Supplemental Material
Enhancement of the electrical characteristics of metal-free
phthalocyanine films using cold isostatic pressing
Toshinori Matsushima,1,2,a) Yu Esaki,1 and Chihaya Adachi1,2,a)
1Center
for Organic Photonics and Electronics Research, Kyushu University, 744 Motooka, Nishi, Fukuoka
819-0395, Japan
2Japan
Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton Engineering Project, 744
Motooka, Nishi, Fukuoka 819-0395, Japan
a)
Author to whom correspondence should be addressed. Electronic mail: tmatusim@opera.kyushu-u.ac.jp and adachi@cstf.kyushu-
u.ac.jp
Experimental procedures
A CIP machine (CPP-28N) manufactured by Sansho Industry was used in this study.
A H2PC film (197 or 2492 nm thick) that was vacuum-deposited on a room-temperature
fused silica substrate at a deposition rate of 0.1 nm/s under a base pressure of 10−4 Pa was
transferred into a thin, flexible polymer bag (Nylon-based multilayer sheets with ≈30 m
thickness), which was then vacuum-sealed. The bag was immersed in a metal vessel filled
with room-temperature water. A pressure of 200 MPa was applied to the film for 60 min
through the water by pushing a metal piston into the vessel with an external pressurization
source. After taking the substrate out into air, film thicknesses were measured with a
surface profilometer (DektakXT, Bruker). To prevent scratching the H2PC surface with
the cantilever of the profilometer, the whole substrate surface was coated with a 20-nm
Al layer using vacuum deposition. The reported thickness values were averages of eight
values and over measured at different positions over the 2.5×2.5 cm2 substrates. Surface
morphologies and structural properties of the films without the Al layer were measured
with an AFM (JSPM-5400, JEOL) in AC mode and an XRD using a 2/ technique
[=1.54 Å (CuK)] (Ultima IV, Rigaku).
The thickness decrease was strongly dependent upon the applied pressure and time
used during CIP. The H2PC thickness linearly decreased as the applied pressure increased
with constant time (60 min). Application of higher pressures of > 200 MPa to the film is
expected to result in a further decrease in the film thickness. However, the maximum
pressure is limited to 200 MPa for our CIP machine. As the applied time was increased
with a constant pressure (200 MPa), the thickness decreased up to 60 min and remained
constant after 60 min. Therefore, we used a pressure of 200 MPa and 60 min in this study.
We further tested CIP using hot water (60 and 90 °C) instead of room-temperature
water. This is termed hot isostatic pressing (HIP). HIP was more effective than CIP to
decrease the thicknesses of some types of organic films but often resulted in the same
effect as CIP. In the case of H2PC, the thickness of the HIP film was confirmed to be the
same as the CIP film.
Hole-only devices with a structure of glass/ ITO anode (100 nm) (GEOMATEC)/H2PC
(197 or 2492 nm)/Al cathode (100 nm) [see the insets of Fig. 4(a) and 4(b)] were
fabricated using vacuum deposition to investigate the electrical characteristics
perpendicular to the substrate. The active device area of the hole-only devices was 4 mm2.
To investigate the electrical characteristics parallel to the substrate, a simple two-terminal
structure was used (see the inset of Fig. 5). To fabricate the two-terminal structure, 200nm-thick Au pads with a pad-pad spacing of 50–60 m and a pad width of 2 mm were
vacuum-deposited on a fused silica substrate through a shadow mask and a 197-nm-thick
H2PC layer was vacuum-deposited on the substrate. The vacuum-deposition rates of Al
and Au were set to 0.5 nm/s. The fused silica substrates and the glass substrates coated
with the 100-nm-thick ITO layer were cleaned with conventional ultrasonication and
ultraviolet-ozone treatment before use. The source material H2PC (Tokyo Chemical
Industry) was purified three times with temperature gradient vacuum sublimation. The
CIP was performed on the devices in the same manner. Current-voltage characteristics of
the devices were measured with a semiconductor parameter analyzer (B1500A or E5250A,
Agilent Technologies) at room temperature. Several current-voltage characteristics
measured from devices made on the same and different substrates are plotted in Figs. 4(a),
4(b) and 5. For TSC measurements, the hole-only devices were cooled to liquid-nitrogen
temperature and biased at 2 mA/cm2 for 2 min to charge hole traps with injected holes
from the ITO electrode. Then, TSC profiles of the devices were measured during device
heating to room temperature with a collecting bias of −0.01 V and a heating rate of
5 °C/min in a TSC measurement chamber (TSC-FETT EL2000, Rigaku). When the
devices were not biased at liquid-nitrogen temperature before the heating process, there
were no TSC peaks from the devices. However, clear TSC peaks were observed after
biasing the devices at 2 mA/cm2 at liquid-nitrogen temperature (Fig. 6), indicating that
TSCs originate from the de-trapping of holes during the heating process.
Estimation of mass densities
A H2PC film was vacuum-deposited on a fused silica substrate that was placed very
close to a quartz crystal microbalance (QCM) inside our vacuum evaporator. We set a
mass density of 1.1 g/cm3, a tooling factor of 100%, and a z-ratio of 1 in the QCM for
this H2PC deposition. The mass density of the H2PC film was estimated by comparing
the thickness measured from the film deposited on the substrate with that displayed on
the QCM. The thickness displayed on the QCM was 208 nm; however, the thickness
measured from the film deposited on the substrate was 284 nm. Since the thicknesses of
the films deposited on the substrate and the QCM are assumed to be the same, this
thickness difference is because the actual mass density of the film deposited on the
substrate is different from the mass density set in the QCM. Therefore, the actual mass
density of the H2PC film deposited on the substrate was calculated to be 0.81 g/cm3 from
the relation: 1.1 g/cm3 (the mass density set in the QCM) × 208 nm (the thickness
displayed on the QCM) ÷ 284 nm (the thickness measured from the film deposited on the
substrate). After the CIP, the film thickness decreased by 34% to 187 nm. Therefore, the
mass density of the CIP film was calculated to be 1.23 g/cm3 from the relation: 0.81 g/cm3
(the calculated mass density of the as-deposited film) × 284 nm (the measured thickness
of the as-deposited film) ÷ 187 nm (the measured thickness of the CIP film). An increase
in mass density of 52% was obtained, probably because of the gap crush. The mass
density of a single crystal of H2PC is 1.44 g/cm3,18 which is higher than the values
measured from the films. The mass densities of the films approach but do not exceed the
single crystal value even after CIP.
(18) J. Janczak and R. Kubiak, J. Alloys Comp. 190, 121 (1992).