## 3 Results and Discussion
### Non-Homogeneous Sailing Effects  
If periodic cleaning is not done, cemented crust formation is inevitable. The effects of cemented material on PV glass installed on a 20\({}^{\circ}\) slope, being the most used around the world, are shown below. In particular, the soiling effect was quantified from measurements made on PV glass samples (A, B, C, and D) with a reference c-Si cell on six different sectors, as shown in red in Figure 6a. There is a reduction in the maximum power due to the deposited dust, mostly because of the current reduction. This effect is shown by the results obtained from IV measurements, as shown in Figure 6. The left, central, and right parts of the glass samples are not similar in regards to IV measurements. This is explained by non-homogeneous soiling deposition on the PV glass surface. However, the 20\({}^{\circ}\) slope effect on dust deposition can be observed. A significant difference is shown among the measurements of the middle and lower quadrants of the four PV pieces of glass, the lower part concentrating the greatest current losses.  
Figure 5: AFM-Raman deposition profile studies: (**a**) Random PV glass sector analysis; (**b**) soiling layers growing one over the other on the lower part of the PV glass.  
The difference between the middle and lower parts of the samples is clearly observed in the percentage difference between a clean and soiled glass. Figure 7 shows the parameters related to the short-circuit current and the open-circuit voltage. The short-circuit current percentage reduction lay between 12.0% and 31.0% for the lower part and between 0.5% and 13.0% for the middle part. This confirms that dust deposition on the lower part is greater than that on the middle part, thereby producing a short-circuit current decrease, which is directly proportional to the incident light [35]. However, the open-circuit voltage percentage reduction shows differences between the lower and middle parts of the samples, but it varies in lower ranges not over 2.6% voltage drop.  
Figure 6: IV curve measurement of the lower and middle parts of glass samples (A, B, C, and D). (**a**) Clean glass divided into sectors to be analyzed; (**b**) IV curves on the middle and lower left sides; (**c**) IV curves on the middle and lower central parts; and (**d**) IV curves on the middle and lower right side.  
Losses due to soiling in each sector are obtained from the power percentage difference between the exposed and a clean glass sample measurement, this is shown in Figure 8. The lower left end shows the highest power reduction values, which are attributed to the predominant wind direction on the site (northeast) [18]. This phenomenon generates soiling patters that are similar to the four PV pieces of glass, thus accumulating more soiling in the sector. For instance, power reduction on the lower left corner of sample A was 27.7 \(\pm\) 0.4%, while it was 14.4 \(\pm\) 1.5% on the lower right corner. This trend is the same as the other three pieces of glass.  
The irregular soiling pattern indicated by losses due to soiling in each sector suggests that solar cells forming a PV module operate in different conditions owing to non-homogeneous PV glass transmissivity and losses due to soiling. This causing the so-called  
Figure 8: Power reduction percentage of glass samples (**A**–**D**) installed northwards at PSDA at a 20\({}^{\circ}\) slope for a one-year exposure (September 2020–2021).  
Figure 7: Short-circuit current and open-circuit voltage percentage reduction for the samples (**A**–**D**).  
mismatch effect that further decreases the PV module power which, together with faulty derivation diodes, may generate hot spots [36, 37].