Applied Physics Express 7, 021602 (2014)
http://dx.doi.org/10.7567/APEX.7.021602

Microscopic description of the current–voltage characteristics of a bulk-heterojunction
organic solar cell under illumination
Osamu Takeuchi1, Noriaki Takeuchi1†, Takahiro Ochiai1,2, Hirokazu Kato1, Shoji Yoshida1, and Hidemi Shigekawa1
1

Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
TAKANO Co., Ltd., Miyada, Nagano 399-4301, Japan

2

Received October 1, 2013; accepted December 28, 2013; published online January 28, 2014
Measurement of the local current–voltage (J–V) characteristics of a MDMO-PPV:PCBM bulk-heterojunction (BHJ) solar cell by using lightmodulated scanning tunneling spectroscopy (LM-STS) revealed that the open-circuit voltage (VOC) differs from one region to another.
Consequently, when the bias voltage is set near VOC, the generated photocurrent in some regions is internally lost in other regions throughleakage. In the same manner, the external J–V characteristics of a BHJ solar cell device results from numerous parallelly connected local solar
cells. LM-STS allows us to interpret the external J–V characteristics of a device in terms of the characteristics of the local solar cells.
© 2014 The Japan Society of Applied Physics

rganic solar cells have attracted considerable interest
from scientists because they are predicted to be cost
efﬁcient, resource saving, lightweight, and mechanically ﬂexible.1–7) Compared to conventional crystalline Si
solar cells, they have many more inhomogeneous structures.
Thus, to improve their generation efﬁciency and lifetime, it isimportant to understand their external characteristics from a
microscopic viewpoint. For instance, in the equivalent circuit
model of a conventional solar cell, a current source, which
generates photocurrent under illumination, is connected inparallel to an internal diode and a leakage resistor as well as
to an output resistor in series. When no external circuit isconnected, the generated photocurrent increases the device
voltage until the leakage current through the internal diodeand resistor balances the generation. In more sophisticatedmodels, additional components are introduced to account forthe deviation of the experimentally observed characteristics
from the simplest model.8–12) As has been suggested in manystudies,13–17) however, the microscopic characteristics of anorganic solar cell vary from one region to another within thedevice. Thus, if one takes this inhomogeneity into account, theequivalent circuit model should be much more complex. In
such a model, numerous small solar cells, the characteristicsof which slightly differ from each other, are all connected inparallel. To obtain such a microscopic view of an organic solarcell, in this study, we investigated the local characteristics of abulk-heterojunction (BHJ) type solar cell18–20) by applying
scanning tunneling microscopy (STM) and light-modulated
scanning tunneling spectroscopy (LM-STS).21,22) The generation of photocurrent and the internal leakage of the generated current are discussed in terms of the local current–
voltage (J–V) characteristics under illumination.
The sample was based on a soluble fullerene derivative
1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61 (PCBM) as
an electron acceptor and poly[2-methoxy-5-(3A,7A-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) as a donor,
which was prepared as follows. First, as a hole transportlayer, PEDOT-PSS23) was spin-coated on an indium tin oxide(ITO) thin ﬁlm on a glass substrate (TOYOBO 300R) andannealed at 140 °C in air for 10 min. Then, a toluene solutionof an MDMO-PPV:PCBM mixture [Fig. 1(a)] at a concentration of 3 mg/mL was spin-coated in a dry-nitrogenenvironment to form the BHJ active layer. STM and STS

O†

Present address: Tokyo Electric Power Company.

(a)

(b)
PCBM

O

n
MeO

MDMO-PPV(c)(d)
Acceptor Molecule
Donor Molecule

STM Tip
Illumination

Metal Electrode
MDMO-PPV +PCBM blend
PEDOT-PSS
ITO
Glass

Illumination

Fig. 1. Schematics of the sample structure. (a) PCBM and MDMO-PPV
molecules. (b) STM topography measured on the MDMO-PPV:PCBM blend
layer. (c) Ordinary BHJ organic solar cell setup. (d) BHJ organic solar cell in
the present study.

measurements (Omicron VT-STM) were performed at room
temperature in an ultrahigh-vacuum (UHV) condition byusing an electrochemically etched tungsten tip that wasannealed by electron bombardment in an ultrahigh vacuumfor oxide removal. An STM image of the topography of
the above-noted sample under illumination is shown in
Fig. 1(b). In agreement with the literature,13,14) the PCBMrich domains with circular or elliptical shapes were selforganized in the MDMO-PPV-rich matrix. The lateral diameters of the PCBM-rich domains ranged from 50 to 150 nm,
and one of them is indicated by the boxed “1” in the ﬁgure. In
contrast, in the MDMO-PPV-rich matrix area, small clusters
with diameters of a few tens of nanometers are imaged, as
seen in the boxed area marked “2”. Compared to the ordinary
solar cell structure shown in Fig. 1(c), in this setup, theplaner metal electrode that gathers electrons from the solar
cell is replaced by the STM tip, as shown in Fig. 1(d). Thus,
the STM tip only gathers the locally generated electrons
beneath the tip via the tunnel junction. Another difference isthat the illumination is from the side of the STM tip in thepresent study, whereas it is from the side of the transparent
substrate in the ordinary solar cell setup.

021602-1© 2014 The Japan Society of Applied Physics

Appl. Phys. Express 7, 021602 (2014)

1.5
1.0

(b)
Raw Current
Dark Current
Illuminated Current
Photovoltage

0.5
0.0

0.1

VOC

Photocurrent

-0.1

-0.5
0.4

JSC

-0.5
Photocurrent
Photovoltage

2

0.0

0

-0.4

-2

-0.8

-4

Photovoltage [V]Photocurrent [nA]

Tunnel Current [nA](a)O. Takeuchi et al.

0.0

0.5

1.0

1.5

(c)
Lens

STM
UHV Chamber

Laser (400 nm)

-3

-2

-1
0
1
Bias Voltage [A]

2

3

Micro
Computer

STM
Electronics

Fig. 2. Typical LM-STS result and setup. (a) Raw J–V curve in chopped illumination and calculated curves from the raw curve. (b) Magniﬁed plot of (a).
(c) LM-STS setup. The solar cell sample is intermittently illuminated synchronously to STM and STS measurement.

100
Photocurrent @ -2V [pA]-20

80
Tunnel Current [pA]

For LM-STS measurement, the sample was illuminated bya violet laser (­ = 400 nm) of ³5 mW, which was electricallychopped synchronously with the STM scan under the control
of a microcomputer, as illustrated in Fig. 2(c). The laser
beam is focused to a diameter of a few tens of micrometers on
the sample surface. A typical LM-STS result is shown in
Fig. 2(a). During the J–V curve measurements with a bias
sweep velocity of 8 V/s, the laser illumination is chopped at
100 Hz. Thus, as seen in the magniﬁed plot in Fig. 2(b), theraw J–V curves (shown in red) oscillate with large amplitudes. To retrieve the current values under illuminated anddark conditions, the tunnel current in each dark and illuminated half-period is averaged, excluding the transient regions(10% of the period). The averaged values are indicated by theblue (dark) and green (illuminated) markers. By smoothly
tracing these points, dark and illuminated local J–V curves
can be simultaneously obtained from one raw J–V curve.
Once the two curves are obtained, the photoinduced current(photocurrent) and the photovoltage can be obtained from thevertical and horizontal separation of the two curves, as
illustrated in Fig. 2(a).
When measured on a solar cell, the dark J–V curves exhibit
rectifying characteristics [the blue curve in Fig. 2(a)] becauseof the existence of the p–n junction in the sample. Almost
negligible current ﬂows when a negative sample bias voltage,i.e., a backward bias voltage, is applied, whereas the current
increases rapidly as the positive bias voltage, i.e., the forward
bias voltage, increases. Under illumination, ﬁnite current
ﬂows at zero bias voltage, as shown in Fig. 2(b). This current
is referred to as the short-circuit current (JSC). The photocurrent decreases when a forward bias voltage is applied. The
bias voltage at which the photocurrent becomes zero is called
the open-circuit voltage (VOC). In between these two points,
the current under illumination ﬂows against the bias voltage;i.e., this region is the solar cell’s working condition.
In an LM-STS measurement, local regions of the solar
cell are biased via a tunnel junction. Thus, if the tunnel
conductance is smaller than that of the solar cell, the observed
current is determined mainly by the tunnel conductance rather
than by the properties of the solar cell. To ensure that thetunnel conductance is large enough for measurement, we

60
40

-15
-10
-5
0
0
20 40 60 80 100
Dark Current @ +3V [pA]20
0

Raw Current
Dark Current
Illuminated Current

-20
-3

-2

-1
0
1
BiasVoltage [V]23

Fig. 3. Dependence of LM-STS result on tip–sample distance measured
on a PCBM-rich cluster. The inset shows the dependence on the tunnel
conductance under the dark condition.

placed the STM tip on a PCBM-rich cluster and conducted
LM-STS measurements with varying tip–sample distances.
The result is shown in Fig. 3. Reﬂecting the variation oftip–sample distance, the dark and illuminated current underpositive bias voltage varied by as much as dozens of times,
whereas the illuminated current under negative bias voltage
does not change as much. Under positive bias voltage, thep–n junction between MDMO-PPV and PCBM is forwardly
biased. Thus, the tunnel conductance becomes relatively
small and governs the total conductance. In contrast, undernegative bias voltage, no dark current is observed because of
the reversely biased p–n junction. When illuminated, photogenerated carriers are collected by the STM tip and a ﬁnite
current ﬂows. Thus, this condition is called the photoamperic
regime.24) When the internal resistance of the solar cell andthe tunnel resistance are both small, a small bias voltage isneeded to collect most of the generated electrons by using
the STM tip. Thus, the photocurrent saturates at small bias
voltage. When the internal resistance is large, the voltage
drop at the resistance limits the current. Thus, the photocurrent increases linearly with the bias voltage. The linear
dependence of the photocurrent illustrated in Fig. 3 corresponds to this situation.

021602-2© 2014 The Japan Society of Applied Physics

Appl. Phys. Express 7, 021602 (2014)

O. Takeuchi et al.

0

−20

−60

Dark Current [pA]

−40

−80

Fig. 4. Dark-current distribution with a sample bias of ¹3.5 V overlappedon an STM topographic image measuring 500 © 500 nm2 with a feedback
condition of 4 V and 1.5 nA. The yellow ellipses are drawn arbitrarily to
guide the eye.

If tunnel resistance is not negligible, the gradient of
the photocurrent will be further decreased depending on thetip–sample distance. Because the negatively biased region
against VOC is the working condition of the solar cells, it
is favorable to have tunnel resistance be negligible in thisregion. To ﬁnd the condition, the photocurrent at ¹2 V isplotted against the dark current at +3 V in the inset of Fig. 3.
One can see that the photoconductance depends on the tunnel
conductance only when the dark current at +3 V is smaller
than 30 pA. Hereafter, we choose the tunnel condition sothat a dark current of ³500 pA ﬂows at +3 V to maintainnegligible tunnel resistance.
As shown in Fig. 4, 1024 J–V curves at 32 © 32 grid
points were measured during the measurement of an STM
topographic image with an area of 500 © 500 nm2. The curve
shown in Fig. 2(a) was measured in the middle of the lowerright yellow ellipse. For the LM-STS measurement, the tip–
sample distance was regulated by a feedback condition of 4 V
and 1.5 nA in the dark condition. In Fig. 4, some ellipticalregions (which are indicated by the yellow ellipses) can be

distinguished in the dark-current distribution. The coloredgrid points in Fig. 4 show the dark current at ¹3.5 V. As
discussed above, when the tip is on the PCBM-rich domains,almost no current ﬂows owing to the rectiﬁcation by the p–njunction at the MDMO-PPV:PCBM interface. When the tip ison the MDMO-PPV-rich matrix, the Schottky barrier possibly
generated between the tip and the sample can also exhibit
rectiﬁcation in the same direction. In the present case, however, the barrier is not thick enough to prevent electron tunneling through the barrier. As a result, ﬁnite current is observed
when the tip is not on the elliptic PCBM-rich domains.
Figure 5 shows the photocurrent distribution as a functionof the sample bias voltage in the same area shown in Fig. 4.
Figure 5(a) corresponds to the JSC mapping. As expected, theregions with large JSC roughly coincide with the yellow
elliptical regions, where the clear rectiﬁcation characteristic
was observed. Upon closer inspection, however, one observes that some parts in the yellow ellipses do not exhibit aslarge a JSC value as other parts. We interpret this as due tohigher internal resistance in such regions. This point will be
discussed in a future publication. The lower part of the lowerright cluster and the upper part of the middle-left cluster
exhibited the largest JSC. Though the magnitude varied
depending on position, almost all the points are colored inblue, i.e., generating photocurrent at zero bias voltage. When
forward bias is applied, the amount of generated photocurrent
decreases. At 0.4 V, some regions are colored in red, whichindicates current in the opposite direction, i.e., the leakage
current. At 0.6 V, almost half of the surface region generates
current and the other half loses current through leakage. Thisis because the local VOC is above 0.6 V in the former region
and is below 0.6 V in the latter region. Interestingly, the lower
part of the lower-right yellow elliptical region now starts
leaking current even though this part exhibited the highest
JSC in Fig. 5(a). At 0.8 V, some portion of the sample still
generates a small photocurrent while the leakage current inother parts overcomes this generation. At 1.0 V, almost all theregions are colored in red.

(a) 0.0 V(b) 0.2 V(c) 0.4 V(d) 0.6 V(e) 0.8 V(f) 1.0 V

60
40

0
−20
−40

Photocurrent [pA]

20

−60

Fig. 5. Sample bias dependence of the photocurrent distribution in the same area as that of Fig. 3. The blue and red colors correspond to photocurrent
generation and leakage, respectively.

021602-3© 2014 The Japan Society of Applied Physics

Appl. Phys. Express 7, 021602 (2014)

Photocurrent [nA]20

O. Takeuchi et al.

Total Current
Generated Current
Leakage Current

10
0
-10
-20
0.2

0.4
0.6
0.8
Sample Bias Voltage [V]1.0

Fig. 6. Total current decomposed into generated and leakage current.

Next, we discuss the macroscopic J–V curve of the devicein terms of the observed microscopic J–V curves of thesample. As illustrated in Fig. 1(d), the STM tip collects onlya small portion of the generated electrons in the device,whereas the planar metal electrode of an ordinary solar cell
device collects all of them, as seen in Fig. 1(c). Thus, we canestimate the total amount of collectable carriers in the device
by summing the observed current at different positions. The
green curve in Fig. 6 shows the total photocurrent in thewhole region observed in Fig. 4 against the bias voltage. This
total current is calculated by simply summing the current
values measured at each grid point. The generated current(blue) and leakage current (red) are also plotted; these arecalculated by summing the current values with negative andpositive polarity separately. Here, the negative current corresponds to photocurrent generation. Thus, below 0.61 V, thiswhole region generates current, whereas, above this bias
voltage, leakage current overcomes the generation. At 0.61 V,a photocurrent of 2.2 nA is internally generated, but exactly
the same amount of leakage current ﬂows in the other part.
Consequently, no current ﬂows out of the device; i.e., thispoint corresponds to the macroscopic VOC.
Let us now brieﬂy discuss the validity of our observation.
Currently, the LM-STS result strongly depends on the individual difference of STM tips. It is known that the generation
efﬁciency of a solar cell depends strongly on the work
function of the metal electrode. Improving the reproducibilityof LM-STS measurements is currently under investigation bydepositing low-work-function materials on the tip apex under
UHV. Regarding the “total current”, the photocurrent measured by an STM tip ﬂows through a very localized area of<0.1 nm2 at the tunnel junction. As pointed out in a previous
study,25) however, the current in the sample spreads over a
wider area of 10–100 nm depending on the shape of the
tip apex and thickness of the ﬁlm. Thus, the area where thegenerated carriers are collected from a single grid point
overlaps the area where the generated carriers are collectedfrom neighboring grid points. Consequently, the total photocurrent plotted in Fig. 6 is overestimated because of multiple
counting of the generated photocurrent. Another concern is
the inner resistance. Because the photocurrent measurement
by STM forces photocarriers that are generated in 10–100 nm
to ﬂow through a very small region beneath the STM tip,

the internal resistance is overestimated in comparison withreal devices. This point will be discussed in detail in a future
publication. Even with these limitations, this kind of microscopic measurement fortiﬁes our understanding of the determination of macroscopic characteristics of a solar cell fromthe microscopic view point and will help improve theirperformances.
In summary, we investigated the microscopic J–V characteristics of a BHJ organic solar cell composed of an
MDMO-PPV:PCBM blend by using LM-STS. Elliptic
PCBM-rich domains are clearly distinguished by the dark
J–V characteristics and they exhibited larger JSC values than
the surrounding MDMO-PPV-rich area. When a forward bias
voltage is applied, some regions of the sample start leaking
the current that is generated in the other regions. The bias
voltage at which all the generated current is lost throughleakage corresponds to the macroscopic VOC. The macroscopic J–V characteristics can be understood as the generated
photocurrent in a part of the device minus the leakage current
in other parts of the device.
Acknowledgments The authors appreciate the fruitful discussion with Dr.
Yasuda of the National Institute for Materials Science, Japan. This work waspartially supported by a Grant-in-Aid for Scientiﬁc Research from the Ministry of
Education, Culture, Sports, Science and Technology.

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021602-4© 2014 The Japan Society of Applied Physics