## 3 Results and Discussion  
To protect solar cells from outer environmental conditions, they are encapsulated for protection and retaining the stability of the solar cells, but the encapsulation process causes optical loss [30]. When incident light arrives at the PV module, some is absorbed, transmitted and reflected at each layer of the module. Optical losses are primarily classified by reflection loss at interfaces and absorption loss at each layer. Due to the optical losses occurring as light passes through each medium, there is an inevitable loss known as cell-to-module (CTM) loss when assembling solar cells into modules [31; 32; 33]. Figure 6 shows the path of incident light and the optical coupling effect inside a module. As it shows, reflection loss occurs at the interfaces of air-glass, glass-encapsulated, encapsulant-white backsheet, as well as the surface of the solar cells, but at the same time, some light is reflected to the solar cells again and induces optical gain. Typically, a material with high reflectance is used for the backsheet; some fraction of the light reflected by the backsheet is absorbed by the solar cells owing to the optical coupling effect, and the amount of reflected light depends on the spacing between solar cells. As the spacing increases, more light is reflected, and the chance of absorption by solar cells also increases, while decreased spacing results in the opposite effect. Therefore, the balance between the spacing of the cells and the amount of reflected light is very important to minimize the CTM loss and maximize the performance of the BIPV modules [34]. To investigate changes in optical characteristics of the shingled strings based on the spacing, different spacings of strings were modeled for the simulation using PSpice. Table 1 shows the simulated results for strings with different spacings, and clearly, the results show a correlation between the spacing and Pm. As the spacing increased, Pm also increased because of more reflected light from the spacing. Figure 6: Path of incident light and the optical coupling effect within a PV module (reflection at the interfaces of (1) air–cover glass, (2) cover glass–encapsulated, (3) surface of the solar cells, (4) encapsulant–white backsheet, (5) encapsulant–cover glass). To verify the simulation results, real solar modules were fabricated for comparison. By analyzing the solar cell string output before and after lamination with changes in string spacing, we could determine how the optical coupling effect of different string spacings affects the output values before and after lamination. The typical structure of a solar module consists of a cover glass, EVA, solar cells, EVA, and a backsheet; in each layer, light transmission and reflection occur. Table 2 shows the measured results before and after lamination of a string with 0.2 cm spacing. On average, there was an efficiency loss of approximately 0.6% for each string, and the open-circuit voltage decreased by approximately 0.95% for all strings, whereas Isc decreased by an average of 1.5%. Table 3 lists the output characteristics of the strings with 0.5 cm spacing. At a spacing of 0.5 cm, there was a decrease of approximately 1.0% in open-circuit voltage characteristics, similarly to the 0.2 cm spacing. However, as the spacing increased to 0.5 cm, the shunt current loss decreased. The shunt current losses were on average 0.2%, which is a decrease of 1.3% compared with the 0.2 cm spacing. As a result, the overall efficiency decreased by an average of 0.448%.