Space Charge Regions at Organic p-i-Homointerfaces from Advanced Modeling of In Situ-Prepared Interfaces Analyzed by Photoelectron Spectroscopy

The electronic level alignment at interfaces plays an important role in the development and optimization of organic semiconductor devices. While organic heterointerfaces are well studied, photoelectron spectroscopy on homointerfaces is challenging. As the emissions of the substrate and the adsorbate layer are dominated by the same spectral features, it is nearly impossible to distinguish their layer origin. In this study, we accept this challenge and analyze the interface between doped and undoped layers consisting of the same hole transport molecule (HTM). We used X-ray and ultraviolet photoelectron spectroscopy to monitor the stepwise thin film deposition of the well-known 4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA) molecule as well as a state-of-the-art triarylamine-based molecule synthesized at Merck KGaA. The interpretation of the data is enabled by a fitting procedure based on an energetic disorder model. First, as a test case, heterointerfaces of step by step deposited, differently p-doped HTMs on indium tin oxide are analyzed, revealing the power of the model for an accurate description of the data while enabling detailed discussions of the model by comparison to classical PES data analysis. Second, homointerfaces of the intrinsic HTMs on their p-doped sublayers are studied. Here, we observe an unexpected space charge region in the p-doped sublayer. Within the boundaries of our model, we obtain good fits of spectra by introducing an increased number of electronic states right at the interface in the undoped layer.