Journal of Electron Spectroscopy and Related Phenomena 197 (2014) 17–21

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Journal of Electron Spectroscopy and
Related Phenomena
journal homepage: www.elsevier.com/locate/elspec

Maximum probing depth of low-energy photoelectrons in an
amorphous organic semiconductor ﬁlm
Yusuke Ozawa a , Yasuo Nakayama a,∗ , Shin’ichi Machida a , Hiroumi Kinjo a , Hisao Ishii a,bab

Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Center for Frontier Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japana r t i c l e

i n f oArticle history:
Received 16 February 2014
Received in revised form 22 June 2014
Accepted 6 August 2014
Available online 17 August 2014
Keywords:
Attenuation length
Photoemission
Photoelectron yield spectroscopy
Ultraviolet photoelectron spectroscopy
Buried interface

a b s t r a c t
The attenuation length (AL) of low energy photoelectrons inside a thin ﬁlm of a ␲-conjugated organic
semiconductor material, 2,2 ,2 -(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), was investigated using ultraviolet photoelectron spectroscopy (UPS) and photoelectron yield spectroscopy (PYS)
to discuss their probing depth in amorphous organic thin ﬁlms. The present UPS results indicated that
the AL is 2–3 nm in the electron energy range of 6.3–8.3 eV with respect to the Fermi level, while the
PYS measurements which collected the excited electrons in a range of 4.5–6 eV exhibited a longer AL of
3.6 nm. Despite this still short AL in comparison to a typical thickness range of electronic devices that are a
few tens of nm-thick, the photoemission signal penetrating through further thicker (18 nm) organic ﬁlm
was successfully detected by PYS. This fact suggests that the electronic structures of “buried interfaces”
inside practical organic devices are accessible using this rather simple measurement technique.
© 2014 Elsevier B.V. All rights reserved.

1. Introduction
Interfaces are crucial places where essential processes that dominate the output performance of organic electronic devices occur.
Because charge carrier injection into organic semiconducting materials is generally essential for the functioning of organic devices [1],the elucidation of interfacial electronic structures within devices
is one of the key issues for understanding and improving device
performance. The electronic structures of a wide range of organic
materials and their interfaces with conductive substrates have been
investigated using photoelectron spectroscopy [2]. The surface sensitivity of this method is, however, problematic if one wishes toprobe the interfaces. The probing depth of conventional ultraviolet
photoelectron spectroscopy (UPS) is approximately 1 nm causedby short attenuation lengths (ALs) of photoelectrons with kinetic
energies of 10–50 eV. This surface sensitivity forces researchers
to investigate model interfaces consisting of a very thin organic
layer deposited on electrodes. Such model systems do not represent the exact interfaces found in organic devices due to probableenergy shift by screening effects of “photo-holes” [3–5] and/or band
bending-like behavior within the organic ﬁlms [6–8], and thus theinvestigation of the ‘buried’ interfaces in a deposited thick organic
layer, such as those found in real devices, is highly desired.

∗ Corresponding author. Tel.: +81 43 207 3894.
E-mail address: nkym@restaff.chiba-u.jp (Y. Nakayama).
http://dx.doi.org/10.1016/j.elspec.2014.08.001
0368-2048/© 2014 Elsevier B.V. All rights reserved.

As a barometer of the probing depth of the electron spectroscopy techniques, the so-called “universal curve (UC)” iscommonly adopted, which exhibits an empirically derived relationship between the kinetic energy (Ek ) and the inelastic mean
free path (IMPF) of electrons in various inorganic and organic solids[9]. Note that the Fermi level is taken as the energy standard of
Ek throughout this article. Because it was predicted that the IMPF
grows proportionally to Ek −2 when Ek is “very low” [9], photons
with an energy lower than several eV are expected as promisingprobes for the investigation of buried interfaces by photoemission[10,11]. In this context, photoelectron yield spectroscopy (PYS),
which measures the total photoelectron yield as a function of theincident photon energy (typically 4–9 eV) [11,12], is potentiallya suitable technique for surveying the interface electronic structures underneath the few tens of nanometers-thick organic ﬁlms
of actual devices. Other advantages of using PYS are its durability
against sample charging [12] and the reduction of “radiation damage” by restricting photoelectron excess energies below 5 eV withrespect to the vacuum level [13,14]. These two drawbacks, sample
charging and radiation damaging, are inevitable problems with UPS
analysis of major organic materials.
The “universality” of the IMFP in the medium- and high-energy
regions has been established for both inorganic and organic materials [15–18]. In the low Ek range, however, the cross section of
the photoelectron energy loss processes has to be inﬂuenced bymiscellaneous material-dependent properties e.g. the energy gap
width and phonon energy [19], it remains controversial whether

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Y. Ozawa et al. / Journal of Electron Spectroscopy and Related Phenomena 197 (2014) 17–21

or not the photoelectron AL actually extends along the universal
trend in the low Ek range [19–21]. In this study, we therefore investigated the AL of low Ek photoelectrons in amorphousﬁlms of a typical organic semiconductor material, 2,2 ,2 -(1,3,5benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) using both
UPS and PYS for the sake of proposing a direct indication of
the accessible probing depth within actual organic devices byphotoemission-related techniques. As a result, it was demonstrated
that PYS can be practically applied for the characterization of buriedinterfaces.
2. Experimental details
UPS measurements were conducted using two types of energy
tunable excitation light sources: synchrotron orbit radiation anda deuterium arc discharge lamp. For the former experiments,
photoelectron energy distribution curves were obtained with aconcentric hemispherical analyzer (CHA) (ARUPS-10, VG) equipped
at the vacuum ultraviolet beam line (BL-8B UVSOR-II), Institute for
Molecular Science (IMS), Japan. The excitation energy used in theseexperiments ranged 20–50 eV. The experimental conditions wereidentical to those reported previously [2]. The latter UPS experiments were carried out in a home-build system equipped with aretarding ﬁeld electron analyzer (RFA), where a sample was placedat the center of a hollow hemispherical anode and photoelectrons
with a certain Ek were selected by sweeping the retarding electric
potential with a small modulation (100 mV, 4.18 Hz) to differentiate the photocurrent using a lock-in ampliﬁer (Stanford Research
Systems, model SR830 DSP) after ampliﬁcation with an ammeter(Keithley, model 6485). The incident photon energy (h) was varied from 6.3 eV to 8.3 eV. The photon incident direction was thesurface normal, and the measurements were conducted under highvacuum (1.0 × 10−4 Pa) conditions.
PYS measurements were carried out with the same RFA–UPS
system [22]. A positive electrostatic potential (+9 V) was applied
to the anode to collect all of the emitted photoelectrons. The incident photon energy was swept from 4.0 eV up to 9.3 eV, and thetotal photocurrent and photon ﬂux were measured using a precise
ammeter (Keithley, model 6430) and photomultiplier (Hamamatsu
Photonics, R6836), respectively, at each photon energy. The photoelectron yield Y was obtained by dividing the photocurrent by thephoton ﬂux.
TPBi was adopted for testing the probing depth inside amorphous organic materials because it is known to generally form
pinhole-free ﬂat overlayers when deposited at room temperature [23]. TPBi was evaporated onto Au-coated Si substrates ina step-by-step manner, and photoemission measurements were
subsequently conducted throughout in vacuum. The evaporation
rate was maintained in the range 0.01–0.03 nm/s, which was monitored with a quartz oscillator and calibrated afterwards using
a step proﬁlometer. The pressure during the sample fabrication
was 4 × 10−3 Pa for the CHA–UPS experiments and was maintained
below 5 × 10−4 Pa for the RFA–UPS and PYS analyses. The ﬁlm morphology was conﬁrmed to be ﬂat and uniform, as shown in Fig. 1,
where no pinhole except several hollows no deeper than 5 nm wasfound in randomly selected ﬁfteen 100 ␮m2 areas over the ﬁlm surface, by means of atomic force microscopy (AFM) (SII, SPA-400) inair.
3. Results and discussion
Fig. 2(a) shows the CHA–UPS spectra of a pristine Au substrate
and a thick (6.7 nm) overlayer taken at h = 30 eV. The thresholdedge due to the highest occupied molecular orbital (HOMO) of TPBi
was located at a binding energy (BE) of 2.4 eV, which corresponds

Fig. 1. Typical AFM images of the Au substrate (upper left) and 18 nm-thick TPBi
overlayer (upper right) represented together with the height histograms of therespective images. Both AFM images are equally scaled. The molecular structureof TPBi is also shown as the middle inset.

to the hole injection barrier from Au to TPBi. Because the energy
gap for TPBi has been reported to be at least 3.5 eV [24], the lowest
unoccupied molecular orbital must be located more than 1 eV above
the Fermi level. Therefore, the photoelectron signals at the Fermi
level (BE = 0 eV) were attributed to the Au substrate exclusively.
As shown in Fig. 2(b), the photoelectron intensity of the Fermiedge decayed with increasing overlayer thickness. A photoelectron excited inside a solid loses its energy due to several scattering
events with certain probabilities on its way toward the surface. The
relative population of the photoelectrons that preserve the primary
excitation energy after passing though the solid of a thickness x canbe expressed as:Nx= exp −
N0




,

(1)

where  is the AL of the primary electrons which is related to thescattering probability. In the present case, N corresponds to the
Fermi edge intensity in the spectrum for the TPBi overlayer witha thickness x, while N0 is given by the spectrum of the pristineAu substrate (x = 0). Fig. 2(c) shows the relative intensity (N/N0 ) of
the Fermi edge derived from the present CHA–UPS spectra plottedin a semi-logarithmic scale as a function of the TPBi thickness forvarious excitation energies. These plots show good linearity, suggesting uniform growth of the overlayer. The ALs were determinedusing the gradients of these plots to be 1.0–1.5 nm for excitationenergies of 20–50 eV. In the present UPS works, we only consider

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19

Fig. 2. (a) CHA–UPS spectra of an Au substrate and a 6.7 nm-thick TPBi overlayer taken at h = 30 eV. (b) Spectral evolution at the Fermi edge region of TPBi layers of different
thicknesses. (c) Relative intensity (N/N0 ) of the Fermi edge taken at various excitation energies plotted as a function of the TPBi thickness. The least-squares ﬁtting lines forthe respective data sets are also displayed. The line N/N0 = 1/e was drawn to estimate the ALs.

the photoelectrons excited from the Fermi level (BE = 0 eV) so thatthe h directly corresponds to the internal kinetic energy of thephotoelectrons Ek . Note that N/N0 decays to less than 1/50 for a
6 nm-thick overlayer even when one assumes the AL to be 1.5 nm.
Because such a faint signal is generally under the detection limit ofconventional UPS systems as indicated in Fig. 1(b), the maximum
accessible probing depth for this technique should be regarded as
no greater than 6 nm.
Fig. 3(a) shows the RFA–UPS spectra of the Au substrate and
TPBi overlayers taken at h = 7.8 eV. From the wide range spectra(not shown), the Au–TPBi hole injection barrier was determined
to be 1.77 (±0.15) eV. The notable deviation of this value from theresult obtained using CHA–UPS may be ascribed to different sample
preparation conditions (e.g., the vacuum pressure).
As observed with CHA–UPS, the photoelectron intensity at the
Fermi level attenuated with increasing TPBi thickness. In this case,however, a signiﬁcant contribution of secondary electrons due toinelastic scattering by the overlayer was superimposed on the primary photoelectron signals, resulting in a monotonous slope tothe left in the RFA–UPS spectra. This situation makes it difﬁcult
to deﬁne the Fermi edge intensity by simple curve ﬁtting with the
Fermi–Dirac distribution function. Consequently, it was assumed
that the Fermi edge intensity could be reasonably derived as theintensity at BE = 0 eV, as indicated on the right axis of Fig. 3(a). In
Fig. 3(b)–(d), N/N0 at BE = 0 eV as a function of the TPBi thickness are
displayed for several excitation energies. The AL values were calculated to be 2–3 nm for excitation energies of 6.3–8.3 eV. It should
be noted that the ALs did not exhibit a regular dependence on the
excitation energy in this case, which is in contrast to the UC thatpredicts a monotonous increase of the AL with decreasing Ek in thisrange.
PYS measurements were also conducted on a sample identicalto that used for the RFA–UPS analysis. Fig. 4(a) represents the PYS
spectra of the pristine Au substrate and deposited TPBi overlayers. Since there is a large energy level offset between the highest
occupied electronic states of Au and TPBi as discussed above, thephotoemission signal in the range h = 4.5–6.0 eV can be attributed
solely to Au. Although the photoelectron yield in this energy range
decayed due to the TPBi overlayer, the photoelectrons from theAu substrate underneath an 18 nm-thick TPBi overlayer were stilldetectable, as clearly seen in Fig. 3(b). It should also be noted that
N/N0 has to decay down to 1/400 for the 18 nm-thick overlayer
if one assumes the AL to be 3 nm based on the aforementioned
RFA–UPS analysis.
The photoelectron yield Y just beyond the onset energy is
known to be proportional to (h − Eth )n , where Eth is the thresholdenergy of the photoemission, and n = 2 and 3 have been empirically adopted for metals and organic materials, respectively [25,26].
For binary systems of metals and organic materials, the yield canbe expressed as a superposition of these two components. In thepresent case, therefore, photoelectron yield can be simulated as:
2

3

Y = NAu (hv − ϕAu ) H(hv − ϕAu ) + NTPBi (hv − ITPBi ) H(hv − ITPBi ),(2)
where NAu and NTPBi are proportional constants indicating thesignal intensity from Au and TPBi, respectively, ϕAu is the work
function of Au, ITPBi is the ionization energy of TPBi, and H(E) is the
Heavyside step function, whose value switches from 0 to 1 when
E turns from negative to positive. For the 18 nm-thick TPBi overlayer (Fig. 3(b)), ϕAu = 4.45 (±0.05) eV and ITPBi = 6.25 (±0.12) eV
are derived from a least-square ﬁtting of the experimental datausing Eq. (2), while the residual in the left side of the onset to the
TPBi contribution may be ascribed to a tail state derived from thehighest occupied state of TPBi. The hole injection barrier of thisburied interface underneath the thick TPBi ﬁlm was determined
to be 1.8 eV (= ITPBi − ϕAu ), which is in good agreement with the
RFA–UPS results for the thinner TPBi overlayers.
The attenuation behavior of the photoelectron yield from Au
is shown in Fig. 4(c) together with the other UPS results. The AL
was estimated to be 3.6 nm based on the PYS data and is indicatedwith the plot as a thick line. It should be noticed that, since theFig. 3. (a) Evolution of the RFA–UPS spectra at the Fermi-edge region taken at h = 7.8 eV. The right axis scale is given to indicate the relative intensity (N/N0 ) at the Fermi
level. (b)–(d) N/N0 plotted as a function of TPBi thickness for excitation energies of (b) 6.3, (c) 7.3, and (d) 7.8 eV.

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Y. Ozawa et al. / Journal of Electron Spectroscopy and Related Phenomena 197 (2014) 17–21

Fig. 4. (a) PYS spectra of an Au substrate and TPBi overlayers. (b) PYS spectra of an 18 nm-thick TPBi overlayer. The vertical scale for the upper plot is extended by ten times
to the lower one. The least-squares ﬁtting result for the experimental data using Eq. (2) is represented by the solid curves. The shaded area can be ascribed to tail states above
the highest occupied state of TPBi. (c) Relative intensity of NAu derived from the PYS spectra plotted as a function of the TPBi thickness. The least-squares ﬁtting result forthis plot using Eq. (1) is indicated by the solid line. Those derived from the present UPS results are also indicated for reference.

present PYS signal from the Au substrate must contain photoelectrons of multiple kinetic energies, this AL value has to be nothing
but a phenomenological reference of the technique and thus does
never correspond to the IMFP of electrons at any speciﬁc Ek .
Finally, AL values having been reported by other groups werecompared to the present results, which are summarized in Fig. 5.
It is clearly seen that, aside from discrepancies to the UC by factorof 2 or less, the present results settle within the range of previous
reports. Opposed to the simple Ek −2 dependence proposed by Seahand Dench [9], two research groups have reported rather shrinkage of ALs for smaller Ek in the range of 4.5–6 eV for alkane-based
molecular layers [19,20]. The conventional UC was also challenged
by Graber and co-workers, where they claimed a constant AL value,
or at least a much moderate power law (Ek −0.1 ), for Ek < 200 eV
based on their results for perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) thin ﬁlms [21]. Our present RFA–UPS results which
revealed the absence of any clear Ek –AL relationship support thisnotion. It is noteworthy that, in spite of considerably weaker Ekdependence for the AL in the low energy region against the orthodox expectation, the present results clearly demonstrate that the
maximum probing depth for PYS reaches at least 18 nm. Since themolecular volume of TPBi is estimated to be approximately 1 nm3 ,this probing depth means that buried interfaces far beneath bulk
organic molecular layers are accessible by this technique. One reason of this long probing depth should be attributed to a principle of
the PYS measurement that it does not discriminate primary photoelectrons from scattered ones; in other words, PYS even collects
electrons no longer preserving the initial Ek which are abandoned
for the UPS measurements. Another important factor is its higher
sensitivity in comparison to the conventional UPS, which is also
represented in Fig. 4.
4. Conclusion
The accessible probing depth of photoemission-based experimental techniques in the low energy region was revisited using ﬂatoverlayers of a typical amorphous organic material TPBi depositedon Au substrates. The UPS results in an excitation energy range
of 6.3–8.3 eV revealed a complex Ek -dependence of the photoelectron attenuation length scattering between 2 and 3 nm as opposedto the so-called “universal curve”, while an expected positive
Ek –AL was exhibited in the conventional excitation energy range(h = 20–50 eV). Despite the fact that the PYS signals (in the excitation energy range of 4.5–6 eV) also decay by the AL of no greaterthan 3.6 nm, the photoelectron yield originating from the Au substrate underneath an 18 nm-thick TPBi overlayer was detected,
and the hole injection barrier at the “buried” Au–TPBi interface
was also successfully determined from a single PYS spectrum. This
result demonstrates that PYS is a potentially promising technique
for surveying the electronic structures of general buried interfaces,
not only organic–electrode interfaces but also e.g. donor–acceptor
organic heterojunctions, inside several tens of nm-thick organic
electronic devices.
Acknowledgements

Fig. 5. The AL values obtained in the present study plotted as a function of Ek . The
values reported in previous studies for organic materials are also shown, where thedensity of TPBi is assumed to be 1.26 g/cm3 to convert the vertical axis of the data setof Seah and Dench [9], the energy difference between the lowest unoccupied energy
and Fermi level (4.4 eV) was added to energy standard for Cartier and Pﬂuger [19],the work function value (4.23 eV) was added to the energy scale for Monjushiro
and Watanabe [20], and the one monolayer thickness of PTCDA was assumed to be
0.322 nm to unify the vertical scale [21]. The vertical error bars of the present data
sets represent the uncertainties of the AL estimation which was caused by scatteringof the data sets, while the left ends of the horizontal bar attached to the PYS results(ours and Ref. [20]) indicates the lowest possible Ek contributing to the PYS signalthat is identical to the work function of the respective samples.

The authors would like to thank Mr. U. Hörmann of the University of Augsburg for his help during the CHA–UPS experiments.
Financial support from the Joint Studies Program [23–551] of IMS,
the G-COE Program (Advanced School for Organic Electronics; G-3)
of Chiba University, JSPS KAKENHI [No. 21245042], and the FIRST
Program of Kyushu University are gratefully acknowledged. This
work was also partially supported by JSPS KAKENHI [23750209],
the TEPCO Memorial Foundation, the Yazaki Memorial Foundationfor Science and Technology, a Sasagawa Scientiﬁc Research Grant[24–233] from JSS, and Izumi Science and Technology Foundation.

Y. Ozawa et al. / Journal of Electron Spectroscopy and Related Phenomena 197 (2014) 17–21

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