Journal
ORGANIC ELECTRONICS
Volume 10, Issue 5, Pages 932-938Publisher
ELSEVIER
DOI: 10.1016/j.orgel.2009.05.007
Keywords
Charge transport; Doping; Electronic structures/processes/mechanisms; Organic electronics; Ultraviolet photoelectron spectroscopy; Inverse photoelectron spectroscopy
Funding
- National Science Foundation [DMR-0705920]
- German Federal Ministry for Education and Research BMBF [13N8166A]
- Deutsche Forschungsgemeinschaft (DFG)
- German Academic Exchange Service (DAAD)
- Division Of Materials Research
- Direct For Mathematical & Physical Scien [819860] Funding Source: National Science Foundation
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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 (alpha-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 similar to 0.5%, which is due to Coulomb-trapping of hole carriers at the ionized dopant sites. (C) 2009 Elsevier B.V. All rights reserved.
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