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Analysis of the Photovoltaic Properties of Single-Walled Carbon Nanotube/Silicon
Heterojunction Solar Cells
Daichi Kozawa1, Kazushi Hiraoka2, Yuhei Miyauchi1;3, Shinichiro Mouri1, and Kazunari Matsuda1
1Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan2Hitachi Zosen Corporation, Osaka 551-0022, Japan
3PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan
Received February 17, 2012; accepted March 12, 2012; published online April 9, 2012
We studied the photovoltaic properties of single-walled carbon nanotube/Si (SWNT/Si) heterojunction cells. We observed an optimal thickness of
the SWNT network film that maximizes the photovoltaic conversion efficiency. The spectra of incident photon to charge carrier efficiency indicate
that the production of carriers in the Si layer mainly contributes to the photovoltaic conversion. The experimental results and loss analysis based
on the equivalent circuit model suggest that the fabrication of a high-density semiconducting SWNT network at the interface of Si is the key to
improving the conversion efficiency of the SWNT/n-Si heterojunction solar cell. # 2012 The Japan Society of Applied Physics
Carbon nanotubes have attracted a great deal of
interest for photovoltaic applications due to their
excellent physical and electronic properties, includ-
ing a band gap that is widely tunable by controlling the tubediameter and high carrier mobility along their one-dimen-
sional axis.14) Recently, the heterostructures of carbon
nanotubes and Si have been extensively studied to realize
highly efficient photovoltaic cells.512) Because commonly
available semiconducting single-walled carbon nanotubes
(SWNTs) are unintentionally hole-doped (p-type semicon-
ductors),13) pn heterojunctions can be formed at the interface
of the SWNT network film and n-type Si, exhibiting diode
currentvoltage characteristics. The SWNT/n-Si pn hetero-
structure is therefore a model device of carbon nanotube solar
cells. In the heterojunction solar cells, microscopic photo-
voltaic conversion processes are complicated because the
SWNT network and Si layers are optically active and can
contribute to photovoltaic processes, including generation
of photocarriers, charge separation, and charge transport.
However, the roles of the SWNT network and Si layer in the
photovoltaic processes are not well understood. Thus, the
photovoltaic properties of SWNT/n-Si solar cells must be
clarified in order to improve the conversion efficiency.
In this letter, we report the photovoltaic properties of
SWNT/n-Si heterojunction solar cells. The photovoltaic
conversion efficiency strongly depends on the thickness of
the SWNT network and shows a maximum value at the
optimized thickness. We observed incident photon to charge
carrier efficiency (IPCE) spectra and found that most of thephotocarriers that contribute to the photovoltaic process are
generated in the n-Si layer. Moreover, detailed analysis of
currentvoltage curves based on the equivalent circuit model
showed that shunt loss and forward-bias current are the
dominant factors influencing power loss in the cell. These
findings provide important information for optimizing
SWNT/n-Si heterojunction solar cells.
For fabricating the heterojunction solar cell devices,
as shown in the inset of Fig. 1(a), CoMoCAT 6; 5-rich
SWNTs were dissolved in dimethylformamide (DMF, 0.5
mg/mL) and treated with a bath sonicator for 60 min and a tip
sonicator for 30 min. The solution was directly sprayed onto
n-type Si wafers and reference glass substrates by means of
an airbrush technique.6,14) A Si wafer, including a window of
a predeposited insulating oxide layer, and a glass substrate
were placed side by side on a heating platform so that the
resulting SWNT network films had the same thickness on
both substrates. The thickness of the SWNT network film was
controlled by varying the number of times the solution was
airbrushed. The fabricated SWNT/n-Si solar cell devices
were electrically contacted using an In and Ag paste.
Optical transmission spectra of the SWNT network films
on the glass substrates were measured using a UV/vis
spectrophotometer (Shimadzu UV-1800). The sheet resis-
tance of the SWNT network was obtained using a digital
multimeter (Keithley 2100) connected to a four-probe
collinear arrangement (Astellatech SR4-S). To perform
photovoltaic testing, the devices were irradiated under a
-10
0
10
CurrentDensityJ
(mA/cm2)
0.40.20
Voltage V (V)
= 2.4%
FF = 0.43VOC = 0.39 V
JSC = 14.6 mA/cm2
Light
Dark
SWNT
n-Si
(a)
300
200
100
0
CurrentDensityJ
(mA/cm
2)
1.20.80.40
Voltage V (V)
Rs = 56
Rsh = 2.0 k
Rs
Rsh Rl
DJph
Measured Curve
Fitted Curve
(b)
Fig. 1. (a) JV characteristics of a typical SWNT/n-Si cell under light
illumination (red solid line) and dark conditions (black solid line). The inset
shows a schematic of the device. (b) Experimental (black solid line) and
simulated (red dotted line) JV curves based on the equivalent circuit
model. The inset shows the schematic of the equivalent circuit model of a
device connected with load (Rl), series (Rs), and shunt (Rsh) resistances and
a diode (D).
E-mail address: [email protected]
Applied Physics Express 5 (2012) 042304
042304-1 # 2012 The Japan Society of Applied Physics
DOI: 10.1143/APEX.5.042304
http://dx.doi.org/10.1143/APEX.5.042304http://dx.doi.org/10.1143/APEX.5.042304http://dx.doi.org/10.1143/APEX.5.042304http://dx.doi.org/10.1143/APEX.5.042304 -
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solar simulator (San-Ei Electric XES-40S1) at AM 1.5
($100 mW/cm2), and current densityvoltage (JV) data
were recorded using a digital multimeter (Keithley 2400).
IPCE spectra were measured with monochromatic incident
light of1 106 photons/cm2 under 100 mW/cm2 (Bunkoh-
Keiki CEP-2000).
Figure 1(a) shows the typical JV characteristics of the
SWNT/n-Si heterojunction solar cell under light illumina-tion (red solid line) and dark (black solid line) conditions,
where the SWNT film has a transmittance T of about 70%
at 550 nm. The JV curve obtained under dark conditions
shows typical diode behavior. In contrast, the JV curve
exhibits typical photovoltaic behavior under white light
illumination. The short-circuit current density JSC, open-
circuit voltage VOC, fill factor FF, and photovoltaic con-
version efficiency were estimated as 14.6 mA/cm2, 0.39 V,
0.43, and 2.4%, respectively, which are consistent with
previously reported values for similar devices.14)
Figure 1(b) shows the JV curve of the device over a
wide voltage range under light illumination. The JV curvewas analyzed using the equivalent circuit model,1517) as
shown in the inset of Fig. 1(b). The total current density J in
the equivalent circuit model can be described as
J J0 expeV RsJ
nkBT
1
V RsJ
Rsh Jph; 1
where J0 is the reverse saturation current density, Rs and Rshare the series and shunt resistances, respectively, e is the
electron charge, kBT is the thermal energy, n is the diode
ideality factor, and Jph is the photocurrent density. The
broken line in Fig. 1(b) shows a curve fitted using eq. (1),
which reproduced the experimental JV curve. The series
and shunt resistances were obtained from this analysis, as
indicated in Fig. 1(b).
Figure 2 shows the conversion efficiency of SWNT/
n-Si solar cells (red circles) as a function of transmittance
T of the SWNT network film at 550nm. The photovoltaic
conversion efficiency strongly depends on the optical
transmission. Because the absorbance is proportional to
the number of SWNTs in a unit area of the network films,
the optical transmission can be used to monitor the film
thickness, assuming a constant SWNT density in a film. The
conversion efficiency significantly increases from $0:4 to
2.4% with increasing transmittance from 20 to $70% (i.e.,
decreasing thickness) and shows a characteristic maximumvalue around 70%. Above 70%, the conversion efficiency
decreases significantly with increasing transmittance. This
characteristic behavior provides important information
regarding photovoltaic conversion processes in SWNT/
n-Si solar cells, as discussed below.
We also measured IPCE spectra of the devices to further
examine the photovoltaic conversion processes. The inset
of Fig. 2 shows typical absorption spectra of the SWNT
network film on a glass substrate (blue line) and the IPCE
spectrum of the device (red line). The small structure of
the E11 transition band of 6; 5 SWNTs ($1000nm) was
observed in the absorption spectrum of the SWNT network
film. In contrast, the IPCE of the device diminishes near the
absorption edge of Si ($1100nm), and no peak feature was
observed at the E11 band of 6; 5 SWNTs. These results
indicate that the photocarriers generated in the Si layer
predominantly contribute to the photovoltaic conversion
processes.
The inset of Fig. 3(a) shows the sheet resistance measured
using the four-probe method for the SWNT network films
(squares). The sheet resistance is nearly constant below a
transmittance of $60% and increases significantly in the
transmittance range of 6090%. We also plotted the net series
E11SWNT/Si
Absorbance
403020100
IPCE(%)
1000500Wavelength (nm)
Absorba
nce
0.30.20.1
2.4
2.0
1.6
1.2
0.8
0.4
0
Efficiency
(%)
80604020
Transmittance T (%)
Fig. 2. Photovoltaic conversion efficiency as a function of transmittance T
of the SWNT network film at 550 nm (solid circles). The inset shows the
optical absorption spectrum of the reference SWNT network film and an
IPCE spectrum of the SWNT/n-Si heterojunction cell.
800
600
400
200
0
SeriesResistance()
80604020
Transmittance T (%)
20
10
0
SheetResistance(k/sq.)
80604020
Transmittance T (%)
(a)
Load
(D+Rsh) Loss
Rs Loss
60
40
20
0
PowerRatio(%)
80604020Transmittance T (%)
(b)
Fig. 3. (a) Series resistance calculated using the equivalent circuit model
(circles) as a function of transmittance. The inset shows experimentally
measured sheet resistance (squares). (b) Power ratios consumed in both the
shunt resistance and the diode, the series resistance, and the external load for
devices with different transmittances.
D. Kozawa et al.Appl. Phys. Express 5 (2012) 042304
042304-2 # 2012 The Japan Society of Applied Physics
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resistance Rs evaluated by fitting the JV curve for each
device (circles) in Fig. 3(a). The Rs behavior is consistent
with that of the sheet resistance of the SWNT network films,
confirming the validity of our JV curve analysis based on
the equivalent circuit model. The significant increase in Rsshould affect the photovoltaic conversion efficiency in the
transmittance range of 6090%, as discussed below.
The power loss of the devices can be derived from theequivalent circuit model.1517) Figure 3(b) shows the power
loss Ploss in both currents through shunt resistance Rsh (shunt
loss) and the diode D (diode loss), as well as the ratio of
obtained power in the external load as a function of T. The
power loss of the devices is dominated by the shunt and diode
losses, because the contribution of power loss in the series
resistance in Fig. 3(b) is negligible (
