Interfacial electronic properties of pentacene tuned by a molecular monolayer of C
60
X. Liu, Y. Zhan, S. Braun, F. Li, and M. Fahlman
Department of Physics, Chemistry and Biology, Linköping University, S-581 83 Linköping, Sweden
Received 23 April 2009; revised manuscript received 9 July 2009; published 1 September 2009
Fine-tuning charge injection barriers between organic materials and electrodes is critical to optimize organic
electronic device performance. Here we demonstrate that by modifying gold substrates with a monolayer of
fullerene, significant decrease in the hole-injection barrier into pentacene films can be achieved. The insertion
of the fullerene monolayer modifies the interfacial dipole and produces an interface where the pentacene
molecules form a standing-up orientation with their long axis parallel to the surface normal. The latter effect
lowers the vertical ionization energy of the pentacene molecules at the interface as compared to the pentacene-
on-Au case, as well as improves the - overlap between the pentacene molecules that will likely enhance the
transport properties in corresponding devices.
DOI: 10.1103/PhysRevB.80.115401 PACS numbers: 73.20.-r, 71.20.Rv, 79.60.Dp, 78.70.Dm
I. INTRODUCTION
Currently, organic semiconducting materials have at-
tracted much attention due to their unique properties in the
prospective applications in organic electronics such as light-
emitting diodes, photovoltaic cells, and field effect
transistors.
1–3
Functionalities of these devices are often de-
termined to a large extent by the energy-level alignment at
the various metal-organic and organic-organic interfaces.
Processes such as, e.g., charge injection as well as exciton
formation, dissociation, and recombination are strongly de-
pendent on the interface energetics.
4–6
Therefore, the energy-
level alignment at heterojunctions is a key property in the
process of designing organic electronic devices. Unfortu-
nately, energy-level diagrams often cannot be obtained
merely by using the values of ionization potential, electron
affinity, and work function of the materials involved, due to
complex interactions at the interfaces.
7–9
For example, depo-
sition of organic semiconductors onto metal surfaces often
results in dipole formation at the interfacial region.
7–18
The
interaction strength at an interface generally determines
which type of processes dominate in determining the ener-
getics and hence which type of regime, vacuum level
alignment,
19
or interface dipole induced off-set of the
vacuum level is observed.
7
For the weakly interacting phy-
sisorption case, i.e., organic-organic interfaces or organic-
metal interfaces where the metal surface is passivated by,
e.g., an oxide, hydrocarbon contaminants, or a dipole-
inducing molecular adsorbant, the so-called integer charge-
transfer ICT model can be applied.
8,14,20–22
In the case of
interfaces with moderate chemical interactions, an induced
density of interface states IDIS model can be applied. The
IDIS model describes the interfaces formed by vapor depo-
sition of -conjugated molecules on clean but nonreactive
metals such as gold, as has been used to model interfaces of
organic molecules adsorbed on self-assembled monolayer
modified metal systems.
9,23,24
In the ICT model, the key in-
put parameters are the substrate work function,
SUB
, as well
as energies of the positive integer charge-transfer state,
E
ICT+
, and the negative integer charge-transfer state, E
ICT-
,
of the deposited organic film. The presence of a passivating
overlayer oxide, hydrocarbon, molecular adsorbant modi-
fies the work function of a metal substrate through the push-
back effect, and some molecules will further modify the
work function due to the charge-transfer effects, intrinsic di-
poles, etc. Hence the resulting “effective” work function of
the metal must be used as
SUB
rather than the work function
of the clean metal surface.
8,14,25
Care must be also taken
when using the E
ICT+
and E
ICT-
values, as they represent
energies of the integer charge-transfer states of the
molecules/polymers at the interface that may differ from the
values in the bulk film, and may be affected as well by order/
disorder in the layers adjacent to the substrate.
8,14
Molecu-
lar order/disorder at the interface also is important in the
context of energy-level alignment in the strongly interacting
chemisorption regimes, as shown in series of recent
articles.
26–28
Furthermore, these studies report dependence of
molecular orientation on the value of ionization energy,
which, in turn modifies charge injection barriers. Engineer-
ing the interface energy-level alignment through the use of
thin insulating barrier layers
29–32
and dipole-inducing mo-
lecular adsorbants
33–37
are consequently quite complex, par-
ticularly in the case of molecular films, as the barrier layer or
dipole-inducing molecular adsorbant may affect the order in
the subsequent molecular overlayer and hence the interface
energetics. We illustrate this general point for charge
injecting/separating interfaces in organic electronic devices
by using pentacene thin films deposited on gold and
C
60
-modified gold substrates, as pentacene films on SiO
x
have been demonstrated to have large difference in the ver-
tical ionization potential IP=0.55 eV depending on the
orientation of the pentacene molecules,
28
whereas the sym-
metry of the C
60
molecule prevents it from undergoing this
effect.
Our choice of material systems is also guided by the great
interest in pentacene and fullerene C
60
for use in organic
electronic devices. Part of that interest is based on their high
field-effect hole and electron mobility, respectively.
38,39
Fur-
thermore, the absorption peak in pentacene is located close
to the maximum of the solar visible spectra, making a bipolar
pentacene C
60
diode promising for solar cell application.
40
The growth process of pentacene films can exhibit thickness-
dependent competitive growth between a thin-film phase of
pentacene with standing-up orientation and a phase with
lying-down orientation.
41
The corresponding interface prop-
PHYSICAL REVIEW B 80, 115401 2009
1098-0121/2009/8011/1154017 ©2009 The American Physical Society 115401-1