P-type doping of organic wide band gap materials by transition metal oxides: A case-study on Molybdenum trioxide

Michael Kröger, Sami Hamwi, Jens Meyer, Thomas Riedl, Wolfgang Kowalsky, Antoine Kahn

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A study on p-doping of organic wide band gap materials with Molybdenum trioxide using current transport measurements, ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy is presented. When MoO3 is co-evaporated with 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP), a significant increase in conductivity is observed, compared to intrinsic CBP thin films. This increase in conductivity is due to electron transfer from the highest occupied molecular orbital of the host molecules to very low lying unfilled states of embedded Mo3O9 clusters. The energy levels of these clusters are estimated by the energy levels of a neat MoO3 thin film with a work function of 6.86 eV, an electron affinity of 6.7 eV and an ionization energy of 9.68 eV. The Fermi level of MoO3-doped CBP and N,N′-bis(1-naphtyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD) thin films rapidly shifts with increasing doping concentration towards the occupied states. Pinning of the Fermi level several 100 meV above the HOMO edge is observed for doping concentrations higher than 2 mol% and is explained in terms of a Gaussian density of HOMO states. We determine a relatively low dopant activation of ∼0.5%, which is due to Coulomb-trapping of hole carriers at the ionized dopant sites.

Original languageEnglish (US)
Pages (from-to)932-938
Number of pages7
JournalOrganic Electronics
Issue number5
StatePublished - Aug 2009

All Science Journal Classification (ASJC) codes

  • Electronic, Optical and Magnetic Materials
  • Biomaterials
  • General Chemistry
  • Condensed Matter Physics
  • Materials Chemistry
  • Electrical and Electronic Engineering


  • Charge transport
  • Doping
  • Electronic structures/processes/mechanisms
  • Inverse photoelectron spectroscopy
  • Organic electronics
  • Ultraviolet photoelectron spectroscopy


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