Direct Determination of Energy Level Alignment and Charge Transport at Metal/Alq$_3$ Interfaces via Ballistic-Electron-Emission Spectroscopy (BEES)

In organic electronic devices, the difference between the electrode work function and the organic lowest unoccupied molecular orbital (LUMO) or highest occupied molecular orbital (HOMO) is a crucial parameter in determining the nature of charge transport. However, experimental determination of LUMO is challenging.\footnote{ J. C. Scott, J. Vac. Sci. Tech, A {\bf21}, 521 (2003).} For the archetypal electroluminescent organic semiconductor tris-(8-hydroxyquinoline) aluminum (Alq$_3$), various techniques gave significantly different HOMO-LUMO gap values.\footnote{I. H. Campbell, D. L. Smith, Appl. Phys. Lett. {\bf74}, 561 (1999); I. G. Hill \emph{et al.} Chem. Phys. Lett. {\bf327}, 181 (2000); S. F. Alvarado \emph{et al.} IBM J. Res. Dev. {\bf45}, 89 (2001).} Using BEES, we directly determined the energy barrier for electron injection at clean interfaces of Alq$_3$ with Al and Fe to be 2.1 eV and 2.2 eV, respectively. We quantitatively modeled the sub-barrier BEES spectra with an accumulated space charge layer, and found that the transport of non-ballistic electrons is consistent with random hopping over the injection barrier. Supported by U.S. DOE Office of Science Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.