Advances in Chemical Engineering and Science, 2012, 2, 461-464 Published Online October 2012 (
Fabrication and Characterization of Phthalocyanine/C60
Solar Cells with Inverted Structure
Kazumi Yoshida1, Takeo Oku1*, Atsushi Suzuki1, Tsuyoshi Akiyama1, Yasuhiro Yamasaki2
1Department of Materials Science, The University of Shiga Prefecture, Hikone, Japan
2Orient Chemical Industries Co. Ltd., Department of New Business, Osaka, Japan
Email: *
Received May 31, 2012; revised June 30, 2012; accepted July 10, 2012
Photovoltaic and optical properties of fullerene/phthalocyanine heterojunction solar cells with normal and inverted
structures were fabricated and investigated. Aluminum and gallium phthalocyanines were used for the n-type semicon-
ductor. The solar cells with inverted structure had more stability compared to that with normal structure in the air.
Nanostructures of the solar cells were investigated by transmission electron microscopy, and energy levels of the mole-
cules were calculated and discussed.
Keywords: Organic Thin Film Solar Cell; Inverted Structure; Phthalocyanine; Fullerene; PCBM; TiO2; Sol-Gel
1. Introduction
Solar cells are expected to solve problems of environ-
mental pollution and exhaustion of fossil fuel, and de-
velopment and practical use of solar energy are needed.
Organic thin film solar cells have an advantage for re-
newable energy resources because of their low cost,
flexible, light weight and fabricate at low temperatures
by spin-coating and printed method [1-3]. Recently,
polymer /fullerene solar cells have been investigated, and
the conversion efficiency of ~5% was obtained [4-6].
Metal phthalocyanines (MPc) are a group of small
molecules with Q-band absorption in the red to near-IR
range, and they have high optical, light stability, chemi-
cal stability and photovoltaic property. Therefore, they
are used for donor materials of organic thin film solar
cells. The heterojunction solar cells using copper
phthalocyanine and fullerene have been fabricated by
evaporation method, and its conversion efficiency was
~3% [7]. The characteristics such as electron conductiv-
ity and absorption range change by changing a central
metal [8-11].
The inorganic solar cells such as using single crystal
silicon have high stability in air. However, the organic
thin film solar cells with normal structures as shown in
Figure 1 (a), have no stability in air. Al metal has often
been used as the back electrode of the organic solar cells
with normal structures, due to its low work function. The
Al is oxidized to insulator Al2O3 at the Al/organic inter-
face and the diffused Al into the active layer acts as a
recombination site. A acidic poly(3,4-ethylenedioxy-
lenethiophene): poly(4-styrene sulfonic acid) (PEDOT:
PSS) would damage the device performance due to cor-
rosion to indium-tin-oxide (ITO). Both of which make
lifetime of the cell very short. An approach to solve these
problems is to use cells with an inverted structure as
shown in Figure 1(b). The cells with an inverted struc-
ture have a TiO2 layer, which work as electron transport
layer. There are some reports of inverted structure, and
improvement of stability has been reported [12-14].
The purpose of the present work is to fabricate and
characterize heterojunction solar cells with normal and
inverted structures using MPc and fullerene. Gold was
used for the electrode instead of aluminum. TiO2 thin
films were fabricated by sol-gel method, and used as
electron transfer layer. Photovoltaic mechanism, the light
induced charge separation and charge transfer of the so-
lar cells with normal and inverted structures will be dis-
cussed on the basis of light-induced current density volt-
age (J-V) curves, and optical absorption. The energy lev-
Organic layer
Al (Electrode)
Au (Electrode)
Organic layer
(a) (b)
Figure 1. Schematic cell structures with (a) normal and (b)
inverted structures.
*Corresponding author.
opyright © 2012 SciRes. ACES
els of the molecules were calculated, and nanostructures
of the solar cells were investigated by transmission elec-
tron microscopy.
300 400 500 600 700 800
Wavelengh (nm)
Al PcCl/C
2. Experimental Procedures
Solar cells with normal structure were fabricated by the
following process. Indium tin oxide (ITO) grass plates
(Geomatec, ~10 /) were cleaned by an ultrasonic bath
with acetone and methanol, and were dried by nitrogen
gas. A thin layer of PEDOT:PSS (Sigma Aldrich) was
spin-coated on the ITO substrates. After annealing at
100˚C for 10 min in N2 atmosphere, metal phthalocya-
nine (metal: Al or Ga) and fullerene (C60) layer were
prepared on a PEDOT:PSS layer by vacuum evaporation.
Finally, aluminum (Al) metal contact were evaporated as
a top electrode and annealed at 140˚C for 10 min in N2
Solar of cells with an inverted structure were fabri-
cated by following process. The TiO2 precursor solutions
were prepared from titanium isopropoxide (TTIP),
2-methoxyethanol and acetylacetone. TTIP (0.46 ml) was
add to 2-methoxyethanol (2.5 ml). After stirred for 1h,
acetylacetone (0.61 ml) as the stabilizer was slowly
added, and stirred for 12h [14]. The TiO2 precursor solu-
tion was spin-coated on fluorine dope tin oxide (FTO)
substrate (Luminescence Technology, ~14 /). After
annealing at 100˚C for 10 min in N2 atmosphere, solution
of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) in
1 ml chlorobenzene on a TiO2 layer by spin-coat method.
Then, gallium phthalocyanine layer were prepared on a
PCBM layer by evaporation. A PEDOT:PSS was spin-
coated onto the active layer. Gold metal contact were
evaporated as a top electrode and annealed at 140˚C for
10 min in N2 atmosphere.
Current density-voltage (J-V) characteristics (Hokuto
Denko Co. Ltd., HSV-100) of the solar cells were meas-
ured both in the dark and under illumination at 100 mW/
cm2 by using an AM 1.5 solar simulator (San-ei Electric,
XES-301S). The solar cells were illuminated through the
side of the ITO substrates, and the illuminated areas were
0.16 cm2. Optical absorption of the solar cells was inves-
tigated by means of UV-visible spectroscopy (JASCO,
V-670ST). Transmission electron microscope (TEM) ob-
servation was carried out by a 200 kV TEM (Hitachi
H-8100). The molecular structures were optimized by CS
Chem3D (Cambridge Soft) and molecular orbital calcu-
lations using Gaussian 03.
3. Results and Discussion
Figure 2 shows UV-visible absorption spectra of AlPc/
C60 and GaPc/C60 heterojunction solar cells. The meas-
urement region is in the range from 300 to 800 nm. The
optical absorption at 350 nm corresponds to of Soret
Figure 2. UV-vis absorption spectra of GaPc/C60 and
AlPc/C60 thin films.
band of Pc. Absorption in the range of 600 - 700 nm and
630 - 700 nm correspond to Q-band for AlPc and GaPc,
respectively. Absorption at ~400 nm is PCBM. Since the
absorption was observed in the whole region, it is con-
sidered that the sunlight is efficiently absorbable. Meas-
ured J-V characteristic parameters of heterojunction solar
cells with a normal structure under illumination are shown
in Table 1. A solar cell with GaPc/C60 structure provided
a power convergent efficiency (η) of 7.9 × 10–3%, fill
factor (FF) of 0.22, open circuit voltage (VOC) of 0.30 V,
and short-circuit current (JSC) of 0.12 mA/cm2, which is
better than those of an AlPc/C60 device. These solar cells
with a normal structure provided a conversion efficiency
of 0% after 24 h. Ta ble 2 shows GaPc/PCBM solar cells
with an inverted structure have more stability in air than
that with a normal structure. Since PEDOT:PSS would
prevented oxygen diffusion into active layers, active lay-
ers did not oxidized.
Figures 3(a)-(c) show TEM image, electron diffrac-
tion pattern and high-resolution image of TiO2 thin films,
respectively. The particle size of TiO2 is 20 - 50 nm from
the TEM image, and the electron diffraction pattern and
high resolution image show formation of TiO2 anatase
structure by annealing at 450˚C.
Table 1. Experimental parameters of MPc/C60 solar cells
with normal structure.
Sample VOC (V) JSC (mA/cm2) FF η (%)
GaPc/C60 0.30 0.12 0.22 7.9 × 10–3
AlPc/C60 0.26 0.0030 0.23 1.8 × 10–4
Table 2. Experimental parameters of GaPc/PCBM solar
cells with inverted structure.
Sample VOC (V) JSC (mA/cm2) FF η (%)
GaPc/PCBM 0.56 0.44 0.240.059
After 2 months0.64 0.25 0.210.033
Copyright © 2012 SciRes. ACES
30 nm
Figure 3. (a) TEM image; (b) Electron diffraction pattern;
and (c) High-resolution image of TiO2 thin films.
An energy level diagram of the heterojunction solar
cells with normal and inverted structures were summa-
rized as shown in Figure 4. Previously reported values
were used for the energy levels of the figures by adjust-
ing to the present work [15-17]. Energy barrier would
exist near the semiconductor/metal interface. In the cells
with a normal structure, electronic charge is transferred
by light irradiation from the ITO or FTO substrate side,
and electrons are transported to an Al electrode, and
holes are transported to an ITO substrate. In the cells
with an inverted structure, electrons are transported to an
FTO substrate, and holes are transported to an Al. When
C60 is used for inverted structure, the energy barrier
would be at the TiO2/C60 interface. To reduce the energy
barrier, PCBM with higher LUMO levels is suitable. Voc
of organic solar cells is related with energy gap between
HOMO of MPc and LUMO of C60 or PCBM, and control
of the energy levels is important to improve the photo-
voltaic performance [15].
-5.0eV hν
-4.4eV -5.0eV
Figure 4. Energy level diagram of solar cells with (a) nor-
mal and (b) inverted structures.
4. Conclusion
Phthalocyanine/fullerenen heterojunction solar cells with
normal and inverted structures were fabricated and char-
acterized. A device with inverted cell using GaPc/PCBM
provided Voc of 0.56 V, Jsc of 0.44 mA/cm2, FF of 0.24,
and η of 0.059%. The solar cell with an inverted struc-
ture has more stability in the air than that of a normal
structure. TEM image, electron diffraction, and high-
resolution image confirmed TiO2 formed anatase struc-
tures and polycrystalline. A carrier mechanism of solar
cells with normal and inverted structures was discussed
based on energy diagram.
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