Film-Forming Properties of Fullerene Derivatives in Electrospray Deposition Method

doi:10.4236/jsemat.2013.31A012

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Journal of Surface Engineered Materials and Advanced Technology, 2013, 3, 84-88

http://dx.doi.org/10.4236/jsemat.2013.31A012 Published Online February 2013 (http://www.scirp.org/journal/jsemat)

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: *[email protected]

Received November 27th, 2012; revised December 28th, 2012; accepted January 4th, 2013

ABSTRACT

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.

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-

cantly.

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

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

Film-Forming Properties of Fullerene Derivatives in Electrospray Deposition Method

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

PC[60]BM PC[70]BM ICMA

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).

PC[60]BM PC[70]BM IC MA

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-

ployed.

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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