Improved green thermal activated delayed fluorescence OLEDs based on thermally evaporated distributed Bragg reflector (DBR) of MgF2/ZnS

Unlike the traditional fabrication of distributed Bragg reflector (DBR) structure via atomic layer deposition or spin-coating, here the 1-6 pairs of magnesium fluoride (MgF2)/zinc sulfide (ZnS) alternative dielectric layers were grown via thermal evaporation. The absorption, transmission, reflection, and photoluminescence (PL) spectra were evaluated. 5 pair MgF2/ZnS denotes the largest reflectance (88.5% at 535 nm) together with a stopband at 450-650 nm among the 1- 6 pair dielectric layers, exhibiting the potential for using as DBR. Relative to the bare 4,4'-bis(carbazol-9-yl)biphenyl(CBP):(4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN) film, the PL intensity of CBP:4CzIPN/5 pair MgF2/ZnS DBR is enhanced and splitted into two peaks. The 5 pair alternative dielectric film presents more uniform aggregation over 4 pair MgF2/ZnS. The cross-sectional scanning electron microscopic image denotes explicit layering for the MgF2and ZnS. The organic light-emitting diode (OLED) incorporating 5 pair MgF2/ZnS DBR layers illustrates significantly improved electroluminescent (EL) performance due to the photons concentrated in the direction perpendicular to the DBR. The slightly narrowed EL spectrum is originated from the microcavity effect between the two Al electrodes. Here we develop a universal method for the DBR fabrication suitable to most of OLEDs.