AVS 56th International Symposium & Exhibition
    Surface Science Wednesday Sessions
       Session SS2-WeM

Paper SS2-WeM6
The Ca / MEH-CN-PPV Interface: Ca Adsorption Calorimetry, LEIS and XPS

Wednesday, November 11, 2009, 9:40 am, Room N

Session: Surface Chemistry and Dynamics
Presenter: J.C. Sharp, University of Washington
Authors: J.C. Sharp, University of Washington
J.H. Baricuatro, University of Washington
F. Bebensee, Universitaet Erlangen-Nuernberg, Germany
H.-P. Steinrueck, Universitaet Erlangen-Nuernberg, Germany
J.M. Gottfried, Universitaet Erlangen-Nuernberg, Germany
C.T. Campbell, University of Washington
Correspondent: Click to Email
Conjugated polymers are of interest for use in photovoltaics and OLEDs because of the tunability of their band gap and ease of processing them into devices. Poly(5-(2-ethylhexyloxy)-2-methoxy-cyanoterephthalylidene) (MEH-CN-PPV) has high photoluminescence efficiency and thus has potential use in OLEDs. The interaction between the conjugated polymer and metal electrode is critical to the performance of these devices. The energetics and structure of calcium growth on MEH-CN-PPV was studied using adsorption microcalorimetry and low-energy ion scattering spectroscopy (LEIS) in ultrahigh vacuum. At 300 K the integrated Ca LEIS peak area grew slowly until ~1.5 ML and the rate of growth increased until a continuous film of Ca was formed by ~15 ML. The slow initial growth rate is due to calcium diffusing subsurface. After ~1.5 ML, Ca forms 3-D islands on the surface. At 130 K, the transitions from subsurface diffusion to 3D cluster growth to film formation occur at lower coverages. The initial heat of adsorption of Ca on MEH-CN-PPV at 300 K is 436 kJ/mol. The heat of adsorption increases to 464 kJ/mol at ~0.1 ML and then decreases to the heat of sublimation of Ca by 4 ML. Similar behavior is seen at 130 K with the initial heat of adsorption being 326 kJ/mol and increasing to 331 kJ/mol at ~0.1 ML and decreasing to the heat of sublimation already by 1.25 ML. The sticking probability (S) at 300 K was 0.70 initially and increased to 0.75 at ~0.1 ML of Ca coverage. After ~0.1 ML, S decreased to 0.64 by ~1.5 ML, then gradually increases to unity. The initial sticking probability at 130 K was 0.92 and increased toward unity. The thermodynamic, sticking and growth behavior below 1.5 ML, where Ca is interacting strongly with specific adsorption sites on the polymer, will be discussed