Optoelectronic Characteristics of Organic Light-Emitting Diodes with a Rb2CO3-Mixed C60 Layer as an Electron Ohmic-Contact

A rubidium carbonate (Rb2CO3)-mixed fullerene (C60) layer was used as the electron ohmic-contact layer and the effect of Rb2CO3 mixing concentration on the optoelectronic properties of organic light-emitting diodes (OLEDs) was investigated. The performance of an electron-only device, with a glass/ITO/Rb2CO3-mixed C60 (10 nm)/Al structure, was strongly dependent on both the thickness and the mixing concentration of the Rb2CO3 layer in the Rb2CO3-mixed C60 thin films. The ultraviolet photoemission spectra of the Rb2CO3-mixed fullerene (C60) layer showed the formation of new Fermi level crossing states, below the Fermi level caused by a chemical interaction between Rb species and C60, and which led the n-mixing effect to the contact. The emergence of new FLCSs contributed to the formation of an electron ohmic-contact between the cathode and the organic layer by increasing the number of electron carriers. The OLED device with Rb2CO3-mixed C60 as an electron ohmic contact layer, and composed of a glass/ITO/MoO3-mixed NPB (25%, 5 nm)/NPB (63 nm)/Alq3 (39 nm)/Rb2CO3-mixed C60 (75%, 3 nm)/Al (150 nm) structure, showed better optoelectronic properties than the OLED fabricated with C60 (5 nm)/LiF (1 nm) as the electron ohmic-contact layer.