Japanese Journal of Applied Physics
Vol. 43, No. 4B, 2004, pp. 2352–2356
#2004 The Japan Society of Applied Physics
Effects of Different Materials Used for Internal Floating Electrode
on the Photovoltaic Properties of Tandem Type Organic Solar Cell
Kuwat T RIYANA, Takeshi Y ASUDA, Katsuhiko F UJITA and Tetsuo T SUTSUI
Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University,
Kasuga, Fukuoka 816-8580, Japan
(Received September 17, 2003; accepted November 29, 2003; published April 27, 2004)
Three thin heterojunctions sandwiched between indium tin oxide (ITO) and the top electrode as triple-heterojunction organic
solar cells have been fabricated. Each heterojunction cell consists of CuPc as a donor layer and perilene tetracrboxylic-bisbenzimidazole (PTCBI) as an acceptor layer. Ultra thin (1 nm average thickness) layers of Ag or Au have been inserted
between two heterojunctions as an internal electrode. Ag and Au were chosen as materials both for internal floating and top
electrodes. Influences of different deposition sequences of the organic layer in each heterojunction cell and different electrode
materials were also investigated. The optimum devices were obtained when the same material was used both as an internal
electrode and a top electrode. When the deposition sequence of the heterojunction is PTCBI/CuPc, the most suitable electrode
is Au and the ITO is negative relative to the top electrode. Meanwhile, Ag is suitable for an electrode when the deposition
sequence is CuPc/PTCBI. In this second deposition sequence, the ITO is positive relative to the top electrode. The open
circuit voltage (Voc ) of both optimum devices is on the order of 1.35–1.5 V. These values are approximately three times higher
than that in single-heterojunction organic solar cells. [DOI: 10.1143/JJAP.43.2352]
KEYWORDS: organic solar cell, internal floating electrode, top electrode, deposition sequence, polarity
1.
Introduction
Due to the potential for very-low-cost solar energy
conversion, organic solar cells based on organic thin films
have attracted much attention. Organic heterojunction cells
using small molecules of 3,4,9,10-perilenetetracrboxylic-bisbenzimidazole (PTCBI) and copper phthalocyanine (CuPc)
as an acceptor and donor respectively, which gave a high
conversion efficiency exceeding 1.0%, have been intensively
studied.1–4) A comprehensive review concerning the application of these materials as a solar cell has also been
presented by Peumans, et al.5) The locations of LUMO and
HOMO of CuPc have been reported to be at 3.5 eV and
5.2 eV, respectively, whereas for PTCBI, they were reported
to be at 4.5 eV and 6.2 eV, respectively.3,4) A single
heterojunction (HJ) solar cell consisting of the deposition
sequence of CuPc/PTCBI on an In2 O3 glass electrode with a
top Ag electrode (In2 O3 /CuPc/PTCBI/Ag) showed 1% of
power conversion efficiency (PCE) and 0.4 V of open circuit
voltage (Voc ) under simulated AM2 illumination.1) The
device with an opposite deposition sequence (PTCBI/CuPc)
on an indium tin oxide (ITO) electrode with a top Au
electrode (ITO/PTCBI/CuPc/Au) showed 1.8% of PCE and
0.50 V of Voc under 15 mW/cm2 xenon white light illumination.2) The PCE was reported to be enhanced up to an
order of 2.4% by incorporating an exciton-blocking layer
and light trapping to an ITO/CuPc/PTCBI/Ag device
between the PTCBI layer and a Ag top electrode.3) The
use of internal floating electrodes in organic solar cells was
proposed as early as 1991, although the PCE was rather
low.4) Further improvement of this approach was carried out
by employing an ultra thin Ag cluster between thin adjoining
HJ cells.6)
The ITO/CuPc/PTCBI/Ag-type cell produces a positive
voltage on the ITO electrode. On the other hand, the ITO/
PTCBI/CuPc/Au-type cell produced a negative voltage on
E-mail address: triyana9@asem.kyushu-u.ac.jp and triyana@ugm.ac.id
the ITO electrode. Thus it is expected that the use of a
different metal either for the internal floating electrode or the
top electrode may provide new features of tandem-type
organic solar cells. In this paper, we report our investigation
on the triple-heterojunction structure based on PTCBI and
CuPc. Two electrode materials, Ag and Au in addition to the
ITO electrode were chosen both as an internal floating
electrode and a top electrode. Ag and Au were chosen as
representative of electrodes which have a work function
lower and higher than that of ITO, respectively. Effects of
the internal floating and top electrode material on photovoltaic characteristics, effects of the location of internal
floating electrode inside the devices on the photovoltaic
characteristics, and effects of the deposition sequence of
organic layers in the individual HJ cells on the polarity of the
device output voltage were studied. In addition, we also
investigated the effect of PEDOT layer thickness on the
photovoltaic characteristics.
2.
Experimental
First, the precleaned ITO substrate was coated with a
polyethylene-dioxythiophene (PEDOT:PSS, Baytron 4083
without modification) layer by spin coating (at 4000 rpm for
40 s), followed by 30 min drying at 110 C. PTCBI and CuPc,
were purified by train sublimation. The PTCBI and CuPc
layers were deposited by thermal evaporation in a vacuum of
about 2 106 Torr, at a rate of 1–2 A/s.
For the purpose of
simplification, we use the notations, type-A and type-B
devices based on the deposition sequence of the organic
layer: The deposition sequence of the HJ cell for type-A
(CuPc/PTCBI) starts with the CuPc layer, followed by the
PTCBI layer, while for the type-B (PTCBI/CuPc), it starts
with the PTCBI layer, followed by the CuPc layer. In the
cases for the fabrication of multiple HJ cells, an ultrathin
(1 nm) Ag or Au layer is inserted as an internal floating
electrode between each single HJ cell. The triple-HJ cells
were fabricated by repeating the deposition of individual-HJ
three times. The top electrode (Ag or Au) was deposited
2352
Jpn. J. Appl. Phys., Vol. 43, No. 4B (2004)
K. TRIYANA et al.
0.004
CuPc
IFE
CuPc
PTCBI
ORGANIC
Type-A
Absorbance triple-HJ without IFE
Absorbance triple-HJ with IFE
Absorbance CuPc
Absorbance PTCBI
ORGANIC
ITO/PEDOT
Type-B
ITO/PEDOT
PEDOT
ITO
GLASS
TE: Top Electrode
light
IFE: Internal Floating Electrode
Fig. 1. Structure of type-A (CuPc/PTCBI) and type-B (PTCBI/CuPc)
organic solar cells based on the deposition sequence of organic layers
(IFE: internal floating electrode, TE: top electrode).
0.4
0.003
Photocurrent
0.3
0.002
0.2
0.001
Absorbance (arb. units)
PTCBI
2
ORGANIC
IFE
0.5
ORGANIC
Photocurrent (mA/cm )
Deposition sequence
TE
2353
0.1
0
450
500
550
600
650
700
0
750
Wavelength (nm)
through a shadow mask, so that the active area of the devices
was 2 2 mm2 . We also fabricated similar devices with a
thinner PEDOT layer at 6000 rpm for 60 s for comparisons.
For the measurement of photocurrent action spectra, an
incident light of a Xenon lamp passing through a monochromator was irradiated onto the device from the ITO
electrode side without any applied bias. The current-voltage
characteristics of the devices were measured with a Keithley
238 source-measure unit under conditions of both darkness
and illumination of 100 mW/cm2 white light provided from
a 1.5 AM solar simulator in the air ambient at room
temperature. Forward bias is defined as positive voltage
applied to the ITO electrode in the type-A device, and to the
top metal electrode in the type-B device. During the
measurement, a positive voltage was applied to the ITO
electrode for type-A device, while a negative voltage was
applied to the ITO electrode for type-B devices. Figure 1
shows two structures of type-A and type-B devices based on
the deposition sequence of organic layers.
3.
Results and Discussion
In order to investigate whether the absorption spectra
changed or not by depositing multiple layers and by inserting a metal internal floating electrode, the total thickness of
the organic layers was kept constant at 100 nm (50 nm each
for CuPc and PTCBI). Beginning from the single-HJ device,
with type B structure for example, ITO/PEDOT/PTCBI
(50 nm)/CuPc(50 nm)/Au(20 nm), the structure of triple-HJ
devices are as follows: ITO/PEDOT/CuPc(12 nm)/PTCBI
(12 nm)/Ag(1 nm)/CuPc(13 nm)/PTCBI(13 nm)/Ag(1 nm)/
CuPc(25 nm)/PTCBI(25 nm)/Ag(40 nm), and ITO/PEDOT/
PTCBI(12 nm)/CuPc(12 nm)/Au(1 nm)/PTCBI(13 nm)/CuPc
(13 nm)/Au(1 nm)/PTCBI(25 nm)/CuPc(25 nm)/Au(20 nm)
for type-A and type-B, respectively. All devices with the
same total thickness of the organic layer showed no change
in absorption spectra although an internal floating electrode
was inserted. It can be concluded that the contribution to the
absorption spectra of an internal floating electrode is
negligible compared to the contribution of organic layers.
Although the photocurrent action spectrum of the triple-HJ
cells was slightly different from that of the single-HJ cell
reported before,1) it may reflect the molecular arrangement
Fig. 2. Photocurrent action spectrum of triple-HJ type-B organic solar cell
and absorption spectra of organic layer incorporating Au internal floating
electrode of triple-HJ cells.
difference in the laminated very thin films. Here, an internal
floating electrode contributes to the photocurrent rather than
absorption of incident light. Figure 2 illustrates these two
types of spectra.
The calculation of the overall energy conversion efficiency, PCE, has been performed using the equation
PCE ¼
Voc Jsc FF
Pinc
where Voc , Jsc , FF, and Pinc are the open circuit voltage, the
short circuit current density, the fill factor and the density of
incident light power, respectively. We determine the value
of the fill factor of a device, FF, from the point (Vmax , Jmax )
in the 4th quadrant of the J-V characteristics with the
maximum electrical power according to
FF ¼
Vmax Jmax
:
Voc Jsc
Figure 3 shows the current-voltage characteristics of typeA devices. The PCE s and FFs of such single-HJ device
were 0.95% and 0.33 respectively (with Ag top electrode)
and 0.14% and 0.23, respectively (with Au top electrode),
while Voc remained unchanged, 0.45 V. In this case, Ag (or
metal with the workfunction less than that of ITO electrode)
is suitable as an electrode of type-A device. The PCE, FF
and Voc of the triple-HJ device were 1.86%, 0.43 and 1.5 V,
respectively for type-A device when both the internal
floating and top electrodes are Ag ones. These parameters
decrease to 0.67%, 0.23 and 0.75 V for PCE, FF and Voc
respectively when the internal floating electrode is a Au one,
while the top electrode is a Ag one.
Figure 4 shows the current-voltage characteristics of typeB devices. The PCEs, FFs and Voc s of such single-HJ
devices were 1.1%, 0.42 and 0.5 V, respectively (with Au
top electrode) and 0.26%, 0.19 and 0.35 V respectively (with
Ag top electrode). In this case, Au (or metal with the work
function more than that of ITO electrode) is suitable as an
electrode of type-B device. The PCE, FF and Voc of the
2354
Jpn. J. Appl. Phys., Vol. 43, No. 4B (2004)
K. TRIYANA et al.
0
−2
−4
−6
5
0
0.2
0.4
0.6
Bias voltage (V)
ITO/PEDOT/D/A/Ag/D/A/Ag/D/A/Ag
ITO/PEDOT/D/A/Au/D/A/Au/D/A/Ag
D: CuPc
A: PTCBI
0
-5
-0.5
ITO/PEDOT/A/D/A/D/A/D/Au
ITO/PEDOT/A/D/Au/A/D/A/D/Au
ITO/PEDOT/A/D/A/D/Au/A/D/Au
ITO/PEDOT/A/D/Au/A/D/Au/A/D/Au
2
2
10
ITO/PEDOT/D/A/Au
ITO/PEDOT/D/A/Ag
2
Current density (mA/cm )
Current density (mA/cm2)
3
2
Current density (mA/cm )
15
0
1
0.5
1.5
Bias voltge (V)
1
0
D: CuPc
A: PTCBI
−1
−2
−3
−4
−0.5
0
0.5
1
1.5
Bias voltage (V)
Fig. 3. Current density vs voltage characteristics of type-A device (tripleHJ). D is donor layer (CuPc), and A is acceptor layer (PTCBI). (Inset):
Current density vs voltage characteristics of type-A device (single-HJ).
2
Current density (mA/cm )
4
2
ITO/PEDOT/A/D/Ag
ITO/PEDOT/A/D/Au
2
2
6
Current density (mA/cm )
8
0
−2
−4
−6
0
0.2
0.4
0.6
Bias voltage (V)
0
D: CuPc
A: PTCBI
-2
ITO/PEDOT/A/D/Au/A/D/Au/A/D/Au
ITO/PEDOT/A/D/Ag/A/D/Ag/A/D/Au
-4
-0.5
0
0.5
1
1.5
Bias voltge (V)
Fig. 4. Current density vs voltage characteristics of type-B device (tripleHJ). D is donor layer (CuPc), and A is acceptor layer (PTCBI). (Inset):
Current density vs voltage characteristics of type-B device (single-HJ).
triple-HJ devices were 1.61%, 0.38 and 1.35 V for type-B
device when both the internal floating and top electrodes
were Au ones. These parameters decrease to 0.61%, 0.36 and
0.85 V for PCE, FF and Voc , respectively when the internal
floating electrode is a Ag one, while the top electrode is a Au
one.
The current-voltage characteristics of four different typeB devices fabricated with different locations of the internal
electrode are shown in Fig. 5. The location of the internal
electrode inside these devices affects their current-voltage
characteristics such as short circuit current density (Jsc ), Voc
and PCE. The first device being considered is a device
without any internal electrode, where the Jsc , Voc , FF and
PCE are 1.05 mA/cm2 , 0.25 V, 0.27 and 0.07%, respective-
Fig. 5. Effect of different location of Au internal floating electrode on the
current density-voltage characteristics.
ly. By inserting the Au internal electrode between the first
and second HJ (first and third HJ correspond to the cell
closest to ITO and top electrode respectively), the Jsc , Voc ,
FF and PCE are improved to 1.81 mA/cm2 , 0.5 V, 0.28 and
0.25%, respectively. However, when the Au internal
electrode is inserted between the second and the third HJ,
the Jsc , Voc , FF and PCE are improved to 2.2 mA/cm2 ,
0.8 V, 0.33 and 0.57%, respectively. The last significant
improvement based on the location of the internal electrode
is when the Au internal electrode is inserted between each
HJ, so that the Jsc , Voc , FF and PCE are improved to
3.18 mA/cm2 , 1.35 V, 0.38 and 1.61%, respectively.
Although generally the Voc of the multiple-HJ device is n
times higher than that of the single-HJ device, the current
density of multiple-HJ is lower than that of the single-HJ
device. This is because the series resistance increases by
forming of a single-HJ in a multiple-HJ. Moreover,
absorption of incident light by the front HJ cell results
attenuation of light in the back cell. This decreases the
terminal current density of multiple-HJ devices. In order to
overcome this problem, we fabricated similar devices with a
thinner PEDOT layer and reduced the thickness of CuPc
layer in each HJ cell. The PEDOT layer was spin coated at a
higher speed and time (at 6000 rpm for 60 s). As a result, the
Vocs remained unchanged, but the Jsc s increased, so that the
PCE was 2.37% and 2.27% for type-A and type-B devices,
respectively (Fig. 6). The fill factors of these devices are
0.34 and 0.36 for type-A and type-B devices, respectively.
It has been assumed from the single-HJ cells consisting of
such organic material, that the absorption of light by both
PTCBI and CuPc layers creates excitons, which can diffuse
and dissociate to be electrons and holes at the interface
between these two organic layers. Thus, upon dissociation of
the excitons at the interface, the holes are collected by the
electrode in contact with the hole transporting layer (CuPC),
while the electrons are transported through the electron
transporting layer (PTCBI) towards the electrode in contact
Jpn. J. Appl. Phys., Vol. 43, No. 4B (2004)
K. TRIYANA et al.
dark-ITO/PEDOT/D/A/Ag/D/A/Ag/D/A/Ag
4
illuminated-ITO/PEDOT/D/A/Ag/D/A/Ag/D/A/Ag
illuminated-ITO/PEDOT/A/D/Au/A/D/Au/A/D/Au
2
Current density (mA/cm )
dark-ITO/PEDOT/A/D/Au/A/D/Au/A/D/Au
2
0
D: CuPc
-2
A: PTCBI
-4
-6
-0.5
0
0.5
1
1.5
Bias voltage (V)
Fig. 6. Current density vs voltage characteristics of triple-HJ organic solar
cells. D is donor layer (CuPc), and A is acceptor layer (PTCBI). PEDOT
layer and organic layers in each HJ cell were prepared thinner than those
of previous devices, under 1 sun, 1.5 AM solar simulator.
with the PTCBI layer. This occurs because the CuPc is
always positive with respect to the PTCBI.1,2) Meanwhile,
from the Fermi level pinning phenomenon of single HJ
cells,7,8) and assuming that the mechanism of this phenomenon is dominant for the contact formation between the
organic layer and top electrode, Ag (because of a low work
function) favors an ohmic contact to PTCBI (electron
transporting layer). On the other hand, Au (because of a
higher work function) favors an ohmic contact with the
CuPc (hole transporting layer). Therefore, in the case of our
devices, Ag is favorable for the top electrode for device
type-A, while Au is favorable for the top electrode for
device type-B (Figs. 3 and 4).
For the triple-HJ device, type-B for example, with an
internal floating electrode, upon absorption of photons by
each organic layer, excitons are generated in each organic
layer, followed by exciton diffusion to the HJ interface, and
subsequent charge separation in this interface. Holes
generated in the third HJ and electrons generated in the
first HJ are collected directly by Au and ITO electrodes,
respectively. Meanwhile, the electrons and holes generated
in the second HJ migrate towards the first and second
internal electrodes, respectively. The first internal electrode
corresponds to the internal electrode closest to the ITO layer.
At the same time, electrons generated in the third HJ and
holes generated in the first HJ migrate towards the nearest
internal electrode.
At the internal electrode, alignment of Fermi levels with
adjacent cells is assumed.4–6,9) When the work function of
the internal electrode is different from that of the top
electrode, alignment of the Fermi level at both electrodes is
essentially not possible, so that the total of Voc is always
lower than that when the internal floating and top electrodes
are of the same material.
By choosing the appropriate thickness of each organic
layer, a balance of the photoresponse can be obtained to
achieve high efficiency. However, the number of excitons, in
2355
principle, generated in each layer is not the same in actual
devices. Due to the differences in the number of excitons
generated, some electrons recombine with holes in an
internal floating electrode, but excess electrons are assumed
to remain near the internal electrode. The internal floating
electrode should act as a location where hole and electron
currents are connected without potential loss due to alignment of the Fermi levels with adjacent organic layers even
though partial loss of current occurs owing to an unbalance
of exciton production in each HJ. A similar explanation can
be applied for the mechanism of type-A devices.
If the Voc of multiple-HJ devices is equal to the sum of the
Voc of a single-HJ device, the Voc of such triple-HJ devices,
both type-A and type-B, should be three times higher than
that of the single-HJ.6) However, care must be taken when
stacking two HJs in series, because an inverse HJ at the
stacking interface may be formed. In this case, the Voc of
triple-HJ does not increase but decreases because of lack of
alignment of Fermi levels between HJ cells. Moreover, it
also causes pile-up of carriers at interfaces. By inserting an
internal electrode at one location between two HJ cells, both
Voc and photocurrent increased. In principle, device performance does not depend on the inserted position of the
internal electrode. However, because of the difference in
organic layer thickness, the insertion between the second and
the third HJ yielded higher Jsc and Voc .
Due to a high degree of roughness of the bare ITO
substrate, planarization using the PEDOT layer is needed to
avoid short circuiting of devices. However, the use of a
PEDOT layer increases the series resistance of devices, or
decreases the current density of device.10) The balance of
light absorption by each HJ cell is also important, so that the
thickness of organic layers should also be rearranged to find
the optimum condition. For this purpose, we fabricated the
following two devices:
ITO/PEDOT/CuPc(10 nm)/PTCBI(10 nm)/Ag(1 nm)/CuPc
(10 nm)/PTCBI(10 nm)/Ag(1 nm)/CuPc(20 nm)/PTCBI(25
nm)/Ag(40 nm) (type-A), and ITO/PEDOT/PTCBI(10 nm)/
CuPc(10 nm)/Au(1 nm)/PTCBI(10 nm)/CuPc(10 nm)/Au(1
nm)/PTCBI(25 nm)/CuPc(20 nm)/Au(20 nm) (type-B). The
performance of these two devices are comparable with those
of similar devices reported by Yakimov and Forrest.6)
4.
Conclusions
The choice of a suitable electrode is correlated with the
deposition sequence of the donor/acceptor organic layer. In
the case of our results, Ag (metal with lower work function
than that of ITO) is suitable for the electrode of type-A
device. Au (metal with higher work function than that of
ITO) is suitable for the electrode of type B device. The use
of different materials between internal floating and top
electrodes decreases the Voc and/or PCE of multiple-HJ
devices. Lastly, the use of a thinner PEDOT layer combined
with optimized organic layer thickness enhances the performance of devices.
1) C. W. Tang: Appl. Phys. Lett. 48 (1986) 183.
2) T. Tsustui, T. Nakashima, Y. Fujita and S. Saito: Synth. Met. 71
(1995) 2281.
3) P. Peumans, V. Bulovic and S. R. Forrest: Appl. Phys. Lett. 76 (2000)
2650.
2356
Jpn. J. Appl. Phys., Vol. 43, No. 4B (2004)
4) M. Hiramoto, M. Suezaki and M. Yokoyama: Chem. Lett. (1990) 327.
5) P. Peumans, A. Yakimov and S. R. Forrest: J. Appl. Phys. 93 (2003)
3693.
6) A. Yakimov and S. R. Forrest: Appl. Phys. Lett. 80 (2002) 1667.
7) C. J. Brabec, A. Cravino, D. Meissner, N. S. Sariciftci, T. Fromherz,
M. T. Ripens, L. Sanchez and J. C. Hummelen: Adv. Funct. Mater. 11
K. TRIYANA et al.
(2001) 374.
8) C. Shen and A. Khan: Org. Electron. 2 (2001) 89.
9) G. G. Milliaras, J. R. Salem, P. J. Brock and J. C. Scott: J. Appl. Phys.
84 (1998) 1583.
10) T. Aernouts, W. Geens, J. Pooertmans, P. Heremans, S. Borghs and R.
Mertens: Thin Solid Films 403 (2002) 297.