Energy level alignment of semiconducting organic electronic devices

Abstract

Understanding the energy level alignment and charge injection mechanism in organic semiconductors (OSCs) are an essential first step to optimize their performance. Traditionally the hole injection barrier is deduced from ultraviolet photoemission spectroscopy (UPS) as the difference between the pinned Fermi level (Ef) of anode and the ionization potential (Ip) of the OSC, which is typically of the order of a few tenths of an eV. In this thesis, I describe electromodulated absorption (EA) spectroscopy of polymer organic devices as a function of temperature and dc bias. From these measurements, the flat-band voltage (i.e., built-in potential Vbi) can be easily obtained as the dc bias required to null the quadratic Stark shift. The Vbi is important not only in light-emitting diodes (LEDs) where it gives the separation of the Fermi levels of the cathode and anode at the onset of injection, but also in photodiodes in which it corresponds to the maximum (open-circuit) output voltage. The values of Vbi have been measured here for a wide variety of polymer organic diodes. A systematic behavior has been found which suggests the existence of well-defined internal energy offsets that enable an operational definition of an effective work-function for the less-reactive metal contacts with the OSC. From the modulation of the sub-gap polaron absorption intensity in these EA spectra, I show further that it is possible to directly measure the interface hole accumulation density, and thus determine that the actual energy offset of the heterojunction in the diode is in fact much smaller than what is given by the UPS results on single heterojunctions. This suggests a considerable energy-level re-alignment in the presence of the cathode that has previously been neglected.