Formation of short three dimensional porous assemblies of super hydrophobic acetylene black intertwined by copper oxide nanorods for a robust counter electrode of DSSCs

Article abstract of DOI:10.1039/c5ra02730f

In this study, we synthesized monolithic copper-oxide nanorods (CuO-NRs) and doped into active super hydrophobic acetylene black (AB) nanocrystals via a fast solvation method. The developed short-dimension assemblies are utilized as counter electrode (CE) materials for energy harvesting in dye sensitized solar cells (DSSCs). Cyclic voltammetry and electrochemical impedance spectroscopy analyses reveal that the hybrid (CuO-NRs/AB) CE displays a fast reduction rate of I₃^- and boosts hole-cascading as compared to those of individual AB CEs, due to the superior catalytic characteristics of CuO-NRs and the good electrical conductivity of AB. The CuO-NRs/AB based DSSCs exhibit an enhanced power conversion efficiency (PCE) of 8.05%, in contrast to AB (6.51%) and pristine Pt (6.96%) CE based DSSCs under the same experimental conditions. The CuO-NRs/AB CE, along with impressive results, has prospective applications in robust DSSCs.

Full text of DOI:10.1039/c5ra02730f

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Formation of short three dimensional porous assemblies of super hydrophobic acetylene
black intertwined by copper oxide nanorods for robust counter electrode of DSSCs
Waqar Ahmada, Liang Chua, Majid Raissan Alꢀbahrania, Zhichun Yanga, Siliang Wanga, Li Luyinga,
Yihua Gao*ab
aCenter for Nanoscale Characterization & Devices (CNCD), Wuhan National Laboratory for
Optoelectronics (WNLO) & School of Physics, Huazhong University of Science and Technology
(HUST), Luoyu Road 1037, Wuhan 430074, China
bHubei Collaborative Innovation Center for Advanced Organic Chemical Materials, 368 Youyi
Avenue, Wuhan 430062, China
Abstract
In this study, we synthesized monolithic copperꢀoxide nanorods (CuOꢀNRs) and doped
into active super hydrophobic acetylene black (AB) nanocrystals via fast solvation method. The
developed shortꢀdimension assemblies are utilized as counter electrode (CE) materials for energy
harvesting in dye sensitized solar cells (DSSCs). Cyclic voltammetry and electrochemical
impedance spectroscopy analyses reveal that the hybrid (CuOꢀNRs/AB) CE display fast
ꢀ
reduction rate of I3 and boost holesꢀcascading as comparing to those of individual AB CE, due
to superior catalytic characteristics of CuOꢀNRs and good electrical conductivity of AB. The
CuOꢀNRs/AB based DSSCs exhibits an enhanced power conversion efficiency (PCE) of 8.05%,
in contrast to AB (6.51%) and pristine Pt (6.96%) CEs based DSSCs under same experimental
conditions. The CuOꢀNRs/AB CE, along with impressive results renders their prospective
applications in robust DSSCs.
Keywords: CuO nanorods, acetylene black, counter electrode and dyeꢀsensitized solar cells.
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1. Introduction
Dyeꢀsensitized solar cells (DSSCs) have appeared to be a promising prospective
applicant among all other photovoltaic techniques due to shortage of energy resources and
environmental pollution problems. DSSCs convert solarꢀtoꢀelectrical energy by an efficient and
facile ecoꢀfriendly manner.1,2 Traditionally, standard DSSC device is the combination of working
electrode, dye molecules, a spacer, electrolyte solution and counter electrode (CE).3 As a key
component of DSSCs, CE should retain minimum resistance, outstanding catalytic property and
high current density to build a complete circuit.4 Owing to the high conductivity and good
electroꢀcatalytic activity, the most favorable CE material in DSSCs is Pt. However, Pt is noble
and scarce earth element that leads significantly high price in the market.5,6 Therefore, it is
essential to develop a cost effective CE for DSSCs.
In the past decades, several viable substitutes of Pt such as carbon based materials,
conducting polymers and transition metal containing constituents have been reported as potential
CEs materials in DSSCs.7 Among them, carbonaceous materials have shown quite promising and
remarkable performance but their conversion efficiency is still insufficient compared to that of Pt.
Although, these materials possess significantly high electrical conductivity but due to low
catalytic activity, capacitance and chemical stability in iodine based electrolytes solution, it is
hard to use as CE in DSSCs.8,9 On the other hand, transition metal oxides show desirable features
ꢀ
such as high catalytic, capacitance and better stability characteristics in I3 /Iꢀ electrolyte, however,
these have poor electronic conductivity and low ion diffusion rate.10,11 Therefore combining
carbonaceous and transition metal oxide material with low dimension has been an effective
strategy for DSSCs materials.12,13
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The four main classifications of composite structure have been examined until now.
These composites i) are made by the mixture of carbon species and oxide based materials
physically.14,15 ii) have cableꢀlike structure fabricated by coating conducting rods or wire with
active oxide species.16,17 iii) have interpenetrative network structure of carbonaceous
materials.18,19 and iv) have sandwich like configuration rather loading layers of graphene and
oxide materials.20,21 All these composites based structures DSSCs retain different characteristics
in terms of ion and charge transportation and showed distinct performances.
However, their extensive efforts for designing new materials in the outline above four
synthetic methods still have shortcomings, e.g. In composite i): the transport pathways for ions
and charges are usually incompetent and poor controlled due to the irregular mixing of binding
oxides and carbonaceous species. ii): the cableꢀlike structure locally provides easiest way for
charge transportation but the oxide coated layer increases the CNTs contact resistance, which
strongly affect the cells performance.14 iii): interpenetrative network of carbon based composite
delivers excellent performances but reduces the stability of electrode and composite iv): the
sandwich like structure composite makes intimate contact between carbonaceous materials and
active oxides, as a result the twoꢀdimensional structure inhibits the charge transport as well as
threeꢀdimensional mass.
In this work, we report the fabrication of novel class nanocomposite architecture with the
aim of increasing efficiency of DSSCs by very simple and efficient solution process. Unlike the
above mentioned approaches, our strategy is based on fast solvation process that uses
hydrophobic materials directly. As illustrated in Fig. 1, first we used nonpolar solvents for
dispersion of hydrophobic materials and secondly with addition of polar solvents, all
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hydrophobic nanocrystals accumulate around the hydrophobic nanorods and gain organized
shaped assemblies. However, calcination creates pores in these 3ꢀD assemblies.
In our experiment, we used acetylene black (AB) nanocrystals with building block copper
oxide nanorods (CuOꢀNRs) as a conceptual model. AB has high conductivity, low cost and
energy storage material while CuO is pꢀtype semiconductor facilitates holes conduction and also
provides intertwined structure to the nanocomposite. First we dispersed hydrophobic AB
nanocrystals and CuOꢀNRs22,23 in toluene and then with addition of methanol both hydrophobic
nanocrystals and nanorods were assembled. However, by calcination composite assemblies
become porous for robust CEs. Our developed 3ꢀD short structure provides distinctive
characteristics which are necessary for high performance CE of DSSCs by following means. The
short closely packed structures not only contact the oxides and AB nanoparticles locally for holes
conduction but also offer excellent network for whole electrode. The porous structure facilitates
effective channel for iodine based electrolyte transportation and active framework offers reduced
ionꢀdiffusion length. Therefore, it comprises excellent chemical and mechanical robustness
which is necessary for stable CEs.
The stateꢀofꢀtheꢀart, our electrochemical and structural properties specify that the as
prepared AB/CuOꢀNRs CEs should be capable of working efficiently as conventional Pt CE. In
particular, the obtained power conversion efficiency (PCE) of CuOꢀNRs/AB under standard
illumination condition (1.5G, 100mW cmꢀ2) was 8.05%, which indicates promising potential
application for electrodes in DSSCs. Here we propose that the high performance of CuOꢀ
NRs/AB based DSSCs is associated with effective synthesis of low dimensional assemblies.
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2. Experimental
2.1 Reagents and materials
All chemicals and reagents were of analytical grade and used without any additional
purification. Copper(II) chloride dihydrate (CuCl2.2H2O), sodium hydroxide (NaOH), ethanol
(C2H6O), terpineol (C10H18O), titanium tetrachloride (TiCl4), ethyl cellulose, tetrabutyl titanate
(C16H36O4Ti) and acetic acid (HAc) were purchased from Sinopharm Chemical Reagent Co., Ltd.
Acetylene (99%) was obtained from Hubei Chuchengwei Chemical Co., Ltd. N719 (cis–bis
(isothiocyanato) bis(2,2'ꢀbipyridylꢀ4,4'ꢀdicarboxylato) ruthenium(II) bisꢀtetrabutylammonium)
was supplied by Solaronix. Fluorine doped tin oxide glass (FTO, 7 ꢁ cmꢀ2) was purchased from
Nippon Sheet Glass Co., Ltd. and washed ultrasonically with laundry soap, acetone, Milliꢀpore
water (18.2 MΩꢀcm) and ethanol for 15 minutes, respectively. Lithium iodide (LiI, 99.999%),
iodine (I2, 99.99%), TiCl4. 3H2O (99.99%), 1ꢀmethylꢀ3ꢀpropylimidazolium iodide (PMII, 98%),
4ꢀtertꢀbutylpyridine (4ꢀTBP, 96%) and tertꢀbutyl alcohol (99.5 %) were obtained from Aladdin.
Acetonitrile (99.8%) and valeronitrile (99%) were bought from Alfa Aesar.
2.2 Preparation of materials
CuO nanorod formation: In a typical procedure, first 0.10 g CuCl2.2H2O (as copper source) was
dispersed in the mixture of 15 mL ethanol and 30 mL distilled water under agitation. After 10
min, 0.55 g solid NaOH particles were directly added and the solution got deepꢀblue color.
Afterward, the total volume of reagents is adjusted to 100 mL by adding distilled water. The
crude solution Cu(OH)2 was separated by centrifugation (at 1600 rpm for 5 min) and decantation,
washed with distilled water and ethanol, respectively. The resultant blue precipitate Cu(OH)2
was dried overnight at 60 °C and the color turned black which is feasible symptom of CuO.
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Finally to obtain the purified black target hydrophobic CuO nanorods, the dried CuO transition
state was heated at 400 °C for 2 h in air.
AB nanoparticle formation: The AB was prepared by the pyrolysis route.24 Typically, 1.5 g
acetylene (C2H2) was heated at 680 °C in Ar atmosphere for 2 h and cooled down to room
temperature naturally to get the super hydrophobic AB nanoparticles (size about 40 nm) with
porous structure.
Synthesis of CuO-NRs/AB nanocomposites: The nanocomposite was prepared by reacting 25 mL
toluene solution and 250 mg AB nanocrystals at room temperature for 1 h with stirring. Then 40
mg CuO nanorods were dispersed in above solution under ultrasonication and stirred. Next to
induce shortꢀdimensions assemblies, 35 mL methanol were added into the mixture and stirred.
Solid/liquid separation was done by centrifugation. Finally the wet products were dried at 60 °C
overnight and then annealed at 400 °C for 2 h in muffle furnace.
2.3 Preparation of counter electrode
The following experimental procedure was adopted for the feasibility of synthesizing
hybrid CuOꢀNRs/AB CEs: 1.0 g synthesized nanocomposite, 0.2 mL acetic acid, 0.50 g ethyl
cellulose, 3.0 g terpineol anhydrous and 10 mL ethanol were mixed together in 25 mL beaker by
tempestuously agitation at room temperature to attain fine dispersion. After vigorous agitation,
the reactant was grounded in a mortar and used ultrasonication for 15 min. Subsequently, the
composite slurry was deposited by mean of doctorꢀblading method onto patterned conducting
layer of FTO glass. The resultant layer was gradually heated at 400 °C for 2 h to remove the
residual organic contents and formed network of 3ꢀD structure using muffle furnace. As
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reference, AB CE was also prepared according to the above experimental approach without
existence of CuO constituents and the conventional Pt CE was prepared as stated by literature 25.
2.4 Fabrication of DSSCs
The mesoporous TiO2 suspension was synthesized by the modified process reported
literature.26,27 In brief, 10 mL tetrabutyl titanate and 15 mL ethanol were mixed together in 100
mL beaker under vigorous stirring at room temperature. Next, 10 mL acetic acid and 40 mL
distilled water were added into the above solution. After 1 h continuous stirring, the
hydrothermal treatment of the solution were carried out at 200 °C for 12 h. Solid/liquid was
separated by centrifugation and the precipitate washed with distilled water and ethanol three
times. Finally, the obtained TiO2 dried at 80 °C for whole night in air oven. Next to acquire fine
suspension, 1.5 g asꢀprepared TiO2 was mixed with 12 mL ethanol and 0.4 mL acetic acid added
drop wise under magnetic stirring. After 10 min, 0.8 g ethyl cellulose and 0.5 g terpineol as
binder and surfactant were added to the above solution respectively under continuous stirring at
room temperature. After agitation, white slurry was grounded in mortar and ultrasonicated for 15
min respectively. For compact layer of TiO2, the already cleaned conducting FTO plate was
immersed into 40 mM TiCl4 aqueous solution at 70 °C in air oven for 0.5 h. Subsequently, the
layer of 20 nm sized TiO2 was coated onto the treated substrate by doctorꢀblade method, using
one layer of magic scotch tape yielding film thickness of ~7 µm and sintered at 130 °C for 0.5 h.
With initial drying, the TiO2 film color turns into light brown/yellow. To obtain a light scattering
layer, 200 nm sized TiO2 single layer having thickness of ~4 µm was then deposited on the
synthesized FTO plate. For the removal of organic materials the asꢀprepared film was sintered at
450 °C for 15 min and next post treated at 500 °C for 1 h in muffle furnace. After thermal
annealing, color of the double layer TiO2 film was changed into milky and finally the asꢀ
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prepared sample was treated with TiCl4 aqueous solution. Before dipping into dye solution, the
prepared sample was reꢀcalcined in furnace at 520 °C for 0.5 h and then slowly cooled to 80 °C.
Finally for dye sensitization, the two layers TiO2 electrode was soaked into 0.5 mM N719 dye in
acetonitrile/tertꢀbutanol solution (V:V/1:1) for 20 h at room temperature. To remove the
physically adsorbed dye molecules the treated sample rinsed thoroughly with ethanol and
allowed in air to dry. The resulting DSSCs were assembled by using dye sensitized photoanode
electrode, electrolyte solution and counter electrode. The electrolyte was composed of 0.6 M
PMII, 0.05 M LiI, 0.03 M I2, 0.1 M GuSCN and 0.5 M 4ꢀTBP in acetonitrile and valeronitrile
(V:V/85:15) as solvents. The projected area of the solar cell was controlled to be 0.15 cm2
without mask.
2.5 Characterization
Xꢀray diffraction (XRD, PANalytical B.V. Netherland, radiation wavelength 1.54060 Å)
system furnished with CuꢀKꢀalpha radiation was used to study crystalline structures of the asꢀ
synthesized samples. The surface morphologies and nanoꢀcomposite structures of the samples
were investigated by a field emission scanning electron microscopy (FEꢀSEM, FEI NOVA
NanoSEM 450), Transmission electron microscopy (TEM, FEI Tecnai G2 20 UTwin) and Raman
spectroscopy (HORIBA Jobin Yvon UVꢀVISꢀNIR LabRAM, 532 nm). Energyꢀdispersive Xꢀray
spectroscopy (EDS) equipped with FEꢀSEM was carried out for observation of various elements
in the prepared films.
2.6 Electrochemical measurements
The photocurrent density versus voltage (JꢀV) curves was measured under dark and
illumination coming from a solar simulator (Newport, USA) with AM 1.5G (100 mW cmꢀ2).
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Electrochemical impedance spectroscopy (EIS) measurements were recorded using an Autoꢀlab
electrochemical workstation (model AUT84315, the Netherlands). Cyclic voltammetry (CV)
measurement was carried out in a threeꢀelectrode system using a CHI660C potentiostate with a
platinum electrode and Ag/AgCl were used as a counter electrode and reference electrode at scan
rate of 50 mVsꢀ1 in acetonitrile solution containing 1 mM I2, 10 mM LiI, and 100 mM
LiClO4.3H2O.
3 Results and Discussion
3.1 Material characteristics
Fig. 2(a) shows the asꢀsynthesized powders XꢀRay diffraction (XRD) pattern of AB,
CuOꢀNRs and CuOꢀNRs/AB nanocomposites. In undoped AB, the diffraction patterns depict
partially graphitized structure of acetylene black and can be readily indexed to reference patterns
(PCPDFWIN, file No. 75ꢀ2078), representing that AB sample is complex carbon materials. All
the XRD reflection peaks of CuO nanorods are indexed and well matched with monoclinic
structure of CuO file (PCPDFWIN, file No. 45ꢀ0937). Figure 2(a) shows the magnified XRD
pattern of CuOꢀNRs doped AB sample revealing the presence of CuO. There is no characteristic
peaks of any impurities such as Cu2O, Cu(OH)2, or precursors used are observed, demonstrating
the formation of a pure CuO phase. The EDX analysis Fig. 2(b, c & d) shows no other elements
are detectable in the above samples.
The surface morphology of nanocomposite CuOꢀNRs/AB CEs are characterized by FEꢀ
SEM. Fig. 3(a) presents the SEM image of AB nanoparticles, prepared by pyrolysis method. The
asꢀprepared nanocrystals show uniform porous structure characteristics with particle size ~40 nm.
Fig. 3(b) shows representative image of individual CuO nanorods used for building 3ꢀD low
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dimensions assemblies. The SEM micrograph also reveals the length and diameter of asꢀprepared
CuO nanorods, about 200 nm and 40 nm respectively. Fig. 3(c) shows the planꢀview FEꢀSEM
images of CuOꢀNRs/AB composite where the circled regions demonstrate the 3D wellꢀconnected
structure of hydrophobic AB nanoparticles and CuO nanorods. The inset in Fig. 3(c) shows the
cross section micrograph of CE, indicating the thickness of porous film is about 11.7 µm.
Overall these shortꢀdimension assemblies are uniformly distributed on FTO substrate and
provide local path ways for iodide/triiodide electrolyte, thus accelerate charge transportation. Fig.
3(d) depicts the high resolution SEM image of asꢀprepared assemblies varied from few hundred
nanometers to few micrometers. The inset in Fig. 3(d) exhibits high porosity with uniform pore
size distribution. Most importantly, the observed interconnected porous structure fortifies strong
adhesion to FTO glass as well as favorable for penetration of redox ions into the film.
In order to study the detailed morphology and further evidence of CuOꢀNRs/AB
composite, the sample is observed by TEM and high resolution TEM (HRꢀTEM). A TEM image
in Fig. 3(e) shows the shortꢀdimension intertwined network of both hydrophobic materials which
might be beneficial for holes transportation. Fig. 3(f) shows the selected area electron diffraction
(SAED) pattern of AB nanocrystals indicating the fringe spacing of 0.334 nm, which matches
well with the interꢀplanar spacing of (111) planes of the reference (PCPDFWIN, file No. 75ꢀ
2078). Fig. 3(g) illustrates the electron diffraction pattern of a CuO nanorod revealing [310] zone
axis, portraying that the nanorod is single crystalline.28 The lattice image with inset of CuO
nanorod in Fig. 3(h) depicts the interplanar distance of ca. 0.253 nm attributed to (002) plane of
monoclinicꢀphase CuO with reference (PCPDFWIN, file No 45ꢀ0937).
Beside surface analysis, both samples are characterized by Raman spectroscopy for
structural changes. The observed Raman spectra of all samples demonstrate two broad peaks as
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shown in Fig. 4(a). The peaks intensity at wave number 1344 cmꢀ1 and 1580 cmꢀ1 correspond to
Dꢀband of disordered carbons and Gꢀband of graphitic carbons, respectively.29 Since relative
intensity ratio of ID/IG is ruler of graphitization degree, the corresponding ID/IG ratio values 1.14
of AB and 1.22 of CuOꢀNRs/AB indicates that graphitization of AB is decreased in the presence
of CuO nanorods. This result can be attributed to the fact that some surface imperfects or defects
are attached with graphitic AB, resulting in the disorder degree.30 Therefore, these added active
sites justify the use of defected AB doped with CuOꢀNRs as an efficient CE material in DSSCs.
3.2 Electrocatalytic behavior
The catalytic performances are responsible for different properties of DSSCs and studied
by various techniques. Three electrode cyclic voltammetry is conducted to evaluate the
electrocatalytic activity of various CEs for the reduction of triiodide into iodide. Notably, the
cathodic current density is strongly dependent on the scan rate of CV measurements.31 Fig 4(b)
shows the CV curves of different CEs over potential range from ꢀ1.5 V to 1.5 V with scan rate of
50 mV sꢀ1. As expected, there are two well resolved redox pair peaks has been observed, i.e low
ꢀ
ꢀ
potential range corresponding to Iꢀ/I3 and high potential range correspond to Iꢀ/I2 . Two critical
parameters influence the DSSCs performance i.e. peak current density and peakꢀtoꢀpeak
separation (Epp). Apparently, the CuOꢀNRs/AB CE exhibits slightly larger redox peak current
density and lower peakꢀtoꢀpeak separation (shown in Table 1) than those of individual AB
electrode. The higher background current response of CuOꢀNRs/AB CE is mainly due to the
capacitance of effective diffusion on electrode/electrolyte interface that promote the adsorption
ꢀ
and desorption of Iꢀ/ I3 redox shuttle. This phenomenon could support the electrocatalytic
ꢀ
activity towards the I3 reduction. However, AB electrodes indicating poor catalytic activity due
to its graphitization structure which limits the sufficient defective sites of amorphous carbon and
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disorder mesoporous channels, confirmed by our Raman analysis. Moreover, the other key
parameter (Epp) exhibits the reversibility of the CEs. With the introduction of CuOꢀNRs to AB,
the CuOꢀNRs/AB shows smaller Epp value and faster reduction reaction than that of AB and
reference Pt electrodes, which is due to high electrical conductivity of AB and high catalytic
activity character of CuOꢀNRs. So the smaller Epp value indicates excellent catalytic reversibility
ꢀ
towards the reduction of I3 that CuOꢀNRs/AB CE has. Moreover, the reduction peaks of AB
and Pt is more negative than CuOꢀNRs/AB electrode, implying more irreversible process for the
triiodide reduction.27 This demonstrates that the CuOꢀNRs/AB CE with corresponding CuO
ꢀ
contents exposed superb catalytic activity towards the I3 reduction and a good photovoltaic
performance of the cells.
Electrochemical impedance spectroscopy (EIS) is another wellꢀknown effective test to
evaluate the electrocatalytic behavior of different CEs. The obtained Nyquist plots with inset
equivalent circuit are shown in Fig. 4(c) and their respective parameters are listed in Table 1. In a
typical Nyquist plot, the intercept of the 1st semicircle within high frequency range (106~105 Hz)
on real axis represents the series resistance (Rs) which is related to the sheet resistance of FTO
glass, contact resistance of the cell and resistance of external circuit. The charge transfer
ct) obtained from 1st semicircle in high frequency range is assigned to the
resistance (
R
C.E/electrolyte interface. The 2nd semicircle in intermediate frequency range ascribes electrons
transport resistance at photoelectrode/dye/electrolyte interface. Usually the Rs and Rct values are
adopted to evaluate the resistance and intrinsic catalytic activity of electrodes.32 The Rs values
are found nearly similar for all CEs, which are related to FTO glass and Rct values for CuOꢀ
NRs/AB, AB and Pt CEs are 1.9 Ω, 2.1 Ω and 3.45 Ω respectively. The CuOꢀNRs/AB CE
exhibits lower
R
ct value than those of AB and Pt CEs indicating excellent electrocatalytic activity.
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The superb catalytic activity also confirms the supremacy of unique lowꢀdimension structure of
CuOꢀNRs/AB on the triiodide reduction. On one hand due to the intertwined porous structure of
low dimensions, CuOꢀNRs provide fast ion transport network and suitable graphitization degree
which are advantageous for electron transportation. On the other hand, the definite synergistic
effect among highly distributed AB nanoparticles and CuOꢀNRs can significantly limits the
electron transfer resistance, yielding in a substantial enhancement of the catalytic activity. The
EIS results of DSSCs are in good agreement with the cyclic voltammetry experiments.
3.4 Photovoltaic performance
The influence of electrocatalytic characteristics on the photovoltaic performance is
examined by fabricating DSSCs with CuOꢀNRs/AB and AB CEs, and subsequently compared
with Pt based DSSCs. Fig 4(d) shows the JꢀV (current density versus voltage) curves of
sandwich type DSSCs fabricated with different CEs under dark and standard illumination. The
derived photovoltaic parameters such as shortꢀcircuit current density (Jsc), openꢀcircuit voltage
(Voc) and fill factor (FF) with their relative efficiencies (η) are summarized in Table 1. Literally,
ꢀ
the I3 /Iꢀ reduction reaction on the interface of electrocatalyst could be affected by these three
fundamental factors i.e. diffusion of electrolyte, electron transport and catalytic active sites. The
best catalytic activity must be the favorable combination of above three factors.
CuOꢀNRs/AB depicts superior PCE (8.05%) over pristine AB (6.51%) and Pt (6.96%)
due to the above stated basic parameters i.e. Jsc, Voc and FF, which are explained subsequently. (i)
Shortꢀcircuit photocurrent: CuOꢀNRs/AB (15.94 mA cmꢀ2) shows higher Jsc than those of pristine
AB (14.84 mA cmꢀ2) and Pt (15.25 mA cmꢀ2). The low Jsc of AB and Pt are due to their poor
electron conductivity. In contrast, amorphous carbon possesses more defective sites or catalytic
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sites as compare to graphitic carbon.27 So the transportation of electrons increases due to the
suitable amorphous structure of amorphous carbon. The graphitic carbon (AB) modified by CuO
nanorods can not only boost electrical conductivity but also provides sufficient catalytic sites.33,34
(ii) openꢀcircuit potential: The Voc of AB (0.74 V) and Pt (0.71 V) CEs are much lower than that
of CuOꢀNRs/AB (0.77 V) based CE DSSCs, using same working electrode and electrolyte. In
fact, the openꢀcircuit voltage is the difference of Fermi level of working electrode TiO2 film and
the redox potential of mediating agent that is influenced by instinct material properties of the
CEs.31 (iii) It is wellꢀknown that, FF is closely related to the Rs and Rct of DSSCs. Here, the Rs of
CuOꢀNRs/AB CE based DSSCs are nearly similar to those of AB and Pt CEs based DSSCs.
Therefore, the superior FF might be due to the lower Rct of CuOꢀNRs/AB CE. Both enhanced
electrical conductivity and abundant active sites of CuOꢀNRs/AB make it attractive CEs material
for high performances DSSCs.
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Table 1: Photovoltaic parameters and electrochemical characteristics of DSSCs based on CuOꢀ
NRs/AB, AB, and conventional Pt CEs.
CE
Voc (V)
FF
PCE (%) Rs (Ω)
Rct (Ω)
Ipc
Epp (V)
Jsc
(mA.cmꢀ2)
(mA.cmꢀ2)
CuOꢀ
NRs/AB
15.94
0.77
0.65
8.05 ± 0.2 21.00
1.90
ꢀ4.117
0.4261
14.84
15.25
0.74
0.71
0.60
0.63
6.51 ± 0.2 20.11
6.96 ± 0.2 22.59
3.45
2.10
ꢀ3.875
ꢀ3.097
0.5668
0.5958
AB
Pt
4. Conclusions
Shortꢀdimension assemblies of CuOꢀNRs/AB composite for high performance DSSCs are
fabricated by a facile approach in the presence of nonꢀpolar and polar solvents. Based on the
proposed method, CuOꢀNRs/AB provides low dimension 3ꢀD assemblies architecture, specific
electrocatalytic activity and high mesoporous channels for CEs, yielding higher solarꢀtoꢀ
electrical PCE than plain AB based DSSCs. The lightꢀtoꢀelectricity power conversion efficiency
of the cell with CuOꢀNRs/AB CE (8.05%) is comparable to that of conventional Pt (6.96 %). It is
demonstrated that the addition of CuOꢀNRs can enhanced the electrocatalytic activity of AB, due
to the synergistic catalytic effect and increased electroconductivity of suitable graphitization
degree of AB. Moreover, our proposed method of CuOꢀNRs/AB not only offers a novel and low
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possible reduction cost pathway to robust CEs but also can expands it to other metal oxides with
carbonaceous materials as an efficient CEs catalysts.
Acknowledgment
This work was supported by the National Basic Research Program (2011CB933300) of China.
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Figure Captions
Figure 1: Schematic of the fabrication procedure of 3D short assemblies for counter electrode of
DSSCs.
Figure 2: (a) XRD Analysis of AB, CuOꢀNRs and CuOꢀNRs/AB nanocomposite electrodes.
EDX spectra of (b) AB nanocrystals (c) CuO nanorods and (d) CuOꢀNRs/AB nanocomposite.
Figure 3: (a) & (b) representative SEM images of AB nanoparticles and CuOꢀNRs, respectively.
(c) Top view surface and cross sectional (inset) SEM images of CuOꢀNRs/AB nanocomposite
film. The circled regions illustrate the short 3D assemblies of super hydrophobic AB
nanoparticles and CuOꢀNRs. (d) high resolution SEM image of shortꢀdimension assembly, inset
shows highly porous structure of assemblies. (e) TEM image of nanocomposite of both
hydrophobic materials. The circled regions show the cluster of super hydrophobic AB
nanoparticles around hydrophobic CuOꢀNRs, (f) SAED pattern of AB. (g) electron diffraction
pattern and (h) HRTEM image of a CuOꢀNR.
Figure 4: (a) Raman spectra of individual AB and nanocomposite CuOꢀNRs/AB films. (b)
Cyclic voltammetry (CV), (c) Electrochemical impedance spectroscopy measurements (EIS) and
(d) typical photocurrent voltage (JꢀV) curves and dark current characteristics of DSSCs with asꢀ
prepared samples.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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