Electrochemical deposition of p-type CuSCN in porous n-type TiO2 films

Article abstract of DOI:10.1016/j.electacta.2007.01.039

We present an energy band model and a method for filling p-type CuSCN in n-type porous TiO₂ film. The energy band model is based on the interface energy levels between TiO₂/CuSCN heterojunction and the aqueous electrolyte. The whole deposition process is divided into three stages: the uniform nucleation on the internal surface at positive potential, the crystal growth with the cathodic potential shifting negatively and the thermal activated growth at constant potential. This was demonstrated by the electrochemical experiment combining the hydrothermal process. It was found that the obtained TiO₂/CuSCN heterojunction exhibited good rectification characteristics, indicating that an intimate electrical contact was formed between the large internal surface of TiO₂ film and CuSCN. This novel hydrothermal-electrochemical method may be valuable for fabricating extremely thin absorber (eta)-solar cells and other semiconductor devices.

Full text of DOI:10.1016/j.electacta.2007.01.039

Electrochimica Acta 52 (2007) 4804–4808
Electrochemical deposition of p-type CuSCN in porous n-type TiO2 films
W.B. Wua, Z.G. Jinb, G.D. Hua,∗, S.J. Buc
a School of Materials Science and Engineering, Jinan University, Shandong 250022, PR China
b Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, PR China
c School of Materials Science and Engineering, Tianjin 300130, Hebei University of Technology, PR China
Received 4 November 2006; received in revised form 8 January 2007; accepted 18 January 2007
Available online 30 January 2007
Abstract
We present an energy band model and a method for filling p-type CuSCN in n-type porous TiO2 film. The energy band model is based on
the interface energy levels between TiO2/CuSCN heterojunction and the aqueous electrolyte. The whole deposition process is divided into three
stages: the uniform nucleation on the internal surface at positive potential, the crystal growth with the cathodic potential shifting negatively and the
thermal activated growth at constant potential. This was demonstrated by the electrochemical experiment combining the hydrothermal process. It
was found that the obtained TiO2/CuSCN heterojunction exhibited good rectification characteristics, indicating that an intimate electrical contact
was formed between the large internal surface of TiO2 film and CuSCN. This novel hydrothermal-electrochemical method may be valuable for
fabricating extremely thin absorber (eta)-solar cells and other semiconductor devices.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: CuSCN; Porous TiO2 film; Energy band model; Hydrothermal-electrochemical method; Eta-solar cell
1. Introduction
trochemical deposition (ED) is considered one standard method
for coating complex shapes [14]. For example, it has been a pre-
ferred technique for depositing metallic copper in the complex
damascene patterns in integrated circuits [15]. The deposition
of CuSCN in porous substrates with the ED method has been
reported [7,12]. It has ever been thought that the organic electro-
chemical deposition solution is better than the aqueous solution
because the aqueous solution is unstable as well as unsuitable
for obtaining fine structure [16]. However, in our previous work,
a stable aqueous solution was prepared. In addition, the fine
CuSCN film was deposited on ITO glass from the aqueous solu-
tion by ED and the formation mechanisms was also discussed
[17]. Further, in the present study, we investigated the deposition
of CuSCN in porous TiO2. A three-stage energetic model was
proposed for filling CuSCN in pores of TiO2 substrate. Further-
more, this was confirmed by a hydrothermal-electrochemical
experiment.
Semiconductor growth on highly structured and porous sub-
strates is of interest for a range of electrical, chemical and
catalytic applications. One typical example is the extremely thin
absorber (eta)-solar cell, mainly consisting of a porous n-type
window layer, an inorganic eta layer, and a wide-band-gap p-
type layer. Highly structural substrates, such as porous TiO2 and
ZnO nanowire films, are used for windows layer to enhance light
trap and improve the separation efficiency of the excited carri-
ers [1–4]. Narrow-band-gap materials, CdTe [5], HgCdTe [6],
CuInS2 [7], CdSe [8], etc., are used for absorbers. Among the
p-type layer materials, CuSCN is considered the most promising
for its stability and technical feasibility in filling pores [9,10].
Conformal deposition of eta layer and p-type layer in pores
of substrate is crucial for obtaining high energy efficiency.
Although some advanced deposition techniques, MBE, MCVD,
PLD and ALD, are the desirable methods for plane substrates,
the problem of non-uniform deposition arises for porous sub-
strates due to the shadow effect [3,4]. Many efforts have been
made to resolve the problems [2,3,11–13]. Among these, elec-
2. Experimental
2.1. Porous TiO2 electrode formation
∗
The porous TiO2 film was deposited directly on conductive
Corresponding author. Tel.: +86 53185175201; fax: +86 53182767915.
E-mail address: mse hugd@ujn.edu.cn (G.D. Hu).
indium tin oxide coated glass (ITO, 10 ꢀ/ꢀ) by a sol–gel dip-
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.01.039

W.B. Wu et al. / Electrochimica Acta 52 (2007) 4804–4808
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coating technology [18]. In details, the precursor sol solution
was obtained by following steps: dissolve 0.5 M Ti(OC4H9)4
and 1 M diethanolamine (DEA) in ethanol, hydrolyze and add
20 g/L polyethylene glycol (PEG, molecular weight 1000) under
magnetic stirring. The precursor sol film was dip coated on ITO
glass. The porous TiO2 electrode was obtained after the precur-
sor film was aged at 100 ◦C for 10 min and calcined at 550 ◦C
for 1 h in air.
2.2. Electrochemical deposition of CuSCN
The CuSCN films were deposited on ITO glass and the
as-prepared TiO2 electrode by the electrochemical method,
respectively. The stable aqueous electrolyte, with 0.06 M
CuSO4, 0.03 M KSCN and 0.06 M EDTA, was prepared based
on our previous study [17]. The pH value was adjusted to 2–2.6
with sulphuric acid (H2SO4). Electrochemical deposition and
measurement were carried out on the LK2005 electrochemi-
cal workstation (Tianjin, China). The normal three-electrode
electrochemical cell was placed in a hydrothermal bath. The
reference electrode and the counter electrode were Ag/AgCl
and Pt, respectively. Potentials mentioned later are all respect
to Ag/AgCl electrode. A thin Au layer was sputtered on the
obtainedheterojunctionfilmforsubsequentcurrent–voltagetest.
Fig. 1. SEM photographs of: (a) the surface and (b) the cross-section of as-
prepared porous TiO2 film.
2.3. Characterization
TiO2 particlesareinterconnectedandpartlyfusedtogetherrather
than individually dispersed as reported by Cao and Hagfeldt
[19,20]. The Efb of −0.07 eV accords with the surface states’
Fermi level (Efs) of TiO2 nanoparticles but is obvious lower
than the Ec (0.51 eV) of nanocrystalline anatase TiO2, implying
that the Efs in the band gap will provide the dominant pathway
for electron transfer between TiO2 nanoparticles and acceptor
species in electrolyte solution [21]. From Fig. 2b, it is found that
CuSCN film is p-type and the Efb is about −0.41 eV, consistent
with that from O’Regan et al. [8]. Since the Efs of TiO2 electrode
is higher than the Efb of CuSCN, electrons will transfer from
TiO2 to CuSCN when the cathodic potential is more negative
than the Efb of CuSCN.
Scanning electron microscope (SEM) images were taken on a
PHILIPS XL-30 environment scanning electron microanalyzer.
N2 adsorption was measured on a Quanta-chrome Nova 2000
specific area instruments. The Mott–Schottky plots and the I–V
plot were measured on the advanced electrochemical system
(PARSTAT 2263, EG&G).
3. Results and discussion
3.1. Structural and electrochemical characteristics of TiO2
and CuSCN films
To elucidate the interface energy levels, the structural and the
electrochemical properties of TiO2 electrode and CuSCN film
were characterized, respectively.
3.2. Energy band model for deposition of CuSCN in porous
TiO2 film
Fig. 1a and b shows the typical morphologies of the surface
and the cross-section of the as-prepared porous TiO2 film. The
TiO2 film exhibits a highly interpenetrated porous structure with
pore size of 200–300 nm and the thickness of ∼1 ␮m. The sur-
face enlargement factor is over 10 for a BET surface area of
72 m2 g−1 [18]. This unique structure is suitable for eta-solar
cells [19].
The flat band potential (Efb) of TiO2 electrode and CuSCN
film can be obtained from the Mott–Schottky plots according
to the M–S equation: 1/(Csc)2 ∝ E − Efb − kT/e, where C is the
differential capacitance of the space charge layer. The measuring
aqueous electrolyte, with 0.2 M KSCN and a pH value of 2.1, is
similar to the electrochemical deposition solution [17]. Efb was
obtained by extrapolating the linear part to the potential axis
where 1/(Csc)2 = 0. The linear part in M–S plot suggests that the
The energy band model is discussed based on the above
energy levels and the Eredox of the electrolyte. As well known,
owing to the band-edge pinning effect, a space charge layer can
be formed at the interface when a semiconductor is in direct
contact with an electrolyte. If the Efb or Efs is higher than
Eredox, electrons in solid will transfer to the acceptor species in
electrolyte, resulting in a depletion layer in the semiconductor.
Otherwise, an accumulation layer will be formed.
For the case of the interface between TiO2/CuSCN hetero-
junction and the electrolyte, the distribution of the energy levels
is much more complex, as shown in Fig. 3. If the CuSCN side
is thick enough, four regions will be formed. In region I, the
quasi-Fermi level for holes (Efp) extends to n-type side. The
width decreases with the negative shift of the cathodic potential.

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the thermal activation of the surface states (CuII(SCN)x) or the
acceptor species (CuII), because the thermal fluctuation of the
surface state or the acceptor species levels (0.5 eV at 55 ◦C) is
much larger than the thermal energy (kT ≈ 0.03 eV at 55 ◦C) of
electrons and holes in CuSCN [17].
According to the above energy model, we propose a three-
stage process for the deposition of CuSCN. In the initial stage,
TiO2 electrode is in direct contact with the electrolyte. For con-
formal deposition on the internal surface of the TiO2 electrode,
small CuSCN crystalline particles are required. To ensure the
size of nuclei as small as possible, the initial deposition poten-
tial must be set lower than the Eredox. Thus, once a thin layer of
p-type CuSCN nuclei is formed on TiO2, the electrons flowing
will be inhibited since the deposition potential is a backward
bias for the just formed junction. Then, the crystal growth is
prevented. The subsequent transfer of electrons will take place
only on the bare TiO2. Consequently, the internal surface will
be completely covered with a thin CuSCN nuclei layer. In addi-
tion, to avoid the crystal growth by the thermal activation of the
acceptor species or the surface states, the deposition must be
performed at a temperature lower than 10 ◦C.
In stage II, corresponding to region II, the formed CuSCN
nuclei will grow by electrons injection through the Efn extending
to CuSCN as the cathodic potential shifts negatively. The final
potential must be under −0.5 V. Otherwise, Cu-rich compounds
or metallic copper will be formed [17]. The process is terminated
until the thickness of CuSCN reaches the maximum width of
region II at the constant potential −0.5 V, when the deposition
current is constant and the very small current is resulted due to
the thermal activation.
Fig. 2. Mott–Schottky plots of: (a) porous TiO2 films and (b) CuSCN films in
0.2 M KSCN aqueous solution, pH 2.1. The measuring frequency is 100 and
500 Hz.
In stage III, combining the above discussion about regions III
and IV, it can be inferred that CuSCN crystals grow only by the
thermal activation of the acceptor species or the surface states.
This can be realized by elevating the deposition temperature.
In region II, the quasi-Fermi level for electrons (Efn) in p-type
side is higher than both of the Eredox and the Efp. In region III,
the Efn is located between the Eredox and the Efp and descends
toward the electrolyte. As for region IV, only the Efp is left. The
widths of regions II and III will increase with the negative shift
of the cathodic potential. According to our previous study, the
deposition reaction in regions III and IV is predominated by
3.3. Deposition and characteristic of CuSCN in porous
TiO2 electrode
According to above discussion, we designed a hydrothermal-
electrochemical process. Firstly, the deposition was performed
at+0.1 Vand10 ◦Candkeptforseveralminutestoproduceathin
CuSCN nuclei layer. Then, the deposition potential was scanned
from +0.1 to −0.5 V by a small step of 1 mV/s to deposit CuSCN
layer by layer. The procedure was maintained at the constant
potential (−0.5 V) until the deposition current was constant. In
the following stage, the temperature was elevated to 45 ◦C by
a step of 2 ◦C/min and held for 15 min. Then, a TiO2/CuSCN
heterojunction film was resulted. The current and temperature
plotsagainsttimeduringthedepositionareshowninFig. 4. Itcan
be found that the current trend is well consistent with the above
discussion. Because the three deposition stages are realized by
five segregated steps, the current density increases sharply at
the very beginning of every step, which is due to the charging
of the double charge layer. In the stage V, the current fluctuates
with the fluctuation of the temperature. The growth rate of the
CuSCN crystal is very correlative with the temperature for the
Fig. 3. Band edges and quasi-Fermi levels for electrons and holes of
CuSCN/TiO2 heterojunction at negative bias with respect to the equilibrium
potential.

W.B. Wu et al. / Electrochimica Acta 52 (2007) 4804–4808
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Fig. 6. Current–voltage characteristic of TiO2/CuSCN heterojunction.
Fig. 4. Plots of current and temperature against time at three deposition stages.
The whole deposition is divided into five segregated steps.
Fig. 6 shows the typical current–voltage curve of the het-
erojunction film. The heterojunction shows good rectifying
behavior with the rectification ratio of ∼33, the reverse satura-
tion current of ∼6 × 10−3 mA/cm2, and the series resistance of
∼1 × 103 ꢀ. The series resistance can be substantially reduced
by adjusting the thickness of CuSCN and TiO2 layers more care-
fully. Our results suggest that a good electronic contact is formed
between the large area TiO2 and CuSCN layer.
current density varies largely with the temperature. It can be
predicted that the junction quality can be further improved if the
deposition is performed at lower temperature close to 0 ◦C.
Fig. 5a and b shows the SEM photographs of the surface and
the cross-section of the TiO2/CuSCN heterojunction. It is found
that, in addition to the dense CuSCN in pores, a pure CuSCN
layer with the thickness of ∼1 ␮m is produced on the surface.
The CuSCN layer is dense besides of several large particles on
top surface, which may originate from the leaking current due
to the polycrystalline structure. Our TiO2/CuSCN heterojunc-
tion film is largely improved compared to that from the organic
electrolyte [22].
4. Conclusions
An energy band model for depositing TiO2/CuSCN het-
erojunction film has been established based on the interface
energy levels between the p–n heterojunction and the aqueous
electrolyte. According to the model, we propose a three-stage
deposition procedure: the uniform nucleation on pore walls at
positive potential, the crystal growth at negative potential and
the thermal activated growth at constant potential. This is fur-
ther confirmed by a hydrothermal-electrochemical process. The
as-prepared TiO2/CuSCN heterojunction exhibits good recti-
fication characteristics. The rectification ratio is ∼33 and the
reverse saturation current is ∼6 × 10−3 mA/cm2. The results
indicate that a good electric contact has been formed between
large area TiO2 and CuSCN. The hydrothermal-electrochemical
technique has potential applications in eta-solar cells and other
p–n heterojunction systems.
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