A simple spin-assisted SILAR of bismuth oxyiodide films preparation for photovoltaic application
Bismuth oxyiodide (BiOI) films have been successfully fabricated from Bi(NO3)3 and KI precursors at room temperature via facile successive ionic layer adsorption and reaction assisted with spin-coating (spin-SILAR). In our work, the uniform and dense BiOI layer was easily formed in a shorter time by more environmentally friendly process. X-ray diffraction, Raman spectroscopy, and field emission scanning electron microscope (FESEM) were used to study the crystal structure and surface morphology of BiOI films. The absorption spectra in the UV–vis–NIR regions were also measured using spectrophotometric measurement in the wavelength range, from 300 to 2000 nm for clarifying the properties of resulted BiOI/FTO films. From the experiment, we obtained the short-circuit current density of BiOI films in the FTO/glass substrates which up to 0.612 mA/cm2. Although the superior solar cell performance was not exhibited by our BiOI films, due to the benefit of spin-SILAR method, it may open up the further application of BiOI films prepared through this method.
KeywordsThin films BiOI Spin-SILAR Optical properties Photovoltaic
BiOI can be considered as the high-tolerant defect semiconductor material [1, 2] which is categorized as p-type semiconductor with the narrow bandgap at ~ 1.8 eV. It has a strong absorption under the visible light irradiation [3, 4]. Owing to its character which is safe to the environment in comparison with the lead-based materials, BiOI has been commonly applied for photocatalytic reaction and photovoltaic devices [5, 6, 7, 8]. Furthermore, BiOI can be successfully synthesized by successive ionic adsorption and reaction with dip-coating (dip-SILAR) [5, 8, 9, 10], chemical bath deposition (CBD) , chemical vapor transport [2, 12], mechanical grinding , and solvothermal reaction [6, 14]. Conventional dip-SILAR, CBD, and slip-casting from BiOI powder  are also common to obtain the BiOI films for photovoltaic application.
In the conventional dip-SILAR, the water rinsing should be included in all steps to maintain the film uniformity. Although good film quality can be acquired, this dip-SILAR consumes more time for producing the films due to the rinsing step. Therefore, it may be difficult to get more BiOI films in a shorter time. If the rinsing step in the dip-SILAR process is removed, the film uniformity may be poor although the dip-SILAR could be modified with different angles in its substrate direction . Furthermore, it is considered that dip-SILAR has less reproducibility because it depends on the rinsing and dipping process even though it is low cost . As we noticed that dip-coating is the most common method for preparing BiOI films, we tried to develop BiOI films preparation through a modified way, namely spin-coating method combined with SILAR (spin-SILAR) for the first time. This work was inspired by the Pb-perovskite synthesis and film preparation.
Spin-coating is the universal method for preparing films, such as P3HT, PEDOT, PSS, and perovskite material [18, 19, 20] for solar cell purposes. In spin-SILAR, this spin-coating method is combined with SILAR, in which its adsorption and reaction steps can occur while the spin-coating process is running. By utilizing this method, the rinsing step and drying process can be also completed when the solution is spun . Then, we suppose that spin-SILAR can deposit flat BiOI films onto the substrates in a shorter time. In addition, only the small amount of precursor solution is required in spin-coating thanks to the less remaining waste solution in spin-SILAR. Due to this reason, it also offers the opportunity that spin-SILAR is more environmentally benign than dip-SILAR.
By the BiOI film deposition via spin-SILAR, we also studied the optical, physical, and photovoltaic properties of BiOI. We characterized the properties of deposited BiOI films via spin-SILAR by changing the number of cycles which follow the direction from the previous experiment . Here, we found the uniformity of the resulted BiOI layer by spin-SILAR although the rinsing step was eliminated. Furthermore, the deposited BiOI films onto FTO/glass substrates could exhibit the performance of solar cell devices with Pt/FTO as counter electrode and I−/I3− solution as electrolyte. We expect that this work can be a beneficial reference for BiOI exploration in BiOI solar cell development.
2 Materials and methods
We utilized Bi(NO3)3·5H2O and KI which acted as the cation and anion sources that were purchased from Nacalai Tesque, Inc (Kyoto, Japan). The ultrapure water from Milli-Q direct water purification system with resistivity 18.2 MΩ·cm at 25 °C was used to make all the solutions during the research.
2.2 Deposition method
2.3 BiOI characterization and solar cell fabrication
The resulted films were characterized using X-Ray Diffraction (Rigaku RINT-2100 diffractometer), Raman Spectrometer (JASCO NRS-2100), FESEM JEOL JSM-7800F, and UV–visible Spectroscopy (JASCO 670 UV). Solar cell structure was prepared by a sandwich structure using FTO glass substrate, adapted from dye-sensitized solar cell devices without involving the dye solution . The photovoltaic structure was arranged by FTO/BiOI photoanode and platinum based as a counter electrode, such as the cell: FTO/BiOI films/Iodine electrolyte/Pt-FTO. Then, this structure was tested using solar simulator (100 mW/cm2; AM 1.5 illumination) with 0.16 cm2 of illumination area.
3 Results and discussion
3.1 Structural analysis
3.1.1 X-ray powder diffraction analysis
3.1.2 Raman study
In addition, the BiOI formation follows the chemical reaction in Eq. 2 . It occurs after the hydrolysis of bismuth precursor. The (001) crystal plane domination can be formed under the strong acidity situation. Otherwise, (110) crystal plane is easier to be obtained under the less acidic solution. If more water is added, the less concentration of Bi(NO3)3 can induce the hydrolysis rate acceleration. As a consequence, the nucleation rate will increase and promote the rapid growth of the crystal along with the (001) surface .
3.1.3 Surface morphology
By the tilting cross-sectional image, the more compact BiOI film by spin-coating is confirmed as shown in Fig. 4d, which tends to be denser than the previous dip-coated BiOI result [16, 27]. Owing to its smaller size, we predict that the prepared BiOI from spin-coating may have the higher surface area which can be expected to result in the better performance for photocatalysis or solar cell application with the suitable hole and electron transport materials. The more uniform films in this work can be a good point of SILAR method without rinsing treatment assisted with spin-coating. Besides, it is easy to produce BiOI via spin-SILAR, and the films can be deposited by the reaction of spread solution onto the FTO substrate.
3.2 Optical properties
The bandgap energy evaluation of BiOI films can be arranged for the following order: 30 cycles (1.95 eV) < 20 cycles (1.95 eV) < 15 cycles (2 eV) < 10 cycles (2.25 eV). The bandgap energy values were less than those in the prepared films by dip-coating. The different sizes may be the reason for this matter since the bigger size could be obtained in the thickening of films, and it changes the bandgap energy. The increase in particle size in our thin film may decrease the bandgap energy since the improvement in thickness and material growth can be generated along with the more reaction cycle. The crystal size reduction has impact on the bandgap energy value. Moreover, the different bandgap energies may be influenced by the different lattice strains, dislocation of film densities, and crystallite size of semiconductor materials.
Related to the increasing of thickness, it also has a correlation with the Raman spectra. The peak intensity of BiOI in the Raman spectra (Fig. 3) shows an increment due to the increasing of SILAR’s cycle reaction number. The highest intensity of BiOI films is shown by the film from 30 reaction cycles via spin-SILAR. Hence, height intensity peak can also reflect its concentration which is in line with its film thickness.
3.3 Photovoltaic cell measurement
Photovoltaic performance of BiOI films prepared from spin-SILAR in the different reaction cycles: 5, 10, 15, 20, and 30 cycles
Rsh (103 × Ω cm2)
Rs (103 × Ω cm2)
Spin- 5 cy
Spin- 10 cy
Spin- 15 cy
Spin- 20 cy
Spin- 30 cy
In addition, the centrifugal force in the spin-coating might play an important role to accelerate the solvent evaporation. Therefore, the density of BiOI films in the FTO substrates can be completed by spin-SILAR. We pointed that the more (001) plane in the BiOI may be unpleasant for solar cell application since the solar cell performance declines due to the film thickening and (001) intensity increment. Since the different crystal type has the influence in the charge photogeneration, it may have the impact on the solar cell performance and the photocatalytic activity of BiOI  in comparison with the previous results [16, 27]. For future work, it may be possible to tailor the facet orientation in BiOI films via spin-SILAR and to study experimentally the effect of facet orientation of BiOI which is more favorable for solar cell application.
In this work, spin-SILAR was proposed for the growth of BiOI films for the first time, and by the solar cell analysis, it achieved the best solar cell parameters of 612 μA/cm2, 0.446 V, and 0.103% for its Jsc, Voc, and PCE, respectively. Good quality of BiOI film can be obtained by SILAR assisted with spin-coating process although the washing step was eliminated. Due to the less time needed for films preparation and its simplicity, it is considered that this method can be aimed to prepare BiOI films for wider application. The crystal quality and quantity in the resulted BiOI films could be modified by the different preparation techniques. To sum up, the SILAR process assisted with the spin-coating could be an alternative way to produce BiOI films, whereas the number of reaction cycles in SILAR resulted in the different solar cell parameters. For the future work, the developed BiOI films prepared by spin-SILAR for solar cell application can be attempted owing to the fact that it has the better compactness over the dip-SILAR result.
A.A.P would like to thank the financial support from MORA Scholarship, Ministry of Religious Affairs, The Republic of Indonesia for the Ph.D. scholarship (No. 36/Dt.I.IV/4/PP.07/01/2017).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no competing interest.
- 12.Ye L, Chen J, Tian L, Liu J, Peng T, Deng K, Zan L (2013) Appl Catal B Environ 130–131:1Google Scholar
- 16.Putri AA, Kato S, Kishi N, Soga T (2019) Jpn J Appl Phys 58 (2019)Google Scholar
- 20.Lehner AJ, Wang H, Fabini DH, Liman CD, Hébert CA, Perry EE, Wang M, Bazan GC, Chabinyc ML, Seshadri R (2015) Appl Phys Lett 107Google Scholar