Berlin 2008 – scientific programme

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SYSA: Symposium Tayloring Organic Interfaces: Molecular Structures and Applications

SYSA 5: Poster Session SYSA

SYSA 5.11: Poster

Tuesday, February 26, 2008, 14:30–20:00, Poster A

Morphology, electronic and crystal structure of Pentacene/Fullerene layered and co-deposited thin films on PEDOT:PSS — •Ingo Salzmann, Steffen Duhm, Ricarda Opitz, Jian Zhang, Jürgen P. Rabe, and Norbert Koch — Humboldt-Universität zu Berlin, Institut für Physik, D-12489 Berlin, Germany

The electronic structure of bilayer and co-deposited thin films of pentacene (P) and fullerene (C60) on the conducting polymer poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been investigated by ultraviolet photoelectron spectroscopy. For a film of C60 on P covered PEDOT:PSS an energy offset between the highest occupied molecular orbital (HOMO) levels of C60 and P of 1.7 eV was found. The P-HOMO was Fermi-level pinned with the HOMO-onset at 0.35 eV binding energy and charge transfer between P and PEDOT:PSS lowered the sample work function by 0.7 eV. The C60 energy levels were vacuum level aligned relative to P. Reversing the deposition sequence leads to an onset of the P-HOMO at 0.3 eV above the Fermi level indicating a lack of thermodynamic equilibrium between PEDOT:PSS and P. Co-deposited films (ratio 1:1) exhibited an offset between the P and C60 HOMO levels of 1.1 eV, which increased to the initial value of 1.7 eV by pre-covering PEDOT:PSS with a thin P layer. Atomic force microscopy and x-ray diffraction investigations revealed phase separation of P and C60 in the co-deposited films as well as crystalline growth of P in all cases, whereas C60 was crystalline only if grown on P. Organic photovoltaic device characteristics could be correlated with the results above.