*2.2. Experiments*

Figure 1 presents an EL image of a 6-inch 54-cell 3BB polycrystalline silicon solar module, where the hose power degrades owing to cell damage. Figure 1a shows the first sample of 190-Wp grade, referred to as 190 A for convenience, and its appearance. Figure 1b–d depict EL images of 190 A, the second sample of the 190-Wp class (190 B), and 190 B, respectively. As shown in images (a) and (c), a weak yellowish appearance, which was not severe, was observed. In addition, approximately six to nine dark areas were observed in the EL image (Figure 1b) and approximately six dark areas were observed at 190 B (Figure 1d). In the green-marked cell of (d), the dark area in the cell occurred because of poor soldering between the mutual connector and the busbar.

**Figure 1.** Module appearance and EL images with degraded power output due to cell breakage. (**a**) is the appearance of 190 A, (**b**) is the EL image of 190 A, (**c**) is the appearance of B, and finally (**d**) is the EL image of 190 B.

The modules used in this study included samples collected from commercially operated power plants; however, the current–voltage (I–V) data at the time of manufacture were unknown. Therefore, the electrical characteristics of the model disclosed by the manufacturer were assumed as the initial electrical performance.

Table 2 lists the initial electrical specifications of samples 190 A and 190 B and the electrical data of the failed samples after a certain period of operation. As confirmed in the EL image, the FF was severely degraded by the damaged cells in the middle of the string. For 190 A and 190 B, the power decreased by −21.69% and −26.47%, respectively.


**Table 2.** Electrical data of the modules in the initial stage and after use.

Figure 2 displays the I–V and voltage–power (V–P) curves of modules 190 A and 190 B. The I–V curves appear step-shaped, while the V–P curves have two or more multi-peaks, which is a typical form caused by the decrease in Isc due to the cracking of a specific cell in a cell string [44].

**Figure 2.** I–V and V–P curves of modules with degraded power output owing to cell breakage.

Figure 3 shows the process of removing the broken cells of 190 A and 190 B cells and replacing them with new cells. (a) First, the module is placed on a hot plate to heat the sun-side and soften the EVA, then, the back sheet is removed from the edge. (b) When the back sheet is completely peeled off, (c) the tape attached to fix the cell-string gap was removed. If it is a material such as EVA, it does not require removal; however, for a tape using polyethylene terephthalate (PET) as a basic material, a gap is formed between the tape and cell owing to the loss of adhesion.

When cleaning the back sheet removal surface or the cell removal area using ethanol or isopropyl alcohol (IPA), the permeated organic solvent may cause solvothermal swelling in the lamination process, or gas may accumulate to cause swelling [32].

**Figure 3.** Module recovery process by replacing broken cells. (**d**) Next, the EVA along the boundary of the cell to be removed was cut, and the tab of the cell to be removed was cut by 15 mm or more for electrical connection during recovery. **(e)** Subsequently, the broken cell was removed using a scraper with a blade. (**f**) The remaining EVA was trimmed to the interface of the adjacent cells, and the removed surface was washed. After cell removal, the module was removed from the hot plate and cooled to room temperature. (**g**) EVA was placed between the glass and the new cell to connect it to the module. Here, the size of the EVA is important because it should be perfectly connected to the first EVA of the existing module without leaving a bubble after lamination of the module. Thus, the EVA should be cut accurately with an error of less than 1 mm. If it is larger than the removal surface, stress is applied to the replacement cell, which can cause the cell edge to crack during the lamination process. (**h**) Subsequently, while electrically connecting the new cell and the existing adjacent cell through re-soldering, insulation was applied to prevent the first EVA from melting in the heat. (**i**) The second EVA was slightly thicker than the original size. (**j**) A margin of less than 5 mm should be given, and if it is more than that, the overlapping part of the edge EVA of the cell replaced after lamination is exposed, causing a repair mark. (**k**) Finally, the EVA and new back sheet covering the entire module were laid up. (**l**) The electrical connection was checked, and the lamination process was completed.
