Physical characterizations of direct and inverted solution-processed Organic Light-Emitting Diodes

In this study, direct and inverted OLEDs (iOLEDs) were fabricated by using the polymer Super Yellow (SY) as the emissive material. By adding a 5 nm-thick layer of polyethylenimine ethoxylated (PEIE) on top of ITO/ZnO (indium tin oxide/zinc oxide) in the iOLEDs, its cathode work function was reduced of 1 eV. A thin layer of 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBi) was further added by wet-process in the iOLEDs, blocking the holes at the PEIE/SuperYellow interface, so that the iOLEDs could finally reach much higher luminance compared to the direct OLEDs, maximum 40000 cd/m(2), with a constant maximum efficiency of 15 cd/A (13 cd/A maximum for the direct OLEDs). Temperature dependent transient electroluminescence measurements were conducted in order to compare charge carriers mobilities and disorder in direct OLEDs and iOLEDs. It was shown that the holes mobilities are higher for direct OLEDs (4 10(-6) cm(2)/(V.s)) than iOLEDs (1.5 10-6 cm(2)/(V.s)) while the width of the distribution of energy states (DOS), sigma, and the positional disorder parameter, Sigma, are comparable for both structures.