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Article

Enhancing the Properties of Nanostructure TiO2 Thin Film via Calcination Temperature for Solar Cell Application

by
Nurliyana Mohamad Arifin
1,2,
Ervina Efzan Mhd Noor
1,2,*,
Fariza Mohamad
3,
Norhidayah Mohamad
1,2 and
Nur Haslinda Mohamed Muzni
2
1
Centre for Manufacturing and Environmental Sustainability (CMES), Multimedia University, Bukit Beruang 75450, Melaka, Malaysia
2
Faculty of Engineering and Technology, Multimedia University, Bukit Beruang 75450, Melaka, Malaysia
3
Faculty of Electric and Electronic Engineering, University of Tun Hussein Onn Malaysia, Parit Raja 86400, Batu Pahat Johor, Malaysia
*
Author to whom correspondence should be addressed.
Energies 2024, 17(14), 3415; https://doi.org/10.3390/en17143415
Submission received: 30 April 2024 / Revised: 27 June 2024 / Accepted: 4 July 2024 / Published: 11 July 2024
(This article belongs to the Special Issue State-of-the-Art Materials toward Efficient Solar Energy Harvesting)
Browse Figures
Versions Notes

Abstract

:
In this study, titanium dioxide (TiO2) was deposited onto a fluorine-doped tin oxide (FTO) substrate using the sol–gel spin coating method. Through the implementation of calcination treatment on the thin film, enhancements were observed in terms of structural, optical, and morphological properties. Various calcination temperatures were explored, with TiO2 annealed at 600 °C identified as the optimal sample. Analysis of the X-ray diffraction spectroscopy (XRD) pattern revealed the prominent orientation plane of (101), indicating the presence of anatase TiO2 with a tetragonal pattern at this temperature. Despite fluctuations in the optical spectrum, the highest transmittance of 80% was observed in the visible region within the wavelength range of 400 nm. The estimated band-gap value of 3.45 eV reaffirmed the characteristic of TiO2. Surface analysis indicated the homogeneous growth of TiO2, uniformly covering the FTO substrate. Cross-sectional examination revealed a thickness of 263 nm with dense and compact nature of TiO2 thin film. No presence of defects or pores reflects a well-organized structure and high-quality formation. Significant electrical rectification properties were observed, indicating the successful formation of a p–n junction. In summary, calcination treatment was found to be crucial for enhancing the properties of the thin film, highlighting its significance in the development of solar cell applications.

1. Introduction

Titanium dioxide (TiO2) stands out among metal oxide thin films due to its versatile nature and distinctive properties, making it a cornerstone of various technological applications. Its utilization spans across electronic devices, such as solar cells [1], sensors [2], memory devices [3], photodetectors [4], and photocatalysis [5]. In solar cell applications, TiO2 serves as an electron-transport material within the photovoltaic mechanism. This material is crucial for facilitating the movement of electrons generated through light absorption, thereby aiding in the conversion of sunlight into electricity [6]. Aside from being insensitive to visible light while absorbing in the near UV region, TiO2 also boasts non-toxicity, excellent durability, a high dielectric constant, and a high refractive index [7]. Typically, TiO2 exists in either an amorphous state or one of three crystalline structures: anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic) [8]. Among these phases, the anatase phase, although metastable, exhibits remarkable optoelectronic activity. Rutile, being the most stable phase, offers a higher refractive index and dielectric constant compared to anatase [9]. The distinct crystal structures of TiO2 thin films profoundly impact their properties, thereby rendering them suitable for specific applications.
Currently, researchers employ various techniques to prepare TiO2 thin films, including chemical vapor deposition (CVD) [10], hydrothermal [11], direct-current (DC) magnetron sputtering [12], and sol–gel processes [13]. Among these, sol–gel spin coating stands out as a versatile and widely utilized method for depositing thin films onto substrates, due to its capability to produce uniform and precisely controlled thin films [14]. This technique involves using a sol–gel solution, which is a colloidal suspension of solid nanoparticles in a liquid, to form a semisolid film through the application of centrifugal force [15]. Compared to other deposition techniques, sol–gel spin coating typically operates at relatively low temperatures. This is supported by a previous study by Yichuan Rui et al., which demonstrated that a compact TiO2 layer can be deposited on an FTO substrate at room temperature [16]. Additionally, Huimin Xiang et al. presented a double-step deposition method for c-TiO2 and m-TiO2 layers onto FTO substrates using the spin-coating technique in MAPbI3-based Perovskite solar cell configurations [17]. Their findings indicate that the spin-coating technique is highly suitable and compatible for the fabrication of TiO2 thin films across various types of solar cell mechanisms. Additionally, it offers the advantage of low-cost equipment, and it often exhibits excellent adhesion to substrates, which is crucial for ensuring long-term durability and performance. The remarkable superiority of TiO2 in self-cleaning procedures stems from its admirable features, uniformity over large areas, and versatility in processing.
Recent years have witnessed a growing interest in the study of TiO2 thin films and their diverse applications. Numerous investigations have been conducted to explore the impact of temperature variations on the internal properties of TiO2. Calcination, also referred to as annealing treatment, is a thermal process involving the heating of materials to high temperatures to induce chemical and physical transformations [18]. At elevated temperatures, atoms within TiO2 become more energetically activated, facilitating their diffusion within the material. This process enables atoms to rearrange themselves, thereby mitigating defects and dislocations while promoting the development of well-defined crystal structures [19]. One of the primary atomic-level alterations observed during calcination is the crystallization of TiO2. In its amorphous state, TiO2 atoms exhibit a disordered or non-crystalline arrangement. Through annealing, these atoms undergo reorganization into a regular and repetitive crystalline lattice structure. The resultant crystal structure is contingent upon the specific annealing conditions employed, potentially leading to the formation of either the anatase or the rutile phase, each characterized by its distinctive crystal lattice arrangement [20].
In 2021, Ofelia Durante et al. demonstrated the formation of TiO2 thin films using a coater equipped with an electron-beam evaporation source. Their study investigated the phonon lifetime’s dependence on TiO2 thickness and annealing temperature, both of which influenced the degree of crystallinity [21]. In addition, Lukong et al. revealed the formation of TiO2 thin films coated on glass substrates in 2022. A morphological analysis indicated the presence of uniformly distributed snowflake shapes on the substrate at 400 °C, with agglomeration improving as the annealing temperature increased [22]. In addition, Lu He et al. showcased TiO2 thin-film fabrication through a two-step spin-coating process in 2023. Their findings indicated a clear trend of decreasing TiO2 thin-film thickness with an increasing annealing temperature. This decrease was attributed to crystal-phase changes and thin-film densification [23]. Additionally, in 2024, Arsha Sunil et al. fabricated TiO2 thin films on soda lime glass and quartz substrates for photocatalytic applications. Their study revealed a remarkable 79.35% efficiency in degrading MB dye when exposed to sunlight. They observed that increasing the annealing temperature led to the formation of larger and smoother anatase TiO2 particles, resulting in an optimized thin film that is highly durable, reusable, and hydrophilic [24]. Thus, the annealing temperature plays a pivotal role in fine-tuning the desired properties of TiO2 thin films.
In this study, TiO2 thin film was deposited onto an FTO glass substrate, which is called TiO2/FTO substrate, by using the sol–gel spin-coating method. After enduring the spin-coating process, the TiO2/FTO substrate underwent calcination treatment at different temperatures. The structural, morphological, and optical properties of the thin film were characterized to obtain the optimized parameters. An illustration of the TiO2/FTO substrate is shown in Figure 1.

2. Methodology and Materials

The TiO2 thin film was deposited onto the FTO substrate by using the sol–gel spin-coating method. The process encompassed four stages: the preparation of the solution, the substrate cleaning process, the spin-coating procedure, and the subsequent calcination treatment. For TiO2 solution preparation, 0.08 M titanium (IV) butoxide, which acts as the precursor, was mixed with 0.89 M n-butanol and stirred continuously for a duration of 30 min. Following this, 0.04 M acetic acid, serving as a catalyst, was introduced slowly to the mixture. Then, deionized water was gradually added, all while maintaining a continuous stirring process. The molar ratio used was 2:20:1:1, which indicated titanium (IV) butoxide, n-butanol, acetic acid, and deionized water, respectively. Regarding substrate preparation, the FTO substrate underwent a comprehensive cleaning procedure that included successive immersions in acetone, ethanol, and distilled water within an ultrasonic bath. The FTO substrate was submerged in an acetone solution and underwent ultrasonic cleaning for 10 min.
Consequently, the spin-coating process commenced with the careful placement of a thoroughly cleaned FTO substrate onto the chuck of the spin coater. Then, a small volume of TiO2 solution was dispensed onto the central area of the substrate. The substrate was then rapidly accelerated to 3000 rpm for 30 s to ensure the uniform distribution of the TiO2 coatings via centrifugal force. This rotational motion persisted until the liquid spread across the entirety of the substrate’s periphery, resulting in the formation of a solid TiO2 film as the solvent evaporated. This sequence was repeated for 8 cycles, followed by a post-heating step on a hot plate set to 100 °C for 2 min. The illustration of the spin-coating procedure for the TiO2/FTO substrate is depicted in Figure 2.
After the TiO2 layer was completely deposited, the thin film was subjected to a calcination process in a furnace at different temperatures of 400, 500, and 600 °C for an hour. The heating ramp was set to 10 °C per minute. The temperature was restricted to 600 °C because the TiO2 thin film exhibited a white precipitate upon visual examination when subjected to calcination temperatures beyond this threshold. This outcome was not aligned with the necessity of the study. The temperature profile during calcination treatment for the n-TiO2/FTO substrate is presented in Figure 3.
Multiple characterizations of the properties were carried out to investigate the characteristics of the TiO2/FTO substrate, including the morphological, structural, and optical properties. The morphological properties were examined using field-emission scanning electron microscopy at an acceleration voltage of 15,000 V, which provided an impressive magnification of up to 100,000×. For assessing its structural properties, X-ray diffraction spectroscopy was employed, covering a 2θ range from 20 to 80 degrees, and Cu Kα radiation, with a wavelength of 1.54056 Å, was utilized to analyze the crystal structure. The crystallite size of the TiO2/FTO substrate was calculated using the Scherrer Equation (1) [25]:
D = k λ β   c o s   θ
where λ is the X-ray wavelength of 1.54056 Å, θ is the Bragg diffraction angle, and β is the full width at half maximum (FWHM) of the corresponding θ. To determine the lattice parameters, the values of ‘a’ and ‘c’ were computed using the interplanar spacing relation (2) designed for tetragonal phases [26]:
1 d h k l 2 = h 2 + k 2 a 2 + l 2 c 2
where d h k l = λ/(2 sin θ) is derivative from the Bragg’s law equation, λ is the X-ray wavelength, and θ is the Bragg’s diffraction angle in radians. In the context of optical properties, ultraviolet–visible spectroscopy was employed to analyze the transmittance spectrum within the wavelength range of 300 to 800, providing insights into its optical characteristics. Subsequently, the sample’s band gap was determined using Equation (3), where α is the absorbance coefficient, hv is the photon energy, and Eg is the optical band gap [27].
( α h v ) = A   ( h v − E g ) 1 / 2

3. Results and Discussion

The morphological, structural, and optical properties of the TiO2/FTO substrate were assessed and characterized. Following the TiO2 deposition onto the FTO substrate, the TiO2/FTO substrate was calcinated using varying calcination temperatures, specifically at 400, 500, and 600 °C.

3.1. Structural Properties of TiO2/FTO Substrate

XRD spectroscopy was employed to characterize the structural properties of the TiO2/FTO substrate. The analysis revealed that the composition of the TiO2/FTO substrate comprised 27% SnO2 and 73% TiO2. A total of 18 distinct peaks were observed in the sample, with 11 peaks for SnO2 and 7 peaks corresponding to TiO2. The SnO2 peaks were detected at the orientation planes of (110), (101), (200), (210), (211), (220), (310), (112), (301), (202), and (321), corresponding to angles of 26.51, 33.76, 37.84, 42.51, 51.61, 54.59, 61.69, 64.50, 65.75, 71.01, and 78.43°, respectively, based on the International Centre for Diffraction Data, ICSD (98-003-9177). The presence of these peaks confirms that the FTO substrate has indeed been coated with a layer of SnO2. Similarly, the TiO2 peaks were identified at the orientation planes of (101), (200), (105), (204), (116), (220), and (215), aligned at angles of 25.28, 48.04, 54.04, 62.77, 68.95, 70.28, and 75.17°, respectively. These findings agree with the anatase and tetragonal pattern, which is in accordance with the ICSD (98-017-2914). Since no other discernible peaks were observed, it indicates the absence of impurities in the sample.
This sample underwent calcination at various annealing temperatures: 400 °C, 500 °C, and 600 °C. Initially, no TiO2 peaks were observed in the as-deposited sample. However, after calcination at 400 °C for 1 h, two distinct peaks corresponding to the anatase phase of TiO2 were detected: (101) and (200) orientation planes at 25.28° and 48.04°, respectively. This finding aligns with Rosniza et al.’s observation that unannealed TiO2 thin films lack clear peaks, indicating their amorphous nature [28]. As the annealing temperature was increased to 500 °C, the intensity of these peaks also increased. Additionally, three smaller peaks, corresponding to anatase TiO2, appeared in the XRD pattern: (105), (204), and (215) orientation planes at 54.04°, 62.77°, and 75.17°, respectively. This observation is consistent with Bakri et al.’s study, which stated that the anatase phase of TiO2 is typically obtained between 400 °C and 800 °C, with a transformation to the rutile phase occurring above 800 °C [29]. Upon reaching 600 °C, all TiO2 peaks showed further enhancement, with the preferred orientation plane of (101) being prominent among the TiO2 peaks. With the rise in annealing temperature, there was a corresponding increase in peak intensity. It was reported that annealing at higher temperatures can greatly enhance the crystal quality of thin-film nanoparticles [30]. At elevated temperatures, amorphous TiO2 initially converts to crystalline anatase, particularly in solution-phase preparations below 600 °C. At higher temperatures, metastable anatase and brookite irreversibly transform into rutile when heated above 800 °C [31]. However, Dorian Hanour et al. stated that transition temperatures can vary depending on the methods used, including the transition temperatures, raw materials, and processing methods. Moreover, the anatase-to-rutile transformation is not instantaneous because it is time-dependent and reconstructive in nature [32].
Using Equation (1) from the Scherrer equation, the crystallite size of the TiO2/FTO substrate was determined at the preferred orientation plane of (101). The calculated crystallite sizes were 20.59, 22.16, and 22.17 nm for annealing temperatures of 400, 500, and 600 °C, respectively. As the annealing temperature increased, the crystallite size also showed an increase. Notably, the sample annealed at 600 °C exhibited the highest peak intensity and a larger crystallite size, which are indicative of a well-defined arrangement and larger crystalline domain, resembling a high-quality crystal structure. Furthermore, lattice parameters were calculated using Equation (2) based on the interplanar spacing, since TiO2 is in the tetragonal phase. Upon calculation, the values for the sample annealed at 600 °C were found to be a = b = 3.7866 Å and c = 9.4915 Å. These values closely corresponded to the standard lattice parameter values based on the ICSD (98-015-4603), which revealed a = b = 3.7850 Å and c = 9.4820 Å. This alignment indicates that the crystal lattice is well-oriented with respect to the substrate and provides insights into the absence of defects in the TiO2 thin film. The XRD pattern and variation results of the TiO2/FTO substrate at different annealing temperatures are presented in Table 1 and Figure 4, respectively.

3.2. Optical Properties of TiO2/FTO Substrate

The light-transmitting capabilities of the TiO2/FTO thin film were evaluated for its potential application as an electron-transport material under light exposure. The transmittance spectrum was characterized using ultraviolet–visible (UV–vis) spectroscopy, revealing fluctuation in the spectrum around the visible region of 400 nm, with a sharp decline at approximately 350 nm. For the sample annealed at 400 °C, the transmittance spectrum exhibited a value of 70%. Subsequently, the transmittance decreased to around 65% for the sample annealed at 500 °C. Remarkably, the highest transmittance spectrum of 80% was observed when the annealing temperature reached 600 °C. Despite some fluctuations in transmittance, the sample annealed at 600 °C showed improvement as the annealing temperature increased. This value aligns significantly with a recent study by Alaya et al., which demonstrated a transmission range between 65% and 95% in the visible and near-infrared regions. Therefore, it can be inferred that these TiO2 thin films may find utility as UV detectors and as optical windows in photovoltaic cells [33]. The notably high transparency of the TiO2 thin film suggests a high-quality surface-layer arrangement, allowing for increased incident light penetration. This characteristic is beneficial, as it requires a significant number of photons to reach the neighboring layer. This observation is consistent with the statement by Miroslav Zeman, who noted that a layer with a high optical transmission in the spectrum can enhance light absorption due to scattering at internal rough interfaces in standard solar cell features [34].
Furthermore, due to the properties of TiO2 thin film, there is a direct band-gap material. The band gap was evaluated using Tauc’s Equation (3). After calculation, the band-gap extrapolation plot was represented as (αhν)1/2 vs. (hν) graph. The graph displays two distinct lines: a blue line representing the bandgap spectrum and a red line, which has been extrapolated to derive the band-gap value. From the result, the band gap of the TiO2/FTO substrate annealed at 600 °C showed a value of 3.45 eV. This estimated value aligns with a typical band-gap value for TiO2 thin film mentioned in Tanski et al., which is 3.32 eV for TiO2 anatase [35]. The value is still within the range of the standard TiO2 band-gap value. The slight difference in value might be due to changes in film density and an increase in grain size [36]. The transmittance spectrum and extrapolated band gap of TiO2/FTO substrates are shown in Figure 5 and Figure 6, respectively.

3.3. Morphological Properties of TiO2/FTO Substrate

The morphology of the TiO2/FTO substrate surface was characterized using field-emission scanning electron microscopy (FE-SEM) at magnifications of 50,000× and 100,000×, focusing on the secondary image mode of TiO2 annealed at 600 °C. The analysis revealed that the TiO2 thin films exhibited homogeneous growth and uniformly covered the FTO surface, with an average grain size of 27.65 nm. The denser and more compact morphology of the TiO2 layer after annealing resembled a closely packed and well-organized structure, akin to a uniform surface. This structure is crucial, as mentioned by Khan et al., as it reduces the scattering and trapping of charge carriers, thereby enhancing electron mobility, which is essential for various electronic applications where efficient charge transport is crucial for device performance [18].
Furthermore, a cross-sectional analysis revealed two distinct layers, namely the FTO layer and the TiO2 layer, with thicknesses of 501 nm and 263 nm, respectively. No defects, pores, or pinholes were detected at the interface between the layers. This observation aligns with previous studies by Pitchaiya et al., emphasizing the importance of heating treatment in producing a good interface layer of TiO2. Such a well-engineered interface layer enhances the stability and reliability of TiO2-based devices by providing a barrier against degradation mechanisms [37]. Additionally, it minimizes charge-carrier recombination at the interface between the TiO2 thin film and other materials, leading to longer carrier lifetimes and enhanced device efficiency by allowing more charge carriers to contribute to the desired electronic processes [38]. Top view and cross-sectional images of the TiO2/FTO substrate annealed at 600 °C are depicted in Figure 7.
Nucleation and growth are fundamental processes in the formation of thin films, including TiO2 thin films. Nucleation marks the initial stage, where small clusters of atoms or molecules begin to form and aggregate on the substrate surface. In the context of thin films, nucleation occurs when atoms or molecules from the precursor solution, as in techniques like spin coating, come into contact with the substrate [39]. Following the nucleation process, the nuclei continue to grow by the addition of atoms or molecules from the solution. During growth, atoms or molecules from the solution condense onto the substrate surface or onto the existing nuclei, gradually increasing the thickness and coverage of the thin film [40]. Subsequently, the thin film undergoes a crystallization process. Crystallization involves the transformation of the initially formed nuclei and growing domains into well-defined, ordered crystal structures. This process entails the rearrangement of atoms or molecules within the thin film into a regular, repeating lattice arrangement that is characteristic of crystalline materials [41]. The illustration of the nucleation, growth, and crystallization process for TiO2 thin-film formation is shown in Figure 8.

3.4. Application of TiO2 Thin Film for Solar Cell

In solar cell application, first-generation solar cells are known as crystalline silicon based. Meanwhile, second-generation solar cells consist of thin-film technologies using various semiconductor materials, such as gallium arsenide (GaAs) [42], cadmium telluride (CdTe) [43], and copper indium gallium sulfide (CIGS) [44]. The advancements in solar cell technology involve the integration of innovative materials and mechanisms that encompass a range of technologies, including perovskite [45], quantum dot [46], dye-sensitized solar cell (DSSC) [47], and multi-junction solar cells [48]. In this study, the application of solar cells was focused on thin-film technologies by utilizing inorganic metal oxide semiconductor thin films. Specifically, the n-type TiO2 thin film was used as the window layer in a heterojunction thin-film configuration. Heterojunction thin-film solar cells involve the combination of two different semiconductor materials with varying types and bandgaps to generate electricity [49]. On the other hand, copper oxide (Cu2O) serves as a native p-type semiconductor, boasting electrical properties, low-cost production, stability, non-toxicity, and high optical absorption in visible light [50]. The n-TiO2 thin film was fabricated with p-Cu2O thin film, which acts as the absorber layer, forming a complete layer arrangement for the heterojunction thin film.
The electrical properties of the n-TiO2/p-Cu2O heterojunction thin film were evaluated, and a current density–voltage (J-V) characteristics graph is presented in Figure 9. Measurements were conducted under both dark conditions and with AM1.5 illumination. Following illumination, significant electrical rectification properties were observed, indicating the successful formation of a p–n junction. The short-circuit current density (JSC), open-circuit voltage (VOC), fill factor, and conversion efficiency were determined to be 0.917 mA/cm2, 0.232V, 23.544, and 0.0501%, respectively. However, the low efficiency observed may be attributed to factors such as the formation of leakage current, high resistivity due to the crystalline nature of the film, and the possible recombination of charge carriers (electrons and holes) [51]. Furthermore, concerns regarding long-term stability due to degradation were noted, which could further impact overall efficiency. Thus, achieving precise control over high-quality interfaces is crucial for enhancing light-capture efficiency and improving device performance in heterojunction thin-film solar cells.

4. Conclusions

In conclusion, the TiO2 thin film was successfully deposited onto an FTO glass substrate using the sol–gel spin-coating method. The characterization of the structural, optical, and morphological properties of the TiO2 thin film indicated characteristics typical of TiO2, reflecting its high-quality formation. Various calcination temperatures were explored, with TiO2 annealed at 600 °C emerging as the optimal sample. Structurally, the presence of several peaks, particularly the prominent orientation plane of (101), indicated the presence of anatase TiO2 with a tetragonal pattern. As the annealing temperature increased, the crystallite size also exhibited an increase, reaching up to 22.17 nm. In terms of optical properties, despite fluctuations, an improvement in transmittance was observed with an increasing annealing temperature, reaching a peak transmittance of 80%. The estimated band-gap value of 3.45 eV is closely aligned with typical TiO2 band-gap values. For morphology properties, the TiO2 thin films demonstrated homogeneous growth, uniformly covering the FTO surface, with an average grain size of 27.65 nm. Cross-sectional analysis revealed thicknesses of 501 nm and 263 nm for the FTO layer and TiO2 layer, respectively. The dense and compact nature of the thin film, devoid of defects or pores, resembled a closely packed and well-organized structure. After depositing the p-Cu2O thin film, significant electrical rectification properties were observed in the heterojunction, indicating the successful formation of a p–n junction. The conversion efficiency was determined to be 0.0501%. Despite the low efficiency, these findings emphasize the crucial role of annealing temperature in tuning the properties of TiO2 thin films, highlighting their potential in solar cell applications.

Author Contributions

Conceptualization, N.M.A. and E.E.M.N.; methodology, N.M.A. and F.M.; validation, N.M.A., E.E.M.N. and F.M.; formal analysis, N.M.A., N.M., E.E.M.N. and N.H.M.M.; investigation, N.H.M.M.; resources, E.E.M.N.; writing—original draft preparation, N.M.A.; writing—review and editing, E.E.M.N. and N.M.; visualization, N.M.A.; supervision, E.E.M.N. and F.M.; project administration, E.E.M.N. All authors have read and agreed to the published version of the manuscript.

Funding

The APC was funded by Multimedia University.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author/s.

Acknowledgments

The authors would like to acknowledge the Microelectronic and Nanotechnology Shamsuddin Research Centre (MiNT-SRC), Universiti Tun Hussein Onn Malaysia, for the technical support.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Illustration of n-TiO2/FTO substrate.
Figure 1. Illustration of n-TiO2/FTO substrate.
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Figure 2. Illustration of spin-coating procedure for TiO2/FTO substrate.
Figure 2. Illustration of spin-coating procedure for TiO2/FTO substrate.
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Figure 3. Temperature profile for the calcination treatment at (a) 400, (b) 500, and (c) 600 °C for the n-TiO2/FTO substrate.
Figure 3. Temperature profile for the calcination treatment at (a) 400, (b) 500, and (c) 600 °C for the n-TiO2/FTO substrate.
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Figure 4. XRD patterns of (a) as deposited TiO2/FTO substrates and after annealing at (b) 400 °C, (c) 500 °C, and (d) 600 °C for 1 h, respectively.
Figure 4. XRD patterns of (a) as deposited TiO2/FTO substrates and after annealing at (b) 400 °C, (c) 500 °C, and (d) 600 °C for 1 h, respectively.
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Figure 5. The transmittance spectrum of TiO2/FTO substrates annealed at (a) 400 °C, (b) 500 °C, (c) 600 °C, respectively.
Figure 5. The transmittance spectrum of TiO2/FTO substrates annealed at (a) 400 °C, (b) 500 °C, (c) 600 °C, respectively.
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Figure 6. The extrapolated band gap of TiO2/FTO substrates annealed for 1 h at 600 °C.
Figure 6. The extrapolated band gap of TiO2/FTO substrates annealed for 1 h at 600 °C.
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Figure 7. Top view images of TiO2/FTO substrates annealed at 600 °C with magnifications of (a) 50,000× and (b) 100,000× and its (c) cross-sectional image.
Figure 7. Top view images of TiO2/FTO substrates annealed at 600 °C with magnifications of (a) 50,000× and (b) 100,000× and its (c) cross-sectional image.
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Figure 8. Illustration of nucleation, growth, and crystallization process for TiO2 thin film formation.
Figure 8. Illustration of nucleation, growth, and crystallization process for TiO2 thin film formation.
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Figure 9. Current density–voltage (J-V) characteristics graph for n-TiO2/p-Cu2O heterojunction thin film under (a) dark and (b) AM1.5 illumination.
Figure 9. Current density–voltage (J-V) characteristics graph for n-TiO2/p-Cu2O heterojunction thin film under (a) dark and (b) AM1.5 illumination.
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Table 1. Variation of crystallite size and lattice parameter of TiO2 thin film at different annealing temperatures.
Table 1. Variation of crystallite size and lattice parameter of TiO2 thin film at different annealing temperatures.
Annealing Temperature
(°C)		(101)-Orientation PlaneLattice Parameter (Å)
2θ (°)FWHM Intensity (%)Crystallite Size (nm)a = b c
40025.340.413331.4820.593.78749.4491
50025.250.383876.6622.163.78589.6673
60025.300.383899.9922.173.78669.4915
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MDPI and ACS Style

Arifin, N.M.; Mhd Noor, E.E.; Mohamad, F.; Mohamad, N.; Mohamed Muzni, N.H. Enhancing the Properties of Nanostructure TiO2 Thin Film via Calcination Temperature for Solar Cell Application. Energies 2024, 17, 3415. https://doi.org/10.3390/en17143415

AMA Style

Arifin NM, Mhd Noor EE, Mohamad F, Mohamad N, Mohamed Muzni NH. Enhancing the Properties of Nanostructure TiO2 Thin Film via Calcination Temperature for Solar Cell Application. Energies. 2024; 17(14):3415. https://doi.org/10.3390/en17143415

Chicago/Turabian Style

Arifin, Nurliyana Mohamad, Ervina Efzan Mhd Noor, Fariza Mohamad, Norhidayah Mohamad, and Nur Haslinda Mohamed Muzni. 2024. "Enhancing the Properties of Nanostructure TiO2 Thin Film via Calcination Temperature for Solar Cell Application" Energies 17, no. 14: 3415. https://doi.org/10.3390/en17143415

APA Style

Arifin, N. M., Mhd Noor, E. E., Mohamad, F., Mohamad, N., & Mohamed Muzni, N. H. (2024). Enhancing the Properties of Nanostructure TiO2 Thin Film via Calcination Temperature for Solar Cell Application. Energies, 17(14), 3415. https://doi.org/10.3390/en17143415

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