We observed very small electronic density of states in the band gap of pentacene thin films deposited on inert surfaces using ultraviolet photoelectron spectroscopy (UPS) with ultrahigh sensitivity. We found, furthermore, that a pentacene film with less density of gap states gives a splitting of the HOMO band in UPS spectra with energy separation of about 0.45 eV due to the band dispersion even for ultrathin polycrystalline films. The results indicate that the gap states do not originate from electronic interaction between pentacene and the substrate surface but from imperfect molecular orientation/packing structure. We confirmed that the Fermi level pinning in the pentacene films originates from the intrinsic gap states depending on their density and energy distribution. The Fermi level position as well as appearance of the band dispersion in pentacene thin films therefore depends sensitively on perfectness of the molecular packing structure in each crystal grain.

Date Published: 17 October 2007
PDF: 9 pages
Proc. SPIE 6656, Organic Photovoltaics VIII, 66560D (17 October 2007); doi: 10.1117/12.731956

Published in SPIE Proceedings Vol. 6656:
Organic Photovoltaics VIII
Zakya H. Kafafi; Paul A. Lane, Editor(s)