Bismuth oxyiodide (BiOI) solar cells have been fabricated using a modified successive ionic layer adsorption and reaction (SILAR) method. To adjust the parameter of reaction, we involved the precursor molarity variation from 2 to 10 mM in our BiOI films preparation. The successful formation of BiOI has been indicated by the existence of tetragonal phase BiOI and BieI internal stretching mode in XRD patterns and Raman spectra, respectively.
Trang 1Original Article
Relevance of precursor molarity in the prepared bismuth oxyiodide
films by successive ionic layer adsorption and reaction for solar cell
application
Anissa A Putria,b,*, Shinya Katoa, Naoki Kishia, Tetsuo Sogaa
a Department of Electrical and Mechanical Engineering, Nagoya Institute of Technology, Nagoya, 466-8555, Japan
b Department of Chemistry, Walisongo State Islamic University, Semarang, 50185, Indonesia
a r t i c l e i n f o
Article history:
Received 7 December 2018
Received in revised form
24 January 2019
Accepted 24 January 2019
Available online 4 February 2019
Keywords:
BiOI
Precursor concentration
Solar cell
p-type semiconductor
Bismuth materials
a b s t r a c t
Bismuth oxyiodide (BiOI) solar cells have been fabricated using a modified successive ionic layer adsorption and reaction (SILAR) method To adjust the parameter of reaction, we involved the precursor molarity variation from 2 to 10 mM in our BiOIfilms preparation The successful formation of BiOI has been indicated by the existence of tetragonal phase BiOI and BieI internal stretching mode in XRD patterns and Raman spectra, respectively By a gradual increase in precursor molarity, the wide ab-sorption and redshift of BiOIfilms are observed in the UV-visible spectra In addition, the large growth of flaky BiOI is displayed in field emission scanning electron microscope (FESEM) image These characters have an impact on the photovoltaic properties of BiOIfilms although a monotonous enhancement of solar cell efficiency cannot be reached by the rising concentration of precursors In this work, we found that the maximum solar cell performance was achieved after an initial concentration increased Then, it showed a decrease in its performance by increasing precursor molarity The IV analysis data confirm that BiOIfilms from 7 mM of precursor have the best Jscand efficiency which up to ~2.2 mA/cm2and 0.318%, respectively Also, this concentration can result in the maximum external quantum efficiency (EQE)
© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
The unique properties of metal semiconductors present the
wider application in some areas, such as energy generation and
environment One of heavy metal-based semiconductor which is
safe, less toxic, and attracting the attention of many researchers is
bismuth oxyiodide (BiOI) [1] Since the last decade, this p-type
semiconductor has been reported as the potential material for
photocatalyst[2e10]and absorber in solar cell device[10e16]due
to its narrow band gap (~1.8 eV) and strong absorption under
visible light region Although BiOI has succeeded to be applied as
the material for waste and water treatment via photocatalytic
re-action, it still has low solar cell performance efficiency (around ~1%)
for BiOI/TiO2/FTOfilm prepared by SILAR[12] We noted that one of
important factors which affects the semiconductor material
performance in solar cell is the condition during the preparation In the wet-synthesis route, the precursor condition (i.e precursor and solvent adjustment[17e20], concentration[21e23], and surfactant selection[18,20]) can be considered as the key factors for control-ling the physical properties (morphology, size, crystallinity, and others) which strongly influence the solar cell performance Normally, there are two general ways to obtain BiOI, i.e sol-ventless reaction and solvo-reaction [10,15,24e27] In the free-solvent process, BiOI films can be prepared by chemical vapor transport of BiI3under Ar/O2atmosphere at around 300 C[10], while the BiOI powder can be produced through low temperature mechanical grinding process Despite the fact that the environ-mental benefits can be yielded by using the free-solvent route to synthesize BiOI, the difficulties in BiOI films mass production; the high temperature needed in the BiOI synthesis route; and the more investigation needed for the BiOI performance evaluation may be the challenge in the BiOI development via dry-synthesis [27] Therefore, in this work, we decided to focus on the wet-synthesis route which is commonly used to prepare BiOI films By the solvo-reaction method, BiOIfilms for photovoltaic devices can be obtained by SILAR and chemical bath deposition (CBD)[12,13,15]
* Corresponding author Department of Electrical and Mechanical Engineering,
Nagoya Institute of Technology, Nagoya, 466-8555, Japan.
E-mail address: anissaputri@walisongo.ac.id (A.A Putri).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
https://doi.org/10.1016/j.jsamd.2019.01.007
2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license
Trang 2which followed the dip-coating principle Although the solvent
usage is required, we noted that both SILAR and CBD techniques
have some advantages, i.e easy, low cost, reproducible, and
controllable In addition, BiOI powder for photocatalytic reaction
can be prepared by hydrothermal and solvothermal methods
[3,25,26]
Reported by Wang and co-workers[15], SILAR has been used to
prepare BiOI solar cell for thefirst time in 2010 We highlighted that
the solar cell performance of SILAR BiOIfilms was affected by the
number of cycles during thefilm preparation In SILAR, the cycle of
reaction controlled thefilm thickness and its physical properties
[12e15], also beside the cycle effect, angle inclination in SILAR has
an impact on the resulted BiOIfilms[28] However, there is a report
about the significant increase of CuO solar cell performance due to
the increase in its precursor molarity in SILAR[29] Then, we
sup-pose that the optimizing of precursor condition in BiOIfilm
prep-aration may be an alternative to screen the better performance of
BiOI photoanode In fact, some prepared nanomaterials with
suit-able morphology, structural and optical properties are controlled
by the condition during the synthesis process, particularly the
precursor concentration[30e32] Moreover, the possibility to get
the desiredfilm, the probability to reduce the amount of solvent
and solute usages, the less-time preparation to get the uniform
films, and the better thicker layer[33]are the benefits which can be
obtained by using the concentrated precursor during the film
preparation
Although the successful BiOI photoanode in solar cell
applica-tion by SILAR with 5 mM of precursors was reported, its
short-circuit current was not more than 1 mA/cm2 [11e15] Here, we
address the possibility of solar cell performance enhancement in
BiOIfilms by varying its precursor concentration and report the
double increment of Jscvalue from our BiOIfilms compared with
the previous results Owing to the increase in the precursor
con-centration, the different properties of BiOIfilms and their solar cell
performance are obtained To the best of our knowledge, there is no
reported study in the precursor concentration effect during BiOI
preparation using SILAR method
2 Experimental
2.1 Synthesis of BiOIfilm
The BiOI deposition in 2 2 cm of FTO substrate was carried out
through modified SILAR[14]using two different precursors as cation
and anion sources In this work, we varied Bi(NO3)3.5H2O and KI
concentrations (i.e 2 mM, 5 mM, 6 mM 7 mM, 8 mM, and 10 mM) to
obtain BiOIfilms Before BiOI deposition process, the cleaning
pro-cess of FTO glass substrates was done by the following steps: 5 min
each of glass rinsing in acetone (twice) and in ethanol (once), N2gas
blowing, and UV/Ozone treatment for 20 min in total During the
deposition process, the withdrawal speed and the cycles were
configured at 0.2 mm/s and 30 cycles respectively To finish the BiOI
preparation, all resultedfilms were dried in air at 100C for 1 h.
2.2 Fabrication and characterization of BiOIfilm
Each resultedfilm was designed as photoanode in the solar cell
device to evaluate the photovoltaic performance In this work, we
used Pt/FTO glass as the cathode and the iodine-based solution
(Solaronix Iodolyte AN-50) as the electrolyte To make the solar cell
device, I/I3 solution was inserted between the photoanode and
counter electrode covered by Himilan polymerfilm as shown in
Fig 1 The solar simulator (100 mW/cm2, AM 1.5 illumination) with
the illuminated area at 0.16 cm2was utilized to test the solar cell
performance To investigate the film characteristics such as
structural, morphology, and optical properties, we analyzed the samples using X-Ray diffraction (Rigaku RINT-2100 diffractometer), FESEM JEOL JSM-7001F, UV-Visible NIR Spectroscopy (JASCO 670 UV), and Raman Spectrometer (JASCO NRS-2100) respectively Since the BiOI particle from 2 mM of precursor solution was not observable in the UV-Visible and Raman spectra, its analysis results are not displayed in this report
3 Results and discussion 3.1 Structural and morphology analysis 3.1.1 XRD analysis
Fig 2represents the diffractogram of prepared BiOIfilms from
5 mM to 10 mM of solutions In this report, we show that the greater bismuth and potassium salt concentrations cause an in-crease in the BiOI crystallinity Apparently, thefilm thickness from the concentrated precursor was thicker in comparison to the pre-pared BiOIfilms from the dilute precursors This phenomenon also can be reflected by the increment of crystal structure intensity in XRD patterns At the higher precursor concentration, the thick-film
of BiOI was obtained easily and the higher intensity of BiOI peaks appear in the XRD patterns as shown in theFig 2 Corresponding to the reported research, our synthesized BiOI is in a good agreement with the JCPDS card no 73-2062 and the previous report[26] The tetragonal phase and attributed peaks which indicate the character
of BiOI in 2qaround 29.6(012), 31.7(110), and 45.5(020) are revealed by the XRD patterns Furthermore, the strongest peak which exists in 2qaround 29.6confirms for (012) plane of BiOI crystal By the displayed data inFig 2, it convinces that there is no
Fig 1 BiOI solar cell illustration.
A.A Putri et al / Journal of Science: Advanced Materials and Devices 4 (2019) 116e124 117
Trang 3any unknown materials detected and rich-oxygen bismuth material
product in our prepared BiOI, such as Bi7O9I3, Bi5O7I, Bi2O3 and
others Generally, the crystal structure type of BiOI both in thefilms
from 5 mM to 10 mM are same Nevertheless, we observed that
there are differences in their full width at half maximum value
(FWHM) and peak intensities By calculating the average crystal
size for this plane using the DebyeeScherrer equation (Eq.(1)), we
obtained the BiOI crystallite size from 5 to 10 mM of precursors
were 16.94 and 18.46 nm respectively Then, we notice that the
higher molarity of salt solutions is able to result in the larger grain
size of material This phenomenon is also similar to the result in the
reported work by Visalakhshi and co-workers[29]
Where,
L: crystallite size
K: constant (0.9)
l: Cu wavelength (0.154 nm)
b: FWHM value in radian
q: Bragg angle
3.1.2 Morphology analysis
Fig 3shows the morphology of synthesized BiOIfilms from
BiOþand Isources concentrations at 5 mM, 6 mM, 7 mM, and
10 mM which are expressed in A, B, C, and D, respectively
Basi-cally, almost of BiOI morphology is found in theflake structure
like informed in the many reported researches Here, we also
show the flaky BiOI from the high concentration of precursor
which is displayed inFig 3 In thisfigure, it can be observed the
evolution of BiOI morphology due to the changing in molarity
precursors By the same cycles, BiOI nanoparticles with general
lateral size around 100e300 nm were produced at the earlier
concentration (5 mM), whilst, at the higher concentration, the
self-assembly of widerflakes BiOI could be obtained The wider
and thicker of BiOIflakes which are shown inFig 3B,C have the lateral size around 500 nm and more than 1mm is for BiOI in
Fig 3D While the molarity increased, we found the more and bigger rod-like material arranged by BiOIflakes This phenome-non is similar to previous research which confirmed that pre-cursor concentration changing influenced the CdS size and morphology[34] Due to this fact, we believed that besides the number of cycle, the precursor molarity has a strong effect on its resulted material size and morphology We display the cross-sectional image of BiOIfilms inFig 3E BiOIfilm with ~3.4mm of thickness was gained by involving the precursor concentration at around 6 mM It can be seen that porous-like BiOI material can be obtained from SILAR Based on this result, we assume that the higher porosity of BiOI may be formed in the thickerfilms which are prepared from the concentrated precursors Since thefilm compactness of BiOIfilms decreases due to the higher porosity, the precursor concentration may turn the character of BiOIfilms from the compact layer to non-compact layer along with the in-crease in the reactants molarities Later, this changing has a
sig-nificant impact on our final result since it drives the optical and physical properties of BiOI films Hence, the decision of mass molarity plays an important role in gaining better crystallinity and suitable characters for solar cell application
Regarding the BiOI morphology evolution due to the precursor concentration effect, we proposed the BiOI growth illustration as shown inFig 4 In low concentration (5 mM), BiOI nanoparticles are formed and in the higher molarities (started from 6 mM), the wider BiOIflakes are obtained Furthermore, the rod-like materials con-sisted of BiOIflakes are produced by the concentrated precursors (7,
8, 10 mM) The increase in the precursor concentration enhances the reaction probability between anion and cation which may be initiated by the collision among reactants in the concentrated so-lution Since the reactant collision improves, the production offlake BiOI rate will be high This might be the reason why the concen-trated precursor resulted in the wider flake of BiOI However,
Fig 2 XRD patterns of BiOI films from the lower and higher molarity of precursors (5 and 10 mM).
Trang 4during the BiOI nucleation step in the concentrated precursor, the
collision also might occur between the precursors and the
previ-ously formed nucleus This circumstance might induce more
chances of interaction between the ion and the formed crystal
Then, it resulted in the different morphology transformation in BiOI
films The morphology transformation in the material growth
sometimes happens to reach the higher stability of solid material
As it is mentioned in the previous works, the BiOIflakes of BiOI are
also able to make a self-assembly formation resulting in the new
morphology likeflower-like structures of BiOI[4,35,36] In
addi-tion, the structural transformation from rod-like material toflakes
morphology could occur electrochemically in the hematite syn-thesis[37] In this work, the reaction between anion and cation in Bi(NO3)3and KI solutions to produce BiOI are shown in Eqs.(2) and (3)
Bi(NO3)3þ H2O/ BiONO3þ 2HNO3 (2)
Fig 3 SEM image of BiOI films in the different concentration: 5 mM (A), 6 mM (B), 7 mM (C), 10 mM (D) and cross-sectional of ~6 mM of precursor (E).
A.A Putri et al / Journal of Science: Advanced Materials and Devices 4 (2019) 116e124 119
Trang 53.2 Optical properties
3.2.1 UV-visible spectral analysis
The light photoresponse of BiOIfilms which corresponds to its
optical properties were studied by UV spectroscopy and the spectra
are shown in Fig 5 From the figure, it is clearly seen that the
precursor concentration influences the absorbance in the visible
spectra of deposited BiOI films Here, the more concentrated
Bi(NO3)3and KI solutions result in the BiOIfilms which have the
wider visible light range absorption and higher absorbance These
responses may be favorable for solar cell application We expect
that the reaction probability between anion and cation improves in
the higher molarities of reactants Consequently, it may attribute to
the further crystal nucleation and enhances the growth rate of BiOI
crystals which persuade the thickerfilms formation Regarding the
increase of BiOI peak intensity in the Raman spectra and XRD
patterns which is parallel to the amount of BiOI in thefilms, we
assumed the thickness of BiOIfilms increased along with the higher
molarity precursor used in this experiment As a result, this greater
thickness and size of BiOI gave the stronger BiOI activity under the
visible light
In this section, we also show the band gap energy calculation
using Tauc plot for indirect band gap estimation of BiOI inFig 6
From thisfigure, it is observed that there is a shift tendency of BiOI
band gap value due to the different concentration According to the
(ahv)1/2vs (hv) plot, the band gap energy of prepared BiOIfilms are
ranging from 1.85 eV to 1.7 eV The shift of band gap value is also in
good agreement with the absorbance data and it informs that more
BiOI has the consequence in its band gap energy decreasing The
increase of BiOI grain size at the higher precursor molarity may
turn the BiOI band gap and it is also in line with the previous report
[38] This is also supported by its SEM image and the crystal size
calculation by DebyeeScherrer equation in XRD patterns
3.2.2 Raman analysis
In this work, we studied the structural and chemical
informa-tion of BiOIfilms using Raman spectroscopy The Raman spectra are
shown inFig 7 All of prepared BiOIfilms from concentrated
re-actants have the stronger peak for BieI vibration stretching mode
(Eg) around 147.91 cm1 This result is in line with the previous
results, as typically, the BieI stretching mode in Raman analysis can
be easily identified by the existence peak around 147-149 cm1
[39e43] Besides the Eg stretching modes, other BiOI vibration types in Raman spectra are notated with A1gand B1gwhich should
be existed in the wavenumber below 100 cm1 However, in our experiment, we could not observe those peaks due to the obser-vation condition in our Raman investigation These similar spectra are also displayed in the previous report[39] Since the increase in the BieI vibration peak is in line to the amount of BiOI in the films,
we believed that the greater molarity of precursor induced the faster agglomeration and aggregation of BiOI particles resulted in a large amount of BiOI[44]
3.3 Photovoltaic properties
Fig 8displays the solar cell performance of synthesized BiOI films by different concentrations We adapted the solar device arrangement like in Dye-Sensitized Solar Cell (DSSC) but we used only the single of BiOI photoanode instead of n-type semiconductor and dye In this work, p-type BiOI layer as a single semiconductor was arranged as photoanode without involving an n-type semi-conductor After the visible light is absorbed by BiOI semiconductor, the excited electron will be injected to the conductive glass sub-strate Additionally, Iin the electrolyte solution can catch the holes from BiOI and it will be followed by the diffusion process with the counter electrode (Pt) Furthermore, this step facilitates the fast reduction and oxidation reaction to complete the cycle process in conventional solar cell
Under the simulated solar illumination, we measured the J-V curve of these photoelectrochemical cells and the data are shown in
Fig 8andTable 1 Generally, from the J-V analysis, it can be seen that the changing of precursor concentration up to 7 mM shows the improvement of power conversion efficiency After concentration increases up to 10 mM, it drops significantly By this work, we also obtained the best efficiency of 0.318% for the independent BiOI working electrode from 7 mM of Bi(NO3)3and KI solutions Further, the best short-circuit photocurrent (Jsc) and open-circuit voltage (Voc) in our cell is 2.292 mA/cm2and 0.447 V respectively Since we did not involve the n-type semiconductor in our work, ourfilm
efficiency is much lower comparing to the previous solar cell per-formance of FTO/TiO2/BiOI films [12] The single material BiOI might have the lower ability of electron transport as its character of
Fig 4 Proposed schematic of BiOI morphology changing due to different concentration.
Trang 6p-type semiconductor Therefore, it exhibited the poor
perfor-mance However, once it contacted with an n-type semiconductor
like TiO2 which could support the better separation of
photo-generated charge, the pen junction structure was formed This pen
junction structure might inhibit the current leakage in the device
As the consequence, its solar performance increased Although our
solar cell performance is still low, we show the photovoltaic
per-formance improvement of BiOI photoanode We obtained the
bet-ter performance of single BiOI photoanode for solar cell in
comparison to the previous results[13,14]
We observed that by using the higher precursor molarity, the films color tended to be more orange This changing might be affected by thefilm thickness since the concentrated precursors bring to the thicker layerfilms which are possible to enhance the visible light harvesting ability in BiOIfilms As a result, the Jscvalue increased Although the increase of thickness can enhance the solar cell performance, the resulted thick-film from 8 mM of precursors shows the lower value in its solar cell parameter This decrease might be caused by the size and morphology of the resulted ma-terials from the concentrated precursors The suitable thickness is
Fig 5 UVeVis transmittance (a) and absorbance spectra of synthesized BiOI films in the different molar concentration (b) Inset: Plots of band gap measurement from reflectance, (ahv) 1/2 vs photon energy (hv).
A.A Putri et al / Journal of Science: Advanced Materials and Devices 4 (2019) 116e124 121
Trang 7an important factor to control the solar cell performance If the thicker layer is formed, it may also result in the non-compact layer which may have the contribution in reducing the solar cell per-formance since the probability of back electron transfer in solar cell device increases This phenomenon might be same as in the pre-vious result which confirmed that by the different angle in the BiOI film preparation could result in the different solar cell performance and the thin BiOIfilm had the better performance of solar cell[28]
In addition, the biggerflake might reduce the electrolyte penetra-tion which strongly affected the charge transfer mechanism in our solar cell device This trend seems similar to the reported re-searches which considered the thickening of BiOI films was the reason to decline in the solar cell parameters [10,12,14e16,45] Apart from this, impressively, the Jscincrement of prepared BiOI films from 6 mM to 7 mM of precursor almost doubled In this case,
we assumed that the light scattering effect might influence the solar cell performance in our work, especially for thosefilms The certain amount of bigger size material also has the ability to pro-mote the back scattering effect which improved the solar cell per-formance like in the mesoporous TiO2solar cell
Moreover, we also analyzed the EQE aspect which is shown in
Fig 9 We display the EQE curve for the synthesized BiOIfilms at
5 mM, 6 mM, 7 mM, and 8 mM of precursors As the Jscresponse in the diode curve of the BiOIfilm from 10 mM was very low, it limited our EQE measurement Then, we are not able to show its EQE character Nevertheless, by the EQE results, we noticed that be-tween EQE peak and Jscshow the same trend To discuss, we agreed with the hypothesis of Hoye and co-workers [10] about the increasing of recombination in BiOI which might be caused by the limited carrier extraction due to the higher photogenerated carrier density This character might easily occur in the thicker layer of BiOI which were represented by the preparedfilms at 8 mM and 10 mM Furthermore, we also realized that the amount of deposited BiOI in
Fig 6 Band gap energy of resulted BiOI films in different concentration.
Fig 7 Raman spectra of BiOI films in different concentration.
Fig 8 IV performance of synthesized BiOI films from different precursor
Table 1 Solar cell parameters of synthesized BiOI films from different precursor concentration.
Sample BiOI J sc (mA/cm 2 ) V oc (V) FF PCE (%)
films from some precursor concentrations.
Trang 8the substrate can be the crucial matter to adjust the BiOI
perfor-mance in order to improve its solar cell efficiency Although the
efficiency of BiOI devices is still low, there are some advantages
which can be attained by BiOI utilization due to its stability
compared with Pb-perovskite There is an evidence that BiOI has
better stability than perovskite material[10]and it is the safe
ma-terial in environment The chemical composition and
crystallo-graphic structure of BiOI are totally different from the perovskite,
however, it has been predicted that this material has a similar
electronic structure replication like in the Pb-perovskite [46]
Therefore, we think that the more optimization and study for the
BiOI application in solar cell are still needed to improve its
per-formance and perovskite development in the future It is expected
that BiOI can be an alternative absorber layer in the solar cell
de-vice Based on this research, we are still focusing to optimize in the
BiOI solar cell application by involving the composited material for
the upcoming work We expect that this study can open up to the
next BiOI solar cell since it is considered that this material
perfor-mance improvement is still challenging
4 Conclusion
In summary, we have demonstrated the effect of precursor
molarity variation in BiOIfilms preparation by SILAR for solar cell
application Different physical properties of as-fabricated BiOIfilms
were obviously observed by varying precursor concentration and it
changed the solar cell performance At a fixed concentration
(7 mM), the best solar cell performance was obtained with the Jsc
value of ~2.2 mA/cm2 However, by the increase in the precursor
molarity up to 10 mM, the decrease in the photovoltaic
perfor-mance was unavoidable Although the thicker layer can be achieved
easily by the higher concentration of Bi(NO3)3and KI, it leads to the
biggerflaky and rod-like BiOI growth which reduced the solar cell
performance Therefore, we found that the high performance of
BiOI in solar cell can be attempted by using the bismuth and iodide
precursors at around 7 mM and it is pointed that the physical
properties of BiOI films prepared by SILAR can be strongly
controlled by the precursor molarity
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