## 3 Results and Discussion
Considering this trade-off, it is  
Figure 8: Efficiency changes according to spacing between strings. Figure 7: Short-circuit current vs. spacing between strings (\(\blacksquare\) before lamination, \(\bullet\) after lamination). evident that optimal string spacing is required to maximize module performance. Future research will focus on determining the optimal string spacing that balances increased light reabsorption and the number of solar cells, ensuring the best possible output for photovoltaic modules. In the previous experiments, the output characteristics of a string were analyzed based on the spacing between strings. To analyze the optical coupling effect of the material that reflects the light entering the module, three modules with the same structure, excluding the backsheet and EVA, were fabricated, and their output characteristics were analyzed. For the experiment, 20.2% Multi PERC cells and four shingled strings of 17 interconnected cells with a 0.14 cm overlap width and 0.5 cm string spacing were prepared as described in the previous experiment. Figure 9 shows the structure of the three modules with partially different layers. The experiment was conducted using a cover glass/ultraviolet transparent (UVT) EVA/string/EVA (UVT)/white backsheet configuration, as well as configurations with cover glass/EVA/string/EVA (white)/white backsheet and cover glass/EVA (UVT)/string/EVA (UVT)/black backsheet. Table 7 lists the measured characteristics before and after the lamination of the shingled string with a white backsheet, which is commonly used in typical solar modules. As shown in the table, the efficiency of the string decreased by an average of 0.448% after lamination, despite the efficiency increase owing to reflection in the white backsheet. Table 8 lists the measured characteristics before and after the lamination of the shingled string with a black backsheet. Black backsheets are commonly used in shingled solar modules to unify the color of the shingled strings and enhance their aesthetics. However, it is difficult to expect reflection from the rear because of the application of black color in the rear. As shown in the results, an average efficiency loss of 1.07% occurred after lamination. This loss is significantly higher than that observed for a typical white backsheet, which exhibits an efficiency loss of 0.6%. Figure 10 shows the reflectance measurements of white and black backsheets. As illustrated in the figure, the black backsheet has a significantly  
Figure 9: Shingled BIPV modules with three different types of backsheet. **Top**: conventional EVA (UVT) -EVA (UVT)—white backsheet structure; **middle**: EVA (UVT)—EVA (white)—white backsheet structure; **bottom**: EVA (UVT)—EVA (UVT)—black backsheet structure. Table 9 lists the results obtained by applying white EVA instead of the commonly used UV-transparent EVA. EVA to the module. As shown in the results, the module with white EVA showed a decrease in efficiency of approximately 0.2% after lamination. However, owing to the increase in reflection from the backside compared with the conventional structure, the short-circuit current increased about 1%, and the efficiency increased about 0.24% after lamination. Figure 11 shows a cross-sectional view of the module with EVA (UVT) applied and the module with EVA (white) applied. As shown in the left figure, in a typical module structure, the incident light is not only reflected at the top of the string but also at the spacing between strings, and part of the reflected light is directed towards the back of the solar cell. However, in the module with white EVA, more reflected light was absorbed by the string owing to the optical coupling effect, and was reflected on the white backsheet with a strong reflection coefficient for the spacing between strings. Compared with a typical module, there is no reflection component from the backsheet to the rear surface of the string. Instead, more light reflected from the white backsheet located between strings is absorbed into the strings.