Depth Profiling of Organic Light-Emitting Diodes by ToF-SIMS Coupled with Wavelet–Principal Component Analysis

The investigation of interface effects in hybrid organic/inorganic electronic and photonic devices is a key step for improving their performance and operation stability. Depth profile analysis by time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a consolidated tool for the molecular analysis and imaging of thin layers, however, the ion beam sources and conditions need to be finely tuned to analyze hybrid stacks. In this work, we apply a low-energy (500 eV) Cs+ sputter beam and a 25 keV Bi3 + analysis beam to successfully depth profile two organic light-emitting diodes (OLED) stacks, including hard inorganic electrodes and soft organic layers. We show that both organic and inorganic materials are profiled and that the characteristic molecular fragments from each organic layer are preserved in these conditions. Despite this the non-negligible molecular fragmentation resulting from Cs+ sputtering sensibly increases the mass spectra complexity. To simplify the data set analysis, we have investigated the application of the wavelet principal component analysis (w-PCA) for data reduction and for assisting at least the first screening of such complex depth profiles.