WDS'05 Proceedings of Contributed Papers, Part III, 569–573, 2005.
ISBN 80-86732-59-2 © MATFYZPRESS
Valence Band States of Conducting Polymer Films
A.T. Wroble, S. Tepavcevic, A. Zachary, and L. Hanley
Department of Chemistry (mc 111), University of Illinois at Chicago, Chicago, Illinois, USA 60607-7061.
Abstract. Surface polymerization by ion-assisted deposition (SPIAD) has been used to
deposit polythiophene-like films (3T SPIAD) on AlxOy, TiO2, and Au substrates for
potential use as an electron donor material in organic solar cells. Ultraviolet
photoelectron spectroscopy (UPS) has been used to determine the energy level alignment
at the interfacial region between 3T SPIAD films and their corresponding substrates. No
changes were observed in the UP spectrum for either the barrier to hole injection (EFVB)
or the Fermi level of the substrate-vacuum level offset (EFVAC) as the work function of
the substrate was changed from 3.5 to 4.8 eV or as the thiophene ion current was varied
from 5 to 25 nA with constant terthiophene flux. The constant values for EFVB and
EFVAC can be explained by the presence of gap states at the interface, resulting in an
interface dipole and the offset of the vacuum levels of the 3T SPIAD film and substrate.
The vacuum level offset allows the Fermi levels of the substrate and 3T SPIAD film to
align without the movement of the 3T SPIAD Fermi level from its initial position.
Introduction
The development of technology for the efficient and cost-effective conversion of solar energy to
electricity has been an important research goal over the past few decades. The non-polluting,
renewable nature of solar energy makes it a viable alternative to the use of traditional energy sources
such as fossil fuels. Conventional silicon-based solar cells have offered solar energy-to-electricity
conversion efficiencies of greater than 24% in laboratory settings [1]. However, organic-based solar
cells composed of conjugated semi-conducting polymers as electron donors and C60 derivatives as
electron acceptors have the potential advantage of facile processing and cheaper cost than
conventional silicon-based solar cells. The major drawback of organic-based solar cells is their
seemingly low efficiency, reaching only 5.7% for a tandem configuration [2]. The energy level
alignment at the interfaces of electrode/electron donor, electrode/electron acceptor, and electron
donor/acceptor in organic-based solar cell systems appears to be one key to understanding factors that
will enhance the efficiency of solar energy-to-electricity conversion. Ultraviolet photoelectron
spectroscopy (UPS) has been extensively used to characterize the energy level alignment at these types
of interfaces for this purpose [3, 4].
The use of surface polymerization by ion-assisted deposition (SPIAD) has been previously
reported as a method for growing polythiophene films (3T SPIAD) [5–7]. In this paper, we will
present the UPS analysis of substrate/3T SPIAD interface to determine the effect of the variation of
substrate work function and deposition parameters on the energy level alignment at this interface.
Experimental
Preparation of Substrates
Silicon wafers (Atomergic Chemetals Corp., Si (100) p-type, boron doped) were coated with 100
nm aluminum or gold films (Evaporated Coatings, Inc.). Aluminum with native oxide and gold
substrates were ultrasonicated in acetone and isopropanol prior to being introduced into the vacuum
chamber. Further, gold substrates were sputtered with 1 keV He+ for 1 hour.
Titanium dioxide films were prepared by doctor-blading a slurry of nanophase TiO2 powder
(Nanophase Technologies Corp.) in distilled water onto a clean indium tin oxide coated glass
substrate. The TiO2 films were dried in air and heated to 450oC for 1 hour to produce anatase TiO2
films.
All substrates were analyzed by UPS prior to deposition of polythiophene film to confirm the
substrate work function.
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WROBLE ET AL.: VALENCE BAND STATES OF CONDUCTING POLYMER FILMS
Deposition Method
Surface polymerization by ion-assisted deposition (SPIAD) was accomplished by simultaneous
deposition of thermally evaporated terthiophene neutrals (3T) and 200 eV thiophene ions (T+) to form
polythiophene films (3T SPIAD). The ions were formed by 80 eV electron impact ionization of
thiophene vapor, accelerated to 1 keV, mass-selected by a Wien filter, decelerated to 200 eV,
refocused, and guided onto the substrate. The most efficient polymerization was previously reported
[6] at an ion-to-neutral ratio of 1/150 and was used in all depositions except where noted otherwise.
All films were grown for two hours. The morphology and thickness of these films has been
investigated previously [8]. Briefly, scanning electron microscopy images of 3T SPIAD films showed
the presence of large rounded features from 10 to 90 µm in diameter surrounded by a featureless
region that is composed of sulfur-containing graphitic carbon. The island layer was estimated to be 4585 nm thick whereas the thickness of the featureless layer was estimated as 7-20 nm.
Ultraviolet and X-Ray Photoelectron Spectroscopies (UPS and XPS)
A helium discharge UV source (Model UV 10, Thermo VG Scientific) operating in He (II) mode
(hν = 40.8 eV) was used to obtain UPS valence band spectra. XPS was performed using a
monochromatic Al Kα x-ray source (15 keV, 25 mA emission current, model VSW MX10 with 700
mm Rowland circle monochromator, VSW Ltd., Macclesfield, UK) and a concentric hemispherical
analyzer (150 mm, model Class 150, VSW Ltd.) with multichannel detector operating in constant
energy analyzer mode. The spectrometer was calibrated by the position of the Au(4f7/2) peak at 83.66
eV and the gold Fermi level at 0 eV binding energy of a He+ sputter-cleaned polycrystalline gold foil.
During analysis of the substrates and 3T SPIAD films, the samples were positioned such that they
were in electrical contact with the gold foil to align their Fermi levels. The location of the deposited
film on the substrate was determined by the position on the sample that gave the highest sulfur signal
as seen in XPS.
Energy level alignment parameters of the substrate/3T SPIAD interface were determined from
UPS as has been described in the literature [3, 4] and is detailed below. Figure 1 shows a generalized
scheme of the energy level alignment between the substrate and the 3T SPIAD film.
The work function (Φ) of the substrate was determined with UPS by subtracting the position of
the secondary electron cutoff (EC) from the excitation energy (hν=40.8 eV):
hν – EC = Φ
(1)
The energy offset between the Fermi level (EF) of the substrate and the vacuum level (VL) of the
3T SPIAD film was determined by subtracting the excitation energy from EC to define the position of
the vacuum level of the 3T SPIAD film:
EC – hν = VL
(2)
VAC
and then subtracting VL from EF to obtain the Fermi level-vacuum level offset, EF :
EF – VL = EFVAC
(3)
The energetic difference between EF of the substrate and the valence band onset (EVB) of the 3T
SPIAD film was determined by identifying the position of EVB, defined as the “intercept between the
tangent to leading edge of the lowest binding energy feature and the zero-intensity background line”
[9], and subtracting EF from EVB:
EVB – EF = EFVB
(4)
VB
EF is often termed the barrier to hole injection. Since EF has been arbitrarily set to 0 eV by
adjusting the spectrometer settings, EFVAC is numerically equal to the absolute value of VL, and EFVB is
numerically equal to EVB. The ionization potential (IP) of the 3T SPIAD film is an inherent property of
the film and should remain constant regardless of the substrate. Furthermore, the ionization potential is
not expected to vary with deposition conditions. The IP was determined by the sum of EFVAC and EFVB:
EFVAC + EFVB = IP
(5)
A combination of UPS and near edge absorption fine structure spectroscopy has previously been
used [10] to compare the electronic structure of 3T SPIAD and 3T evaporated films by concentrating
on changes in valence band features, band gap, and EFVB.
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WROBLE ET AL.: VALENCE BAND STATES OF CONDUCTING POLYMER FILMS
Figure 1. General scheme of energy level alignment at interface between substrate and 3T SPIAD
film. Vacuum levels of substrate and 3T SPIAD film may or may not be aligned depending on
substrate.
Results
Variation of Substrate Work Function
3T SPIAD films were deposited onto AlxOy, TiO2, or Au substrates and were subsequently
analyzed by XPS and UPS. UPS determination of the work functions of aluminum, gold, and titanium
dioxide substrates prior to deposition shows agreement of our data (see Table 1) with values reported
in the literature for similar substrates: Φ(AlxOy) = 3.9 eV [3], Φ(TiO2) =4.1 eV [11], Φ(Au) = 4.7-5.4
eV [3, 4]. Figure 2a shows the UP spectra for 3T SPIAD films deposited on each of these substrates.
These spectra contain a continuous emission band that begins at 1.2-1.4 eV, corresponding to the π*
anti- bonding orbitals of 3T SPIAD films, which is consistent with previously reported spectra for
polythiophene and longer chain thiophene oligomers [12–14]. A peak at ~4 eV is assigned to the n
non-bonding orbitals, and a group of peaks from ~6-10 eV represents the π bonding orbitals. Figure 2b
focuses on the π* anti-bonding region of each spectrum and shows the method for determination of the
valence band onset position. The position of EVB is shown to be near 1.4±0.2 eV for all 3T SPIAD
films, which suggests that EFVB does not shift with respect to the substrate work function for 3T
SPIAD films. Table 1 summarizes the parameters EFVB, EFVAC, IP, and Φ for 3T SPIAD films grown
on various substrates.
6000
2500
3T SPIAD on Au
3T SPIAD on AlxOy
3T SPIAD on TiO2
π
3T SPIAD on Au
3T SPIAD on AlxOy
3T SPIAD on TiO2
2000
4000
Intensity (cps)
Intensity (cps)
5000
3000
n
2000
π*
1000
1500
1000
500
EVB~1.4eV for all substrates
0
0
0
2
4
6
8
10
0.0
Binding Energy (eV)
0.5
1.0
1.5
2.0
2.5
3.0
Binding Energy (eV)
(a)
(b)
Figure 2. a) UP spectra of 3T SPIAD films on AlxOy, Au, and TiO2 substrates.
b) π* anti-bonding region of UP spectra showing determination of EVB.
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3.5
4.0
4.5
5.0
WROBLE ET AL.: VALENCE BAND STATES OF CONDUCTING POLYMER FILMS
Table 1. EFVB, EFVAC, IP(3T SPIAD), and Φ(substrate) as calculated from UP spectra of substrates and
3T SPIAD films.
Substrate Φ (eV) EFVB (eV) EFVAC (eV) IP (eV)
AlxOy
3.5±0.3 1.3±0.2
4.0±0.2
5.3±0.3
TiO2
4.0±0.3 1.4±0.2
4.0±0.2
5.4±0.3
Au
4.8±0.3 1.3±0.3
4.1±0.2
5.4±0.3
Variation of Ion Current
3T SPIAD films were deposited onto AlxOy substrates with ion currents of 5, 15, and 25 nA and
constant 3T flux and were subsequently analyzed by XPS and UPS. Figure 3a shows the UP spectra of
3T SPIAD films grown at various ion currents. All characteristic emissions of 3T SPIAD films are
present in the spectrum for each ion current as discussed above. However, the peak due to emission
from π* anti-bonding orbitals is not as pronounced for the film grown at 5 nA ion current as compared
to the emission seen for films grown at 15 and 25 nA. The decreased emission can be more clearly
seen in Figure 3b. EFVB, EFVAC, IP, and Φ for 3T SPIAD films grown at various ion currents are
summarized in Table 2. EFVB is within the range of 1.3-1.4 eV for all 3T SPIAD films grown on AlxOy
regardless of the deposition conditions.
2500
1000
3T SPIAD on AlxOy, 15 nA
3T SPIAD on AlxOy, 25 nA
3T SPIAD on AlxOy, 5 nA
3T SPIAD on AlxOy, 15 nA
3T SPIAD on AlxOy, 25 nA
3T SPIAD on AlxOy, 5 nA
800
Intensity (cps)
Intensity (cps)
2000
1500
1000
500
600
400
200
0
0
0
2
4
6
8
10
0.0
Binding Energy (eV)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Binding Energy (eV)
(a)
(b)
Figure 3. a) UP spectra of 3T SPIAD films on AlxOy substrate grown at 5, 15, and 25 nA ion current.
b) π* anti-bonding region of UP spectra showing determination of EVB.
Table 2. EFVB, EFVAC, IP(3T SPIAD), and Φ(substrate) as calculated from UP spectra of AlxOy
substrate and 3T SPIAD films. Φ(AlxOy) = 3.6±0.3 eV.
Ion Current (nA) EFVB (eV) EFVAC (eV) IP (eV)
5
1.4±0.2
4.0±0.2
5.4±0.3
15
1.4±0.2
4.1±0.2
5.5±0.3
25
1.3±0.3
4.0±0.2
5.3±0.3
Discussion
Surface polymerization by ion-assisted deposition has been used to grow polythiophene-like films
onto AlxOy, TiO2, and Au substrates to observe the effect of substrate work function and deposition
conditions on the interfacial energy level alignment. UPS analysis has shown that EFVB and EFVAC
remain the same for films grown on substrates with work functions in the range 3.5-4.8 eV and for
films that have been grown with various thiophene ion currents and constant terthiophene flux.
Different degrees of variation of EFVB and EFVAC were reported in the literature for semiconducting
polymers such as 4,4’-N,N'-dicarbazolyl biphenyl, tris (8-hydroxyquinoline) aluminum, (N,N’diphenyl- N,N’-bis (1-naphthyl)- 1,1’-biphenyl-4,4”-diamine, 3,4,9,10 perylenetetracarboxylic
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WROBLE ET AL.: VALENCE BAND STATES OF CONDUCTING POLYMER FILMS
dianhydride [9, 15], and poly(9,9-dioctylfluorene) [3] with respect to substrate work function. Direct
correspondence between changes in substrate work function and energy level alignment was seen for
poly(9,9-dioctylfluorene) [3], whereas no shift was observed for 3,4,9,10 perylenetetracarboxylic
dianhydride [9, 15]. This phenomenon appears to be a property of the film, and an explanation for the
constant value of EFVB with respect to substrate was given by Hill and coworkers [9]. The ability of the
Fermi level to move within the band gap of the semiconductor – the 3T SPIAD film in this case – is
reduced due to the presence of states within the gap. As the Fermi level of the semiconductor attempts
to align with the Fermi level of the substrate, the gap states are more or less populated depending on
the direction the Fermi level attempts to move. An interface dipole is formed as a result of the net
space charge at the substrate/semiconductor interface, which causes the vacuum levels of the substrate
and semiconductor to offset. The vacuum level offset makes it possible for the Fermi levels of the
substrate and semiconductor to align without the semiconductor Fermi level moving from its initial
position.
Acknowledgements. This work is funded by the National Science Foundation under grant no. CHE0241425. ATW’s visit to Charles University, Prague, Czech Republic was funded by Kontakt 1PO4ME754.
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