Fabrication of high-performance fluorine doped–tin oxide film using flame-assisted spray deposition.

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Thin Solid Films 520 (2012) 2092–2095

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Thin Solid Films
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Fabrication of high-performance fluorine doped–tin oxide film using flame-assisted
spray deposition
Agus Purwanto a,⁎, Hendri Widiyandari b, Arif Jumari a
a
b

Department of Chemical Engineering, Faculty of Engineering, Sebelas Maret University, Jl. Ir. Sutami 36 A, Surakarta, Central Java 57126, Indonesia
Department of Physics, Faculty of Mathematics and Natural Sciences, Diponegoro University, Jl. Prof. Dr. Soedarto, Tembalang, Semarang 50275, Indonesia

a r t i c l e

i n f o

Article history:
Received 5 September 2010
Received in revised form 6 August 2011
Accepted 9 August 2011
Available online 17 August 2011
Keywords:

Fluorine–doped tin oxide
Flame-assisted spray deposition
Atmospheric pressure
Transparent conductive oxide
Sheet resistance
X-ray diffraction

a b s t r a c t
A high-performance fluorine–doped tin oxide (FTO) film was fabricated by flame-assisted spray deposition
method. By varying the NH4F doping concentration, the optimal concentration was established as 8 at.%. X-ray
diffractograms confirmed that the as-grown FTO film was tetragonal SnO2. In addition, the FTO film was
comprised of nano-sized grains ranging from 40 to 50 nm. The heat-treated FTO film exhibited a sheet
resistance of 21.8 Ω/ with an average transmittance of 81.9% in the visible region (λ = 400–800 nm). The
figures of merit shows that the prepared FTO film can be used for highly efficient dye-sensitized solar cells
electrodes.
© 2011 Elsevier B.V. All rights reserved.

1. Introduction
Fluorine–doped tin oxide (FTO) film is widely used for optoelectronic applications such as displays, solar cells, light emitting diodes,
smart windows, etc. [1]. Compared to indium–doped tin oxide (ITO),

FTO has many advantages such as low cost, chemical resistance, and
high availability of precursor sources [1,2]. In addition, ITO suffered
from low electrical conductivity after annealing treatment in an
oxygen atmosphere during fabrication of dye-sensitized solar cells
(DSSCs) [3,4]. This case was not found in the application of FTO as
electrodes for DSSCs. Therefore, FTO would be an economically viable
and a best candidate for DSSCs electrodes.
The fabrication of FTO thin films has been reported by various
methods, which includes spray pyrolysis deposition (SPD) [2,5,6],
sputtering [7], and chemical vapor deposition [8]. The SPD method is
the most popular due to its simplicity, low cost, and scale-up ease
[2,5,9,10]. Several modifications of the SPD method have been
reported, such as the application of lamp radiation for a substrate
heater [11], electric field induction [10], and the use of an ultrasonic
nebulizer as a droplet generator [12,13]. Recently, a class of deposition
technique of flame-assisted spray deposition (FASD) has been
introduced as an alternative low-cost method for film deposition
[4,14–18]. Several materials have been fabricated using this method,

⁎ Corresponding author. Tel./fax: + 62 271 632112.

E-mail address: Aguspur@uns.ac.id (A. Purwanto).
0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2011.08.041

such as ITO [4], Pt–ZnO [18], TiO2 [17], and yttria–stabilized zirconia
[16]. However, the preparation FTO film using FASD method was not
found in the literatures.
In this paper, we report the fabrication of FTO film using the FASD
route. The effect of flow configuration, doping concentration,
substrate temperature, and heat-treatment were investigated systematically. An ultrasonic nebulizer was used to atomize an aqueous
precursor with a controllable droplet speed (size of 5 μm) [19]. The
configuration of precursor droplet flow controlled the characteristic of
the prepared FTO film. In addition, substrate temperature and heat
treatment were highly affected on the electrical conductivity of asprepared FTO film. In this investigation, the high performance FTO
films that showed good electrical conductivity, transparency, and film
adhesion could be produced by FASD method.
2. Experimental details
The schematic diagram of the FASD system is shown in Fig. 1. In
accordance with the FASD technique, the substrate (soda lime glass)
was located above a burner. The temperature of the substrate was

monitored using a thermometer (K-type thermocouple, Fluke 51,
Fluke Corporation, Washington, United States). The substrate temperature was controlled from 300 to 420 °C. This temperature range is
the operation temperatures of the experimental devices. The droplet
was transported through the center of the burner and deposited on
the bottom-side of glass substrate (Fig. 1, configuration I). In
configuration II, droplets were transported to the upper-side of glass

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3. Results and discussion

Fig. 1. Schematic diagram of the FASD method.

substrate. The heating of substrate was conducted from the belowside of substrate. Prior flowing of droplets to the substrate, the
substrate was heated to the designated temperature. When the
desired temperature achieved, the droplets were then transported to
the upper side of the glass substrate. For all experiments, an ultrasonic

nebulizer was used to generate droplets of the precursor at a
frequency of 1.7 MHz (Omron NE-U22V, Tokyo, Japan). The precursor
was an aqueous solution of SnCl2. 2H2O (98%, Merck Ltd., Germany)
and NH4F (98%, Merck Ltd., Germany) in 0.5 N HCl. The FTO films were
prepared at various NH4F concentrations (6 at.%–14 at.%). To improve
the conductivity of the films, annealing was conducted at elevated
temperatures (450–550 °C).
The sheet resistance (Rs) of the FTO films was evaluated using
four-probe measurements (Jandel Engineering, Bedfordshire, UK).
The optical properties of the FTO films were evaluated using a UV–Vis
spectrophotometer (UV 2450, Shimadzu, Kyoto, Japan). The figure of
merit was calculated using a Haacke equation (ΦTC = T 10/Rs) [20] and
MTC for application in dye sensitized solar cells (DSSCs) [13]. This
figure of merit was calculated using equation:

The investigation of FTO fabrication using FASD method was
started by the evaluation of droplets flow configurations as indicated
in Fig. 1. In configuration I, droplets were suplied to the bottom side of
the glass substrate. The substrate was positioned at 10 cm above the
burner with the deposition temperature at 420 °C for 25 min. The

resistivity of the as-prepared FTO film can be found in Table 1. All
samples had resistivity in the MΩ order and the film was
heterogeneous in surface. In addition, the X-ray characterization
indicated that the FTO film was amorphous (Fig. 2). Annealing at
550 °C for 1 h did not improve the crystallinity significantly. The
unexpected characteristics of FTO film prepared by FASP-configuration I can be concluded from imperfect crystalization FTO film.
Since FASD-configuration I did not produce FTO film with the
expected characteristics, FASD-Configuration II was then examined.
Using this configuration, good characteristics of FTO film can be
fabricated. The dependency of sheet resistance (Rs) as a function of
doping concentration is shown in Fig. 3. The present study was
conducted using a precursor concentration of 0.5 M. The deposition
process was conducted for 10 min. To evaluate the effect of doping
concentrations (6–14 at.%), the substrate temperature was maintained at 420 °C. As Fig. 3 shows, the optimal doping concentration
was 8 at.%. The conductivity of the FTO film originated from the
substitution of fluorine atoms in the SnO2 lattice. The substitution of
oxygen ions with fluorine ions leaving an oxygen vacancy suggests the
conduction of electrons by the SnO2 film [6,11]. When doping
concentration is above the optimal level (8 at.%), the Rs increase.
The high Rs originated from an accumulation of fluorine in the grain

boundaries [6]. Then, this fluorine concentration level was used in all
experimentation.
Substrate temperature is an important parameter in controlling
FTO film characteristics. Fig. 4 shows the dependency of Rs and
transmittance on substrate temperatures. Fig. 4a showed a decrease in
Rs when the substrate temperature was elevated. The optimal
temperature for FTO film preparation ranged from 410 to 420 °C
(Rs = 20.5–21.5 Ω/ ). Unfortunately, the light transmittance (T) of the
as-prepared FTO films deposited for 10 min was low (21.7–25.9%), as

Table 1
The FTO film characteristics prepared by FASD-configuration I route.
Doping level (at.%)

Resistivity (MΩ, measured in 1 cm apart)

10
12
14
16

20
25

Not conductive
35 MΩ
35 MΩ
36 MΩ
33 MΩ
Not conductive

MTC = ð0:769−0:0868T−0:000287Rs −0:00207 T:Rs Þð1:23 + 18:99T Þ

where T is transmittance. To evaluate the crystal structure, the films
were characterized using an X-ray diffractometer (XRD, RINT 2200V,
Rigaku-Denki Corp., Tokyo, Japan). The XRD measurements were
carried out using nickel-filtered CuKα radiation (λ = 0.154 nm) at
40 kV and 30 mA. The XRD measurements were carried out using
nickel-filtered Cu Kα (λ = 0.154 nm) at 40 kV and 30 mA with a scan
step of 0.02° and a scan speed of 4°/min. The morphology of the FTO
film was evaluated using field emission-scanning electron microscopy

(FE-SEM) observations with an S-5000 at 20 kV (Hitachi Ltd., Tokyo,
Japan). Prior to FE-SEM characterization, the sputtering treatment
was not conducted because the FTO film is conductive.

Fig. 2. XRD diffractograms of FTO film prepared using FASD-configuration I.

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Fig. 3. Sheet resistance of FTO film prepared using FASD-configuration II as a function of
doping concentration.

shown in Fig. 4b. The increasing deposition temperature improved the
crystallinity of FTO film. As a result, electron mobility and conductivity
are improved due to less grain boundaries [21]. For comparison, the
optimal temperature for FTO deposition using SPD method ranges
from 400 to 450 °C [13,21].
To increase the light transmittance, the deposition time and

precursor concentration were reduced to 5 min and 0.1 M (doping
concentration 8 at.%), respectively. Using this strategy, film with a
high transmittance (T = 81.9%) was produced, as shown in Fig. 5b.
However, the Rs of the as-prepared FTO film dropped significantly to
254.2 Ω/ (Fig. 5a). To improve the electrical conductivity, an
annealing treatment under atmospheric conditions was conducted
(450–550 °C; 30 min). As expected, the Rs of the FTO film was enhanced

Fig. 5. Effect of the annealing temperature on (a) sheet resistance and (b) light
transmittance (inset: high transmittance of FTO film).

Fig. 4. Sheet resistance (a) and light transmittance (b) of FTO film as a function of
substrate temperature.

to 21.8 Ω/ (500 °C, Fig. 5a) with no loss of light transmittance (Fig. 5b
(inset: high transmittance of FTO film)). Thus, the combination of FASD
and annealing treatment could be used to produce high-performance
FTO film. The figure of merit (ΦTC) of the FTO film annealed at 500 °C
was 1.1 × 10−2 Ω− 1, which is comparable to other methods (in the
order 10 −2 Ω− 1) [6,11,22]. Evaluation using figures of merit for a DSSC
application (MTC) showed a value of 11.5. For comparison, the MTC value
of FTO prepared by spray pyrosol technique can reach approximately
11.2 which can be used to produce DSSC solar cell with the efficiency
around 7% [13]. This result indicated that the prepared FASD-FTO film
was ideal for a high-efficiency DSSC solar cell electrode [13].
Fig. 6 shows an XRD pattern on the films before and after
annealment at 500 °C for 30 min. The crystalline structure of the FTO
films was tetragonal SnO2 (JCPDS no. 770451, and space group: F42/
mnm(136)). The XRD pattern also confirmed that annealing in
elevated temperatures improved the crystallinity of the FTO films.
The crystallite sizes FTO film were 27 nm, and 34 nm for the film
without, and after annealing at 500 °C, respectively. This produced
better electrical conductivity, as shown in Fig. 5a. From XRD
diffractograms, it is also shown that the crystal orientations of the
film were same. This suggested that improvement of electrical
conductivity only originated from the enhancement of the FTO film
crystallinity.
The morphology of the as-prepared FTO films was characterized
using FE-SEM. Fig. 7a and b shows FE-SEM images of FTO film
prepared by FASD before and after annealing, respectively. The FTO
film was composed of small grains ranging from 40 to 50 nm. This size
was larger compared to the XRD calculation. This discrepancy may
originate from the different basis of calculation. Using XRD diffractograms, crystal size of FTO was calculated using Scherer's equation. On
the other hand, grain size measured from the SEM images predicted
the particle size regarding the crystal nature of the films. Annealing at
500 °C for 30 min did not alter the grain size much as indicated in

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4. Conclusion
A high performance of FTO film was fabricated using a FASD
technique. The droplet supplied technique controlled the performance of the as-prepared FTO film. Droplet supplying to the top-side
of substrate produced FTO film with good characteristics. With this
method, the optimal doping concentration of fluorine was 8 at.%. The
FASD method combined with post-treatment (500 °C for 30 min)
produced a highly conductive FTO film (sheet resistance 21.8 Ω/ )
with high light transmittance (81.9%). A figure of merit calculation
showed a value that is incomparable with other methods. We showed
that the FASD method is a method that can be applied in the
production of relatively large-area, high-performance FTO films.
Fig. 6. XRD pattern of FTO films before and after annealed at temperature 500 °C (for
30 min).

Fig. 7b. The cross-sectional images showed that the thickness of the
FTO film was approximately 260 nm. The calculated specific resistivity
of the heat treated-FTO film was 5.67 × 10 − 5 Ω cm. For comparison,
the specific resistivity of FTO film prepared by pyrosol method was
averagely 5 × 10 − 4 Ω cm [12]. Thus, the specific resistivity of FASDprepared FTO film is in comparable with the other method.

Acknowledgment
The authors wish to thank the DGHE-Ministry of National
Education of Indonesia for providing the research funding through
the Hibah Kompetitif Penelitian Sesuai Prioritas Nasional Batch II,
2009–2010 with contract number 22/SP2H/PP/DP2M/VI/2009–2010.
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Fig. 7. FE-SEM images of FTO film (inset: cross-section image): (a) before annealed, and
(b) after annealed at temperature 500 °C (for 30 min).

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