OPEN
SUBJECT AREAS:
SOLAR CELLS
TWO-DIMENSIONAL MATERIALS
Received
21 July 2014
Accepted
12 February 2015
Published
11 March 2015
Correspondence and
requests for materials
should be addressed to
B.K.M. (bkmin@kist.re.
kr) or D.-K.L. (dklee@
kist.re.kr)
* These authors
contributed equally to
this work.
Monolithic DSSC/CIGS tandem solar
cell fabricated by a solution process
Sung Hwan Moon1*, Se Jin Park1,2*, Sang Hoon Kim3, Min Woo Lee4, Jisu Han4,5, Jin Young Kim4,
Honggon Kim4, Yun Jeong Hwang1, Doh-Kwon Lee4 & Byoung Koun Min1,5
1
Clean Energy Research Center, Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul, 136–791,
Republic of Korea, 2Department of Chemical and Biological Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul
136–713, Republic of Korea, 3Center for Materials Architecturing, Korea Institute of Science and Technology, Hwarang-ro 14-gil 5,
Seongbuk-gu, Seoul, 136–791, Republic of Korea, 4Photo-electronic Hybrids Research Center, Korea Institute of Science and
Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul, 136–791, Republic of Korea, 5Green School, Korea University, 145,
Anam-ro, Seongbuk-gu, Seoul 136–713, Republic of Korea.
Tandem architecture between organic (dye-sensitized solar cell, DSSC) and inorganic (CuInGaSe2 thin film
solar cell, CIGS) single-junction solar cells was constructed particularly based on a solution process.
Arc-plasma deposition was employed for the Pt interfacial layer to minimize the damage to the layers of the
CIGS bottom cell. Solar cell efficiency of 13% was achieved, which is significant progress from individual
single-junction solar cells (e.g., 7.25 and 6.2% for DSSC and CIGS, respectively).
T
andem architecture of solar cells, wherein two or more sub-cells with complementary absorption characteristics are stacked and connected either in series or in parallel, is one of the exciting approaches to increase the
efficiency effectively of solar cells beyond the Shockley-Queisser limit of single-junction devices1–3. For
example, dual- and triple-junction solar cells based on III–V compound semiconductor materials such as
InGaP/GaAs/InGaAs have been demonstrated to show over 37% power conversion efficiency (PCE)4.
Moreover, several other photovoltaic materials, including a-Si, CIGS, and various polymers, have also been
explored in order to fabricate efficient tandem solar cells5–8.
In addition to the tandem structure achieved with similar class materials, two substantially different solar
cell materials have also been incorporated into tandem solar cell construction. For example, tandem solar cells
fabricated with a-Si and a polymer have been demonstrated to show enhanced voltage and power conversion
efficiency compared to conventional single-junction solar cells9–11. Furthermore, the tandem architecture of a
DSSC with GaAs or polymer is reported to show high voltages of 1.8 and 1.36 V, respectively12,13. Recently,
Liska et al. and Wenger et al. suggested the possibility of a tandem type of architecture with DSSC and CIGS
based on mechanical stacking as well as a monolithic conjunction14,15. Such a tandem construction of DSSC
with CIGS seems to be an ideal design owing to the optical band-gaps of DSSC (,1.7 eV) and CIGS
(,1.1 eV), making them suitable for use as top and bottom cells, respectively (see Fig. 1). Furthermore,
CIGS offers the feasibility of tuning the band-gap by adept control of its composition. This property is
particularly beneficial for tandem solar cell applications. For example, by substituting In with Ga and/or
Se with S, the band-gap of CIGS can be gradually increased in the range between 1.0 (CuInSe2) and 2.4 eV
(CuGaS2)16. In addition, from the viewpoint of manufacturing costs, the DSSC/CIGS tandem solar cell would
be a competitive option, given the advantage of the economic viability of DSSCs prepared by solution
processes. However, to realize a further reduction in the cost of DSSC/CIGS tandem solar cells, it is necessary
to develop low-cost and high-throughput solution processing methods (e.g., printing) for the fabrication of
the bottom CIGS cell, which is currently fabricated by a vacuum-based method such as co-evaporation.
Recently, solution-based fabrication methods for CIGS thin films have attracted much attention due to their
potential for realizing low-cost and printable solar cells17. The highest efficiencies of solution processed CIGS
thin film solar cells have been reported to be 15.2% (hydrazine based)18 and 12% (non-hydrazine based)19.
Among the various solution-based synthesis methods, the fabrication of CIGS thin films using nanoparticle
ink has potential. This method offers the advantage of fabricating CIGS thin films with uniform and wellcontrolled compositions by starting from stoichiometry-controlled CIGS nanoparticles, thereby eliminating
the impurity phases that could significantly deteriorate the device performance20. With subsequent sintering
of the as-prepared film, the nanoparticles are converted to uniform films of high crystallinity and a large grain
SCIENTIFIC REPORTS | 5 : 8970 | DOI: 10.1038/srep08970
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damage to the AZO and CIGS absorber films. The DSSC/CIGS tandem solar cell was fabricated via a simple solution-based process by
preparing the CIGS absorber film for the bottom cell using CIGS
nanoparticle ink. We have therefrom demonstrated a DSSC/CIGS
tandem solar cell that shows a higher solar cell performance (PCE of
13.0%) than the corresponding single-junction devices of DSSC
(,7.25%) or CIGS (,6.2%). Note that this value is higher than the
champion cell efficiencies of the constituent single-junction solar
cells in the literature, i.e., 11.2% of N719 dye-based DSSC23 and
12% of nanoparticle ink-based CIGS19.
Figure 1 | Schematic of the DSSC/CIGS tandem solar cell structure
(upper), and spectral irradiance of solar light adapted from reference 3
and external quantum efficiencies of a DSSC (red) and a CIGS (blue)
single-junction solar cell used in the study (bottom).
size, which in turn lead to a high solar-to-electricity conversion
efficiency of 12%, as demonstrated by Guo et al.19
One of the major issues in the fabrication of efficient tandem
DSSC/CIGS cells (Fig. 1) is to control the electrical/optical properties
of the interface between the DSSC and the CIGS. A Pt-based catalytic
layer is deposited on the transparent conducting oxide (TCO) layer
of the CIGS thin film solar cell such as Al-doped ZnO (AZO) and Sndoped In2O3 (ITO)21. The role of the Pt catalytic layer is effectively to
reduce the overpotential for the reduction of the I3 ions to I- ions by
the photoelectrons coming out from the underneath CIGS solar cell.
However, the existence of the Pt catalytic layer inevitably decreases
the optical transmittance, which should lead to a decrease in the
photocurrent from the bottom cell and to a photocurrent mismatch
in the tandem solar cell. Therefore, it is desirable to prepare Pt
catalysts with high catalytic properties and high optical transmittance simultaneously. The Pt catalytic layer in a DSSC is conventionally prepared via a simple thermal decomposition method using a Pt
precursor (e.g., H2PtCl6), followed by sintering at 380–450uC under
ambient conditions22. However, this method cannot be applied during the fabrication of a DSSC/CIGS tandem cell, as the bottom CIGS
cell, fabricated prior to the Pt layer, is vulnerable to high-temperature
treatments. Therefore, it is crucial to identify a suitable low-temperature preparation method for the fabrication of the Pt layer.
In this study, a Pt interfacial layer was deposited by the arc-plasma
deposition (APD) technique, which is considered to be a lowtemperature and softer deposition method that could minimize the
-
SCIENTIFIC REPORTS | 5 : 8970 | DOI: 10.1038/srep08970
Results
As mentioned earlier, one of the most important components in the
DSSC/CIGS tandem architecture is the Pt catalyst deposited on the
AZO layer of a CIGS thin film solar cell. The APD method24,25
involves the direct evaporation of a solid metal target into a highly
ionized plasma pulse in vacuum (,3x1025 Torr). The kinetic energy
of the evaporated metal nanoparticles is relatively low (tens of eV),
and hence these nanoparticles can be deposited onto a substrate
without causing significant damage to the substrate surface. The
pulsed deposition mode (i.e., enough idle time between plasma
pulses) along with the low kinetic energy of the evaporated nanoparticles also prevents the temperature from increasing in the samples
(,100uC). Another advantage of the APD method is that the amount
of metal deposits can be controlled easily and accurately by changing
deposition parameters such as the number of plasma pulses (n), the
discharge condenser voltage (V), and the discharge condenser capacitance (C). Figs. 2a–e show transmission electron microscopy
(TEM) images of the Pt catalysts deposited using the APD method
with different values of n (10, 20, 40, 60, and 80), where glass substrates with an amorphous carbon film were used for the coverage
estimation. Isolated Pt nanoparticles with a diameter of ,2 nm were
observed after 10 plasma pulses (Fig. 2a), and the density of Pt
nanoparticles increases with an increase in the number of plasma
pulses, resulting in an almost continuous Pt film after 60 pulses. The
catalytic properties of the APD-deposited Pt layer was investigated in
terms of the conversion efficiency of DSSCs incorporating APD-Pt
catalysts with different values of n (Fig. 2f, filled squares), where the
conversion efficiency increases with an increase in the number of
plasma pulses. This result shows that more Pt catalysts with more
plasma pulses would be beneficial to enhance the device performance
of the single-junction DSSC. In the case of a DSSC/CIGS tandem
solar cell, however, the optical transmittance of the catalytic layer is
also very important, because it directly influences light harvesting by
the bottom CIGS solar cell, and the resulting photocurrent generation. In this context, the ultraviolet-visible (UV-Vis) spectra of Pt
layers deposited on glass substrates using the APD method with
different values of n have been investigated. Given that the bottom
CIGS cell can only absorb photons that have passed through the top
DSSC, transmittance values at 800 nm are compared (see Fig. 1). As
shown in Fig. 2f (open squares), the optical transmittance at 800 nm
decreases with an increase in the number of plasma pulses, especially
after 40 pulses, which is consistent with the TEM analysis. Owing to
the trade-off between the DSSC performance and light harvesting by
the bottom CIGS, the optimal condition for the Pt deposition by
APD was set to 40 pulses. It should be noted that the conversion
efficiency of a DSSC with a Pt layer deposited by APD (40 pulses, 7.25
(60.26)%) was slightly higher than that of a DSSC with Pt layer
deposited by the conventional thermal sintering method (7.16%,
see Supplementary Information, Fig. S1).
In order to realize the solution processed fabrication of a DSSC/
CIGS tandem solar cell, we adopted the nanoparticle ink-based coating method for the deposition of CIGS absorber film. In this study,
the method that was originally developed by Guo et al. was slightly
modified for the fabrication of the absorber film of the CIGS bottom
cell (see Methods section). Briefly, the CIGS film was formulated
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Figure 2 | Transmission electron microscopy (TEM) images of Pt nanoparticles deposited on amorphous carbon films by the arc-plasma deposition
(APD) method with different plasma pulses (a) 10, (b) 20, (c) 40, (d) 60, and (e) 80, and optical transmittance at 800 nm of APD-Pt layers on glass
substrates (left y-axis in (f)) and conversion efficiencies of DSSCs with APD-Pt catalyst (right y-axis in (f)) as a function of the plasma pulses.
from CIGS nanoparticles, which were synthesized via the hot injection route using Cu, In, and Ga acetylacetonate precursors and elemental S. Subsequently, ink composed of CIGS nanoparticles and
hexanethiol was doctor-bladed and preheated under ambient conditions. After a sodium treatment, the film was annealed under selenium vapor to allow the substitution of Se for S and to facilitate
crystal growth19.
Fig. 3a shows the cross-sectional configuration of the CIGS bottom cell. As shown in the figure, the CIGS thin film shows large-sized
grains (,1 mm) with a very low degree of porosity (Figs. 3a and b).
Moreover, we could observe an amorphous layer with a thickness of
540 nm at the interfacial region of the Mo substrate, as marked by the
dashed lines in Fig. 3a. This amorphous layer could be a residual
carbon layer of the type often observed in solution-processed CIGS
thin films26,27. This hypothesis was further substantiated by Auger
electron spectroscopy (AES) for an elemental analysis, which confirmed the presence of carbon near the Mo layer (Supplementary
Information, Fig. S2). In addition, the selenization of the Mo substrate resulted in the formation of a MoSe2 layer with a thickness of
580 nm. Furthermore, as shown in Fig. 3a, a CdS buffer layer and
i-ZnO and AZO window layers were deposited onto the top surface
of the CIGS absorber film. Owing to the deposition of the AZO film,
the top surface of the CIGS bottom cell appears to be very dense
(Fig. 3c). In particular, we deposited a thicker AZO film (2500 nm),
approximately five times thicker than the conventional CIGS solar
cell (500 nm), in order to protect the underneath CdS/CIGS layers
during the deposition of Pt. Pt deposition by APD (40 pulses)
resulted in the formation of smaller Pt nanoparticles on the top of
the AZO surface, corresponding to the single-layer thickness of Pt
particles (Fig. 3d). The representative single-junction CIGS thin film
solar cell (closest one to the average efficiency value, 6.1 6 0.5%,
among the five cells) formed using the ZnO/CdS/CIGS films deposited onto Mo-coated glass shows a power conversion efficiency of
6.2%, with JSC, VOC, and FF values of 26.3 mA/cm2, 450 mV, and
52.3%, respectively (see Fig. 4).
The DSSC top cell was fabricated according to the conventional
procedures reported in the literature28. As in the typical process, a
SCIENTIFIC REPORTS | 5 : 8970 | DOI: 10.1038/srep08970
mesoporous TiO2 film of thickness 20 mm was prepared by a pastecoating method, followed by sintering at 500uC. A TiO2 blocking
layer was also deposited between the FTO and mesoporous TiO2
film in order to enhance the transmission of light29. N719 dye and
I3 /I- ions were employed as a sensitizer and redox couples, respectively. As mentioned earlier (Fig. 2), the efficiency of a DSSC varies
with respect to the deposited amount of the Pt layer. For tandem cell
fabrication, we used a DSSC top cell with an average efficiency of
,7.25%, as shown in Fig. 4.
-
Figure 3 | Cross-sectional scanning electron microscope (SEM) image of
bottom CIGS cell (AZO/i-ZnO/CdS/CIGS/Mo) (a), top-view SEM images
of the CIGS film only (b) and AZO/i-ZnO/CdS/CIGS film (c), and TEM
image of Pt nanoparticles deposited on AZO/i-ZnO/CdS/CIGS film by
APD.
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Figure 4 | Current density-voltage (J–V) characteristics of DSSC/CIGS
tandem solar cell and DSSC and CIGS single-junction solar cells under 1
sun illumination.
Fig. 4 shows the J–V characteristics of the DSSC/CIGS tandem
solar cell along with the DSSC and CIGS single-junction solar
cells. The as-fabricated DSSC/CIGS tandem cell showed a power
conversion efficiency of 13.0% (champion device) with an opencircuit voltage (VOC) of 1.17 V, a short-circuit current density
(JSC) of 14.6 mA/cm2, and a fill factor (FF) of 0.77. The measured
VOC of the tandem cell (1.17 V) was in fairly good agreement with
the sum of the individual average VOC values for DSSC and CIGS
single-junction cells (1.18 V). The plateau value of JSC for the
tandem device (14.2 mA/cm2) was slightly lower than that of
the DSSC (14.7 mA/cm2). The JSC of CIGS bottom cell in the
tandem configuration cannot be measured with two terminal electrodes (on top of the DSSC and at the bottom of the CIGS). It is
also far from simple to estimate it by covering the CIGS with a
dummy DSSC cell, due to the difficulties in applying DSSC components identical to those used for the top cell in the tandem
structure. Nevertheless, the observation that the JSC of tandem cell
is quite close to that of the DSSC suggests that the JSC values from
both sub-cells are well balanced.
The FF of the tandem cell (0.77) was found to be much higher
than those of the constituent DSSC (0.68) and CIGS (0.52) samples. This can be explained in part by considering the series and
shunt resistances (Rs and Rsh) connected in series. In principle, the
Rs of a series-connected tandem device should be the sum of the Rs
values for DSSC and CIGS samples. The slope values, dJ/dV, at
VOC of the tandem and single-junction devices (7.59 Vcm2 for
DSSC and 4.46 Vcm2 for CIGS) were thus compared as approximations to Rs values. The measured dJ/dV value at VOC of
the tandem device (7.41 Vcm2) was found to be smaller than the
sum of dJ/dV values for the constituent single-junction cells,
12.05 Vcm2. The discrepancy is likely due to the absence of the
FTO contribution to Rs in the tandem configuration (Fig. 1). The
Rs evaluation from the dJ/dV value at high V regime of dark J–V
curves (Fig. S3) also showed that the Rs of the tandem cell
(1.1 Vcm2) is smaller than the sum of the Rs values for DSSC
(0.55 Vcm2) and CIGS (1.2 Vcm2). Similarly, it was proved that
the series-connected shunt resistance (Rsh 5 (1.2 6 0.5) 3
104 Vcm2) is also determined by the higher value of the two
sub-cells, i.e., Rsh of DSSC ((1.4 6 0.8) 3 104 Vcm2). In comparison, Rsh of CIGS was (3.1 6 0.5) 3 102 Vcm2. As a consequence,
the slope of the tandem device in the range of 0 to 1.0 V looks as
flat as a DSSC, in contrast to a CIGS cell. This feature is an
additional advantage of a tandem cell, particularly when employing
a single cell with high shunt conductance, such as a CIGS cell.
SCIENTIFIC REPORTS | 5 : 8970 | DOI: 10.1038/srep08970
Discussion
A hypothetical tandem cell with two subcells having exactly complementary absorption profiles that do not overlap at all would have the
efficiency close to the sum of the individual cell efficiencies. It is clear
that this is not the case with the present combination of DSSC and
CIGS solar cells as there is some amount of overlap in the absorption
profiles of DSSC and CIGS, as shown in Fig. 1. Although we could not
measure how high the CIGS bottom cell performs in the tandem
configuration, we could estimate each device parameter of the bottom cell as follows. The VOC of CIGS bottom cell (operating in the
tandem cell) is rather straightforward to estimate, that is, VOC(CIGS)
5 VOC(tandem) 2 VOC(DSSC) 5 1.170 V 2 0.725 V 5 0.445 V.
This value is slightly smaller than that of the single-junction CIGS
cell (0.450 V), which is reasonable. Assuming that the subcell current
densities are well matched, the JSC of the CIGS bottom cell should be
that of the tandem cell, i.e., 14.6 mA/cm2. Namely, the CIGS bottom
cell loses its current by ,45% in comparison with the CIGS single cell
(26.3 mA/cm2). If the FF of CIGS bottom cell improves to such
an extent that compensates for the current loss, the bottom cell
may operate at the efficiency of PCE 5 VOC 3 JSC 3 FF 5
(0.445 V)(14.6 mA/cm2)(0.79) 5 5.1% having a J-V behavior shown
in Fig. S4, where the FF of CIGS bottom cell, 0.79, was assumed to be
slightly higher than that of tandem cell (0.77). The significant
enhancement in FF value of CIGS bottom cell can be attributed to
the reduction of shunt conductance that is in turn due to the series
connection of shunting path of CIGS bottom cell to that of DSSC top
cell as well as to the reduction of series resistance due to the absence
of lateral collection path along the TCO layer. The sum of the resulting individual cell efficiencies is, thus, 7.3% (DSSC) 1 5.1% (CIGS)
5 12.4% (Tandem). If we used the highest efficiency value of our
single junction DSSC (7.5%) the expected tandem cell efficiency can
increase up to 12.6%. Although this estimation is very rough, we
think it is enough to show that the measured tandem cell efficiency
(13.0%) is not unrealistic. Besides this, the subcells used in tandem
construction can be different from each single cell due to the insufficient reproducibility, as is implied by efficiency deviation (6.1 6
0.5%) of single junction CIGS thin film solar cells.
It is worth noting that the present PCE (13.0%) is the highest value
among the monolithic DSSC/CIGS tandem solar cells reported thus
far15. This is significant progress in that the present device fabricated
with solution-processed CIGS surpasses the DSSC/CIGS tandem
devices based on the vacuum-processed CIGS15. However, we should
also note that our current tandem solar cell devices are not stable
enough mainly due to usage of corrosive idodine electrolyst (Fig. S5).
The improvement of stability of the tandem cell device is under
investigation in our lab by applying less corrosive electrolyte.
In brief, we present a strategy to construct an efficient DSSC/CIGS
tandem architecture by solution-based coating method. A Pt-based
interfacial catalytic layer is the most important component in a
DSSC/CIGS tandem cell; therefore, its catalytic properties and
optical transmittance should be well optimized to achieve a highly
efficient tandem solar cell. The APD method was successfully
employed for the Pt interfacial layer, showing best solar cell performance with 40 pulses. Notably, the DSSC/CIGS tandem solar cell
revealed much higher solar cell performance (PCE of 13.0%) than
the corresponding single-junction devices of DSSC (,7.25%) or
CIGS (,6.2%) cells. Moreover, these are higher values than the
champion cell efficiencies of the constituent single-junction solar
cells, i.e., 11.2% from a N719 dye-based DSSC and 12% from a
nanoparticle ink-based CIGS cell.
Methods
Fabrication of the DSSC cell. Fluorine-doped tin oxide (FTO) glasses (Pilkington,
TEC-8) were washed in ethanol using an ultrasonic bath. To create a working
electrode, the FTO layer was initially covered with 7.5 wt% Ti(IV) bis(ethyl
acetoacetato)-diisopropoxide solution by a spin-coating method, after which TiO2
paste was deposited by doctor blade method. The paste was prepared using
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nanocrystalline TiO2 (25 nm in diameter) that was synthesized by a hydrothermal
method28. The coated film was sintered at 500uC for 30 min in air. The sintered TiO2
film was dipped into a TiCl4 solution (0.04 M) at 70uC for 30 min and was resintered
at 500uC for 30 min. The thickness of the prepared film is 20 mm, as measured by an
Alpha-Step IQ surface profiler (KLA Tencor). After cooling down to 80uC, the film
was dipped into a N719 dye solution (0.5 mM) at 40uC for 3 hr.
Pt-based catalytic layer as counter electrode was prepared by two methods: arc
plasma deposition (APD) and thermal sintering. The APD method used a coaxial
pulsed APD system (ULVAC, ARL-300) at room temperature under a 1025 Torr
vacuum to deposit platinum nanoparticles onto FTO glass, whereas the thermal
sintering method was carried out by spreading a 7 mM solution of H2PtCl6 dissolved
in 2-propanol onto FTO glass and followed by drying and annealing at 450uC for
15 min.
Two small holes were drilled into the counter electrode to introduce the electrolyte,
and 60 mm thick surlyn (DuPont 1702) was used to assemble the finished counter
electrode with the working electrode. The electrolyte was introduced into the
assembled device through a hole on the counter electrode, where the electrolyte
solution was composed of PMII (0.7 M), I2 (0.03 M), guanidine thiocyanate (0.05 M)
and 4-tert-butylpyridine (0.5 M) in acetonitrile and valeronitrile (85515 v/v).
Preparation of the CIGS film and solar cell fabrication. For the preparation of CIGS
film, CIGS nanoparticles were synthesized using the method proposed by Guo et al.
3 mmol of copper (II) acetylacetonate, 2.1 mmol of indium(III) acetylacetonate,
0.9 mmol of gallium(III) acetylacetonate and 15 ml oleylamine were mixed in a
three-necked flask and heated to 130uC using a heating mantle under Ar. The solution
was pumped under a vacuum at 130uC for 30 min for degassing and heated to 285uC
under Ar. At 285uC, 9 ml of 1 M solution of elemental sulfur in oleylamine was
swiftly injected in a three-necked flask under vigorous magnetic stirring and the
reaction was kept at 285uC for 30 min before cooling down. After cooling down, the
nanoparticles were precipitated with isopropyl alcohol (IPA) through centrifugation
at 12000 RPM for 5 min. The supernatant was discarded and the precipitate was
redispersed in hexane. This purification process was repeated several times to remove
excess oleylamine. The precipitate was then dried using an evaporator under vacuum
and redispersed in hexanethiol to form an ink with concentration of ,200 mg/ml.
To make a film, the ink was coated onto Mo-coated soda-lime glass by a doctor
blade method under ambient conditions. After coating, the film was annealed on a hot
plate preheated to 300uC for 1 min and the coating and annealing steps were repeated
two times to obtain a suitable film thickness. Prior to selenization, the nanocrystal
films were soaked in a 1 M NaCl solution for 10 min followed by selenization with
elemental Se under Ar at 500uC for 20 min in a quartz tube.
A solar cell device was fabricated with the conventional Mo/CIGS/CdS/i-ZnO/
AZO/Ni/Al structure and the case of a tandem solar cell was excepted Ni and Al
electrodes. A 60-nm-thick CdS buffer layer was prepared on 0.5 M aqueous potassium cyanide (KCN) solution-treated CIGS film by chemical bath deposition
(CBD), and i-ZnO (50 nm)/Al-doped ZnO (2500 nm) were deposited by the radiofrequency magnetron-sputtering method. A Ni (50 nm) and Al (500 nm) grid were
prepared as a current collector by thermal evaporation. The active area of the
completed single cell was 0.44 cm2.
Fabrication of DSSC/CIGS tandem device. Tandem device was fabricated by
assembling a working electrode (FTO/TiO2-dye) and a counter electrode (Pt/AZO/
i-ZnO/CdS/CIGS/Mo-coated glass) into a sandwich type using 60 mm thick surlyn
(DuPont 1702). The working electrode was prepared by the identical method to DSSC
as mentioned above. Also, the counter electrode was fabricated similar to that of single
junction CIGS solar cell except for adding the Pt layer on top of the AZO film.
Characterization. Structural characterization of the films was performed using a
scanning electron microscope (SEM, Nova, Nano200) with an acceleration voltage of
10 kV, transmission electron microscopy (TEM, FEI Inc., Tecnai G2), and X-ray
diffraction (XRD, Shimadzu, XRD-6000) with Cu-Ka radiation (l 5 0.15406 nm).
The optical properties were measured by ultraviolet-visible spectroscopy (UV-Vis,
Varian, Cary 5000). Composition analysis was carried out by an electron probe
microanalyzer (EPMA, Jeol, JXA-8500 F) and energy-dispersive X-ray spectroscopy
(EDX, E2V Tech. Inc.). Depth profiling was conducted by Auger electron
spectroscopy (AES, Ulvac-Phi). Device performances were characterized using a class
AAA solar simulator (Wacom, WXS-155S-L2) and an incident photon-to-current
conversion efficiency (IPCE) measurement unit (PV measurement Inc.).
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Acknowledgments
This work was supported by the ‘‘National Agenda Project’’ program of the Korea Research
Council of Fundamental Science & Technology (KRCF) and by the KUUC program and
University-Institute cooperation program funded by the Minister of Science, ICT and
Future Planning (MSIP) through the National Research Foundation of Korea.
Author contributions
B.K.M. and D.L. planned the project. S.H.M. and S.J.P. managed and performed most
detailed experiment, and S.H.K., M.W.L., J.H., J.Y.K., H.K. and Y.J.H. helped data analysis
and manuscript preparation. All the authors discussed the results and commented on the
manuscript.
Additional information
Supplementary information accompanies this paper at http://www.nature.com/
scientificreports
Competing financial interests: The authors declare no competing financial interests.
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www.nature.com/scientificreports
How to cite this article: Moon, S.H. et al. Monolithic DSSC/CIGS tandem solar cell
fabricated by a solution process. Sci. Rep. 5, 8970; DOI:10.1038/srep08970 (2015).
SCIENTIFIC REPORTS | 5 : 8970 | DOI: 10.1038/srep08970
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