Supplementary Material
Electron affinity of pentacene thin film studied by
radiation-damage free inverse photoemission spectroscopy
Weining Han, 1 Hiroyuki Yoshida, 2 Nobuo Ueno, 1 and Satoshi Kera 1
1
Graduate School of Advanced Integration Science, Chiba University
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
2
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
Ionization energy of pentacene on ITO studied by ultraviolet
photoemission spectroscopy
1. Experimental
Ultraviolet photoemission spectroscopy (UPS) experiments were performed with He I
resonance radiation (h= 21.22 eV). UPS spectra were measured using an apparatus
described elsewhere.1 The angle between the incident photons and the detected
photoelectrons is fixed to 45°with an acceptance angle of ± 12°. The spectra are measured at
a photoelectron take-off angle of 0ºnormal. The total instrumental energy resolution of the
measurements was set to be less than 60 meV, as measured from the Fermi edge of an
evaporated Au film. The sample was biased at -5.0 V so that the secondary-electron cutoff of
the spectrum gives the vacuum level (Evac). The purified pentacene2 was evaporated on ITO
substrate at room temperature. The deposition amount and deposition rate (0.05 ~ 0.1
nm·min-1) were measured with a quartz microbalance.
1
2. Results
Fig. S1 shows thickness dependence of UPS spectra of pentacene thin films (thickness =
1 nm, 10 nm and 20 nm) on ITO substrates. The spectra clearly show that the highest
occupied molecule orbital (HOMO) feature has single peak with sharp onset without serious
tailing into the band gap, and the secondary electron cut-off (SECO) is extremely sharp with
a sharp peak at the cut-off followed by typical conduction band feature at ~1.7 eV above the
vacuum level (Evac). The single HOMO feature represents that the film consists with the bulk
phase crystal structure (Fig.9 in Ref. 3, which shows that the single HOMO peak from the
bulk phase pentacene),3 while the film with thin film phase of standing molecules gives two
HOMO features.4 Furthermore, a sharp SECO down to the baseline demonstrates that the
films consist of well-packed standing molecules.4
The HOMO onset was determined to be 0.60 ± 0.04 for 1-nm thick pentacene thin film,
and 0.64 ± 0.04 eV for 10- and 20-nm pentacene with a commonly used method as shown in
Fig. S1. The work function was obtained to be 4.28 ± 0.03 eV for the 1-nm film and 4.26 ±
0.03 eV for 10- and 20-nm films from the onset of SECO spectra. From these values the
ionization energy (IE), which is defined as the energy difference between the HOMO onset
and Evac, is 4.90 ± 0.05 eV for 10- and 20-nm-thick bulk phase pentacene films on ITO, while
4.88 ± 0.05 eV for the 1-nm film that may involve effects of the substrate-mediated
imperfectness of the molecular orientation/packing.
2
Pentacene/ITO
Intensity (a.u)
HOMO
(nm)
SECO
20
0.64
4.26
IE
0.64
4.26
IE
0.60
10
1
0
4.28
IE
4.53
-2.0
-1.5
-1.0
-0.5
0.0
3
4
5
6
7
8
9
Energy from EF (eV)
Fig.S1 He I UPS spectra (HOMO and SECO regions) of pentacene thin films on ITO as a
function of film thickness (). The HOMO onset and the vacuum level (Evac) are shown by
vertical bars with their energies. The ionization energy (IE) is given by the energy difference
between the HOMO onset and the vacuum level.
Reference
1
H. Yamane, H. Fukagawa, S. Nagamatsu, M. Ono, S. Kera, K. K.Okudaira, N. Ueno, IPAP Conf.
Series, 6,19 (2005)
2
H. Yamane, S. Nagamatsu, H. Fukagawa, S. Kera, R. Friedlein, K. K. Okudaira, N. Ueno, Phys. Rev.
B 72, 412 (2005)
3
H. Yoshida and N. Sato, Phys. Rev. B 77, 235205 (2008).
4
H. Fukagawa, H. Yamane, T. Kataoka, S. Kera, M. Nakamura, K. Kudo, and N. Ueno, Phys. Rev. B
73, 245310 (2006).
3