Highly Sensitive Evaluation of Density of States in Molecular Semiconductors by Photoelectron Yield Spectroscopy in Air

Despite the critical importance of the density of states (DOS) to the electrical properties of molecular semiconductors, there are few reliable measurement methods for the DOS, especially for the edge regions with low DOS that determine the charge conduction. Here, we evaluate the DOS for occupied states with high sensitivity (down to 10(-5) relative to the peak maxima) using a commercially available photoelectron yield spectroscopy (PYS) system operated in air. The conduction edges of the DOS for both the semiconducting polymers and molecules are expressed as a single Gaussian with no gap states with one exception. There are subtle differences in the widths of the Gaussian for the DOS edges between the surface of the polymer films and the interface with the substrate, which result in the large difference in the lateral charge carrier mobilities at each interface. The charge carrier mobilities of the semiconducting polymers have a correlation with the widths of the DOS. PYS in air is a powerful tool for studying the relationship among the structure and electronic and electrical properties of molecular semiconductors.