Determination of dopant concentration in co-deposited organic thin films by using RBS and X-ray fluorescence combined techniques

Organic light emitting diodes using phosphorescent dyes (PHOLEDs) have excellent performance and an internal quantum efficiency approaching 100%. To maximize performance, PHOLED devices use a conductive organic host material with a phosphorescent guest that is sufficiently dispersed to avoid concentration quenching. One of the most widely used organic compounds, green phosphorescent fac-tris(2-phenylpyridine)iridium, or [Ir(ppy)3], can be used to produce PHOLEDs with very high external quantum efficiency by doping host material at different nominal concentrations. In this study, a methodology to accurately establish dopant concentration in co-deposited organic layers is proposed and discussed. X-ray fluorescence (XRF) and Rutherford backscattering (RBS) analyses were performed in co-deposited organic thin films and then combined to provide an accurate methodology. [Ir(ppy)3] was used at different concentrations in two different hosts - 2,7-bis(9-carbazolyl)-9,9-spirobifluorene (Spiro2-CBP) and copper phthalocyanine (CuPc) - to test the proposed methodology. As Cu peak is easily detected by RBS, the CuPc host was chosen for calibration purposes, allowing more accurate determination of [Ir(ppy)3] concentration. A linear correlation between the RBS and the XRF measurement data was found allowing the drawing up of a calibration chart used to determine the [Ir(ppy)3] mass content in co-deposited films.