This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 9501–9503 9501

Cite this: Chem. Commun., 2012, 48, 9501–9503

Employing electrostatic self-assembly of tailored nickel sulfide

nanoparticles for quasi-solid-state dye-sensitized solar cells with Pt-free

counter electrodesw

Won Seok Chi, Joung Woo Han, Sungeun Yang, Dong Kyu Roh, Hyunjoo Lee* and

Jong Hak Kim*

Received 26th June 2012, Accepted 7th August 2012

DOI: 10.1039/c2cc34559e

A low cost, low-temperature processable, highly efficient nickel

sulfide counter electrode is demonstrated. Using the tailored,

preformed nickel sulfide nanoparticles and electrostatic self-

assembly, a novel counter electrode was fabricated that exceeded

the efficiency of a conventional Pt-based cell.

Dye-sensitized solar cells (DSSCs) are promising photovoltaic

devices due to their high efficiency and low cost.1 Considerable

efforts have been made to boost the efficiency and stability of

cells based on structural control of the TiO2 photoelectrode,2–4

development of efficient redox couples,5,6 synthesis of new

sensitizers7,8 and development of a stable solid electrolyte.9–11

One of the current issues of DSSCs is the use of platinum (Pt) as

a counter electrode, which is a noble metal with low abundance

and high cost, limiting large-scale manufacture. Furthermore,

heat treatments are needed to increase the electrocatalytic activity

and conductivity of Pt, precluding the use of flexible plastic

substrates. Thus, the development of new, Pt-free counter

electrodes has recently received considerable attention.12–23

Carbon-based materials with a high surface area such as

mesoporous carbon or carbon nanotubes (CNTs),12,13 and

conjugated conducting polymers are representative alternative

materials to Pt, but their electrocatalytic activities are lower

than that of Pt.14,15 Recently, inorganic compounds such as

metal oxides,16,17 nitrides,18 carbides19 and sulfides20–23 have

been investigated due to the possibility of generating diverse,

new nanomaterials with simple fabrication processes. Among

them, nickel sulfide is one of the most efficient catalytic

substances. Meng’s group prepared nickel sulfide layered on

F-doped tin oxide (FTO) glass via an electro-deposition

method and obtained an efficiency (6.82%) slightly lower than

that of a Pt-based cell (7.00%) upon using a liquid electrolyte.20

Lam’s group also prepared Ni3S2-coated counter electrodes

through an annealing process after drop-casting a nickel and sulfur

precursor mixture solution; they obtained an efficiency (7.01%)

slightly lower than that of the comparable Pt-based cell (7.32%)

with a liquid electrolyte.21 These previous papers have proved

that nickel sulfide is a prospective stable catalyst that can

substitute for the Pt counter electrode. However, there have

been no reports on the efficiency of metal sulfide counter

electrodes exceeding that of conventional Pt electrodes. This is

presumably because (1) the adhesion properties between the

FTO substrate and metal sulfides were not strong enough to

reduce the interfacial resistance of the electrode/electrolyte, and

(2) the metal sulfides were not small enough to provide high

electrocatalytic activity.

Here, we report a low cost, low-temperature processable,

highly efficient nickel sulfide counter electrode exceeding the

efficiency of the comparable Pt-based cell. Nickel sulfide

nanoparticles with two different atomic ratios and morphologies

were synthesized through wet-chemistry, which enables mass

production of electrode materials and facile fabrication of the

electrode. The preformed nanoparticles turned out to have distinct

surface charges, enabling facile deposition on the electrode

through electrostatic self-assembly of nanoparticles, which is

responsible for strong adhesion between the FTO substrate

and the nickel sulfide nanoparticles.

Fig. 1a and b shows transmission electron microscopy

(TEM) images of Ni3S2 and NiS nanoparticles, respectively.

Most Ni3S2 nanoparticles have an octahedral shape with an

average size of 30 nm, whereas the rod-shaped nanoparticles

with an average length of 50 nm were dominantly found in the

NiS. The insets in Fig. 1a and b are the high resolution (HR) TEM

images of Ni3S2 andNiS, which more clearly show their shape and

size. Energy dispersive X-ray spectroscopy (EDX) analysis with

TEM revealed that the atomic ratio of Ni to S was almost 3 : 2

and 1 : 1 for Ni3S2 and NiS, respectively (Table S1, ESIw).Interestingly, the surface charges of the preformed nickel

sulfide nanoparticles were significantly different, depending on

the atomic ratio and morphology. The zeta potential values of

the nickel sulfides were measured, as shown in Fig. S1 and

Table S2 (ESIw). The zeta potential of NiS nanoparticles was

+28.4 mV in ethanol solution, which was more than two-fold

greater than that of the Ni3S2 nanoparticles (+11.3 mV).

Nickel sulfide nanoparticles with a positive surface charge

would effectively attach to the negatively charged FTO glass

Department of Chemical and Biomolecular Engineering,Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749,South Korea. E-mail: azhyun@yonsei.ac.kr, jonghak@yonsei.ac.krw Electronic supplementary information (ESI) available. See DOI:10.1039/c2cc34559e

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9502 Chem. Commun., 2012, 48, 9501–9503 This journal is c The Royal Society of Chemistry 2012

via electrostatic interaction.24 Thus, the NiS nanoparticles

with a higher positive surface charge were expected to be more

strongly adhered to the FTO glass than the Ni3S2 nano-

particles. These speculations were visually confirmed in the

SEM images (Fig. 1c and d) and photos (Fig. 1f). Under the

same experimental conditions, the loading of NiS nano-

particles (0.02 � 0.005 mg cm�2) was always greater than that

of Ni3S2 (0.01 � 0.007 mg cm�2) due to the greater surface

charge of the former (Scheme S1, ESIw). It should be noted

that the Pt counter electrode is formed through a chemical

reduction process directly from the precursor (H2PtCl6), which

follows a completely different mechanism compared to that of

the nickel sulfide electrodes (Fig. 1e).

The detailed properties of nickel sulfides such as crystallinity

and element composition were characterized using X-ray

diffraction (XRD) and X-ray photoelectron spectroscopy

(XPS), as shown in Fig. S2 and S3 (ESIw), respectively.25

The nickel sulfides were utilized as a catalyst in counter

electrodes for quasi-solid-state DSSCs (qssDSSCs), which

have advantages in terms of lighter weight and improved

long-term stability compared to liquid DSSCs. Furthermore,

the process temperature (200 1C) of nickel sulfides is much

lower than that of Pt (450 1C), which enables the use of flexible

substrates, which are preferred for qssDSSCs. It should also be

noted that the interfacial contact between electrode and

electrolyte plays a very crucial role in determining the perfor-

mance of solid or qssDSSCs.4,9

The qssDSSC fabricated with NiS displayed higher efficiency

(6.8% at 100 mW cm�2), which is one of the highest values

observed for qssDSSCs4,9,10 and approximately 1.2-fold greater

than that of a conventional Pt electrode (5.8%). As far as we

know, this is the first report of nickel sulfide counter electrodes

exceeding the efficiency of Pt-based cells (Fig. 2 and Table 1).20,21

The efficiency improvement is mostly due to the increased FF

value, resulting from the reduced interfacial resistance of the cell

and larger electrocatalytic activity, which will be characterized

in detail. Jsc values were not different among the cells, but the

Voc values of nickel sulfides were slightly lower than that of Pt.

This might result from recombination due to the presence of

some free nickel sulfide nanoparticles less strongly bound to

the FTO substrate.20

The internal resistance and charge transfer kinetics of

qssDSSCs fabricated with different counter electrodes were

investigated using electrochemical impedance spectroscopy (EIS)

analysis. EIS was performed under dark conditions, as shown in

Fig. S4 (ESIw). The dark EIS data indicate that the recombination

resistance of the Pt counter electrode is greater than that of the

nickel sulfide electrode, which explains the slightly lower Voc

values of nickel sulfide-based cells compared to Pt-based cells.26

EIS was also performed under 1 sun in order to investigate the

interfacial resistance of the cells, as shown in Fig. S5 (ESIw).27,28 Asmaller Rs value indicates that the catalytic material is more

strongly attached to the FTO glass substrate.19 The Rs values of

NiS, Ni3S2 and Pt were 13.8, 14.7 and 16.4 O, respectively,indicating that the strength of adhesion to the FTO is NiS >

Ni3S2 > Pt, which is consistent with the zeta potential results.

The Rct1 value can be related to the electrocatalytic activity

of materials. Cyclic voltammetry (CV) was performed on the

catalysts to investigate the electrocatalytic activity for the

reduction of I3 at the counter electrode, as shown in Fig. 3.

Fig. 1 TEM images of (a) Ni3S2 and (b) NiS nanoparticles. The insets

in (a) and (b) show HR-TEM images of each particle. SEM images of

(c) Ni3S2, (d) NiS and (e) Pt on FTO glass. (f) Photos of three counter

electrodes (Ni3S2, NiS, Pt).

Fig. 2 J–V curves of qssDSSCs with different counter electrodes

under one sun and dark conditions.

Table 1 Photovoltaic parameters of DSSCs with different counterelectrodes

SampleVoc

a

(V)Jsc

b

(mA cm�2) FFcZ(%)

Rsd

(O)Rct1

e

(O)CPE1

f

(mF)

NiS 0.75 13.5 0.65 6.8 13.8 8.5 55.6Ni3S2 0.75 13.4 0.59 5.9 14.7 22.1 26.7Pt 0.77 13.5 0.55 5.8 16.4 19.4 24.6

a Voc: open circuit voltage. b Jsc: short circuit current density.c FF: fill

factor. d Rs: series resistance. e Rct1: charge transfer resistance 1.f CPE1: constant phase element of capacitance corresponding to Rct1.

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 9501–9503 9503

All three electrodes showed two similar pairs of redox peaks,

indicating their similar roles as a catalyst.

I3� + 2e� 2 3I� (1)

3I2 + 2e� 2 2I3� (2)

Reaction (1) corresponds to the cathodic peak around �0.3 V

and an anodic peak around 0.0 V, whereas reaction (2)

corresponds to a cathodic peak around 0.25 V and an anodic

peak around 0.43 V. The cathodic peak of reaction (1) is the

most important peak, indicating the reduction of I3 at the

counter electrode with electrolyte in DSSCs.20,21 The cathodic

peak position in reaction (1) appeared at a more positive potential

for NiS compared to Pt, demonstrating superior catalytic activity of

NiS compared to Pt. TheRct1 values of NiS, Ni3S2 and Pt were 8.5,

22.1 and 19.4 O, respectively, indicating that NiS has the greatest

electrocatalytic activity, which is in good agreement with the CV

results. The CPE1 values were ordered as follows: NiScNi3S2 >

Pt, based on FTO surface coverage and consistent with the SEM

images. The small interfacial resistance at the counter electrodes

as well as the greater electrocatalytic activity of NiS is responsible

for a high FF of 0.65, resulting in higher efficiency (6.8%) than

the Pt-based cell. On the other hand, better adhesion of N3S2to the FTO glass compensated for its lower electrocatalytic

activity relative to Pt, leading to an efficiency (5.9%) compar-

able to that of a Pt-based cell (5.8%). We also found that the

monolayered, deep coverage of nickel sulfides on the FTO

glass via the electrostatic self-assembly using the drop-casting

method is most important for improving the cell performance,

as seen in Fig. S6 and Table S3 (ESIw).In conclusion, we have demonstrated a low cost, low-

temperature processable, highly efficient nickel sulfide counter

electrode. The counter electrode comprised of the tailored,

preformed nickel sulfide nanoparticles via electrostatic self-

assembly showed an efficiency of 6.8% at 100 mW cm�2. The

observed efficiency is one of the highest values for qssDSSCs4,9,10

and much greater than that of a conventional Pt electrode

(5.8%). In particular, the rod-shaped NiS nanoparticles exhibited

higher electrocatalytic activity than Pt. Furthermore, the electro-

static self-assembly method allowed the preformed nanoparticles

to attach more strongly to the FTO glass, leading to reduced

interfacial resistances of the cells. The low-cost, highly catalytic

nanomaterials and facile fabrication methods should have

enormous potential to pioneer nano-scale energy devices with

high energy conversion performance.

We acknowledge the financial support of a National Research

Foundation (NRF) grant funded by the Korean government

(MEST) through No. 2012R1A2A2A02011268 and the Korea

Center for Artificial Photosynthesis (KCAP) (NRF-2009-

C1AAA001-2009-0093879).

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Fig. 3 Cyclic voltammograms of Ni3S2, NiS and Pt electrodes in

10 mM LiI, 1 mM I2, 0.1 M LiClO4 in acetonitrile solution at a scan

rate of 50 mV s�1.

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