Preparation and Comparison Between Bi₂WO₆ and ZnO Photoanodes in Dye-sensitized Solar Cells

Abstract

This study presents a comparative analysis of two photoanode materials such as bismuth tungstate (Bi₂WO₆) and zinc oxide (ZnO) for application in dye-sensitized solar cells (DSSCs). The Bi₂WO₆ and ZnO films were coated onto fluorine-doped tin oxide (FTO) glass substrate via the doctor blade method. The film samples were characterized by using XRD, SEM, TEM, XPS, FT-IR and UV-Vis-NIR techniques. The photovoltaic performance of DSSCs using MO and RhB dyes as sensitizers was assessed under solar light irradiation. Results revealed that Bi₂WO₆ and ZnO film photoanodes sensitized with MO dye showed superior photovoltaic efficiency compared to when sensitized with RhB dye. Moreover, the ZnO film photoanode achieved an efficiency of 1.24%, outperforming the Bi₂WO₆ film photoanode which had an efficiency of 0.85% under similar conditions. A probable mechanism for photogenerated electron transfer and charge carrier separation in DSSCs was proposed.

Keywords:
Bi₂WO₆; ZnO; Dye-sensitized solar cells

1. Introduction

Dye-sensitized solar cells (DSSCs) are sustainable and clean technologies for converting solar energy into electrical energy, first developed by O’Regan and Grätzel in 1991¹ and Grätzel in 2001². DSSCs have gained great popularity due to their attractive features and advantages such as easy production process, low cost, high stability, and environmental friendly³,⁴. In general, DSSCs preparation is comprised of an oxide semiconductor film coated on a conductive glass substrate with sensitizing dyes as a photoanode, carbon powder or metal particles used as a counter electrode, and iodide/triiodide (I^-/I^3-) redox couple applied as an electrolyte in DSSCs⁵. In the past decade, the varieties of oxide semiconductors have been considered as photoanodes such as TiO₂, MgO, MnO₂, SnO₂, WO₃, ZnO and so on⁶-¹¹. Among these oxide semiconductor materials, ZnO is one of the most significant oxide semiconductors, drawing wide attention of researchers for potential applications in DSSCs due to its high efficiency, lower cost, high physicochemical stabilities, small particle size, and wide band gap energy, which is similar to TiO₂¹²,¹³. Moreover, ZnO has a porous-like structure, leading to the adsorption of more dye molecules and the conduction of photoinduced electrons from dyes on the surface area¹⁴,¹⁵, resulting in its enhanced photoconversion performance in DSSCs. Among all the proposed oxide semiconductors, the efficacy of Bi₂WO₆ particles in DSSCs has been reported. Bi₂WO₆ is one of the simplest Aurivillius oxide family with the general formula Bi₂A_n-1B_nO_3n+3 (A = Ca, Sr, Ba, Pb, Bi, Na, K and B = Ti, Nb, Ta, Mo, W, Fe), which has orthorhombic layered structure of the perovskite-like slaps of octahedral (WO₄)^2- layers and alternating bismuth oxide (Bi₂O₂)²⁺ layers¹⁶-¹⁸. To date, Bi₂WO₆ has gained great attention because it shows excellent properties for significant applications, including catalysts, optical fiber, ferroelectric piezoelectricity, pyroelectricity, magnetic devices, and so on¹⁸. Moreover, various methods have been employed to synthesize Bi₂WO₆ particles resulting in different structures such as hydrothermal, co-precipitation, sol-gel, microwave-assisted, solid-state reaction, flux growth, etc¹⁹-²⁴.

On the other hand, Bi₂WO₆ and ZnO films on the FTO glass substrate are used as photoanode materials in DSSCs. There are various methods such as screen-printing process²⁵, sol-gel process²⁶, physical vapor deposition process²⁷, electro-hydrodynamic deposition process²⁸, spin coating process²⁹, dip-coating process³⁰, and doctor blade process³¹. Especially, the doctor blade process is mainly mentioned as a preferable technique for preparing films owing to their easier production techniques and low cost. In addition, a film can be quickly prepared up to several meters per minute and the film thickness can be tuned from twenty nanometers to one hundred micrometers on the FTO glass substrate³².

In this research, the doctor blade technique is used to produce Bi₂WO₆ and ZnO films on FTO glass substrate, as a photoanode for application in DSSCs. Moreover, the as-prepared films were coated with two synthetic dyes, MO and RhB for the use as sensitizers in DSSCs. Furthermore, the photovoltaic efficiency and mechanism of DSSCs were investigated by irradiating solar light.

2. Experimental Section

2.1. Preparation of Bi₂WO₆ particles

In a typical hydrothermal synthesis procedure, bismuth nitrate pentahydrate (Bi(NO₃)₃•5H₂O) and sodium tungstate dihydrate (Na₂WO₄•2H₂O) in a molar ratio of 2:1 were dissolved in 50 ml of diluted nitric acid (HNO₃) and deionized water, respectively. The solutions were mixed with a magnetic stirrer before adding ammonium hydroxide (NH₄OH) solution to adjust pH 7. Then, the as-prepared suspension was poured into 100 ml Teflon-lined stainless autoclave vessel and heated at 180 °C for 24 hours. After that, the suspension was washed with deionized water using a centrifuge to separate the precipitate and the solution, and then dried in an electric oven at 80 °C for 24 hours.

2.2. Preparation of ZnO particles

The ZnO particles were synthesized via a co-precipitation method. The zinc acetate dehydrate (Zn(CH₃COO)₂·2H₂O) and potassium hydroxide (KOH) were used as starting materials. Firstly, Zn(CH₃COO)₂·2H₂O and KOH with 0.1:0.2 M ratio were dissolved in deionized water of 50 ml. The solutions were then mixed with vigorous magnetic stirring to form a homogeneous solution. The 6 M of nitric acid was diluted with deionized water to adjust the pH 7. The obtained precipitates were collected and filtered by a centrifuge and then dried with a muffled oven at 80 °C for 24 hours. Finally, the white ZnO particles were heated at 500 °C for 1 hour.

2.3. Preparation of DSSCs

First of all, the FTO glass substrate was cleaned with deionized water and ethanol in an ultrasonic bath to eliminate contaminants from the glass. Besides, the pastes were prepared by using 0.5 g of particle powders, 1 ml of acetic acid, 0.5 ml of Triton x-100, and 5 ml of absolute ethanol. Then the mixtures of the paste preparation were ground continuously in a mortar to form homogeneous pastes. The as-prepared pastes were coated on FTO glass substrate by the doctor blade method. Final films were annealed at a temperature of 450 °C for 1 hour to eliminate Triton x-100.

After that, the as-prepared films were soaked in dye solution for 24 hours to ensure absorption-desorption equilibrium for the use as a photocathode. For the part of the photoanode, the carbon powder decorated on FTO glass substrate was used. To estimate the photovoltaic efficiency, photocathode and photoanode were assembled to form a sandwich type.

2.4. Characterization

The phase structure of all film samples was recorded by X-ray diffraction (XRD, BRUKER, D8 Advance) using Cu Kα radiation, with the 2θ of samples ranging from 10° to 80°. The morphological characterization of as-prepared film samples was analyzed using scanning electron microscopy (SEM, HITACHI S-3400N). The actual microstructure of film samples was observed via transmission electron microscopy (TEM, JEOL model JEM 2100). The specific surface area of film samples was determined through Brunauer-Emmett-Teller (BET, 3 Flex Micromeritics). The chemical composition and surface electronic states of film samples were studied through X-ray photoelectron spectroscopy (XPS, AXIS ULTRA^DLD). The identification of functional groups of film samples was studied through Fourier transform infrared spectroscopy (FT-IR, PerkinElmer Scientific) technique. The optical absorption properties and band gap energies of film samples were measured via UV-VIS-NIR spectrophotometer (UV-vis, Shimadzu 3600).

3. Results and Discussion

3.1. XRD analysis

The crystallographic structure of Bi₂WO₆ and ZnO films was analyzed by using XRD technique. Figure 1 shows the XRD patterns of Bi₂WO₆ and ZnO films. The structure and phase purity of Bi₂WO₆ and ZnO films were confirmed. The XRD pattern of Bi₂WO₆ film typically indicated characteristic peaks that corresponded to the orthorhombic phase with a JCPDS file no. 73-1126³³. The main diffraction peaks of Bi₂WO₆ film were observed at 2θ of approximately 28.31°, 32.87°, 47.13°, 55.91° and 58.56°. These peaks matched well with (113), (200), (220), (313) and (226) planes, respectively. For XRD pattern of ZnO film, the diffraction peaks were indexed as the hexagonal wurtzite crystal structure which related to a JCPDS file no. 36-1451³⁴. The crucial diffraction peaks of ZnO film located at 31.79°, 34.44°, 36.27°, 47.54°, 56.57°, 62.87°, 67.93° and 69.09°. These intensities corresponded to (001), (002), (101), (102), (110), (103), (112) and (201) planes, respectively. Moreover, it can be seen that there was no other impurity phases, indicating that both orthorhombic Bi₂WO₆ and hexagonal wurtzite ZnO structures had high crystalline quality and purity.

Figure 1
XRD patterns of Bi₂WO₆ and ZnO films.

3.2. SEM-TEM analysis

SEM technique was employed to investigate the morphology of ZnO and Bi₂WO₆ films, as shown in Figure 2. Figure 2a reveals the SEM image of ZnO film. The results demonstrated that the as-prepared ZnO film had a rod-like structure, with average particle size from ca. 20 to 50 nm. For SEM image of Bi₂WO₆ film, a plate-like structure with a large diameter of ca. 100 to 200 nm was found, as displayed in Figure 2b. To confirm the actual microstructure and particle size of ZnO and Bi₂WO₆ films, TEM technique was used. Figure 2c exhibits the microstructure of ZnO film. The structure was rod-like with ca. 50 to 100 nm in particle diameter. In part of Bi₂WO₆ film, a plate-like structure with a diameter ranging from ca. 100 to 200 nm was observed, as represented in Figure 2d.

Figure 2
SEM and TEM images of (a,c) ZnO and (b,d) Bi₂WO₆ films.

3.3. XPS analysis

To study the information on the elemental composition of Bi₂WO₆ and ZnO film photoanodes, the X-ray photoelectron spectroscopy (XPS) technique was used and shown in Figure 3. It can be seen that the XPS spectrum of Bi₂WO₆ mainly composed of Bi 4f, W 4f and O 1s. As shown in Figure 3a, the intensity of Bi 4f was comprised of two peaks at 159.23 eV and 164.53 eV, which can be ascribed to Bi 4f_7/2 and Bi 4f_5/2 of Bi³⁺ ions in [Bi₂O₂]2+ ³⁵,³⁶. In Figure 3b, the peaks of W 4f located at a binding energy of 35.44 eV and 37.61 eV correspond to W 4f_7/2 and W 4f_5/2, respectively, which can be attributed to W⁶⁺ ions in [WO₄]2- ³⁵,³⁶. The O 1s peaks of Bi₂WO₆ had four binding energies which appeared at 530.16 eV, 531.35 eV, 532.95 eV, and 534.32 eV, matching Bi-O (lattice oxygen), W-O, OH groups, and oxygen species on the Bi₂WO₆ surface³⁷-³⁹, respectively, as displayed in Figure 3c. For XPS spectrum of ZnO, it constituted the chemical states of Zn 2p and O 1s. In Figure 3d, it shows the XPS peaks of Zn 2p, which included two peaks at binding energy of 1021.68 eV for Zn 2p_3/2 and 1044.70 eV for Zn 2p_1/2, which can be assigned to Zn²⁺ ions in the form of ZnO⁴⁰,⁴¹. The XPS peaks of O 1s displayed in Figure 3e can consist of four binding energies at 530.49 eV, 531.53 eV, 532.44 eV, and 533.65 eV, which identified chemisorbed O₂ species, OH groups in wurtzite hexagonal ZnO structure and lattice oxygen species in chemisorbed H₂O, respectively⁴⁰,⁴¹. Therefore, the results of XPS analysis can be concluded that the existences of Bi, W, and O elements in Bi₂WO₆ and Zn, O elements in ZnO, correspond to the results of XRD analysis.

Figure 3
XPS spectra of (a) Bi 4f, (b) W 4f and (c) O 1s in Bi₂WO₆ film and (d) Zn 2p and O 1s in ZnO film.

film.

3.4. FT-IR analysis

Figure 4 displays the FT-IR spectra of Bi₂WO₆ and ZnO films, which were measured in the wavenumber region of 400-4000 cm^-1 through the KBr method. From the FT-IR analysis results, the absorption bands of Bi₂WO₆ film appeared at 572 cm^-1 could be attributed to the asymmetric stretch of Bi-O-Bi⁴²,⁴³, while the peaks at 729 cm^-1 and 1030 cm^-1 were stretching of W-O, and the peak at 876 cm^-1 was stretching mode of Bi-O⁴²,⁴³. The two absorption bands located at 1618 cm^-1 and 3425 cm^-1 could be matched well with the stretching and bending modes of O-H on the surface of Bi₂WO₆⁴⁴. For the FT-IR peaks of ZnO film, the band of functional groups was showed at the wavenumber range of 832 cm-¹, 983 cm^-1, 1386 cm^-1, and 1519 cm^-1, corresponding to the bending vibration of C-H, stretching of C-N, bending vibration of C-H of an alkane group and stretching of C=C of an aromatic ring⁴⁵,⁴⁶, respectively. The peaks at 2363 cm^-1 and 3405 cm^-1 corresponded to the vibration of H-O-H of water molecules and the stretching vibration of O-H of hydroxyl compounds⁴⁶,⁴⁷, respectively.

Figure 4
FT-IR spectrum of Bi₂WO₆ and ZnO film photoanodes.

3.5. UV-VIS-NIR analysis

The optical absorption and energy band structure of Bi₂WO₆ and ZnO films were carried out by using UV-vis NIR spectroscopy technique, as shown in Figure 5. The absorbance of Bi₂WO₆ and ZnO films can be calculated by using transmittance and reflectance measurements according to the equation as follows⁴⁸:

$$\%A=100-\left(\%R-\%T\right)$$ (1)

where $A$is the absorbance, $R$is the reflectance and $T$is the transmittance, respectively. From experimental results, the absorption edges of Bi₂WO₆ and ZnO films showed the UV light range of ca 363 nm and 382 nm, respectively, as displayed in Figure 5a and 5b. The band gap energy of Bi₂WO₆ and ZnO films can be expressed via Tauc's formula as follows:

$$\left(\alpha hv\right)=A{\left(hv-E_g\right)}^n$$ (2)

where α is the absorption coefficient, h is the Planck’s constant,$A$is the constant,$v$is the frequency of photons, E_g is the band gap energy of film samples and n is the type of optical transition of film samples: $n=\frac{1}{2}$ for direct optical transition and n = 2 for an indirect optical transition⁴⁹-⁵¹. The band gap values of Bi₂WO₆ and ZnO films were plotted between (αhv)ⁿ versus$hv$of the film samples, as represented in Figure 5c and 5d. The band gap energies of Bi₂WO₆ and ZnO films were found to be 3.39 eV and 3.24 eV, respectively. The absorption edge and band gap energy are key factors that affect the improved efficiency of DSSCs.

Figure 5
Optical absorption and band gap energy of (a,c) Bi₂WO₆ and (b,d) ZnO film photoanodes.

3.6. Photoabsorption properties of synthetic dyes

Figure 6 shows the photoabsorption analysis of synthetic dyes such as methyl orange (MO) and rhodamine B (RhB), in which UV-vis spectrophotometer technique was employed. For the photoabsorption of MO and RhB dyes, maximum intensities appeared at 466 nm and 553 nm, respectively. Furthermore, the UV-vis spectra of MO dye located at 350-550 nm, which was in the wavelength range of UV and visible light. For the absorbance of RhB dye, it was in the visible light region of ca. 450-600 nm. The obtained results of photoabsorption in Figure 6 affected the π-π^* transition of electrons on the dye surface. Moreover, it was found that MO dye showed a broader absorption region compared to RhB dye, indicating that MO dye showed the highest π-π^* transition of electrons from LUMO of dye to conduction band of Bi₂WO₆ and ZnO film photoanodes as compared to RhB dye⁵²,⁵³. Therefore, it can be concluded that using MO dye as a sensitizer on the surface of Bi₂WO₆ and ZnO film photoanodes is more suitable than RhB dyes for potential application in DSSCs.

Figure 6
Photoabsorption spectra of synthetic dyes.

3.7. Photovoltaic efficiency of Bi₂WO₆ and ZnO film photoanodes

The photovoltaic efficiency was investigated by using Bi₂WO₆ and ZnO film photoanodes coated with two synthetic dyes as photoelectrodes in DSSCs. Figure 7 shows the plot of current density (J_SC) versus open-circuit voltage (V_OC). All parameters of photovoltaic efficiency study of Bi₂WO₆ and ZnO film photoanodes in DSSCs are shown in Tables 1 and 2. In addition, the solar cell conversion efficiency (η) of DSSCs based on Bi₂WO₆ and ZnO film photoanodes with synthetic dyes can be calculated from the following equation⁵⁴-⁵⁶:

$$FF=P_{max}/J_{SC}x{V}_{OC}$$ (3)

$$\eta \left(\%\right)=J_{SC}x{V}_{OC}xFFx100/P_{in}$$ (4)

where P_max is the ratio of the maximum power of solar cells per unit area divided by the J_SC and V_OC, J_SC is the short circuit current, V_OC is the open-circuit voltage, FF is the fill factor and P_in is the input power (100 mW/cm²). The results of Bi₂WO₆ film photoanode-based DSSCs with MO and RhB dyes showed 0.85% (J_SC is 6.103 mA/cm², V_OC is 0.500 V and FF is 0.28) and 0.45% (J_SC is 4.054 mA/cm², V_OC is 0.533 V and FF is 0.21), respectively, as shown in Figure 7a and Table 1. In the part of ZnO film photoanode-based DSSCs with MO and RhB dyes, the photovoltaic efficiency was equal to 1.24% (J_SC is 9.56 mA/cm², $V_{OC}$is 0.450 V and FF is 0.29) and 0.98% ($J_{SC}$is 8.1 mA/cm², $V_{OC}$is 0.450 V and FF is 0.27), respectively, as displayed in Figure 7b and Table 2. From the experimental results, it was found that using MO dye as the sensitizer in DSSCs showed the highest efficiency as compared to other dyes. This phenomenon may be due to MO dye having a broader photoabsorption in UV and visible light region than that of RhB dye⁵⁷-⁶⁰, as represented in Figure 8. For the efficiency comparison study between Bi₂WO₆ and ZnO film photoanodes in DSSCs, it was found that ZnO film photoanode-based DSSCs exhibited higher efficiency than Bi₂WO₆ film photoanode-based DSSCs, which might be attributed to the transformation of electron from dye molecules to conduction band of ZnO film photoanode-based DSSCs, which was higher than Bi₂WO₆ film photoanode-based DSSCs⁶¹. Moreover, enhanced photovoltaic efficiency does not only depend on photoabsorption region and transformation of electrons, but also on crystal structure, specific surface area, microstructure, and so on⁵⁸.

Figure 7
J-V curves of DSSCs using (a) Bi₂WO₆ and (b) ZnO film photoanodes with two synthetic dyes.

Table 1
Photovoltaic parameters of Bi₂WO₆ film photoanode-based DSSCs with synthetic dyes.

Synthetic dyes	JSC (mA/cm2)	VOC (V)	FF	ç (%)
Methyl orange (MO)	6.103	0.500	0.28	0.85
Rhodamine B (RhB)	4.054	0.533	0.21	0.45

Table 2
Photovoltaic parameters of DSSCs based on ZnO photoanode with synthetic dyes.

Synthetic dyes	JSC (mA/cm2)	VOC (V)	FF	ç (%)
Methyl orange (MO)	9.56	0.450	0.29	1.24
Rhodamine B (RhB)	8.10	0.450	0.27	0.98

Figure 8
Comparison of photovoltaic efficiencies.

3.8. A possible Mechanism of DSSCs

A possible mechanism of Bi₂WO₆ and ZnO film photoanodes-based DSSCs was studied and shown in Figure 9. The values of the conduction band (CB) and valence band (VB) offsets can be estimated by using the equation as follows⁶²:

$$E_{CB}=\chi -E^C-0.5E_g$$ (5)

$$E_{VB}=E_{CB}+E_g$$ (6)

where E_CB is the energy level of the conduction band edge of a semiconductor, E_VB is the energy level of the valence band edge of the semiconductor, χ is the absolute electronegativity of the semiconductor (χ is 5.89 eV for ZnO⁶³, $\chi$is 6.36 eV for Bi₂WO₆⁶⁴), E^C is the energy of free electrons on the hydrogen scale (∼4.5) and$E_g$is the band gap energy of the semiconductor. The values of CB and VB edges of Bi₂WO₆ derived from equations 3 and 4 were 0.17 eV and 3.56 eV, and of ZnO were -0.23 eV and 3.01 eV, respectively. The basic compositions of DSSCs are mainly composed of FTO glass substrate, semiconductor oxide photoanodes, dyes, electrolytes, and counter electrodes. A likely mechanism of DSSCs was illuminated by solar light. The synthetic dye sensitizer has higher absorbance than photon energy or equal to the band gap energy of the dye sensitizer to excite the photogenerated electron transfer from HOMO to LUMO level of the dye sensitizer and finally become photoexcited. The separation of photogenerated charge carriers occurred in the dye sensitizer and interface of semiconductor oxide photoanodes when the electrons and holes are located at the semiconductor oxide photoanodes and oxidized dye sensitizer. The electrons on LUMO of the dye sensitizer are injected into the CB of semiconductor oxide photoanodes, which is lower in energy than the excited state of the dye sensitizer. Then the photogenerated electrons flow into the photoelectrode to transfer positive charges to the counter electrode, which is connected to the working electrode through the external load. Furthermore, electrons from the dye sensitizer can be injected from iodine electrolyte through iodide (I^-)ions to become triiodide (I^3-)ions, releasing photogenerated charges. Finally, the photogenerated charges flow from the external load and reverse to DSSCs via the carbon counter electrode on FTO glass substrate⁶⁵,⁶⁶. This process is repeated to generate electric power by irradiating solar light.

Figure 9
Schematic diagram of a possible mechanism of DSSCs.

4. Conclusions

In summary, two synthetic dyes, MO and RhB were employed as the sensitizers for DSSCs. The Bi₂WO₆ and ZnO films decorated on FTO glass substrate as photoanodes were prepared through the doctor blade method. A comparison of photovoltaic efficiency of film photoanodes and two synthetic dyes under similar conditions was investigated. The result indicated that coating MO dye on film photoanodes in DSSCs gave better efficiency than RhB dye owing to its broader photoabsorption region. Moreover, it was found that ZnO film photoanode showed better photovoltaic efficiency than Bi₂WO₆ film photoanode. As a result, it can be concluded that the ZnO film photoanode as a photoelectrode and MO dye as a sensitizer are suitable for application in DSSCs due to better bonding of dye molecules to ZnO film photoanode and better transitions of electrons from LUMO of dye molecules to conduction band edge of ZnO film photoanode. Therefore, to maximize the potential for improving the efficiency of DSSCs in the future, we may be considered the modifying Bi₂WO₆ through doping, morphology control, or hybrid designs.

5. Acknowledgements

The authors are appreciative to the Department of Science and Mathematics, Faculty of Industry and Technology, Rajamangala University of Technology Isan Sakonnakhon Campus, and the Division of Physics, Faculty of Science, Nakhon Phanom University for supporting this research.

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• Data Availability
  The data that support the findings of this study are available from the corresponding author upon reasonable request.

Edited by

• Associate Editor:
  Jose Eiras.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Publication Dates

• Publication in this collection
  14 July 2025
• Date of issue
  2025

History

• Received
  07 Jan 2025
• Reviewed
  08 May 2025
• Accepted
  18 May 2025