Electroluminescent mechanism of organic light-emitting diodes with blue-emitting Alq

We study on aluminum (III) bis(2-methyl-8-quinolinate)-4-phenylphenolate (BAlq) organic light-emitting diodes (OLEDs) with various BAlq thicknesses. The electroluminescence (EL) due to N-N′-di(1-naphtyl)-N-N′-diphenyl-benzidine (NPD) increased with decreasing BAlq thickness for indium-tin-oxide (ITO)/NPD/BAlq/LiF/Al. The increase in electron density in BAlq near the NPD/BAlq interface enhances the electron injection from BAlq into NPD. Despite Al cathode is used instead of LiF/Al, however, the EL spectrum of the OLED with Al cathode where electron injection from the cathode is suppressed is shifted to the shorter wavelength region than that of the OLED with LiF/Al cathode. This means that the ratio of the EL due to NPD increases to that of BAlq. The decrease in the EL due to BAlq is suggested to relate with enhanced electric field in the BAlq layer (electric field quenching).