FeSe2 films with controllable morphologies as efficient counter electrodes for dye-sensitized solar cells

Article abstract of DOI:10.1039/c3cc49175g

The FeSe₂ films with controllable morphologies (including 3D flower-like and sphere-shaped) have been applied as the counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). It is found that 3D flower-like FeSe₂ CEs perform c

Full text of DOI:10.1039/c3cc49175g

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FeSe2 films with controllable morphologies as
efficient counter electrodes for dye-sensitized
solar cells†
Cite this: Chem. Commun., 2014,
50, 2618
Received 3rd December 2013,
Accepted 10th January 2014
Wenjun Wang,a Xu Pan,*a Weiqing Liu,a Bing Zhang,b Haiwei Chen,a Xiaqin Fang,a
Jianxi Yaob and Songyuan Dai*ab
DOI: 10.1039/c3cc49175g
www.rsc.org/chemcomm
The FeSe2 films with controllable morphologies (including 3D flower- special shapes for application in the development of advanced
like and sphere-shaped) have been applied as the counter electrodes functional devices. In recent years, much effort has been devoted to
(CEs) for dye-sensitized solar cells (DSSCs). It is found that 3D flower- the synthesis of FeSe2 films, low-dimensional FeSe2 micro/
like FeSe2 CEs perform comparably to conventional platinum CEs nanostructures and anisotropic three-dimensional (3D) FeSe2 nano-
(power conversion efficiencies of 8.00 and 7.87%, respectively).
structures. However, to the best of our knowledge, no research on
the use of FeSe2 as CEs in DSSCs has been reported so far.
Herein, as the electrocatalytic activity may depend on the
Dye-sensitized solar cells (DSSCs) are promising alternatives to
conventional solid-state photovoltaic devices based on materials morphology, we have prepared the FeSe2 films with controllable
such as Si, CdTe and CuIn1ꢀxGaxSe2, owing to their low cost, ease morphologies, including 3D flower-like FeSe2 assembled with nano/
of production and high efficiency.1 In general, a typical DSSC microrods (Fr) and sphere-shaped FeSe2 assembled with sphere-
usually comprises three main components: a dye-sensitized shaped particles (Fs), via a facile hydrothermal synthetic approach,
porous nanocrystalline titanium dioxide (TiO2) film as the photo- and used them as Pt-free electrocatalysts for DSSCs, respectively.
anode, an electrolyte traditionally containing an iodide/triiodide Electrochemical results of the Fr CE showed Pt-like catalytic activity
(Iꢀ/I3ꢀ) redox couple and a counter electrode (CE).
for triiodide reduction, surpassing that of the Fs CE. As a result, the
As a crucial component, CE plays an important role in the power conversion efficiency (Z) of the DSSC with the Fr CE was
performance of a DSSC in that it collects electrons from external 8.00%, which was comparable to that of the Pt CE (7.87%), while the
circuit and fulfills electron transfer from ꢀthe CE interface to the Z of the Fs CE was 7.38%. Details of the fabrication method are
electrolyte by catalyzing the reduction of I3 to Iꢀ. Usually, fluorine shown in the experimental section of the ESI.†
doped tin oxide (FTO) glass loaded with noble metal platinum (Pt) is
The crystallographic structures of the as-synthesized FeSe2 pro-
the preferred material for the CE due to its superior conductivity, ducts were characterized by X-ray diffraction (XRD). From the XRD
electrocatalytic activity and stability.2 However, in view of the low patterns (Fig. 1), all diffraction peaks of Fr and Fs can be readily
abundance and high cost of platinum, much incentive exists in indexed to an orthorhombic phase of FeSe2 (JCPDS No. 65-2570).
developing other cost-effective alternatives for Pt. For instance, carbon The morphologies of the FeSe2 samples (Fr and Fs) were observed
materials,3 conducting polymers4 and inorganic compounds5,6 have using typical scanning electron microscopy (SEM) and transmission
been utilized as CEs in DSSCs. In particular, some metal chalcogen- electron microscopy (TEM). As shown in Fig. 2a, the as-obtained Fr
ides among these Pt-free electrocatalysts have been shown to be quite CE in the reaction system without citric acid consists almost entirely
effective and have exceeded the performance of Pt.7–10 As another of the flower-like FeSe2 assembled with micro/nanorods with an
compound in the metal chalcogenide series, iron diselenide (FeSe2) is
an important VIII–VI transition metal selenide semiconductor and
has drawn enormous attention owing to its unusual structure and
electronic properties, as well as the opportunity it offers to engineer
a Key lab of Novel Thin Film Solar Cells, Institute of Plasma Physics,
Chinese Academy of Sciences, Hefei, PR China. E-mail: sydai@ipp.ac.cn,
mars_dark@hotmail.com; Fax: +86 (0)551 5591377; Tel: +86 (0)551 5591377
b State Key Laboratory of Alternate Electrical Power System with Renewable Energy
Sources, North China Electric Power University, Beijing 102206, China
† Electronic supplementary information (ESI) available: Full experimental proce-
dures. See DOI: 10.1039/c3cc49175g
Fig. 1 XRD patterns of the as-synthesized FeSe2 products.
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Table 1 Photovoltaic and EIS parameters of the DSSCs with different CEs
Voc
/
Jsc/
Rs/
Rct/
ZN/
Epp/
CEs mV mA cmꢀ2 FF
Z/% O cm2 O cm2 O cm2 mV
Fr
Fs
Pt
744 14.93
724 14.60
741 15.13
0.721 8.00 16.82
0.698 7.38 27.05
0.702 7.87 17.01
0.53
0.96
0.78
0.43
0.79
0.59
252
302
262
Fig. 2 (a) Low-, (b) high-magnification and (c) cross-section SEM images
of Fr grown in situ on the FTO glass. The inset shows the TEM image of Fr.
(d) Low and (e, f) high-magnification SEM images of Fs grown in situ on the
FTO glass. The inset shows the TEM image of Fs.
average diameter of 0.5–3 mm. Only a small portion of single micro/
nanorods can be found. As shown in Fig. 2(b) and (c), the 3D flower-
like microconstruction is composed of many 1-dimensional subunits
(micro/nanorods) that connect with each other by self-assembly. The
inset shows the TEM image of Fr. Fig. 2d shows the SEM image of
the as-obtained Fs CE in the same reaction system with additional
citric acid. The surface is partially covered with single relatively
uniform sphere-shaped particles (Fig. 2e). Meanwhile, other parts of
the film have large near-spherical particles. The sizes of the large
near-spherical particles are found to be in micrometer-scale. As can
be seen at higher magnification (Fig. 2f), the large near-spherical
particles are composed of lots of nanoparticles. The inset of Fig. 2d
shows the TEM image of single sphere-shaped particles with a mean
size of 200–400 nm.
Fig. 4 (a) Cyclic voltammograms for Fr, Fs and Pt CEs obtained at a scan
rate of 50 mV sꢀ1. (b, c) CV curves of the Fr and Fs CEs at various scan rates.
The inset shows the redox peak current of eqn (1) as a function of scan
rate. (d) Nyquist plots obtained by EIS analysis of the symmetrical cells. The
inset gives the equivalent circuit.
To inꢀvestigate the reaction kinetics of various electrodes in
the Iꢀ/I3 electrochemical system, cyclic voltammetry (CV) was
carried out in a three-electrode system. Fig. 4(a) shows the CV
curves of the Fr, Fs and Pt CEs, respectively. Two pairs of redox
peaks (peak I/peak I0 and peak II/peak II0) are observed for all of
the materials. The left redox peaks correspond to eqn (1) and
the right ones correspond to eqn (2)11,12
The performances of the DSSCs with Fr and Fs CEs were
studied in comparison to that of the DSSC with the Pt CE. The
photocurrent–voltage (I–V) curves of the DSSCs applying Fr, Fs and
Pt CEs are shown in Fig. 3 (under simulated solar light illumina-
tion of 100 mW cmꢀ2). The corresponding photovoltaic para-
meters are summarized in Table 1. As shown in Table 1, when
the Fr was used as the counter electrode, the DSSC exhibits a short-
circuit current ( Jsc) of 14.93 mA cmꢀ2, an open-circuit voltage (Voc)
of 744 mV, a fill factor (FF) of 0.721 and an Z of 8.00%. The DSSC
ꢀ
I3 + 2eꢀ 2 3Iꢀ
(1)
(2)
ꢀ
3I2 + 2eꢀ 2 2I3
with the Pt CE, on the contrary, renders a Jsc of 15.13 mA cmꢀ2
,
Since the counter electrode of a DSSC is responsible for catalyzing
the reduction of I3ꢀ to Iꢀ, the characteristics of peak I and peak I0 are
the focus of our analysis. The peak current and the peak-to-peak
separation (Epp), which is negatively correlated with the standard
electrochemical rate constant of the redox reaction, are two critical
parameters for comparing electrocatalytic activities of different CEs.13
As shown in Fig. 4(a), the lower Epp and higher current densities of
the Fr electrode indicate that Fr produces higher reversibility of the
I3ꢀ/Iꢀ redox reaction and remarkable electrocatalytic activity for I3ꢀ
reduction, even better than Pt, which is advantageous for the applica-
tion of the Fr electrode as an efficient CE in DSSCs. Meanwhile, the
lower peak current densities and larger Epp of the Fs electrode reveal
that the electrocatalytic activity of the Fs electrode is inferior to that of
Fr and Pt electrodes. The conclusion from the CV data is consistent
with previous energy conversion efficiency results. The Z of the
Fs-based DSSCs are lower than those of Fr and Pt-based DSSCs, which
stems from the inferior electrocatalytic ability of the Fs electrodes.
a Voc of 741 mV, a FF of 0.702 and an Z of 7.87%. The Fr-based cells
showed a comparable, even slightly higher, Z than Pt, which is
mainly attributed to the slight improvement in the FF. Meanwhile,
the Z of 7.38% has been obtained by the DSSCs applying Fs CEs,
indicating a poorer catalytic activity for the reduction of I3ꢀ. The
incident photon-to-current conversion efficiency (IPCE) spectra of
these DSSCs were also collected (see ESI†).
Fig. 3 I–V curves of DSSCs with Fr, Fs and Pt electrodes.
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Fig. 4(b) and (c) represent the typical CV curves of Fr and Fs Meanwhile, with irregular surfaces, the electrons or holes must
electrodes in the Iꢀ/I3 system with different scan rates. As shown move through many interfaces before they can meet and recom-
ꢀ
in Fig. 4(b) and (c), the peak current densities (Ip) changed with the bine with each other,10 which may result in the inferior electro-
scan rate. Also, the cathodic peaks gradually and regularly shifted catalytic activity of Fs. However, the Fs CE is also covered with
negatively, and the corresponding anodic peaks shifted positively single sphere-shaped particles that may be active sites to improve
upon increasing the scan rate. The inset of Fig. 4(b) and (c) the electrocatalytic activity. As a consequence, the Fs CE still
illustrates a linear relationship between the redox peak current exhibits good electrocatalytic activity.
density and the square root of scan rates. According to the Langmuir
In conclusion, we have shown that FeSe2 films with control-
isotherms principle, this linear relationship indicates the diffusion lable morphologies (including 3D flower-like and sphere-shaped)
limitation of the redox reactions (eqn (1)) on the surface of Fr and Fs can be used as electrocatalysts for triiodide reduction in DSSCs.
CEs.14 This phenomenon also shows that the adsorption of iodide The Fr CE has been successfully demonstrated to be an efficient
species is little affected by the redox reaction on the surface of Fr electrocatalyst with low charge-transfer resistance (Rct) and fast
ꢀ
and Fs CEs and there is no specific interaction between the Iꢀ/I3
redox couple and Fr and Fs CEs as is the case of the Pt CE.15,16
reaction rates for the reduction of I3ꢀ ions, even slightly superior
to Pt. As a consequence, the DSSC with the 3D flower-like FeSe2
To further elucidate the electrocatalytic activity of the FeSe2 CEs CE shows an Z of 8.00%, slightly higher than that of the DSSC
ꢀ
in the reduction of I3 ions, electrochemical impedance spectro- using a Pt CE (7.87%). In contrast, the Fs CE is inferior to the Fr
scopy (EIS) was conducted on the symmetrical Fr–Fr, Fs–Fs and CE. Furthermore, the simple preparation procedure and inexpen-
Pt–Pt electrochemical cells. Fig. 4(d) shows the Nyquist plots sive cost properties of the FeSe2 CE allow the development of a
obtained by the EIS analysis of symmetric cells and the equivalent high-performance FeSe2 CE to replace the expensive Pt CE
circuit diagram used for the simulation. Generally, the high- catalyst in large-scale industrial DSSC production.
frequency intercepts on the real axis represent the series resistance
This work was financially supported by the National Basic
(Rs), which is mainly composed of the bulk resistance of CE Research Program of China under Grant No. 2011CBA00700,
materials, resistance of FTO glass substrates, contact resistance, the National High Technology Research and Development
etc. The high frequency semicircle indicates the electrochemical Program of China under Grant No. 2010AA050510, and the
charge transfer resistance (Rct) at the interface of CE/electrolyte for National Natural Science Foundation of China under Grant No.
ꢀ
I3 reduction and the corresponding constant phase element 21273242, 21173227, 21103197.
(CPE) describing deviation from the ideal capacitance, due to the
electrode roughness, whereas the low frequency semicircle indꢀi-
cates the Nernst diffusion-limited impedance (Zn) of the Iꢀ/I3
redox species in the electrolyte.16 The Nyquist plots were fitted and
the impedance parameters of various electrodes obtained by fitting
are shown in Table 1. The Fr CE possessed the lowest Rct (0.53 O
cm2), so it had a relatively high catalytic activity. The conclusions
derived from the EIS and CV data are consistent. Furthermore,
previous analyses have indicated that the total series resistance
(Rseries) of solar cells strongly influences the FF and Z.16,17 In
DSSCs, the Rseries is mainly related to Rs, Rct, the diffusion
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