Probing the effect of relative molecular orientation on the photovoltaic device
performance of an organic bilayer heterojunction using soft x-ray spectroscopies.
S.W. Cho, A. DeMasi, A.R.H. Preston, and K.E. Smith†
Department of Physics, Boston University, 590 Commonwealth Ave, Boston, MA 02215, USA.
L.F.J. Piper
Department of Physics, Applied Physics, and Astronomy,
Binghamton University, State University of New York, Binghamton, NY 13902, USA.
K.V. Chauhan and T.S. Jones
Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom
†Corresponding author: ksmith@bu.edu
SUPPORTING INFORMATION:
Figure 1 illustrates the procedure used for the determination of energy-level alignment at the
interface. The basic equation used in interpreting photoelectron spectra is:
E B = hn - E k - F
(3)
The photon energy (hν) is known and the photoelectron kinetic energy (Ek) is measured in order
to deduce the binding energy (EB) referenced to EF. When hν is known, the work function (Φ)
can be obtained from the measured energy of the secondary-electron cut-off (Ecut-off)., i.e.:
F = hn - E cut-off
(4)
The change in the work function; ΔΦ, can then be tracked by measuring Ecut-off after a deposition
step. Therefore, the shift of this Ecut-off indicates the magnitude of the interfacial dipole, which is
equal to increasing or decreasing the work function.1,2 Similarly, the ionization potential (IP) can
be obtained from Ecut-off and the highest occupied molecular orbital (HOMO) onset (EHOMO):
IP = hn - (E cut-off - E HOMO )
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(5)
(b)
(a)
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"
EVAC
substrate
"
EA
substrate
IP
LUMO
EF
HOMO
EHOMO
Substrate
Organic molecule
Figure 1. (a) Schematic illustration of some of the important parameters derived from PES characterization of surfaces
and interfaces. (b) An energy-level diagram for a generic junction formed between an organic film and an ITO
substrate.
Figure 2. (a) Change in the onset of secondary electron PES spectra after the deposition of each layer of
C60/ClAlPc/MoO3. (b) Valence band PES spectra recorded near EF after the deposition.
Figure 2(a) shows how the work function (as measured from Ecut-off in PES) varies with the
thickness of a ClAlPc film and with subsequent deposition of a C60 film on the ClAlPc layer. The
cut-off position shifted toward lower work function immediately after 1 nm ClAlPc was
deposited. The abrupt shift of the secondary cut-off in the earliest stages of film growth in
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attributed to formation of an interface dipole.3 As more ClAlPc was deposited, the cut-off
position moved a little more toward lower work function and the total shift of the cut-off position
of ClAlPc was 1.6 eV. The magnitude of the dipole was estimated to be 1.4 eV at the
ClAlPc/MoO3 interface after subtracting the contribution of downward band bending. Subsequent
deposition of C60 resulted in a shift of the interface dipole between C60 and ClAlPc of 0.3 eV to
lower energy.
Figure 3. (a) Change in the onset of secondary electron PES spectra after the deposition of each layer of
C60/ClAlPc/PTCDA/MoO3. (b) Valence band PES spectra recorded near EF after the deposition.
Figure 2(b) presents valence band photoemission spectra collected within 8 eV of EF from MoO3,
from ClAlPc films grown on MoO3, and then from the C60/ClAlPc/MoO3 multilayer. As thicker
ClAlPc films are deposited, it is clear that the ClAlPc HOMO shifts toward higher binding
energies and the total energy shifts reaches 0.2 eV. This confirms that downward band bending
occurs at the ClAlPc/MoO3 interfaces as implied by the data in figure 2(a). By contrast, there was
no shift in the HOMO energy for the C60/ClAlPc interfae. The HOMO onset of the C60 layer
deposited on the ClAlPc layer was measured as 1.5 eV. We can also estimate the lowest
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unoccupied molecular orbital (LUMO) onset of the C60 layer (0.5 eV) from the previously
reported band gap of 2.0 eV.4
Figure 4. (a) Change in the onset of secondary electron PES spectra after the deposition of each layer of
C60/ClAlPc/Pentacene/MoO3. (b) Valence band PES spectra recorded near EF after the deposition.
Figure 3 shows the valence band and cut-off spectra of the C60/ClAlPc/PTCDA/MoO3 interfaces.
In this system, the band bending at the ClAlPc/MoO3 interface was not observed. The HOMO
onsets of the ClAlPc and C60 layer were 0.6 eV and 1.45 eV, respectively. It also shows the work
function changes as a function of the ClAlPc and C60 thickness from the measured Ecut-off.
Figure 4 also shows the valence band and cut-off spectra of the C60/ClAlPc/pentacene/MoO3
interfaces. The HOMO onsets of the ClAlPc and C60 layer were 0.35 eV and 1.2 eV, respectively.
Using the same method, we can confirm the work function changes as a function of the ClAlPc
and C60 thickness.
References
1
S. T. Lee, X. Y. Hou, M. G. Mason, and C. W. Tang, Appl. Phys. Lett. 72, 1593 (1998).
2
C. Shen and A. Kahn, Org. Elec. 2, 89 (2001).
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3
H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater. 11, 605 (1999).
4
M. Brumbach, D. Placencia, and N. R. Armstrong, J Phys Chem C 112, 3142 (2008).
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