Recovery of Pure Silicon and Other Materials from Disposed Solar Cells

Department of Chemical Engineering, KPR Institute of Engineering and Technology, Coimbatore 641407, India Department of Electrical and Electronics Engineering, KPR Institute of Engineering and Technology, Coimbatore 641407, India Department of Electronics and Communication Engineering, KPR Institute of Engineering and Technology, Coimbatore 641407, India Department of Mechanical Engineering, KPR Institute of Engineering and Technology, Coimbatore 641407, India Faculty of Engineering, Kuwait College of Science and Technology, Doha, Kuwait Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam


Introduction
The use of solar cells has been tremendously growing around the world as it is a nonpolluting device for power generation. In contrary, the quantity of end-of-life of solar cells and damaged solar cells is drastically increased [1]. The average life expectancy of solar panel is about 25-30 years. In worldwide, around 60 million tons of solar panels have become wastes as those are in the stage of end-of-life [2]. Therefore, an efficient method for recycling of disposed solar cells is a necessity as it causes serious environmental pollution.
A few works have carried out for recycling of photovoltaic panels by using various methods [3][4][5][6][7][8][9]. Doi et al. [3] have recovered solar cells from EVA laminated solar cells by dissolution method. Later, it is found that thermal treatment is more economical to remove EVA from solar cells [4,5]. Further, dissolution of EVA of solar cells in various solvents with ultrasonic irradiation has been analyzed by Kim and Lee. [6]. Dias et al. [7] have used chemical and thermal treatments to separate silver from the disposed solar cells.
To extract pure silicon from the solar cell, various chemical treatments have been used [4,5,8]. Hydrofluoric acid was the most common chemical used for separating silicon from the solar cell [4,5]. However, the usage of hydrofluoric acid has to be eliminated as it is a highly toxic and corrosive chemical. Therefore, the main objective of this work is to replace hydrofluoric acid with other chemicals and efficiently recover pure silicon from the disposed solar cells. In addition, the other materials present in the solar cells such as aluminium, silver, and lead have to be recovered.

Recovery of Pure Silicon
2.1. Experimental Procedure. The outer part of the PV panel contains various materials such as glass, ethylene vinyl acetate glass, copper, steel, aluminium, and plastics. The outer part can be removed by thermal degradation, and the inner solar cell can be separated. Here, the recovery of pure silicon from inner solar cell is conducted. The solar cell mainly contains silicon (about 91.586% by weight) ( Table 1). It also contains other materials such as aluminium at bottom layer, silver electrodes at top layer, and lead and antireflecting coating (Si 3 N 4 ) at top layer. The p-type and n-type silicon wafers are situated in between antireflecting coating and aluminium layer.
The considered solar cell wafer sample for the experiment is shown in Figure 1. The blue color layer is the antireflecting coating (SiNx), and the white lines are the silver electrode. Figure 2 shows the bottom aluminium layer of solar cell. To remove these materials (aluminium, silver, lead, and antireflecting coating), chemical treatment method is used. Earlier, hydrofluoric acid or mixture of hydrofluoric acid and other chemicals was used to remove the antireflecting coating, silver, lead, and p-n junction of the solar cell. The use of hydrofluoric acid is eliminated in the present work as it is highly toxic and corrosive chemical. Each layer is removed one by one using chemical treatments.
A few researchers have used KOH as a reagent to remove aluminium layer [4,5]. In this work, NaOH has been used to remove aluminium layer as it is more economical than KOH. Similarly, silver electrodes are removed by treating with HNO 3 [4,5]. Klugmann-Radziemska et al. [4,5] have used mixture of hydrofluoric acid and other acids to remove antireflecting coating. In the present work, the hydrofluoric acid is replaced with phosphoric acid as it is highly toxic. In addition, the use of other acids is eliminated. Therefore, the separation of unreacted acid and product will be easier by using the present method. The chemical reactions involve in the present work are described next.

Chemical Treatments.
The aluminium layer is removed by treating the solar cell with 10 M NaOH solution for 5 minutes at 63°C temperature, and the chemical reaction is given below.
The silver electrodes and lead from the solar cell are removed by treating with 6 M HNO 3 for 5 minutes at 70°C, and the chemical reactions are given below. Pb The antireflecting coating (Si 3 N 4 ) is removed by treating with 90% phosphoric acid for 45 minutes at 70°C. The volume of reagents, temperature, and time duration for treating the wafer at each stage are presented in Table 2.

Experimental Results
The aluminium layer is removed after treating with concentrated NaOH solution (Figure 3). The hydrogen formed during the reaction (Eq. (1)) is collected in a chamber and can be used as fuel. The aluminium hydroxide (Al(OH) 3 ) from NaOH solution is separated by filtration. Similarly, the silver and lead are removed by treating with HNO 3 (Figure 4). The released NO 2 is collected and can be used for the production of nitric acid. To separate silver nitrate (AgNO 3 ) (Eq. (2)), the solution is treated with HCl, and silver chloride (AgCl) is formed. The precipitated silver chloride is filtered out. Further, the solution is treated with NaOH to separate lead compound (PbO) from the solution. The lead oxide is converted to lead hydroxide, and the precipitated lead hydroxide is filtered.   The antireflecting coating (blue in color) and p-n junction are the only impurities at present stage, and these impurities are removed by treating with concentrated phosphoric acid ( Figure 5). The pure silicon silicon (99.9984%) is obtained after this treatment. The weight percentage of each element present in the wafer before and after chemical treatments is analyzed using ICP-MS, and the data are tabulated in Table 1.

Profit Analysis
The profit analysis has been carried out based on recycling of 1 kg of solar cell. The masses of products obtained after recycling 1 kg solar cell are tabulated in Table 3. The lead compound is neglected in calculation as it is very low in quantity. Similarly, cost of each product in market is also tabulated, and the profit obtained by each product is calculated.
The total profit by recycling 1 kg of solar cell is calculated by the following formula.
Here, P T is the total profit, P S is the profit obtained by silicon, P Al is the profit obtained by Al(OH) 3 , P Ag is the profit obtained by AgCl, and R C is the recycling cost.

Conclusions
A method for recovering pure silicon from the disposed solar cell using chemical treatments has been presented in this work. The use of highly toxic chemical such as hydrofluoric acid is eliminated, and other chemicals have been effectively used for treatments. The pure silicon (99.9984%) has been recovered using this method, and the response of recovered silicon towards DC voltage has been analyzed. The silicon is not conducting electric current as it is pure silicon. The recovered silicon can be used for manufacturing new solar cell or electronic components such as diode, transistor, and microchip. The other materials present in solar cell such as aluminium, silver, and lead are recovered as aluminium hydroxide, silver chloride, and lead hydroxide, respectively. The sequential chemical treatments of solar cell are used in this method. Therefore, the compounds from the treated solution are easily separated. The profit analysis for recycling 1 kg of solar cell has been conducted, and it is observed that the solar cell recycling is highly profitable.

Data Availability
The data obtained in this work by ICP-MS analysis is performed at South India Textile Research Association-Coimbatore.