Relevance of precursor molarity in the prepared bismuth oxyiodide films by successive ionic layer adsorption and reaction for solar cell application

In summary, we have demonstrated the effect of precursor molarity variation in BiOI ﬁlms preparation by SILAR for solar cell application.. Different physical properties of as-fabricated [r]

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Original Article

Relevance of precursor molarity in the prepared bismuth oxyiodide

ﬁlms by successive ionic layer adsorption and reaction for solar cell

application

Anissa A Putri

, Shinya Kato

, Naoki Kishi

, Tetsuo Soga

aDepartment of Electrical and Mechanical Engineering, Nagoya Institute of Technology, Nagoya, 466-8555, Japan bDepartment of Chemistry, Walisongo State Islamic University, Semarang, 50185, Indonesia

a r t i c l e i n f o

Article history:

Received December 2018 Received in revised form 24 January 2019 Accepted 24 January 2019 Available online 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 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 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

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which 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 mM of precursors was reported, its short-circuit current was not more than 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 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 mM, mM, mM mM, 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: each of glass rinsing in acetone (twice) and in ethanol (once), N2gas blowing, and UV/Ozone treatment for 20 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 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/I3solution was inserted between the photoanode and counter electrode covered by Himilan polymerﬁlm as shown in

Fig 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 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 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 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 2

q

Fig BiOI solar cell illustration

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any 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 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 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]

L¼ K

l

b

q

Where,

L: crystallite size K: constant (0.9)

l

b

q

3.1.2 Morphology analysis

Fig 3shows the morphology of synthesized BiOIlms from BiOỵand Isources concentrations at mM, mM, 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 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 1

m

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 ~3.4

m

Regarding the BiOI morphology evolution due to the precursor concentration effect, we proposed the BiOI growth illustration as shown inFig In low concentration (5 mM), BiOI nanoparticles are formed and in the higher molarities (started from 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,

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during 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

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Bi(NO3)3ỵ H2O/ BiONO3ỵ 2HNO3 (2)

BiONO3ỵ KI / BiOI ỵ KNO3 (3)

Fig SEM image of BiOIﬁlms in the different concentration: mM (A), mM (B), mM (C), 10 mM (D) and cross-sectional of ~6 mM of precursor (E)

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3.2 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 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 From thisﬁgure, it is observed that there is a shift tendency of BiOI band gap value due to the different concentration According to the (

a

[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 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 Generally, from the J-V analysis, it can be seen that the changing of precursor concentration up to 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 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

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p-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 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 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)1/2vs photon energy (hv).

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an 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 mM to 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 We display the EQE curve for the synthesized BiOIfilms at mM, mM, mM, and 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 mM and 10 mM Furthermore, we also realized that the amount of deposited BiOI in

Fig Band gap energy of resulted BiOIﬁlms in different concentration

Fig Raman spectra of BiOIﬁlms in different concentration

Fig IV performance of synthesized BiOI ﬁlms from different precursor concentration

Table

Solar cell parameters of synthesized BiOI ﬁlms from different precursor concentration

Sample BiOI Jsc(mA/cm2) Voc(V) FF PCE (%)

5 mM 0.123 0.456 0.411 0.023

6 mM 1.246 0.431 0.430 0.231

7 mM 2.292 0.447 0.310 0.318

8 mM 1.087 0.418 0.401 0.182

10 mM 0.033 0.032 0.232 0.000249

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the 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 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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