Bismuth chalcogenide iodides Bi13S18I2 and BiSI: Solvothermal synthesis, photoelectric behavior, and photovoltaic performance

Article abstract of DOI:10.1039/c9tc05139b

Bismuth chalcogenide iodides Bi₁₃S₁₈I₂ and BiSI were synthesized and characterized as light absorbers for solar cells. The formations of Bi₁₃S₁₈I₂ and BiSI can be controlled via a facile solvothermal process. BiSI is formed under low S/Bi mole ratio conditions but it easily converts to Bi₁₃S₁₈I₂ under high S/Bi mole ratio conditions. Bi₁₃S₁₈I₂ is an n-type semiconductor with a narrow bandgap of 0.75 eV and exhibits a strong light absorbance in the wide wavelength range from UV to NIR, making it a promising light absorber material for solar cells. A uniform B₁₃S₁₈I₂ nanorod film was fabricated on a porous TiO₂ electrode via a modified solvothermal process for a photovoltaic study. A Bi₁₃S₁₈I₂-based solar cell has demonstrated a power conversion efficiency of 0.85% for the first time, revealing the potential of Bi₁₃S₁₈I₂ as a new type of light absorber material for solar cells which can be fabricated via a low-cost solution process.

Full text of DOI:10.1039/c9tc05139b

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J Po lue ran sael od fo Mn aot te rai ad l jsu Cs ht emm ai rs gt ri yn sC
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DOI: 10.1039/C9TC05139B
Journal Name
ARTICLE
Bismuth Chalcogenide Iodides of Bi S I and BiSI: Solvothermal
Synthesis, Photoelectric Behavior, and Photovoltaic Performance
1
3 18 2
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 00.0000/x0xx00000x
a
a
b
a
a
a
Sen Li, Linfeng Xu, Xingang Kong, Takafumi Kusunose, Noriaki Tsurumach, and Qi Feng *
Bismuth chalcogenide iodides of Bi13
for solar cells. The formations of Bi13
S
S
18
I
2
and BiSI were synthesized and characterized as a light absorber
and BiSI can be controlled via a facile solvothermal process. BiSI
18
I
2
is formed under the lower S/Bi mole ratio conditions but it easily converts to Bi13
mole ratio conditions. Bi13 is an n-type semiconductor with a narrow bandgap of 0.75 eV and
possesses a strong light absorbance in the wide wavelength range from UV to NIR, suggesting the
promising light absorber material for solar cells. A uniform B13 nanorods film was fabricated on a
porous TiO electrode via a modified solvothermal process for the photovoltaic study. The Bi13 -based
18 2
S I under higher S/Bi
18 2
S I
18 2
S I
2
18 2
S I
solar cell demonstrates a power conversion efficiency of 0.85% for the first time, revealing the potential as
a new type of light absorber material for solar cells which can be fabricated via a low-cost solution
process.
environmental risk due to leaching of toxic Pb.10-12 In order to
solve these problems, there is an urgent need to explore
alternative light absorber materials consisting of nontoxic
Introduction
As the global demand for energy is expected to grow rapidly,
the securing of clean energy is essential for sustainable
economic growth. The solar energy is the most promising clean
elements. The substitution of Pb(II) with Bi(III) in the hybrid
perovskites is attractive because both Bi(III) and Pb(II) have
2
0
the same 6s 6p electronic configuration, thus ensuring that
their compounds will share similar properties. Some studies
suggest that Bi-based hybrid perovskites are promising as the
1
, 2
energy source for the next-generation.
At present, the
photovoltaic (PV) market is dominated by crystalline silicon
panels which possess relatively high efficiency, but the high
cost has prompted the development of low cost high
performance PVs. Some new types of PVs, such as the thin
film type of CuIn1-xGa
developed to reduce the cost and increase power conversion
efficiency (PCE). Although the PCE of these PVs having
recently reached 22%, their potential and future prospect are
limited by the high cost of indium and scarcity of tellurium.4
PV light absorber, but the success on the synthesis of Bi-based
13-16
hybrid perovskites has not been reported.
bismuth-halide double perovskites A Bi
A non-toxic
3
+
3
-
2
X
9
(A = Cs , Ag,
x
Se
2
(CIGS) and CdTe PVs, have been
+
2+
-
-
3 3 2
CH NH and HC(NH ) ; X = I , Br , Cl ) have been much
investigated and employed for solar cells, just possessing a
16
17
PCE of 3.2% up to now.
, 5
Recently, some V-VI-VII compounds (V=Sb, Bi, VI=O, S, Se,
VII= Cl, Br, I), have attracted much attention as potential
electronic materials. These compounds exhibit a variety of
Recently the Pb-based hybrid halide perovskites, such as
3 3 3
CH NH PbI (MAPI), have attracted much attention as a
physical characteristics, including high photoconductivity,
promising solar light absorbers for PV due to the high PCE and
18
ferroelectric
electrooptical effect,
photovoltaic
effect,
piezoelectricity,
6
, 7
the low cost of the solution fabrication process. Although it
was first reported only 10 years ago, the PCE of the PVs using
a hybrid perovskite as a light absorber has reached to 24.2%,8
surpassing all other third-generation PVs, and this fast-growing
PCE is a new record in the history of PV technology.9
Unfortunately, in addition to humidity and thermal instability,
the Pb-based hybrid halide perovskites pose a potential
19, 20
13, 14
and structural diversity.
BiSX
compounds have attracted significant attention as PV materials
because of their excellent semiconducting properties and
narrow bandgaps, which can absorb the solar spectrum in a
21, 22
wide wavelength range.
Another important advantage is
that the bandgaps are tunable via substitution between halogens
23
each other, such as BiSBr1-x
I
x
solid solutions.
Recently,
Hahn et al. have described a solar cell using a BiSI
semiconductor thin film as a lighter-absorber, which yielded a
a. Department of Advanced Materials Science, Faculty of Engineering and design,
PCE of 0.25%.24 Tiwari et al. described a BiSI-based solar cell
Kagawa University, 2217-20 Hayashi-cho, Takamatsu 761-0396, Japan
School of Materials Science and Engineering, Shaanxi University of Science and
with a record PCE of 1.32% by employing p-type organic
b.
25
semiconductor as hole transport material. BiOX compounds
Technology, Weiyang, Xi'an, Shaanxi, 710021 PR China
Electronic Supplementary Information (ESI) available: Bismuth Chalcogenide
possess tunable bandgaps from 3.46 eV for BiOCl to 1.77 eV
18 2
Iodides of Bi13S I
and BiSI: Solvothermal Synthesis, Photoelectric Behavior, and for BiOI, and exhibit superior photocatalytic activities.2
1,
26
Photovoltaic Performance. See DOI: 000.0000/x0xx00000x
BiSX compounds are synthesized generally using a vapor
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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J Po lue ran sael od fo Mn aot te rai ad l jsu Cs ht emm ai rs gt ri yn sC
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transport process at high temperature above 400 °C in a Synthesized bismuth chalcogenide iodide powder sampVileewwAratisclegOennltinlye
vacuum tube.27 BiSI can also be obtained through a ground into fine powder. This powder samDOplI:e10w.1a0s39t/hCe9nTCp0r5e1s3s9eBd
solvothermal process and melt growth process by the using a pallet press mold with a diameter of 10 mm at 30 MPa for 3
2
0, 28
Bridgeman-Stockbarger technique.
min. Subsequently, cold isostatic press (CIP) treatment was
employed on the pellet sample with a pressure of 200 MPa for 5 min
Fewer studies have been reported on Bi19
27
S X
3
(X=Cl, Br, I) to increase sample density. The thickness of the pellet was about 1
compounds. Traditionally, these bismuth sulfohalogenides are mm.
synthesized using a vapor transport process and flux process
under high temperature conditions over 600°C.2
9,
30
A
Fabrication of TiO Electrode.
2
solvothermal process has also been employed to obtain
3
1-33
Bi19S
27
X
3
nanocrystals.
The Bi19
S
27Br
3
and Bi19
S27Cl
3
A TiO2 nanocrystal paste was prepared by mixing a P25 TiO2
nanocrystals exhibit efficient photocatalytic degradation of 2,4- nanocrystal sample (0.5 g), ethanol (2.5 g), α-terpineol (2.0 g), a 10
3
2, 33
dichorophenol and the reduction of Cr(VI).
The wt % solution of ethyl-cellulose 10 (1.4 g), and a 10 wt % solution
characteristics and structure of Bi19 remain unclear. Ho et of ethyl-cellulose 45 (1.1 g). The mixture was dispersed by
27 3
S I
al. have reported that this compound possesses an indirect ultrasonication for 30 min and then ball-milling for 72 h. After
bandgap at 0.73 eV and a direct bandgap at 1.08 eV, ball-milling, the ethanol was removed from the mixture using a
respectively.30 However, Groom et al. have reported that this rotary evaporator.35
compound is an indirect bandgap semiconductor based on the
single crystal structure study and electronic structure The TiO
2
electrode was prepared in the following manner.36 The
calculations, furthermore, the result also revealed that this fluorine-doped tin oxide (FTO) conducting glass plate was cleaned
compound can be expressed as a new composition formula of in distilled water and acetone by ultrasonication for 10 min,
2
9
Bi13
Bi19
S
S
18
I
I
2
.
3
Very recently, Yan et al. have fabricated the consecutively. Then the FTO glass plate was dipped in 0.1 M
nanorod cluster films through a solvothermal process titanium tetraisopropoxide (TTIP) solution for 1 min and washed
27
3
4
and applied the film to a photodetector. However, to the best with distilled water and ethanol, dried at room temperature, and
of our knowledge, the Bi13
S I
18 2
-based solar cells have not been calcined at 480 °C for 1 h to coat the FTO glass surface with a dense
TiO thin film. The prepared TiO paste was then coated on the
dense TiO thin film on the FTO glass plate by a screen printing
Herein, we report a facile solvothermal process for the technique and dried at 120 °C for 10 min. This process was repeated
synthesis of bismuth chalcogenide iodides of Bi13 and BiSI to obtain TiO nanocrystal film of about 10 μm in thickness. After
and systematical characterization of their photoelectric behavior the TiO paste coating, the TiO film was calcined at 450 °C for 30
reported yet.
2
2
2
18 2
S I
2
2
2
as a light absorber for solar cells. A possible formation min to obtain a TiO nanoporous electrode. The TiO nanoporous
2
2
mechanism for Bi13
conditions is proposed. Both Bi13
S
18
I
2
and BiSI under the solvothermal electrode was dipped in the 0.1 M TTIP solution for 1 min, washed
and BiSI are n-type with distilled water and ethanol, dried at room temperature, and
18 2
S I
semiconductors with the bandgaps of 0.75 and 1.57 eV, calcined at 480 °C for 1 h again.
respectively. We modified the solvothermal process to fabricate
Bi13
S
18
I
2
nanorod film on a porous TiO
new type of Bi13 -based solar cells. The preliminary PV
study gave a PCE of 0.85%, suggesting the potential of the 0.4 mmol BiI (0.2359 g), 0.6 mmol MAI (0.0954 g), and 1.6 mmol
2
nanocrystal film for a Fabrication of Bi13
18 2 2
S I film on TiO Electrode.
18 2
S I
3
18 2
4 2
Bi13S I -based solar cells which can be fabricated by the CH N S (1.2179 g) were dissolved in 1 mL ethylene glycol solvent
low-cost solution process.
3 4 2
by stirring for 1 h, and then a red BiI -MAI-CH N S mixed solution
was obtained. The mixed solution was coated on the TiO
nanoporous electrode surface. After standing for 2 h, the coated TiO
2
Experimental details
Chemicals and reagents.
2
nanoporous electrode was placed on a shelf set in a Teflon-lined
stainless steel autoclave with an internal volume of 80 mL. After
adding 1 mL ethylene glycol solvent to the autoclave bottom, the
o
autoclave was sealed and solvothermally treated at 195 C for 12 h.
3
Hydroiodic acid (HI, 57% in water), and BiI (98%) were purchased
from Sigma-Aldrich. CH N S (99.0%) from TCI chemistry and
4 2
ethylene glycol, acetone and dimethyl sulfoxide (DMSO) were
bought from Wako Chemicals. All chemicals and reagents were
analytical grade and used as received.
After cooling to room temperature, a black Bi13 -coated TiO
S
18
I
2
2
electrode was obtained. The Bi13 -coated TiO electrode was
S
18
I
2
2
o
washed with ethanol and then dried at 60 C for 12 h in a drying
oven.
Fabrication and characterization of solar cells.
Synthesis of bismuth chalcogenide iodide powder samples.
18 2 18 2 2
The Bi13S I -solar cell was comprised of the Bi13S I -coated TiO
electrode as an anode, Pt-coated FTO glass as a cathode, and an
electrolyte solution between the anode and the cathode. The
3 3
CH NH I (MAI) was synthesized by the reaction of 24 mL of
methylamine and 10 mL of HI solution at 0 °C for 2 h in a rotary
evaporator at 120 rpm, and then evaporated at 80 °C. The precipitate
was washed three times with ethanol and then diethyl at room
temperature and then dried at 60 °C for 24 h. For the synthesis of a
electrolyte solution contained 0.1 M LiI, 0.01 M I
2
, 0.6 M
0.4
-tert-butylpyridine (TBP), and 0.1 M guanidine thiocyanate (GT) in
1
4
-butyl-3-n-propylimidazolium
iodide
(BMII),
M
bismuth chalcogenide iodide powder sample, BiI
3
(2 mmol), MAI (3
a mixed solvent of acetonitrile and valeronitrile (v/v = 85:15). The
photocurrent−voltage characteristic curve for the Bi13 -solar cell
mmol), and a desired amount of CH S were added into 5 mL of
4
N
2
18 2
S I
ethylene glycol solvent. The mixture was solvothermally treated at a
desired temperature for 12 h under stirring conditions. The obtained
sample was filtrated, washed with ethanol, and then dried in a drying
was measured using a Hokuto-Denko BAS100B electrochemical
analyzer under irradiation with simulated sunlight of AM 1.5 (100
2
o
mW/cm ), using a sunlight simulator (YSS-E40, Yamashita Denso).
oven at 60 C for 12 h.
A light-passing mask was fixed on the surface of the FTO glass of
Fabrication of bismuth chalcogenide iodide pellet samples.
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the anode to set the effectively irradiating area on the cell at 0.25
cm .
View Article Online
DOI: 10.1039/C9TC05139B
2
Physical analysis.
The crystal structures of the samples were investigated using a
powder X-ray diffractometer (Shimadzu, model XRD-6100) with Cu
Kα (= 0.1542 nm) radiation. The morphology of the samples was
observed using field emission scanning electron microscopy
(FE-SEM) (JEOL, ISM-7001F) and the chemical composition was
measured using energy dispersive X-ray spectroscopy (EDX)
attached to the FE-SEM at 15 kV of acceleration voltage.
Transmission electron microscopy (TEM) and high-resolution TEM
(HR-TEM) observations were performed on a JEOL Model
JEM-3010 system at 300 kV. The Mott-Schottky plot was measured
using an electrochemical analyzer (Shanghai Chenhua, CHI660E).
The ultraviolet-visible-near infrared (UV-Vis-NIR) absorption
spectrum was obtained on a Shimadzu solidspec-3700 UV-Vis-NIR
spectrophotometer at room temperature. The TG-DTA analysis was
carried out using a thermal analyzer (Shimadzu, DTG-60H) at a
o
heating rate of 10 C/min. For the conductivity measurement, a
voltage of 100 V was applied to the pellet sample, and the current
was measured using a ferroelectric testing system (Toyo Corporation,
FCE3-4KVSYS) at room temperature. X-ray photoelectron
spectroscopy (XPS) was performed using a Kratos AXIS SUPRA
X-ray photo-electron spectrometer. The XPS spectra were calibrated
to the adventitious carbon peak at 284.8 eV.
Fig. 1. XRD patterns of products obtained by solvothermal
treatments of solutions with CH
a) 1:2:3, (b) 2:2:3, (c) 3:2:3, (d) 4:2:3, and (e) 8:2:3 at 195 C
for 12h, respectively.
4 2 3
N S/BiI /MAI mole ratios of
o
(
3 4 2
To provide a phase diagram for the BiI -CH N S-MAI
solvothermal reaction system, the solvothermal treatments were
carried out at different temperatures and the XRD patterns of
the products are shown in Fig. S1. Based on the XRD results,
Results and discussion
Solvothermal synthesis of bismuth chalcogenide iodide
compounds.
3 4 2
the phase diagram for the BiI -CH N S-MAI solvothermal
reaction system was obtained as shown in Fig. 2. BiSI is
preferably formed in low S/Bi mole ratio range below 1.5,
namely the Area-A. By increasing the S/Bi mole ratio, a
The bismuth chalcogenide iodide compounds of Bi13
BiSI were synthesized by solvothermal treatment of a mixture
of BiI , CH S, and CH NH I (MAI) in an ethylene glycol
solvent. The formation of the bismuth chalcogenide iodide
compounds is strongly dependent on the CH S/BiI /MAI
18 2
S I and
mixture of the BiSI, Bi13
unknown phase is formed in Area-B. The single Bi13
can be obtained in Area-C. By further increasing the S/Bi mole
ratio, Bi is formed in Area-D, and the Bi single phase is
formed in Area-E, suggesting that Bi13 is unstable under
-CH S-MAI
S
18
I
2
, small amount of metal Bi, and an
3
4
N
2
3
3
18 2
S I
phase
4
N
2
3
S
2 3
2 3
S
mole ratio and reaction temperature. Fig. 1 shows the XRD
patterns of the samples synthesized under the conditions of
18 2
S I
the high S/Bi mole ratio conditions in the BiI
solvothermal reaction system.
3
4 2
N
o
different CH
BiI
the iodine amount was excessive for the formation of Bi13
and BiSI because the mole ratios of Bi/S/I are 13:18:2 in
and 1:1:1 in BiSI, respectively. The XRD pattern of
the sample prepared at a CH S/BiI mole ratio of 1:2 can be
4
N
2
S/BiI
3
mole ratios at 195 C, where the
3
/MAI mole ratio was fixed at 2:3. In this reaction system,
18 2
S I
18 2
Bi13S I
4
N
2
3
well indexed to an orthorhombic BiSI phase with a space group
of Pnma and lattice constants of a = 0.8519, b = 1.0177 and c =
0
.4172 nm (JCPDS File No.73-1171). By increasing
CH S/BiI mole ratio, the Bi13 phase (JCPDS File No.
3-1157) was formed and became main phase with small
amount of metal Bi phase at CH S/BiI = 3:2, and then the
single Bi13 phase was formed at CH S/BiI = 4:2. The
structure of the Bi13 phase belongs to a hexagonal system
with a space group of P6 and lattice parameters of a = 1.5612
and c = 0.4017 nm.2 When the CH
S/BiI mole ratio was
increased to 8:2, the Bi13 phase disappeared, where a Bi
4
N
2
3
18 2
S I
7
4
N
2
3
S
18
I
2
4
N
2
3
18 2
S I
3
9
4
N
2
3
S
18
I
2
2 3
S
single phase was formed. The above results reveal that the
products are sensitive to the Bi/S mole ratio in this
solvothermal reaction system.
Fig. 2. Phase diagram for BiI
reaction system in ethylene glycol solution with mole ratio of
BiI /MAI = 2:3. Area-A: BiSI; Area-B: mixture of BiSI, Bi13 , Bi
and unknown phase; Area-C: Bi13 ; Area-D: mixture of
and Bi ; Area-E: Bi
3 4 2
-CH N S-MAI solvothermal
3
18 2
S I
18 2
S I
18
Bi13S I
2
S
2 3
2 3
S .
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We also tried to synthesize bismuth chalcogenide iodide rod-like Bi13
compounds in other solvents, such as acetone, DMF, and rod-like crystal axis direction correspondsDOtoI:
DMSO. The XRD patterns of the solvothermal products other words, Bi13 crystals grow in the direction c-axial.
prepared at CH S/BiI /MAI = 4:2:3 (S/Bi = 2:1) mole ratio Zhou et al. have proposed that such ribbon structural geometry
and 195 C are shown in Fig. S2. Although the single phase of can exhibit a high conductivity along covalently bonded
S I particles were single crystals, where the
18 2
View Article Online
10.1039/C9TC05139B
[001] (Fig. S3); in
18 2
S I
4
N
2
3
o
Bi13
Bi13
S
18
I
2
can be formed in acetone solvent, the yield of [Bi
is disappointing, due to the low reactivity in this gives several advantages as light absorbers for solar cells.
phase
and metal Bi phase is formed. However, an unknown phase
without Bi13 is formed in the DMSO solvent. These results
4 6
S ]-ribbons direction, i.e, the direction of the c-axis, which
37
S
18
I
2
18 2
solvent. In the DMF solvent, a mixture of the Bi13S I
18 2
S I
suggest that the solvothermal formations of bismuth
chalcogenide iodine compounds are sensitive to the solvents.
Compared with the high temperature reaction process,2
9, 30
the
solvothermal process described above is a facile method for the
synthesis of the bismuth chalcogenide iodide compounds.
FE-SEM was employed to investigate the morphology of
products obtained by the solvothermal reaction and the results
o
for the samples prepared at 195 C are shown in Fig. 3 The
4 2 3
sample obtained at a mole ratio of CH N S/BiI = 1:2 possesses
a fibrous particle morphology with a diameter of about 500 nm
and a length of about 20 m (Fig. 3(a)). The fibrous particles
correspond to BiSI because this sample is a BiSI single phase
(
Fig. 1(a)). The fibrous particles and urchin-like particles were
observed in the sample of mixed BiSI and Bi13 phases
obtained at a mole ratio of CH S/BiI = 2:2 (Fig. 3(b)),
where the fibrous particles can be attributed to the BiSI phase
and the urchin-like particles to the Bi13 phase (Fig. 1(b)).
18 2
S I
4
N
2
3
18 2
S I
The Bi13 single phase sample obtained at a mole ratio of
S
18
I
2
CH S/BiI = 4:2 exhibits a rod-like particle morphology with
4
N
2
3
a diameter of about 400 nm and a length of about 5 m (Fig.
(c)). The Bi single phase sample obtained at a mole ratio of
CH S/BiI = 8:2 shows fibrous and rod-like particle
morphologies (Fig. 3(d)).
3
2 3
S
4
N
2
3
Fig. 4. Structures of (a) hexagonal Bi13
S I
18 2
and (b)
orthorhombic BiSI.
2
In the BiSI structure, double chain subunit of [(BiSI) ] is
formed by linking Bi with S and I together by strong Bi-S and
Bi-I bonds along the direction of c-axis, and the
[
2
(BiSI) ]-chains are held together by weak Van der Waals type
38
bonds, as shown in Fig. 4 (b). TEM and HRTEM results
reveal that rod-like BiSI particles were also single crystals and
the rod-like crystals grew along the direction of c-axial in the
18 2
similar manner as the Bi13S I structure (Fig. S3).
Formation reactions of bismuth chalcogenide iodide
compounds
The BiI
3
-CH
4 2
N S-MAI mixed solution with a mole ratio of
o
2:4:3 CH
4
N
2
S/BiI /MAI was solvothermally treated at 195 C
3
for different reaction times to investigate the formation process
of bismuth chalcogenide iodide compounds. XRD results
indicate that a binary mixture of BiSI and Bi13
Fig. 3. FE-SEM images of products synthesized by solvothermal after the reaction for 0.5 h (Fig. 5). By prolonging the reaction
treatments of solutions with CH S/BiI
/MAI mole ratios of time, the fraction of BiSI decreased and that of Bi13
a) 1:2:3, (b) 2:2:3, (c) 4:2:3, (d) 8:2:3 at 195 C for 12 h, increased, then the BiSI disappeared completely after 6 h
respectively.
reaction, and finally the Bi13 single phase was formed after
2 h reaction. This result suggests that first BiSI is formed,
18 2
S I was formed
N
4 2
3
18 2
S I
o
(
18 2
S I
1
The Bi13
S
18
I
2
possesses an interesting structure, as shown in which then converts to Bi13
S
18
I
2
under the solvothermal
2
7
reaction conditions.
Fig. 4(a). In this structure, ribbon-like subunits of [Bi
form six spokes around a central hexagonal channel at the
corners of the unite cell. The [Bi ]-ribbons spokes are
4 6
S ]
4 6
S
connected together to form a triangular tunnel structure in the
-
direction of c-axial, where I located in the tunnel site of the
18 2
Bi13S I structure. TEM and HRTEM results revealed that the
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DOI: 10.1039/C9TC05139B
Fig. 6. (A). FE-SEM images of products synthesized by
solvothermal treatments of solution with CH S/BiI /MAI
4
N
2
3
o
mole ratios of 4:2:3 at 195 C for (a) 0.5, (b) 1, (c) 2, (e) 6 h
respectively. (B). Schematic illustration of formation reaction
Fig. 5. XRD patterns of products synthesized by solvothermal
treatments of solution with CH S/BiI /MAI mole ratios of
:2:3 at 195 C for (a) 0.5, (b) 1, (c) 2, (d) 4, (e) 6, (f) 12 h,
respectively.
18 2
mechanism of BiSI and Bi13S I .
N
4 2
3
o
4
Based on the results described above, we deduced a reaction
mechanism for the formation of BiSI and Bi S I in the
13 18 2
solvothermal reaction system, as shown in Fig. 6(B). In the first
18 2
The conversion reaction of BiSI to Bi13S I
4 2
was investigated step, S2 is formed by the decomposition reaction of CH N S
-
by FE-SEM observation (Fig. 6(A)). In the sample obtained under solvothermal conditions and then BiSI is formed by the
after 0.5 h reaction, large fibrous particles and small urchin-like solution reaction of Bi(III), S2 and I , resulting in the fibrous
particles were observed simultaneously. Energy dispersive BiSI particles. In the second step, the crystal nuclei of Bi S I
3 18 2
-
-
1
X-Ray spectroscopy (EDS) analysis reveals that the large are formed on surface of the fibrous BiSI particles (Fig. 6
fibrous particles and small urchin-like particles have element (A)(a)) and then they grow up to the small urchin-like Bi S I
13
18 2
compositions close to the stoichiometric compositions of BiSI particles through
a
BiSI-dissolution reaction and
a
(
Bi/S/I =1:1:1) and Bi13
S
18
I
2
(Bi/S/I =13:18:2), respectively Bi S I -deposition reaction. In the third step, the small
13 18 2
(
Figure S4 and Table S1); that is to say, they correspond to the urchin-like Bi S I particles further grow up and finally
13 18 2
18 2
13 18 2
BiSI phase and Bi13S I phase, respectively. By increasing the collapse, becoming the individual rod-like Bi S I particles
reaction time up to 4 h, the urchin-like Bi13
up and their fraction in the product increased, accompanying Bi S I -deposition reaction. Thus, the BiSI phase is converted
the consumption of the fibrous BiSI particles. After 6 h to the Bi S I phase in this reaction system.
18 2
reaction, the urchin-like Bi13S I particles became to rod-like
S I
18 2
particles grew through
the
BiSI-dissolution
reaction
and
the
13 18 2
13 18 2
in shape and the fibrous BiSI particles disappeared completely. Characterization of bismuth chalcogenide iodide compounds
This reaction process can be confirmed by element composition
analysis results of SEM-EDS in Fig. S4 and Table S1, and also
corresponds to the XRD results in Fig. 5.
It has been reported that BiSI can be used as a light absorber for
solar cells. In the present study, therefore, we investigated the
4
thermostabilities and photoelectric properties of Bi13
BiSI to evaluate the potential of Bi13 as a light absorber for
a new type of solar cells. The Bi13 and BiSI samples
synthesized by solvothermal
CH S-BiI -MAI mixed solution with CH
18 2
S I and
18 2
S I
S I
18 2
treatment
of
the
4
N
2
3
4
N
2
S/BiI
3
/MAI
o
mole ratios of 4:2:3 and 1:2:3 at 195 C for 12 h, respectively,
were used for the characterization. The TG-DTA analysis and
XRD results (Fig. S5 and S6) suggest that Bi13
S
18
I
2
is stable up
o
o
to 350 C, decomposes to BiOI at around 380 C and finally to
o
Bi
4
2 3 18 2
O at above 500 C. Similar to Bi13S I , BiSI is stable up to
o
o
00 C, decomposes to BiOI at around 400 C and finally to
o
Bi
2
O
3
at above 500 C (Fig. S5 and S6). BiSI exhibits a slightly
in air.
higher thermostability than Bi13
18 2
S I
18 2
The pellet samples of Bi13S I and BiSI were used for the
conductivity measurement at room temperature (Fig. S7). The
electrical conductivity (σ) was calculated from the applied
voltage (V), measured current (I), thickness of pellet (L), and
area (S) of the pellet sample using formula (1).
푰 ∗ 푳
훔 =
(1)
푽 ∗ 푺
The calculated electrical conductivity of the Bi13
sample is 8.85×10 S/cm, which is much lower than the
S
18
I
2
pellet
-7
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30
reported value of 9×10 S/cm for a single crystal sample. The composition of the Bi13S I and BiSI samples. The XPS survey
18 2
View Article Online
lower conductivity of the Bi13
attributed to the low density of the pellet sample and the grain and iodine, as well as a small amount of carbon and oxygen
boundaries because of the rod-like particle morphology of impurities in both Bi13 and BiSI samples (Fig. S9). The
, which can be confirmed in its FE-SEM image (Fig. carbon and oxygen may correspond to the residual orgonic
S8). The BiSI pellet sample has an electrical conductivity of precursor and adsorbed oxygen molecules on the particle
18 2
DOI: 10.1039/C9TC05139B
S I pellet sample can be spectrum reveals the predominant presence of sulfur, bismuth
18 2
S I
18 2
Bi13S I
-
7
6
.24×10 S/cm.
surface. The Bi spin-orbit doublet peaks of BiSI is consistent
with the XPS result of BiSI reported by Grigas et al., where all
and BiSI samples are black crystals. The Bi are trivalent in BiSI.42 The Bi-4f7/2 and Bi 4f5/2 peaks were
The Bi13
S
18
I
2
18 2
UV-Vis-NIR absorption spectra of Bi13S I and BiSI are observed at 158.7 and 164.0 eV for BiSI and at 158.0 and 163.3
shown in Fig. 7A. A strong and broad absorption peak around eV for Bi S I . The different binding energies between
13 18 2
5
00 nm in the visible light wavelength range and another strong Bi S I and BiSI may account for the existence of bivalent
13 18 2
peak at 1115 nm in the near-IR wavelength range were Bi(II) in Bi S I . It has been reported that the bivalent Bi(II)
13 18 2
18 2
S I
13 18 2
absorption spectrum. A strong flat as Bi24+ dimers is possible in Bi S I , which results in the
observed in the Bi13
3+
4+
2-
-
absorbance in the wavelength range of 300 to 800 nm was formation of (Bi ) (Bi ) (S ) (I ) rather than Bi S I or
observed in the BiSI absorption spectrum. The absorption (Bi(Bi S ) I ) . The XPS result indicates that the atomic ratio
12
2
0.5
18
2
19 27 3
29
2
3 9 3 2/3
spectra findings reveal that Bi13
18 2
13 18 2
S I has a smaller bandgap of I/Bi/S in the Bi S I sample is 1:6.5:8.9, which is very
energy than that of BiSI and is a promising light absorber for close to the stoichiometric mole ratio of 1:6.5:9.0 of
3+
4+
2-
-
solar cells.
12 2 0.5 18 2
(Bi ) (Bi ) (S ) (I ) . A single peak of S-2s was observed
at 225.0 and 225.2 eV for Bi13
) of a semiconductor can be estimated I-3d3/2 and I-3d5/2 spin-orbit spin components were observed at
using Tauc’s formula (Ahν) = B(hν-E ), where A, B, and hν 630.1 and 618.6 eV for Bi S I and 630.2 and 618.7 eV for
g
13 18 2
18 2
S I and BiSI, respectively. The
The bandgap energy (E
g
1
/n
are the absorption coefficient, a constant, and the incident BiSI, respectively. The binding energies of S-2s, I-3d3/2 and
photon energy, and the exponent n is equal to 1/2 or 2 for the I-3d5/2 of Bi S I are very close to those of BiSI.
13
18 2
3
9
allowed direct or indirect transition, respectively. Because
2
9, 38, 40
Bi13S
18
I
2
and BiSI are indirect semiconductors,
Taus’s Fig. 8 shows the valance-band spectra of Bi S I and BiSI.
13 18 2
formula can be represented as the Kubelka–Munk function A = The valence band maximum is located at 0.61 and 0.89 eV
2
19, 41
C(hv-E
g
) /hv, where C is the absorption constant.
The below the Fermi level for Bi S I and BiSI, respectively.
13 18 2
bandgap energy can be estimated from the transformed Therefore, the conduction band minimum of Bi S I will be
13 18 2
Kubelka–Munk function versus the photon energy (Fig. 7(b) located at 0.14 eV above the Fermi level because the band gap
and (d)). The estimated bandgap energy of the Bi13 sample of Bi S I is 0.75 eV. As the Femi level is close to the
was 0.75 eV by extrapolating the straight line to zero conduction band minimum, Bi S I should be an n-type
18 2
S I
13 18 2
13 18 2
semiconductor.43 This is different from the result that Bi13
18 2
S I
absorption intercepting the (h ) axis. It would be noticed the
ν
is a p-type semiconductor based on a Hot-probe measurement
result.30 The n-type and p-type semiconductors can be
distinguished also by Mott-Schottky plot, where n-type
semiconductor shows positive slope value and p-type
absorption does not fall to zero at 0.75 eV, which suggests that
the true bandgap may be lower than 0.75 eV as Groom et al.
29
reported. The bandgap value corresponds that reported by Ho
et al’s.30 Yan et al. reported a direct band gap of 0.83 eV that is
negative slope value.45 The
18 2
S I shows a positive slope value, as
3
4
semiconductor shows
Mott-Schottky plot of Bi13
shown in Fig. S10, namely it is an n-type semiconductor.
a
slightly larger than our result. The BiSI sample has a bandgap
energy of 1.57 eV, which agrees with reported values of
.57-1.59 eV.4, 22, 40
1
There is also an argument about Bi13
semiconductor or a direct bandgap semiconductor.
arguments suggest that the properties of Bi13 are strongly
18 2
S I is an indirect bandgap
29, 30
These
18 2
S I
dependent on its synthesis conditions, maybe slightly different
in the chemical composites, due to formation of the bivalent
4+
2
[Bi ]
dimer. Therefore, Bi13
S
18
I
2
may be a tunable
semiconducting material. On the other hand, the conduction
band minimum of BiSI will be located at 0.68 eV above the
fermi level because its bandgap energy is 1.57 eV. Therefore,
BiSI should be also an n-type semiconductor because the Femi
level was close to the conduction band minimum, which is
agreement with the results reported.4
Fig. 7. UV-Vis-NIR absorption spectra and corresponding plots
of transformed Kubelka-Munk function versus hv for (a, b)
Bi13S I and (c, d) BiSI powder samples, respectively.
18 2
X-ray photoelectron spectroscopy (XPS) was used to
investigate the surface valence state and the chemical
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DOI: 10.1039/C9TC05139B
Fig. 9. (a) Schematic illustration of Bi13 -based solar cell
S
18
I
2
structure and (b) I-V curves of eight Bi13S I -based solar cells.
18 2
2 18 2
The FE-SEM images of FTO/TiO /Bi13S I electrode reveal
that the thickness of the porous TiO
2
film was about 10 μm
(Fig. 10(a)). A uniform black Bi13
S
18
I
2
film was formed on the
porous TiO
2
film after solvothermal treatment (Fig. 10(d)).
nanocrystals were formed in the nanopores
and surface of the porous TiO film (Fig. 10(b, c)). The
diameter of the Bi13 nanorods in the nanopores was about
0 nm, which is slightly smaller than those on the surface,
18 2
Rod-like Bi13S I
Fig. 8. Valance-band XPS spectra of (a) Bi13
Solar cell performance of Bi13 as light absorber
holds promise as a light absorber for solar cells
because it exhibits strong light absorption in the whole visible
light wavelength range as shown in Fig. 8(a). However, to the
best of our knowledge, no any study has been reported on the
18 2
S I and (b) BiSI.
2
18 2
S I
18 2
S I
5
indicating that the crystal size in the nanopores is controlled by
the pore size.
18 2
Bi13S I
Fig. 9(b) shows the photocurrent-voltage (I-V) characteristic
curves of eight solar cells under simulated AM 1.5 spectrum.
The cell performance parameters of energy conversion
efficiency (η), short-circuit photocurrent density (Jsc),
open-circuit voltage (Voc), and fill factor (FF) evaluated from
the I-V curves were summarised in Table S2. These cells
application of Bi13
18 2
S I as a light absorber for solar cells.
Therefore, we carried out a preliminary PV study using
Bi13
Bi13
S
S
18
I
I
2
as a light absorber to confirm its possibility. First,
solar cells with a cell structure as shown in Fig. 9(a)
18
2
were fabricated. In the solar cell, a TiO
on the surface of FTO conducting glass as a hole blocking layer
and a TiO porous film was coated on the TiO dense film. A
film was fabricated on the TiO porous film using a
modified solvothermal process. An electrolyte solution of I
redox couple was used as the hole-transporting material and a
Pt-coated FTO conducting glass was used as
counter-electrode similar to the dye-sensitized solar cells
DSSCs).
2
dense film was coated
exhibited η, Jsc, Voc, and FF values in the ranges of 0.53 to 0.85
2
%
, 3.22 to 3.82 mA/cm , 0.51 to 0.58 V, and 29.3 to 38.3 %,
2
2
respectively. The relatively high repeatability of the cell
performance can be attributed to the high stability of Bi13
Bi13S
18
I
2
2
-
-
18 2
S I .
3
/I
The highest η value is 0.85 % with cell parameters of Jsc=3.82
2
mA/cm , Voc=0.58 V, and FF=38.3 %. Although the PCE of
a
these preliminary solar cells are still low, these results reveal
the potential of Bi13S I as a light absorber for photovoltaics
18 2
and photoelectric devices. The low PCE can be attributed
(
mainly to low Jsc and FF values of the cell. Many factors, such
as the cell structure, the thickness of the Bi13
18 2 2
S I and TiO
films, crystallinity of the Bi13 , and hole-transporting
18 2
S I
material, can affect the Jsc and FF values. The optimization of
these factors will further improve the solar cell performance.
Moreover, the Bi13
ideal bandgap of 1.4 eV for a single p-n junction solar cell,
which also limits the maximum PCE value. To give the precise
evaluation of the ultimate potential of Bi13 as light
18 2
S I band gap of 0.75 eV is smaller than the
46
18 2
S I
absorber for the photovoltaics, the further detailed studies, such
as the lifetime, photoluminescence quantum efficiency, and
Urbach energy, are expected.
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The solvothermal process is a facile method for the synthesis of
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The formations of Bi13 and BiSI are strongly dependent on
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18 2
of the Bi13S I light absorber for a new type of solar cell that
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This work was supported by The Murata Science Foundation and
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Journal of Materials Chemistry C
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DOI: 10.1039/C9TC05139B
TOC GRAPHICS
Bi S I -based solar cell
1
3 18 2
1