# Optimization of Laser Fixed Contact Process for the Formation of an Al-BSF Layer
## 3 Results
In the case of the Al-BSF layer, it is critical to collect the minority electrons in the solar cell of the p-type substrate, as the BSF layer, the p+ region at the back, pushes electrons to the opposite side, thus facilitating the collection of holes due to the field effect. This reduces the probability of electron/hole recombination, and increases the number of electrons collected, resulting in improved electrical properties--hence, the thickness of the BSF layer is important. Figure 4: SEM imagery of wafer surfaces and cross-sections in response to varied laser speeds, at 5 W laser power: (a) 10 mm/s; (b) 100 mm/s; (c) 500 mm/s; (d) 1000 mm/s. Figure 5: SEM imagery of wafer surfaces and cross-sections in response to varied laser speeds, at 6 W laser power: (a) 10 mm/s; (b) 100 mm/s; (c) 500 mm/s; (d) 1000 mm/s. Figure 6: Laser track widths for 5 and 6 W laser power settings. Figure 7: SEM image of the Al-BSF layer in relation to operating laser speed: (**a**) 10 mm/s; (**b**) 50 mm/s; (**c**) 100 mm/s; (**d**) 500 mm/s. It can be seen in Figure 7 that the Al-BSF layer was not observed at a laser speed of \(\geq\)500 mm/s. In this case, sufficient heat was not applied to the Al layer, due to the relatively low energy, and only the surface of the Al layer was processed to confirm that the alloy was not formed. However, the Al-BSF layer was \(\sim\)1.82 \(\upmu\)m at 10 mm/s laser speed, \(\sim\)1.41 \(\upmu\)m at 50 mm/s, and \(\sim\)1.22 \(\upmu\)m at 100 mm/s. Figure 8 shows the thickness and SEM image of the Al-BSF layer according to the laser speed with a fixed laser power of 5 W. As shown in Figure 8, the thickness of the Al-BSF layer gradually decreased as the laser speed increased, and a thick Al-BSF layer was observed at laser speeds of 10 mm/s and 50 mm/s. At lower laser speed, the thermal energy absorbed by the metal electrode material increased, making it more affected by the laser process. When the laser speed increased, the residence time of the laser beam in the process area of the cell was less, and the interaction time between the materials of the metal electrodes and laser was also reduced. This confirmed that the thickness of the Al-BSF layer decreased at higher laser speeds because the energy required for BSF layer formation had not been transferred sufficiently to the process area. When analyzing the surface image, it can be seen that the silicon wafer has the greatest damage due to the HAZ on both sides of the laser track. However, the Al-BSF layer of adequate thickness was formed in compliance with a laser speed of 100 mm/s, in which case, SEM imagery confirmed that the associated thermal damage was minimal. Based on these results, the laser characteristics optimized for LFC--5 W laser power, laser speed of 100 mm/s, and frequency of 400 kHz--were fixed, and electrical characteristics were then analyzed. To fabricate a solar cell with an LFC structure, electrical analyses were conducted by varying the laser pitch through 250, 500, 750, and 1000 \(\upmu\)m, and conducting the laser processes between one and three times. Figure 9 shows the external quantum efficiency (EQE) graph measured after fabricating the solar cell using one, three, and five treatment repeat numbers, and four different laser pitches. If the pitch of the laser was too wide, the contact area of the electrode was narrow, causing less BSF layer to be formed. In contrast, with too narrow a pitch of the laser, an overlapping portion may result, because of the thermal effect of the laser, and efficiency may be reduced. Figure 9a shows the EQE graph for solar cells fabricated against the number of processing repeats and confirms that the EQE decreased as the number of laser  
Figure 8: The thickness and SEM image of the Al-BSF layers created using various laser speeds, with laser power fixed at 5 W.  
process repeats increased. This reduction may be caused by laser-induced damage, which generates recombination.