Fabrication and characterization of microcavity organic light-emitting diode with CaF2/ZnS distributed Bragg reflector

We fabricated alternately multilayered films using CaF2 and ZnS with a different number of pairs by thermal evaporation method and evaluated their reflectance as a distributed Bragg reflector (DBR). A 7-pair CaF2/ZnS multilayered film had an enough high reflectance for use as DBR in the wavelength region of 400–600 nm, which is good agreement with the theoretically calculated reflectance. The fabricated CaF2/ZnS multilayered film was incorporated to the organic light-emitting diode (OLED) devices. We found that the inversed-type OLED (iOLED) structure is suitable for achieving the working device. From the result of electroluminescence (EL) measurement, the spectral linewidth of EL of inversed OLED with DBR (μ-cavity iOLED) was significantly narrowed compared to that of conventional OLED (cOLED). The EL peak wavelength of μ-cavity iOLED was centered around 520 nm, which was well corresponded to the designed cavity length. The observed narrowing of EL spectra for μ-cavity iOLED was originated from the microcavity effect. Color coordinate of the emission were changed from cOLED: (0.31, 0.54) to μ-cavity iOLED (0.20, 0.72), which indicates a significant improvement of color purity of green emission.