Small molecular organic light-emitting diodes (OLEDs) can be fabricated by spin-coating and thermo-evaporation of organic materials. In this work, X-ray photoelectron spectrometry (XPS) and secondary ion mass spectrometry (SIMS) are used to examine the microstructure of emissive layers in organic LEDs prepared with different technique using in situ C 60 + sputtering. It is found that the Ir-containing guest molecule is enriched at the bottom-interface and a concentration gradient results from spin-coating while the opposite molecular distribution is observed with thermo-evaporation. Such a difference in microstructure affects the carrier-injection and carrier-transportation and is key to the dramatic difference (∼3.5×) in the efficiency of the resultant OLED devices.
