# Applications of Cu@C Nanoparticles in New Dye-Sensitized Solar Cells

Authors: Chang-Yu Liao; H. Paul Wang; F.-L. Chen; C.-H. Huang; Y. Fukushima
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

To enhance the efficiency of a newly developing dye-sensitized solar cell (DSSC), the Cu@C (Cusize=7 and 20 nm) core-shell nanoparticles-dispersed molten salt-conjugated electrolyte has been studied.  Experimentally, the efficiencies (η) of the DSSC are in the range of 2.70–4.09% with a short-circuit photocurrent density (JSC) of 5.775–9.910 mA/cm2. Interestingly, it is found that dispersion of 1% of the Cu@C (Cu size=7 nm) nanoparticles in the molten salt-(1,2-dimethyl-3-propylimidazolium iodide (DMPII)) conjugated electrolyte results in an enhancement (about 11%) of the η (4.06%). Greater fractions (3–10%) of the Cu@C nanoparticles dispersed in the molten salt cause a poor performance (lower JSC and η) of the DSSC possibly due to interference of the internal electron transportation routes in the DSSC by oxidation of Cu with I2 originally in the electrolyte.


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## Body

## 1. Introduction

Dye-sensitized
solar cells with advantages of low manufacturing cost and comparable
light-to-electricity conversion efficiency to conventional silicon-based
photovoltaic cells have been considered as the next-generation clean energy sources [1].
However, incorporation of liquid solvents such as
acetonitrile and iodide/triiodide redox mediators in the cell has been facing
problems, for instance, solvent evaporation (reducing life-time), inconvenience
in the module sealing, and carcinogenic concerning of chemicals used in the
electrolyte.  Alternatively, a
quasisolid-like composite electrolyte turns out to be a
solution of those problems [2–8].  Hiroki et al. found that
incorporation of carbon materials in a molten salt could improve the electric
conductivity of the quasisolid-like electrolyte, which also had better photon conversion efficiency [3].  In the present work, Cu@C (copper
encapsulated in the carbon shell) nanoparticles were dispersed in a molten salt
(1,2-dimethyl-3-propylimidazolium iodide (DMPII)) as a
nanocomposite electrolyte utilized in
DSSCs.  X-ray absorption near edge
structure (XANES) spectroscopy was also used to reveal speciation of copper in
the Cu@C nanoparticles dispersed in the molten salt.


## 2. Experimental

The core-shell (Cu@C) nanoparticles were synthesized
via carbonization of Cu2+-starch complexes at 673 K for 12 hours. By TEM and XRD, it is clear that the Cu@C
nanoparticles have the (core) metallic copper (Cu) sizes of 7–20 nm.  The Cu@C  nanoparticles (0.3–10 wt%) were dispersed
in the DMPII  (Solaronix  SA, Switzerland)  (0.6 M)
in an electrolyte which contains 0.1 M of LiI  (Aldrich, USA),  0.05 M of I2  (Riedel-de Haën, Germany),  and 0.5 M of 4-tert-butylpyridine (Aldrich, USA) in
acetonitrile (Mallinckrodt, USA).To fabricate the photoelectrode of the DSSC, 6 g of nanosize TiO2 (P25 nanograde, Degussa, Germany)  were mixed with a solution containing 0.1 mL of Triton X-100
(Sigma-Aldrich, USA),  0.2 mL of acetylacetone (Fluka, Switzerland), and 10 mL of deionized water to
yield a TiO2 paste.  Three
drops of the TiO2 paste were spread on the Scotch magic tape-(3 M) masked fluorine-doped tin oxide
substrate (8 ohm/square, Solaronix  SA, Switzerland)  (2.5 cm × 2.5 cm)
by the doctor-blade method.  The
as-coated glass was dried and scratched out for a working area of 0.25 cm2, preheated at 323 K
for 15 minutes, and calcined at 723 K for 30 minutes.  The TiO2-coated glass was immersed
in a 3×10−4 M of N3 (cis-di(thiocyanato)-N,N′-bis(2,2′-bipyridyl-4,4′-dicarboxylate)
ruthenium(II), ruthenium 535, Solaronix  SA, Switzerland)  anhydrous ethanol solution for 24 hours
in the dark.  The stained photoelectrode
was dried in N2 for the assemblage of the DSSC.The counterelectrode was prepared by distribution
of three drops ofH2PtCl6⋅6H2O
(Alfa Aesar, USA)  on the conducting tin oxide electrode (10 ohm/square, RITEK, Taiwan)  with
a spin-coater (SP-M1-S, APISC, Taiwan)  at 1500 rpm for 15 seconds.  The electrode was preheated at 323 K for 10 minutes and calcined at 658 K for 10 minutes.The photoelectrode and counterelectrode were
sandwiched together with a 60μm thick hot-melt thermal foil (SO-SX1170, Solaronix  SA, Switzerland)  as a spacer.  The Cu@C
dispersed electrolyte was introduced into the cell and sealed with AB epoxy that
was cured for 30 minutes.  A 300 W xenon arc
lamp solar simulator (no. 91160A,
Oriel, USA) conjugated with an AM 1.5 Globe filter (no. 59044, Oriel, USA) was employed to
measure the I-V characteristics of the quasisolid-state DSSC.  The illumination was fixed at 100 mW/cm2 using a reference solar cell and meter (no. 91150, Oriel, USA).  In each experiment, at least five solar
cell measurements were carried out to ensure the acceptable data being obtained.The copper K-edge XANES spectra of the Cu@C
nanoparticles dispersed (1–10%) in the electrolyte were determined on the
Wiggler BL17C1
beam line at the Taiwan National Synchrotron Radiation Research Center.  The electron storage ring was operated at
energy of 1.5 GeV.  A Si(111)
double-crystal monochromator was used for selection of energy with an energy
resolution (△E/E) about 1.9×10−4 (eV/eV).  The incident photon energy was
calibrated using a standard copper foil with a characteristic absorption edge
at 8979 eV.  The XANES spectra of copper
in the Cu@C nanoparticles were analyzed with the UWXAFS 3.0 program [9].  Semiquantitative analyses of the XANES
spectra of copper were conducted by the least-square fitting of linear
combinations of standard spectra (Cu (Merck, Germany,  99.7%), Cu2O (Aldrich, USA,
97%), CuO (Merck, Germany,  99%), and NanoTeKR CuO (Kanto, Japan,  99.3%)) to the
spectra of samples.  On the average, an
uncertainty limit of 5% corresponds to an error of about c.a.
2.0% in the fitting results.


## 3. Results and Discussion

TheI-V characteristics of the quasisolid-state
DSSC enhanced with the Cu@C nanoparticles (Cu size = 7 nm) dispersed molten
salt (DMPII) in the electrolyte are shown in Figure 1.  The DSSC containing 1% of the Cu@C
nanoparticles dispersed in the molten salt exhibits a maximum short-circuit photocurrent
density (JSC = 9.910 mA/cm2),
moderate open-circuit photovoltage (VOC = 0.748 V) and an enhancement (about 11%) of the conversion efficiency (η = 4.06%).  A greater fraction (3%) of the Cu@C
nanoparticles, on the contrary, leads to a decrease of the conversion
efficiency (η = 3.53%).Figure 1
Photocurrent densit (J) of the DSSC containing (a) 0, (b) 1, and (c) 3% of the Cu@C
nanoparticles (Cu size = 7 nm) dispersed in the molten salt.Figure2
shows the dependence of the Cu sizes in the core of the Cu@C nanoparticles on JSC and η of the DSSC.  It seems that JSC and η decrease with an increase of the Cu size in
the Cu@C nanoparticles.  For the large
Cu@C nanoparticles-(Cu size = 20 nm) dispersed molten salt in the DSSC, the JSC (Figure 2(a)) and η (Figure 2(b)) are dramatically reduced.Dependence of Cu sizes in the Cu@C
nanoparticles ((a) 1 and (b) 3%) dispersed molten salt in the electrolyte on
the short-circuit photocurrent density (JSC)
and conversion efficiency (η) of the DSSC.
(a)(b)In Figure3, the DSSC containing 1% of the Cu@C (Cu size = 7 nm) nanoparticles dispersed in
the molten salt in the electrolyte has a desirable JSC (9.910 mA/cm2) and η (4.06%).  In the separate experiments, we also found
that the self-diffusion coefficients of cations and anions in a room
temperature ionic liquid ([bmim][BF4])  dispersed with 0.08% of Cu@C nanoparticles were
increased by 4.3–6.6 times [10].  The
electrical conductivity of the Cu@C dispersed [bmim][BF4] was
increased by as high as 3.7 times in comparison with that without the Cu@C
nanoparticles [10].  Nevertheless, in the
present work, it is found that greater fractions (3–10%) of the Cu@C
nanoparticles dispersed in the molten salt, on the contrary, may interfere the
internal electron transportation routes or result in the electron back
transportation from photoelectrode to counterelectrode, which cause a poor
performance (lower JSC and
η) of the DSSC.  A short-circuit in the
presence of a large amount of carbon nanotubes and/or carbon nanoparticles in the
quasi-solid-state DSSCs was also found by Usui et al. [3].Dependence of fractions of Cu@C nanoparticles-(Cu size = 7 nm) dispersed in the molten salt on the (a) short-circuit photocurrent
density (JSC) and (b)
conversion efficiency (η) of the DSSC.
(a)(b)Molecular-scale
data of select compounds in the complex matrix determined by XANES turn out to
be very useful in revealing chemical structure and composition of key species
in the reaction pathways [11, 12].
Figure 4 illustrates the XANES spectra of copper in the Cu@C
nanoparticles-dispersed molten salt.  The
Cu@C nanoparticles contain mainly nanosize Cu (63%), Cu2O (13%), and
CuO (24%).  Note that after the repeatedly
efficiency measurements of the quasisolid-state DSSC, in the presence of 0.05 M of I2 (in the
electrolyte), 17–32% of Cu encapsulated in the carbon shells are oxidized to Cu2O
(10–17%) and CuO (5–17%) (Figure 4).  The
oxidation of Cu with I2 may interfere the internal electron transportation
routes in the DSSC.  As shown in Figure
3, greater fractions (3–10%) of the Cu@C nanoparticles dispersed in the molten
salt in the electrolyte result in a poor performance (lower JSC and η) of the DSSC.The least-square fitted XANES spectra of
copper in the Cu@C nanoparticles-(Cu size = 7 nm) (fractions of (a) 0, (b) 1,
(c) 3, (d) 5, and (e) 10%) dispersed in the molten salt.
(a)(b)(c)(d)(e)Generally, the ratio of surface-to-total atoms for a Cu
nanoparticle having a size of 7 nm is about 0.11.
It seems that oxidation occurs mainly on the surfaces of the Cu
nanoparticles, and further oxidation down to subsurfaces may be limited to about 40%
of the total Cu atoms.  The CuO or Cu2O layer on the surfaces of the core Cu in the Cu@C may not perturb
directly the electron transportation routes in the DSSC.  Alternatively, the electron-rich surfaces of
the carbon shell consisted of diamond and graphite carbons may play the main
role in the electrical conduction.


## 4. Conclusions

Enhancements of the short-circuit photocurrent
density (JSC) and solar
conversion efficiency (η) of the DSSC with the Cu@C nanoparticles-dispersed
molten salt-conjugated electrolyte have been observed.  The DSSC containing 1% of the Cu@C (Cu size = 7 nm)
nanoparticles dispersed in the molten salt has a desirable JSC (9.910 mA/cm2) and η (4.06%).  Greater fractions (3–10%) of the Cu@C
nanoparticles dispersed in the molten salt cause a poor performance (lower JSC and η) of the DSSC possibly
due to interference of the normal internal electron transportation routes in
the DSSC by oxidation of Cu with I2 originally in the electrolyte.



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*Source: 698501-2009-04-06.xml*