of Al-based TOLEDs could be attributed to the weaker ionic bindings of carboxylates on Al and the weaker microcavity effect resulting from the inferior reflectivity of Al as compared to Ag.