Journal of Surface Engineered Materials and Advanced Technology, 2013, 3, 84-88 Published Online February 2013 (
Film-Forming Properties of Fullerene Derivatives in
Electrospray Deposition Method
Kazumasa Takeshi1,2, Kenji Takagi1,2, Takeshi Fukuda1,2, Teiji Chihara1, Yusuke Tajima1,2*
1Department of Functional Materials Science, Graduate School of Science and Engineering, Saitama University, Saitama, Japan;
2RIKEN, Saitama, Japan.
Email: *
Received November 27th, 2012; revised December 28th, 2012; accepted January 4th, 2013
Thin films of three types of fullerene derivatives were prepared through the electrospray deposition (ESD) method. The
optimized conditions for the fabrication of the thin films were investigated for different types of fullerene derivatives:
[6,6]-phenyl-C61-butyric acid methyl ester, [6,6]-phenyl-C71-butyric acid methyl ester, and indene-C60-monoadduct. The
spray diameter during the ESD process was observed as a function of the supply rate achieved by changing the applied
voltage. In all cases, the spray diameter increased with increasing applied voltage, reaching the maximum diameter
(Dmax) in the voltage range 4 to 6 kV. It was clear that Dmax was influenced by the dipole moments of the fullerene de-
rivatives (as calculated by density functional theory methods). Scanning electron microscopy observation of the fabri-
cated thin films showed that imbricated structures were formed through the stacking of the fullerene-derivative sheets.
Atomic force microscopy images revealed that the density of the imbricated structure was dependent on the spray di-
ameter during the ESD process, and the root-mean-square roughness of the film surface decreased with increasing ap-
plied voltage. These findings suggest that the ESD method will be effective for the preparation of fullerene-derivative
thin films for the production of organic devices.
Keywords: Electrospray Deposition Method; Fullerene Derivative; Thin Film; Scanning Electron Microscope;
Imbricated Structure; Atomic Force Microscope; Root-Mean-Square Roughness
1. Introduction
Organic devices are of interest owing to their special
advantages over inorganic semiconductor devices, such
as their light weight, flexibility, large device area, low-
cost manufacture, and the applicability of solution-proc-
essing techniques. Solution processes have already been
demonstrated with organic semiconductors for the fabric-
cation of light-emitting diodes [1], thin-film transistors
[2], and photovoltaic cells [3]. On the contrary, a bilayer
heterojunction photovoltaic system [4-6], in which donor
and acceptor films are stacked sequentially on top of one
another, is difficult to form practically with different or-
ganic compounds using normal solution processes such
as spin-coating, dip-coating, screen printing, and inkjet
printing. This is because these solution processes cause
the dissolution of the underlying organic layer while
coating the upper organic layer. In addition, the forma-
tion of a single layer of fullerene or a fullerene derivative
through a solution process has innate drawbacks: the
thickness and surface roughness cannot be controlled
because of the poor film-forming ability due to strong
cohesive forces, and these species have low solubility in
common organic solvents. Therefore, the considerable
amount of research on the fabrication of fullerene films
has dealt almost exclusively with as-deposited films,
consisting of vacuum-sublimed C60 on top of a spin-
coated polymer film [4-6]. However, most fullerene de-
rivatives cannot be deposited by vacuum processes, and
there have been few reports on the fabrication of ful-
lerene-derivative films by the spin-coating process [7].
For the fabrication of fullerene-derivative-based bilayer
heterojunction photovoltaic devices, an appropriate solu-
tion process for fullerene-derivative thin-film deposition
is desirable.
Recently, our research group has investigated a new
electrospray deposition (ESD) method as a film-forma-
tion process for organic semiconductors, which can be
adapted for the fabrication of organic photovoltaic (OPV)
devices with regioregular poly(3-hexylthiophene-2,5-diyl)
and fullerene derivatives [8]. In this ESD process, a high
voltage of several kilovolts is applied to the organic solu-
tion, which is then divided into innumerable droplets
with diameters of a few micrometers [9]. As a result,
following solvent evaporation, organic semiconductor
*Corresponding author.
Copyright © 2013 SciRes. JSEMAT
Film-Forming Properties of Fullerene Derivatives in Electrospray Deposition Method 85
nanoparticles are formed by the agglomerating solute
from the solution before it reaches the substrate. The
ESD method is as good as the evaporation method for the
formation of nanoparticulate films for successful multi-
layer fabrication without dissolution of the underlying
organic layer. Furthermore, the method has the advan-
tage that film formation is possible from very dilute or-
ganic semiconductor solutions (less than 100 μg/mL).
To investigate how the homogeneous fullerene-deri-
vative films are formed in this ESD process, we deter-
mined the controllable factors: the applied voltage and
the spray diameter (signifying the spray angle of the
electrospray). In this study, we show the electrospray
deposition of films of several soluble fullerene deriva-
tives that are well-known electron-acceptor materials for
OPV devices, and discuss the relationship between the
film-growth mechanisms of the fullerene derivatives and
their molecular structures.
2. Experimental
2.1. Materials
[6,6]-Phenyl-C61-butyric acid methyl ester (PC[60]BM)
[10] and [6,6]-phenyl-C71-butyric acid methyl ester
(PC[70]BM) [11] (purity, 99%) were purchased from
Frontier Carbon. Purified indene-C60-monoadduct (ICMA)
[12] (99.9%) was obtained from FLOX corp., and o-dich-
lorobenzene (o-DCB: Wako Pure Chemical Industries)
and acetonitrile (Wako Pure Chemical Industries) were
used as received.
2.2. ESD Process
The fullerene-derivative powder (1.0 mg) was mixed
with o-DCB (1 mL). Then, acetonitrile was added to the
resulting solution as an additional solvent. The concen-
tration of the additional solvent was 10 vol%. Figure 1
shows the experimental setup for the ESD process. We
used a glass capillary for the ESD process because of the
controllability of its diameter, which leads to the forma-
tion of an organic thin film with a smooth surface [13].
The glass capillary (inner diameter 50 μm) was fabri-
cated using a puller (Narishige PC-10) and a microforge
(Narishige MF-900). A high-voltage source (Matsusada
Precision HJPQ-30P1) was used to apply a positive high
voltage to a copper wire in the fullerene-derivative solu-
tion in the glass capillary. The earthed line was con-
nected to an indium tin oxide (ITO) layer on top of the
glass substrate.
The ITO-coated glass substrate was cleaned in solvent
(isopropyl alcohol, acetone, and ethanol) and deionized
water with ultrasonic treatment. The spray diameter and
supply rate of the ESD process were measured with a
charge-coupled device (CCD) camera (Watec WAT-902B),
as shown in Figure 1. The distance from the glass capil-
Figure 1. Experimental setup for ESD method.
lary to the substrate was 100 mm, and the applied high
voltage was varied from 3.5 to 8.0 kV. The surface
roughness of the neat film was estimated by atomic force
microscopy (AFM; Seiko SPA-300) and scanning elec-
tron microscopy (SEM; Hitachi Science Systems S-4100).
The dipole moments of the fullerene derivatives were
determined by using the semiempirical PM3 Hamiltonian
and the B3LYP/6-31G(d) method, as implemented in
Gaussian 09.
3. Results and Discussion
3.1. Film Formation by ESD Method
In our previous study, we found that the dielectric con-
stant of the solvent used in the ESD process influences
the spray diameter significantly [14]. Solvents with low
dielectric constants such as o-DCB showed relatively
small spray diameters. However, the electrospray was
spread significantly owing to the increasing Coulomb
repulsion when another solvent with a high relative di-
electric constant was added. As a result, the surface rou-
ghness of the organic thin film was also reduced drastic-
cally by the dense deposition of fine particles [15].
Therefore, we adopted the abovementioned mixed-sol-
vent technique to achieve fullerene-derivative film depo-
sition by the ESD method.
Solutions of three types of fullerene derivatives were
prepared for investigation of the solute-structure de-
pendence on the ESD conditions (Figure 2). The changes
in supply rate of each solution to the glass capillary with
the voltage applied during the spray deposition of the
fullerene-derivative solutions are shown in Figure 3. The
solution supply rate increased linearly with increasing
voltage in all cases. The structures of the fullerene de-
rivatives did not affect the slopes of the plots signifi-
Figure 4 shows the influence of the applied voltage
on the spray diameter measured 4 mm below the tip of
the glass capillary. In all cases, the spray diameter in-
creased with increasing applied voltage, and reached a
Copyright © 2013 SciRes. JSEMAT
Film-Forming Properties of Fullerene Derivatives in Electrospray Deposition Method
PC [60] BM PC [70] BM ICMA
Figure 2. Fullerene derivatives used in this study.
Figure 3. Influence of applied voltage on supply rate of
fullerene solution.
Figure 4. Spray diameter 4 mm far from glass capillary as a
function of applied high voltage.
maximum value at 4.0 (ICMA), 4.5 (PC[60]BM), or 5.0
kV (PC[70]BM). These results imply that a stable Taylor
cone was formed at the tip of the glass capillary at these
voltages (VDmax), causing the maximum spray diameter
(Dmax) to be achieved under these deposition conditions
[16,17]. PC[60]BM, PC[70]BM, and ICMA had Dmax
values of 4.3, 7.6, and 6.2 mm, respectively, at VDmax. It
should be noted that Dmax of PC[70]BM was almost dou-
ble that of PC[60]BM. The Dmax values of each fullerene
derivative and their calculated dipole moments are sum-
marized in Table 1 . It is clear that the increase in Dmax is
due to the increase in the dipole moment. The dipole
moments of the fullerene derivatives affect the charge
densities of the droplets generated from the solution. Our
previous study revealed that the spray diameter was di-
rectly proportional to the dielectric constant of the sol-
vent. This is because with a high relative dielectric con-
stant of the solvent, the polar surface of the droplets
causes a spread in the spray diameter owing to the in-
creased Coulomb repulsion [14]. These results indicate
that the droplets containing fullerene derivatives showed
their maximum spread at VDmax, and the Coulomb repul-
sion between droplets was at its maximum in the direc-
tion perpendicular to the movement of droplets toward
the substrate.
3.2. SEM and AFM Observations
The fullerene-derivative films formed by the ESD proc-
ess were observed by SEM. In all cases, an imbricated
structure, formed by the stacking of fullerene-derivative
pieces, was obtained in the vicinity of VDmax (Figure 5).
In the case of PC[60]BM, when the applied voltage was
increased to 4.5 kV, the number of fullerene-derivative
pieces in an area of 100 × 100 μm increased, reaching a
maximum value of 120, with an average piece size of
about 10 μm. As the applied voltage was increased be-
yond VDmax, the number of pieces decreased to less than
100 in the same area. The average size of the PC[60]BM
pieces was found to be greater than 10 μm at 4.5 kV. The
observations for PC[70]BM and ICMA were very similar
to results in the case of PC[60]BM. Accordingly, it is
supposed that the size of the imbricated pieces corre-
sponds to the diameter of the droplets coming into con-
tact with the substrate. At 6.0 kV, the imbricated struc-
tures were not observed, but the results showed aggrega-
tion defects of the fullerene derivatives PC[60]BM and
PC[70]BM or no film area for ICMA. This was because
the wet droplets came into contact with the substrate in
high-voltage regions (high supply rate), resulting in ag-
glutinated fullerene derivatives that were poorly adhered
to the substrate.
For investigation of the surface nanostructures of the
imbricated pieces, the fullerene-derivative films were
observed by AFM in an area of 10 × 10 μm (Figure 6).
The surface roughnesses of the deposited films were
obtained from the AFM images; a planarizing surface
was observed with increasing applied voltage for all
samples. AFM observation of the ICMA and PC[70]BM
films formed at VDmax showed that their surfaces were
comprised of 5 - 80 nm particles, but with PC[60]BM, no
such nanoparticles were observed under any conditions
(Figure 6). The nano surface structures were probably
dependent on the self-organization behaviors of the
fullerene derivatives. The relationship between the ESD
applied voltage and the root-mean-square (RMS) rough-
nesses of the fullerene-derivative films is shown in
Figure 7. It is noted that the RMS roughness of the
surface was reduced dramatically around VDmax to less
Copyright © 2013 SciRes. JSEMAT
Film-Forming Properties of Fullerene Derivatives in Electrospray Deposition Method
Copyright © 2013 SciRes. JSEMAT
Table 1. Summarized dipole moment, Dmax, and VDmax values of fullerene derivatives.
dipole Momenta (debye) Dmax (mm) VDmax (kV)
PC[60]BM 2.34 4.33 4.5
ICMA 3.15 6.27 4.0
PC[70]BM 3.32 7.97 5.0
aCalculated the geometry and the electron density by DFT-method.; Gaussian 09 (RB3LYP/6-31G(d)).
3.5 kV
4.5 kV (VDmax)
6.0 kV6.0 kV6.0 kV
5.0 kV(VDmax)4.0 k V(VDmax)
3.5 kV3.5 kV
Figure 5. SEM images of fullerene derivatives thin films fabricated by ESD with several voltages; PC[60]BM (left),
PC[70]BM (center) and ICMA (right).
Figure 6. AFM images of fullerene derivatives thin films fabricated at VDmax by ESD method.
1) The ESD process enables the fabrication of single
films of various fullerene derivatives that are used as
organic semiconductor materials.
than 10 nm.
4. Conclusions 2) The applied voltage giving the maximum spray di-
ameter varies according to the fullerene derivative em-
In conclusion, we investigated the film-forming proper-
ties of various fullerene derivatives in the ESD process,
and obtained the following results: 3) SEM studies showed that, at the maximum spray
Film-Forming Properties of Fullerene Derivatives in Electrospray Deposition Method
Figure 7. Relationship between applied voltage and RMS
roughness estimated from AFM image.
diameter, the deposited films consist of the imbricated
structure formed by the stacking of 10 μm fullerene-de-
rivative sheets.
4) The RMS roughness of the film surface drops shar-
ply when the maximum spray diameter is reached.
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