Equivalent ambipolar carrier injection of electrons and holes with Au
electrodes in air-stable field effect transistors
Thangavel Kanagasekaran,*,† Hidekazu Shimotani,*,‡ Susumu Ikeda,† Hui Shang,‡
Ryotaro Kumashiro,† and Katsumi Tanigaki*,†,‡
†
WPI - Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, 21-1 Katahira, Aoba, Sendai, 980-8577, Japan
‡
Department of Physics, Graduate School of Science, Tohoku University, 6-3 Aoba,
Aramaki, Aoba, Sendai, 980-8578, Japan
S1. Band Diagrams determined by Photoelectron Yield Spectra
In order to acquire unambiguous spectroscopic evidence on the metal semiconductor
(MS) contact in addition to phenomenological understanding, we carried out
photoelectron yield spectroscopy and evaluated activation energies from the temperature
dependence of the Au/BP2T interface fabricated on both PMMA-SiO2 and TTC-SiO2.
The photoelectron yield spectra of Au and thin film fabricated on a PMMA-SiO2 substrate
are shown in FIG. S1(a). Important information can be obtained from the kinks appearing
in the spectra at 4.51 and 5.48 eV, which correspond to the work function (φm) of Au and
the ionization potential of BP2T, respectively. Given the optical band gap of BP2T of 2.54
eV obtained from the absorption spectrum of BP2T, the LUMO of BP2T can be evaluated
as –2.94 eV. When the same experiments were conducted on a Au/BP2T bilayer (30 nm
in thickness), two kinks were clearly observed as shown in Fig. 1(b). Considering the
general interpretations regarding photoelectron spectra of Au/organic semiconductor
bilayer thin films as reported elsewhere, the first and the second kinks correspond to the
Fermi level of Au and the HOMO of BP2T, respectively. The difference between these
two values then gives the injection barrier φb for holes at the Au/BP2T contact. The
observed barrier height (φb) increased with an increment in film thickness of Au and
saturated when the thickness reached 0.5 nm on both PMMA-SiO2 and TTC-SiO2. Since
these changes reflect the coverage of Au thin film on the BP2T thin film, the φb values at
the Au/BP2T interface on PMMA-SiO2 and on TTC-SiO2 can be evaluated as 0.96 and
1.10 eV, respectively, under such saturation conditions.
Considering the band gap of 2.54 eV for BP2T as described earlier, the LUMO with
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MS contact was evaluated as the difference between the gap energy and φb, and found to
be 1.44 and 1.58 eV for TTC-SiO2 and PMMA-SiO2, respectively. The band diagrams
with MS interface were deduced from these experiments and are shown in Fig. 1(g) (two
right figures). The experimental observation that a small vacuum level shift Δ = 0.01 eV
was observed at the interface on PMMA-SiO2, i.e. the φb for hole injection was almost
equal to the difference between φm of the metal electrode and the HOMO of the organic
semiconductor without contact, is unambiguous spectroscopic evidence of the Schottky
limit. On the other hand, in the case of the Au-BP2T interface on TTC-SiO2, a large
vacuum level shift of Δ = –0.13 eV was evaluated, and the Fermi level of Au electrode
shifted towards the middle of the BP2T band gap at the interface. This gives the evidence
that the MS contact has switched from the Schottky limit to the Bardeen limit.
When contact is made between a metal and a semiconductor, charge transfer between
them generally occurs, resulting in the alignment of their Fermi levels. The transferred
charges in a semiconductor are partly accommodated in the interface gap states, and the
remaining charges are accommodated in the space charge region close to the interface.
They cause a vacuum level shift at the interface leading to a band bending within the
space charge region. If the density of the interface gap states (DIGS) is low as in the case
of Au/BP2T on PMMA-SiO2, a small charge transfer to the gap states occurs and
Δ is
consequently small. Then, the potential drop predominantly occurs in the space charge
region as illustrated in the left panel of Fig. S1(h). In such a case, φb approaches the
Schottky limit with a value of φm – χsc, where χsc is the electron affinity of the
semiconductor. On the other hand, if DIGS is high as in the case of Au/BP2T on TTCSiO2, a significantly large potential drop occurs at the interface, and therefore φb differs
considerably from φm – χsc. This limit is the Bardeen limit, where φb is insensitive to φm.
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FIG. S1. Band diagrams of Au and BP2T without contact and their interfaces. (a)
Photoelectron yield (Y) spectra of Au (red circles) and BP2T (orange circles) thin films.
(b) Photoelectron yield spectra of Au thin film (0.5 nm) on BP2T thin film (30 nm) on
PMMA-SiO2 (green-yellow circles) and TTC-SiO2 (purple circles). (c-f) Activation
energy (Ea) of carrier injection (blue circles) and carrier transport in the channel (black
circles) of holes and electrons in FETs on TTC-SiO2 (c and d) and PMMA-SiO2 (e and f)
as a function of gate voltage (Vg). (g) Band diagram of Au and BP2T without contact
(left), and Au-BP2T interface on PMMA-SiO2 (center) and TTC-SiO2 (right). It is
assumed that BP2T is an intrinsic semiconductor. EF and Δ denote the Fermi level and the
potential drop at the interface, respectively. The red lines inside the zone indicate the gap
states. The energy is expressed with the unit of eV. (h) Band diagrams of a metal (M) and
a semiconductor (SC) before and after contact. The left and right figures illustrate the
cases of low- and high-density gap states, respectively. Red and black circles denote
electrons in the interface gap states and positive charges induced by the charge transfer,
respectively. (Band bending at a semiconductor surface without contact with metal is not
shown for simplicity).
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S2. X-ray diffraction measurements of BP2T thin films on PMMA-SiO2 and on
TTC-SiO2
The quality of BP2T thin films grown on PMMA-SiO2 and onTTC-SiO2 was evaluated
by X-ray diffraction. Fig. S2 shows X-ray diffraction peaks observed at 2θ = 13.46º. The
width (B) of BP2T thin films on PMMA-SiO2 and TTC-SiO2 was 0.201º (0.00350 rad)
and 0.230º (0.00402 rad), respectively. Using the Scherrer equation of D=Kλ/(Bcosθ),
where K = 4/3 (the Scherrer constant) and λ = 0.1541 nm (the wavelength of X-ray), the
volume-weighted average crystal size (D) was calculated to be 59.0 and 51.5 nm,
respectively. These results indicate that BP2T thin films on TTC-SiO2 have lower
crystallinity than thin filmson PMMA-SiO2.
FIG. S2. X-ray diffraction spectra of BP2T thin films. Green and purple circles show the
X-ray diffraction peaks of BP2T deposited on PMMA-SiO2 and TTC-SiO2 substrates,
respectively.
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S3. SEM analysis
Figure S3 shows SEM images of BP2T deposited on TTC-SiO2 and PMMA-SiO2. The
grain size of thin films on TTC-SiO2 is smaller than that on PMMA-SiO2. The results
indicate that the crystallinity of BP2T thin films on TTC-SiO2 is lower than those on
PMMA-SiO2.
FIG. S3. SEM images of thin film BP2T on (a) TTC-SiO2 and (b) PMMA-SiO2.
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