Paper EM2-ThA9
The Structure and Energetics of the Calcium / Phenyl-C₆₁-butyric Acid Methyl Ester Interface

Thursday, November 13, 2014, 5:00 pm, Room 314

The vapor deposition of metal films onto the surfaces of organic semiconductors, especially fullerene derivatives and π-conjugated polymers, plays an important role in the fabrication and long-term stability of organic electronic, optoelectronic and photovoltaic devices. Nevertheless, the strength and structural details of metal-organic bonding at such interfaces is not well known. We report here measurements of the interface structure and adsorption energies of calcium metal films grown by vapor deposition on phenyl-C₆₁-butyric acid methyl ester (PCBM). PCBM is a much studied electron acceptor material used in a vast array of organic electronic devices, most notably organic photovoltaics, and Ca metal is often used as the cathode material for such devices due to efficient electron collection at the cathode-active layer interface. Structural details of the interface and the morphology of the evolving metal film were measured using low-energy ion scattering spectroscopy and X-ray photoelectron spectroscopy. The energetics of interfacial bonding were measured using an adsorption microcalorimeter which is unique in the world, and which has been applied previously to the adsorption of Ca on common and well-researched electron donor materials including derivatives of poly-(phenylene-vinylene), polyfluorene, and polythiophene. Spin-cast PCBM samples were prepared under nitrogen environment and transferred to our ultrahigh vacuum chamber without exposure to atmosphere. There, a pulsed beam of calcium atoms was directed at the sample surface. By simultaneously measuring the heat of adsorption and the sticking probability of the metal atoms as a detailed function of metal coverage, we obtain interfacial bonding energies for Ca on PCBM.

Work supported by the National Science Foundation under grant CHE-1010287