Impact of partial nitrogen and fluorine substitution on the interface properties of π-conjugated molecules adsorbed on inorganic substrates. A combined photoelectron-spectroscopy and X-ray-standing-waves study

Abstract

Organic semiconductors are carbon-based materials with promising features aimed at substituting and/or complementing their inorganic counterparts (e.g. silicon, germanium or gallium arsenide). They can be cheaply mass-produced, they are flexible and their properties can be chemically tuned, all attractive characteristics for the consumer-electronics market. In recent years, remarkable progress has been made and one can already buy smartphones and TVs equipped with organic LEDs. Also, interesting prototypes of roll-up organic solar cells have been presented. Nonetheless, the real breakthrough for these materials is yet to come since fundamental aspects regarding the charge transport through metal electrodes (among others), important for the device circuitry, still limit the overall efficiency. In this context, the energy-level alignment (ELA) between the molecules and the electrode determines the charge injection/extraction energy barriers and, therefore, is responsible for an optimum charge transfer across the interface. A proper rationalization of the ELA requires a full description of the interface properties: electronic, chemical as well as structural, including adsorption distances and molecular distortions. Chemical functionalization of polymers or small molecules by adding side groups with strong electron donor or acceptor behavior has been a way to optimize the ELA. In this work, we employ element-specific techniques such as X-ray photoelectron spectroscopy (XPS) and X-ray standing waves (XSW) to infer how partial nitrogen and fluorine substitution in prototypical and well studied π-conjugated organic molecules affects the geometrical, chemical and electronic properties of these when deposited on metal single-crystal substrates with different reactivities. We show that the combination of high-resolution XPS with XSW is a powerful method to tackle this issue since the adsorption distance and molecular distortion can be readily connected to feature changes in the XP spectra. In particular, we deposit several perylene and pentacene derivatives on different metal and semiconductor surfaces, namely the (111) surface of gold, silver and copper and the polar surfaces (000±1) of zinc oxide (ZnO). Finally, we also show that this method can be extended successfully to study the evolution of the bare ZnO surface under different treatments in real-time. ZnO is a promising transparent inorganic semiconductor that can be engineered easily in different nanostructures. Its polar surfaces represent a conundrum, as the actual composition and conformation of the surface are still a topic of debate. Having extracted the chemical and structural information with a combination of XPS and XSW, we provide new results about the surface and its respective behavior under different treatment conditions.