3.1 Particles after the combustion of EVA
As mentioned before, solar cells are thin and brittle, and they are capsulated by the glass and back-sheet. The solar cells, glass, and back-sheet are crosslinked by EVA (ethylene-vinyl acetate copolymer). So when the solar cells are recycled, EVA should be removed. EVA is a kind of polymer having three kinds of elements, carbon, hydrogen, and oxygen. So when the EVA was combusted, CO2 and H2O will produce without toxic substance generation. But unexcepted particles with diameters of about 1 mm appeared after the combustion of EVA. The XRD result of the particles’ powder is shown in Fig. 3.
In Fig. 3, the black curve shows the XRD result of the particles. The blue lines show the standard XRD result of Si (PDF # 99 − 0092), and the red lines show the standard XRD result of Al (PDF # 01-1180). The black curve fits the blue lines and the red lines well, and it indicates that the particles are a mixture of silicon and aluminum.
The unsupposed particles come from the break-up of the solar cells. As mentioned before, solar cells are brittle. When they are broken, the silicon particles are produced from the wafers, and the aluminum particles are produced from the Al back electrode. When the EVA is combusted, the encapsulation disappears, and the particles of aluminum and silicon show.
3.2 The morphology of the sample after Al-Si solvent refining
Fig.4 (a) shows the cutting surface of sample No.2 after Al-Si melt solidification. There are primary silicon phases, eutectic Al-Si matrix, and cavities in this macro-scope.
The primary silicon phases in Fig. 4 (a) have a relatively uniform distribution. It indicates that the temperature of the melt during the solidification is even, and nucleation takes place randomly.
Figure 4 (c) shows the image of primary silicon phases after the treatment of HCl. It shows that the primary silicon phases have the shape of a flake. In the hyper-eutectic Al-Si melt, the primary silicon phase grows from the liquidus temperature to the eutectic temperature. Besides, there is no rapid cooling in this work, and a flake primary silicon comes into being after nucleation.
Figure 4 (b) shows the image of the eutectic silicon phase, which was obtained by the HCl treatment of the eutectic matrix. The eutectic matrix forms at the eutectic reaction, and it is a mixture of eutectic silicon and eutectic aluminum. During the HCl treatment, the eutectic aluminum dissolves, and eutectic silicon stays. It can be found in Fig. 4 (b) that the eutectic silicon phases are powder-shaped. According to the Al-Si phase diagram, the eutectic reaction takes place at 850 K, and the composition of Al-12.6%Si [21]. The eutectic silicon phases do not have enough time to grow and appear as powders at last.
Besides, there are unsupposed cavities in Fig. 4 (a). They are caused by the vapor in the surroundings during the solidification. The vapor reacts with aluminum at high temperatures, and hydrogen atoms are produced [22]. The hydrogen atoms are wrapped by the melt and shown as cavities after solidification.
In addition, compared to the Al-Si solidification without impurities [23], the morphologies of Al-Si macrostructure, primary silicon, and eutectic silicon are similar. It indicates that the concentrations of boron and phosphorus in this work have little effect on the solidification behavior of Al-Si melt.
3.3 The microstructure of the sample after Al-Si solvent refining
The SEM image of sample No.2 is shown in Fig. 5 (a). The primary silicon (A), eutectic silicon (B), and eutectic aluminum (C) can be found. The eutectic silicon and eutectic aluminum appear as the Al-Si matrix in the macrostructure in Fig. 4 (a).
Besides, SEM line scanning analysis was conducted at the same time. Figure 5 (b) and (C) show the line scanning results of Si and Al respectively, and the line scanning results fit the different phases in the microstructure of Fig. 5 (a).
In addition, there are no other phases and impurities atoms found in SEM measurement even though impurity atoms boron and phosphorus exist. It may be because the impurity concentrations are low and under the limit of SEM detection.
3.4 The impurity removal during Al-Si solvent refining
The purpose of Al-Si solvent refining is to remove boron and phosphorus in silicon. The lattice parameter of silicon is 0.5431 nm [24], and the atomic radii of silicon, boron, and phosphorus are 0.11, 0.085, and 0.1 nm respectively [25]. Boron and phosphorus are substitution impurities in silicon, and they occupy the silicon atoms’ position in the silicon lattice and distribute in the body of the silicon crystal. So, boron and phosphorus contents in silicon are tested by ICP-MS, which can measure boron and phosphorus concentrations in the silicon crystal.
Based on ICP-MS measurement, the boron and phosphorus contents in silicon from the end-of-life solar module in this work are 59 and 41 ppmw respectively.
Figure 6 shows the boron and phosphorus contents in the primary silicon (purified silicon) after Al-Si solvent refining, and the dates are tested by ICP-MS. The black line and square show the boron contents in purified silicon, and it can be found that the boron contents after Al-Si solvent refining are much lower than 59 ppmw. The red line and dot show phosphorus contents, and phosphorus contents in purified silicon are much less than 41 ppmw. It indicates that boron and phosphorus atoms in silicon can be removed by Al-Si solvent refining.
The purification mechanism of Al-Si solvent refining is discussed following. On the one hand, the segregation of impurity atoms during solidification affects. In the hyper-eutectic Al-Si melt, the primary silicon grows when the temperature decreases below liquidus. At the same time, the impurity atoms, such as boron and phosphorus atoms, are rejected by the growing solid silicon and segregated into the remaining melt. As a result, the silicon after Al-Si solvent refining is purified. On the other hand, the mixture of aluminum and silicon plays a role. Boron and phosphorus are impurities in silicon, but there are few boron and phosphorus atoms in high-purity aluminum. When aluminum and silicon are mixed in the melt, the boron and phosphorus concentrations are diluted.
3.5 Effect of kinetics on impurity removal during Al-Si solvent refining
To explore the effect of kinetics during Al-Si solvent refining, samples with different cooling rates during the solidification are designed and carried out. The results are shown in Fig. 6. It can be found that the cooling rate affects boron and phosphorus contents in purified silicon. When the cooling rate decreases, the boron and phosphorus contents in purified silicon decrease.
It can be explained by the migration of the impurity atoms during solidification. When the solid silicon grows in the hyper-eutectic Al-Si melt, impurity atoms are rejected by the solid silicon and enriched at the solid-liquid (S-L) interface. The gathered impurity atoms migrate into the remaining melt. If the cooling rate is low, the growing speed of the solid silicon is low, and the impurity atoms at the S-L interface have time to migrate into the melt. As a result, the impurity concentration at the S-L interface is low. The impurity concentration in purified silicon is determined by the impurity segregation ratio and impurity concentration at the S-L interface [26]. As a result, the impurity concentration in the purified silicon is low.
In addition, sample No.3 has the lowest boron and phosphorus concentrations because of its slow cooling rate. So the primary silicon flakes in sample No.3 were collected and mixed with aluminum for the second time of Al-Si solvent refining under the same experimental condition. The results are shown as the blue square and green dot in Fig. 6. The boron and phosphorus contents in purified silicon after the second time of Al-Si solvent refining are 0.28 and 0.03 ppmw respectively, which satisfies the standard of solar-grade silicon. In other words, the silicon in the module was recycled successfully.