Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies

TLDR: This study reveals the existence of a surface dipole built into molecular layers and offers design guidelines for improved organic-organic heterojunctions, hole- or electron-blocking layers and reduced barriers for charge-carrier injection in organic electronic devices.

Abstract: Although an isolated individual molecule clearly has only one ionization potential, multiple values are found for molecules in ordered assemblies. Photoelectron spectroscopy of archetypical pi-conjugated organic compounds on metal substrates combined with first-principles calculations and electrostatic modelling reveal the existence of a surface dipole built into molecular layers. Conceptually different from the surface dipole at metal surfaces, its origin lies in details of the molecular electronic structure and its magnitude depends on the orientation of molecules relative to the surface of an ordered assembly. Suitable pre-patterning of substrates to induce specific molecular orientations in subsequently grown films thus permits adjusting the ionization potential of one molecular species over up to 0.6 eV via control over monolayer morphology. In addition to providing in-depth understanding of this phenomenon, our study offers design guidelines for improved organic-organic heterojunctions, hole- or electron-blocking layers and reduced barriers for charge-carrier injection in organic electronic devices.

Figures

Figure 6 Energy-level diagram for the 6T/DH6T heterostructures on Ag(111).

Full text

S. Duhm et al., Nature Materials, accepted
Orientation-dependent ionization energies and interface dipoles in
ordered molecular assemblies
Steffen Duhm
1
*, Georg Heimel
1
**, Ingo Salzmann
1
, Hendrik Glowatzki
1
, Robert L.
Johnson
2
, Antje Vollmer
3
, Jürgen P. Rabe
1
, and Norbert Koch
1
1
Institut für Physik, Humboldt-Universität zu Berlin
Newtonstr. 15, D-12489 Berlin, Germany
2
Institut für Experimentalphysik, Universität Hamburg
D-22761 Hamburg, Germany
3
Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung m.b.H.
D-12489 Berlin, Germany
* E-mail: duhm@physik.hu-berlin.de
** E-mail: georg.heimel@physik.hu-berlin.de
1

S. Duhm et al., Nature Materials, accepted
While an isolated individual molecule clearly has only one ionization potential,
multiple values are found for molecules in ordered assemblies. Photoelectron
spectroscopy of archetypical π-conjugated organic compounds on metal substrates
combined with first-principles calculations and electrostatic modeling reveal the
existence of a surface dipole built into molecular layers. Conceptually different from
the surface dipole at metal surfaces, its origin lies in details of the molecular
electronic structure and its magnitude depends on the orientation of molecules
relative to the surface of an ordered assembly. Suitable pre-patterning of substrates
to induce specific molecular orientations in subsequently grown films thus permits
adjusting the ionization potential of one molecular species over up to 0.6 eV via
control over monolayer morphology. In addition to providing in-depth
understanding of this phenomenon, our study offers design guidelines for improved
organic/organic heterojunctions, hole- or electron-blocking layers, and reduced
barriers for charge-carrier injection in organic electronic devices.
2

S. Duhm et al., Nature Materials, accepted
It is well established that the work function (Φ) of metals depends on the crystal face
1-3
.
Φ is defined as the energy difference between the Fermi level (E
F
) and the electrostatic
potential above the sample, the vacuum level (V
vac
). For, e.g., copper, Φs of the (100),
(110), and (111) surfaces are spread over a range of 0.5 eV
1, 2
. As E
F
is constant, this
observation has been explained by the difference in the intrinsic “surface dipole”:
Differences in the geometric and, consequently, electronic structure cause a different
amount of the electronic cloud to spill out of the bulk into the vacuum
3, 4
. The resulting
dipole raises V
vac
to a larger or smaller extent and thus impacts Φ
4, 5
. Note that this effect
can only be observed for laterally extended surfaces, as the spatial region above the
sample where V
vac
is raised reaches farther away from the surface with increasing sample
size (i.e., area of the exposed surface)
6, 7
. Small metal clusters with multiple facets of
different crystal orientations have only one well-defined work function
8, 9
.
For van der Waals (i.e., non-covalent) crystals of non-dipolar molecules, surface
dipoles and work-function anisotropy have not yet been explored
6
. While variations of
the ionization potential (IP; the molecular equivalent of the work function) depending on
the molecular orientation on a substrate have been reported before
10-16
, the prevalent
interpretation in terms of variable photo-hole screening could never be satisfactorily
quantified. Here, we propose a qualitatively different and novel explanation for the
intriguing observation that one and the same molecule can have different - still well-
defined - IPs if part of an ordered supramolecular structure.
3

S. Duhm et al., Nature Materials, accepted
We performed X-ray photoelectron spectroscopy (XPS) and ultraviolet
photoelectron spectroscopy (UPS) on α,ω-dihexyl-sexithiophene (DH6T) and α-
sexithiophene (6T), on Ag(111). The IPs of the molecules change by up to 0.6 eV
depending on whether they are lying down flat on the substrate or standing upright. In
contrast to prior attempts
10-16
, we rationalize these observations in terms of the collective
electrostatic effect of the highly anisotropic intra-molecular charge distribution based on
density-functional theory (DFT) calculations and electrostatic modeling. Supplementary
studies on different substrates and molecules underline the universality of the observed
effects and their explanation. We stress that the general concept is valid also for single
crystals and ordered polymers.
Since 6T and DH6T are used in organic field-effect transistors (OFETs)
17-23
, we
discuss the immediate practical relevance of our findings in terms of the hole-injection
barrier (HIB), a crucial parameter in organic electronic devices
6, 24, 25
. Pre-patterning an
electrode with films of lying or standing DH6T allows for subsequent growth of films of
likewise lying or standing 6T molecules and thus permits lowering the HIB at the Ag/6T
contact by 0.4 eV. Furthermore, we derive conceptual guidelines for molecular design to
optimize the energy-level alignment at inorganic/organic and organic/organic
heterojunctions. Our findings thus open new routes towards organic electronic devices
with improved performance and functionality, not only OFETs but also organic light
emitting diodes (OLEDs) and organic solar cells.
4

S. Duhm et al., Nature Materials, accepted
In general, the orientation of molecules in mono- and multilayers with respect to the
substrate critically depends on the relative strengths of molecule-substrate interaction vs.
intermolecular interaction
26, 27
. In the case of DH6T (and similar thiophene derivatives
23
),
the molecules in the monolayer adsorb lying flat on metal surfaces
28, 29
, whereas
molecules in subsequent layers are "standing" with their long axis close to the surface
normal
28, 30
. The experimental UPS spectra of DH6T on Ag(111) in the monolayer (L for
"lying") and multilayer (S for "standing") regime are shown in Figure 1a, and Figure 1b
displays the corresponding simulated spectra (vide infra). In the former, three low
binding energy (BE) peaks can be clearly distinguished with their respective maxima at
1.6 eV, 2.3 eV, and 2.9 eV in the L-regime, and at 1.0 eV, 1.7 eV, and 2.3 eV in the S-
regime. These peaks are derived from the highest occupied molecular orbital (HOMO),
the HOMO-1, and the HOMO-2
31
. All peaks are at 0.6 eV lower BE for the (standing)
multilayer (S) compared to the (lying) monolayer (L). Note that the change in the
intensity ratio between the HOMO and HOMO-1 peak from 1:1 (L) to almost 1:2 (S) is
indicative of the different orientation of the long molecular axes with respect to the
surface normal due to photoemission selection rules
31
. As commonly observed
6, 24, 32
, the
adsorption of a molecular monolayer leads to a decrease of Φ. In our case, ΔΦ≈-0.7 eV as
determined from the secondary electron cutoff (SECO) (see Methodology section). This
lowering of V
vac
above the sample surface is often termed "interface dipole" (ID)
6, 24, 32
.
No further reduction of Φ is observed upon subsequent deposition of the multilayer
(Figure 1a). Consequently, the -0.6 eV BE shift of the molecular levels directly translates
into a reduction of the molecular IP (i.e., the energy difference between HOMO and V
vac
)
by this amount. In order to understand the physical origin of this shift, it is indispensable
5

Frequently asked questions

Q1. How does the first 6T layers adsorb?

The first 6T layers adsorb lying flat on Ag(111)31, 33 and only a slow, gradual transition to almost standing molecules was suggested for very thick (> 200 nm) films31, 33. 

Q2. How does the ionization potential of a molecular species change over time?

Suitable pre-patterning of substrates to induce specific molecular orientations in subsequently grown films thus permits adjusting the ionization potential of one molecular species over up to 0.6 eV via control over monolayer morphology. 

Q3. How is the polarization energy of the photo-hole analyzed?

It may be speculated, however, that the photo-hole is more efficiently screened bysurrounding standing molecules than by surrounding flat-lying molecules and, for similar organic compounds, the impact of molecular orientation on the IP has indeed been qualitatively rationalized in terms of the polarization energy depending on the packing density and/or morphology10, 11, 15, 16. 

Q4. Why do the authors overestimate the shifts in IP?

The authors attribute the overestimation of the shifts in IP to the high degree of order and uniformity in the simulations (not necessarily present in experiment) and to possible discrepancies between the structures assumed for the calculations and the actual structures probed in experiment. 

Q5. What is the -electron system above and below each ring?

The π-electron system above and below each ring is clearlynegatively charged; this is represented by negative point charges of -0.5 e (elementary charge) placed 0.5 bohr above and below the molecular plane. 

Q6. Why do the authors not include photo-hole screening in their calculations?

As photo-hole screening is not included in standard DFT calculations,our calculated shifts in IP have to be regarded as shifts of the initial electronic states prior to removal of the photo-electron. 

Q7. What is the energy scale of a single layer of 6T?

DFT calculated density-of-states (DOS) of a single layer of lying (red) and standing (green) 6T molecules; the origin of the energy scale is the respective vacuum level. 

Q8. What is the difference in the IP between standing and lying DH6T?

the presence or absence of neighboring molecules in the upper half-space must have a stronger effect on the polarization energy (and thus the measured IP) than differences in the orientation of neighboring molecules. 

Q9. What is the energy resolution of the spectrawere?

The spectrawere collected with a hemispherical electron energy analyzer (Scienta SES 100) with 120 meV energy resolution at 20 eV pass energy. 

Q10. What is the effect of the photo-hole on the kinetic energy of photoelectrons?

To understand their observations, it is important to consider that the kinetic energyof photoelectrons and thus the measured IP is affected by the polarization of neighboring matter by the photo-hole. 

Q11. How much does the BE shift of the molecular levels directly affect the molecular IP?

the -0.6 eV BE shift of the molecular levels directly translates into a reduction of the molecular IP (i.e., the energy difference between HOMO and Vvac) by this amount.