In this study, we modified the spin-coating device to coat a ZnO solution on a solar cell for enhancing its efficiency. Initially, zinc acetate hydrate, methoxyethanol, and monoethanolamine solutions were stirred on a hotplate at a temperature of 75°C. After 150 min, the solution became milky, and after another 24 h, it became clear and transparent. Finally, we obtained the ZnO solution, which was injected into the solar cell through a micropipette. The motor speed was increased from the initial level to 2000 rpm. This process was performed thrice. To obtain a thin-film coating on the solar cell, it was dried. During the coating process, vibration occurs; thus, a vibrometer was used to analyze the vibration of the coating machine. The motor speed was controlled using the subsynchronous resonator (SSR) model. A thermocouple was used to maintain a constant temperature. The annealing process was performed in a separate oven. After the annealing process, a vacuum pump was used to remove the moisture inside the drum. In this modified device, a heating unit was fabricated for the drying process, which provides the required heat through a blower gun. This increases the evaporation rate, leading to smooth and uniform film layers. The Taguchi DOE is widely used to reduce the occurrence of defects and failures in the production of goods. It improves the quality and reduces the number of experiments required. Higher values of the signal-to-noise ratio (SNR) find control factors and minimize the effect of noise.
The results of the experiments were converted into the Taguchi S/N, and the optimal parameters were obtained. ANOVA statistics were used to determine the contribution rates of the components. The target film thickness of 2.4 µm was achieved with good uniformity for the production of solar panels using the proposed spin-coating device.
3.1 Spin coater
The spin-coating process uses the principle of centrifugal force. When the disk is rotated in any direction, a centrifugal force is generated, which draws the rotating body away from the center of rotation. The spin coater is fabricated according to the requirements of the thin-film thickness of the solar panel. The spin-coater modeling is shown in Figs. 1 and 2. Many parameters affect the film thickness. In this study, the evaporation rate is considered as an important factor. Typically, an annealing process, which is also called the evaporation rate, should be performed on the coated substrate. The annealing process was performed in a separate oven. When the substrate is placed in an oven, the atmospheric pressure or temperature affects the layer [18]. In the modified device, a heating unit was fabricated for the drying process, which provides the required heat through a blower gun. This increases the evaporation rate, leading to smooth and uniform film layers.
Table 1
Specifications of the fabricated spin coater
Materials | Rating |
Spinning speed | 800 rpm (max) |
Motor rating | 220 V |
Mode | Programmable Human Interface |
Speed controller used | SSR |
Spin plate | 6 mm |
Material | Acrylic |
Size of pipette | 0.5 mm |
3.2 Coating material
In this study, the coating material was a zinc oxide (ZnO) solution, which is a highly conductive medium. The preparation of the ZnO solution is described in detail in the following subsection.
3.3 Preparation of ZnO solution
The most common semiconductor compounds are ZnO and titanium dioxide (TiO2) [19]. ZnO has a higher conductivity than TiO2 [20]. In this experiment, a ZnO solution was used to coat the solar panel. Zinc acetate hydrate was used as a starting material, methoxyethanol was used as the dissolvent, and monoethanolamine was used as a stabilizer. These three solutions were stirred on a hotplate at 75°C for 150 min, yielding a milky solution, which was then kept for 24 h to let it become clear. Figure 3 shows a flow diagram of the preparation of the ZnO solution. Following the preparation of the ZnO solution, the coating was injected using a micropipette. The motor speed was increased to 2000 rpm. This process was repeated thrice. To obtain a thin-film coating on the solar cell, it was dried. A thermocouple was used to maintain a constant temperature. After the annealing process, the moisture inside the drum was removed using a vacuum pump. As shown in Fig. 4, the ZnO solution was prepared using a magnetic stirrer with a hotplate.
3.4 Characteristics of thin film
In thin-film coating[22], the thickness of the ZnO layer was measured via SEM according to the cross section of the layer, and the surface of the layer was also examined. The presence of ZnO was confirmed using an energy-dispersive detector[23]. Figure 5 shows the surface of the thin-film layer after the annealing process. The grey color indicates the growth of crystals in the solution. Figure 6 shows the ZnO layer, with the yellow dots representing the atoms in the ZnO.
3.5 Taguchi DOE
DOE is an important method for determining the relationships between the factors affecting a process [21]. The Taguchi DOE is widely used to reduce the occurrence of defects and failures in the production of goods. It improves the quality and reduces the number of experiments performed. Higher values of the SNR find control factors and minimize the effect of noise.
The three parameters examined in this study were the spinning speed, spinning time, and quantity of solution. Three levels were used for each experimental parameter. The three levels of the spinning speed were 1000, 1500, and 2000 rpm. The spinning times were 3, 6, and 9 s, and the solution quantities were 3, 4, and 5 µL. The process parameters, levels, and factors of the spin coater are presented in Table 2. Here, three parameters and three levels are given, and there is no interaction among the parameters. Therefore, in the experiment, an L9 OA was utilized to find a smaller film thickness of the spin coater, as shown in Table 3.
Table 2
Spin-coating process parameters, levels, and factors
Parameter | Unit | 1 | 2 | 3 |
Spinning speed | rpm | 1000 | 1500 | 2000 |
Spinning time | s | 3 | 6 | 9 |
Quantity of solution | µL | 3 | 4 | 5 |
Table 3
Spin-coating process parameters
Experiment number | Spinning speed (rpm) | Spinning time (s) | Quantity of solution (µL) |
1 | 1 | 3 | 3 |
2 | 1 | 6 | 4 |
3 | 1 | 9 | 5 |
4 | 2 | 3 | 4 |
5 | 2 | 6 | 5 |
6 | 2 | 9 | 3 |
7 | 3 | 3 | 5 |
8 | 3 | 6 | 3 |
9 | 3 | 9 | 4 |
3.6 ANOVA
ANOVA is a method that is utilized to find significant differences between two or more independent variables. It is a statistical test that generalizes the t-test with means. The F-test of by and large criticalness demonstrates whether the direct relapse ideal gives a preferred fit to the data over a model that holds no autonomous factors. The optimized process parameters of the spin coater were subjected to an ANOVA.
3.7 SNR analysis
In the Taguchi approach, the SNR was employed to optimize the coating conditions. In the analysis of the SNR, the optimal parameter was utilized to optimize the ZnO value of the coatings. This resulted in SNR optimization of the sol-gel process for coating metallic substrates. The average % of coated area figures for the nine experimental settings are shown here.
$$S/N=-10\text{L}\text{o}\text{g}\left[\frac{1}{n}\left(\sum _{i=1}^{n}\frac{1}{{y}_{i}^{2}}\right)\right]$$
Here, \({y}_{i}\) represents the value of the quality characteristic, and\(n\) represents the number of measurements in each experiment.