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

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.

Journal of Science: Advanced Materials and Devices (2019) 116e124 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd 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 a, b, *, Shinya Kato a, Naoki Kishi a, Tetsuo Soga a a b Department of Electrical and Mechanical Engineering, Nagoya Institute of Technology, Nagoya, 466-8555, Japan Department of Chemistry, Walisongo State Islamic University, Semarang, 50185, Indonesia a r t i c l e i n f o a b s t r a c t Article history: Received December 2018 Received in revised form 24 January 2019 Accepted 24 January 2019 Available online February 2019 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 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 By a gradual increase in precursor molarity, the wide absorption and redshift of BiOI films 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 BiOI films 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 BiOI films from mM of precursor have the best Jsc and efficiency which up to ~2.2 mA/cm2 and 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/) Keywords: BiOI Precursor concentration Solar cell p-type semiconductor Bismuth materials 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 reaction, it still has low solar cell performance efficiency (around ~1%) for BiOI/TiO2/FTO film prepared by SILAR [12] We noted that one of important factors which affects the semiconductor material * 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 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 controlling 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 solventless reaction and solvo-reaction [10,15,24e27] In the freesolvent process, BiOI films can be prepared by chemical vapor transport of BiI3 under Ar/O2 atmosphere at around 300  C [10], while the BiOI powder can be produced through low temperature mechanical grinding process Despite the fact that the environmental 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, BiOI films for photovoltaic devices can be obtained by SILAR and chemical bath deposition (CBD) [12,13,15] 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 (http://creativecommons.org/licenses/by/4.0/) A.A Putri et al / Journal of Science: Advanced Materials and Devices (2019) 116e124 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 the first time in 2010 We highlighted that the solar cell performance of SILAR BiOI films was affected by the number of cycles during the film preparation In SILAR, the cycle of reaction controlled the film thickness and its physical properties [12e15], also beside the cycle effect, angle inclination in SILAR has an impact on the resulted BiOI films [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 suppose that the optimizing of precursor condition in BiOI film preparation may be an alternative to screen the better performance of BiOI photoanode In fact, some prepared nanomaterials with suitable 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 desired film, 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 application by SILAR with mM of precursors was reported, its shortcircuit current was not more than mA/cm2 [11e15] Here, we address the possibility of solar cell performance enhancement in BiOI films by varying its precursor concentration and report the double increment of Jsc value from our BiOI films compared with the previous results Owing to the increase in the precursor concentration, the different properties of BiOI films 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 Experimental 2.1 Synthesis of BiOI film The BiOI deposition in  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 BiOI films Before BiOI deposition process, the cleaning process of FTO glass substrates was done by the following steps: each of glass rinsing in acetone (twice) and in ethanol (once), N2 gas 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 resulted films were dried in air at 100  C for h 2.2 Fabrication and characterization of BiOI film Each resulted film 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, IÀ/IÀ solution was inserted between the photoanode and counter electrode covered by Himilan polymer film as shown in Fig The solar simulator (100 mW/cm2, AM 1.5 illumination) with the illuminated area at 0.16 cm2 was utilized to test the solar cell performance To investigate the film characteristics such as 117 Fig BiOI solar cell illustration 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 Results and discussion 3.1 Structural and morphology analysis 3.1.1 XRD analysis Fig represents the diffractogram of prepared BiOI films from mM to 10 mM of solutions In this report, we show that the greater bismuth and potassium salt concentrations cause an increase in the BiOI crystallinity Apparently, the film thickness from the concentrated precursor was thicker in comparison to the prepared BiOI films 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 the Fig 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 2q around 29.6 (012), 31.7 (110), and 45.5 (020) are revealed by the XRD patterns Furthermore, the strongest peak which exists in 2q around 29.6 confirms for (012) plane of BiOI crystal By the displayed data in Fig 2, it convinces that there is no 118 A.A Putri et al / Journal of Science: Advanced Materials and Devices (2019) 116e124 Fig XRD patterns of BiOI films from the lower and higher molarity of precursors (5 and 10 mM) 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 the films 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] L ¼ Kl/bCosq (1) 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 shows the morphology of synthesized BiOI lms from BiOỵ and I sources concentrations at mM, mM, mM, and 10 mM which are expressed in A, B, C, and D, respectively Basically, almost of BiOI morphology is found in the flake structure like informed in the many reported researches Here, we also show the flaky BiOI from the high concentration of precursor which is displayed in Fig In this figure, 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 wider flakes BiOI could be obtained The wider and thicker of BiOI flakes which are shown in Fig 3B,C have the lateral size around 500 nm and more than mm is for BiOI in Fig 3D While the molarity increased, we found the more and bigger rod-like material arranged by BiOI flakes This phenomenon is similar to previous research which confirmed that precursor 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 crosssectional image of BiOI films in Fig 3E BiOI film with ~3.4 mm of thickness was gained by involving the precursor concentration at around 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 thicker films which are prepared from the concentrated precursors Since the film compactness of BiOI films decreases due to the higher porosity, the precursor concentration may turn the character of BiOI films from the compact layer to non-compact layer along with the increase in the reactants molarities Later, this changing has a significant 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 in Fig In low concentration (5 mM), BiOI nanoparticles are formed and in the higher molarities (started from mM), the wider BiOI flakes are obtained Furthermore, the rod-like materials consisted of BiOI flakes 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 solution Since the reactant collision improves, the production of flake BiOI rate will be high This might be the reason why the concentrated precursor resulted in the wider flake of BiOI However, A.A Putri et al / Journal of Science: Advanced Materials and Devices (2019) 116e124 119 Fig SEM image of BiOI films in the different concentration: mM (A), mM (B), mM (C), 10 mM (D) and cross-sectional of ~6 mM of precursor (E) during the BiOI nucleation step in the concentrated precursor, the collision also might occur between the precursors and the previously 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 BiOI flakes of BiOI are also able to make a self-assembly formation resulting in the new morphology like flower-like structures of BiOI [4,35,36] In addition, the structural transformation from rod-like material to flakes morphology could occur electrochemically in the hematite synthesis [37] In this work, the reaction between anion and cation in Bi(NO3)3 and KI solutions to produce BiOI are shown in Eqs (2) and (3) Bi(NO3)3 ỵ H2O / BiONO3 ỵ 2HNO3 (2) BiONO3 ỵ KI / BiOI ỵ KNO3 (3) 120 A.A Putri et al / Journal of Science: Advanced Materials and Devices (2019) 116e124 Fig Proposed schematic of BiOI morphology changing due to different concentration 3.2 Optical properties 3.2.1 UV-visible spectral analysis The light photoresponse of BiOI films 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)3 and KI solutions result in the BiOI films 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 thicker films formation Regarding the increase of BiOI peak intensity in the Raman spectra and XRD patterns which is parallel to the amount of BiOI in the films, we assumed the thickness of BiOI films 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 in Fig From this figure, it is observed that there is a shift tendency of BiOI band gap value due to the different concentration According to the (ahv)1/2 vs (hv) plot, the band gap energy of prepared BiOI films 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 information of BiOI films using Raman spectroscopy The Raman spectra are shown in Fig All of prepared BiOI films from concentrated reactants have the stronger peak for BieI vibration stretching mode (Eg) around 147.91 cmÀ1 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 cmÀ1 [39e43] Besides the Eg stretching modes, other BiOI vibration types in Raman spectra are notated with A1g and B1g which should be existed in the wavenumber below 100 cmÀ1 However, in our experiment, we could not observe those peaks due to the observation 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 displays 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 semiconductor After the visible light is absorbed by BiOI semiconductor, the excited electron will be injected to the conductive glass substrate Additionally, IÀ in 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 and Table 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)3 and KI solutions Further, the best short-circuit photocurrent (Jsc) and open-circuit voltage (Voc) in our cell is 2.292 mA/cm2 and 0.447 V respectively Since we did not involve the n-type semiconductor in our work, our film efficiency is much lower comparing to the previous solar cell performance of FTO/TiO2/BiOI films [12] The single material BiOI might have the lower ability of electron transport as its character of A.A Putri et al / Journal of Science: Advanced Materials and Devices (2019) 116e124 121 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)1/2 vs photon energy (hv) p-type semiconductor Therefore, it exhibited the poor performance However, once it contacted with an n-type semiconductor like TiO2 which could support the better separation of photogenerated 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 performance improvement of BiOI photoanode We obtained the better 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 the film thickness since the concentrated precursors bring to the thicker layer films which are possible to enhance the visible light harvesting ability in BiOI films As a result, the Jsc value 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 materials from the concentrated precursors The suitable thickness is 122 A.A Putri et al / Journal of Science: Advanced Materials and Devices (2019) 116e124 Table Solar cell parameters of synthesized BiOI films from different precursor concentration Fig Band gap energy of resulted BiOI films in different concentration Fig Raman spectra of BiOI films in different concentration Fig IV performance of synthesized BiOI films from different precursor concentration Sample BiOI Jsc (mA/cm2) Voc (V) FF PCE (%) mM mM mM mM 10 mM 0.123 1.246 2.292 1.087 0.033 0.456 0.431 0.447 0.418 0.032 0.411 0.430 0.310 0.401 0.232 0.023 0.231 0.318 0.182 0.000249 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 performance since the probability of back electron transfer in solar cell device increases This phenomenon might be same as in the previous result which confirmed that by the different angle in the BiOI film preparation could result in the different solar cell performance and the thin BiOI film had the better performance of solar cell [28] In addition, the bigger flake might reduce the electrolyte penetration which strongly affected the charge transfer mechanism in our solar cell device This trend seems similar to the reported researches 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 Jsc increment 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 those films The certain amount of bigger size material also has the ability to promote the back scattering effect which improved the solar cell performance like in the mesoporous TiO2 solar cell Moreover, we also analyzed the EQE aspect which is shown in Fig We display the EQE curve for the synthesized BiOI films at mM, mM, mM, and mM of precursors As the Jsc response in the diode curve of the BiOI film 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 between EQE peak and Jsc show 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 prepared films at mM and 10 mM Furthermore, we also realized that the amount of deposited BiOI in Fig EQE curve of synthesized BiOI films from some precursor concentrations A.A Putri et al / Journal of Science: Advanced Materials and Devices (2019) 116e124 the substrate can be the crucial matter to adjust the BiOI performance 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 material in environment The chemical composition and crystallographic 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 performance and perovskite development in the future It is expected that BiOI can be an alternative absorber layer in the solar cell device 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 performance improvement is still challenging Conclusion 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 BiOI films 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 performance was unavoidable Although the thicker layer can be achieved easily by the 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contribution in reducing the solar cell performance since the probability... 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