## 2 Bifacial PV Module Structure and Resulted Incident Irradiance Evaluation  
### Bifacial PV Structure  
Bifacial and monofacial photovoltaic modules are based on the photoelectric phenomenon, which explains how electromagnetic radiation incident on a material can be absorbed and converted into an electric current (usually called light-induced current). Unlike mo-PVs, bi-PVs can also convert incident radiation on the rear surface. Only minor changes to the wafer structure are required to enable this capability. In fact, as seen in Figure 1, the two PV technologies share most of their components. A first difference can be seen in the rear contact of the solar cells. In mo-PVs, the rear contact consists of a foil, while in bi-PVs, it takes the form of a mesh, as seen in [17; 18; 19; 20]. Both contact configurations are usually made of aluminum. The choice of a foil is because it ensures the shortest distance between the generated ion and the electric pole, reducing the likelihood of ion recombination [21]. In addition, the foil provides a larger cross-sectional area for the moving electrons, resulting in lower electrical resistance [22]. This structure and its advantages are abandoned in bi-PVs in favor of allowing the light to pass through the contact and possibly be harvested. The second relevant difference lies in the rear covering surface. While a white, highly reflective backsheet is commonly used in mo-PVs, a transparent backsheetis used in bi-PVs [23; 24; 25]. In mo-PVs, the backsheet is designed to reduce the operating temperature of the module. Such a surface is chosen to be highly reflective in order to reflect as much as possible of the incident radiation that would otherwise heat the module. In contrast, for bi-PV, the surface must allow solar radiation to reach the cell. For this reason, the backsheet is usually made of glass covered with antireflective coating. It is evident that these choices, which reduce the conversion efficiency of the module, are justified by the fact that bi-PVs achieve an energy yield up to 11% higher than their monofacial counterparts, as shown in [26; 27]. From the basic features described so far, various implementations can be derived. The most relevant ones are summarized in Figure 2. This figure provides a schematic view of the wafer cross-section of the most popular silicon-based bifacial solar cells presented in the literature [8]. Each structure is accompanied by a brief description and summary of key performance indicators, such as front and rear conversion efficiencies, cost of ownership where available, and bifaciality factor \(\varphi_{\text{mod}}\). \(\varphi_{\text{mod}}\) is a representation of the different results that irradiance can have when incident on the front surface rather than the rear surface, i.e., the difference in energy conversion efficiency of the two sides, and is defined as  
\[\varphi_{\text{mod}}=min\left(\frac{I_{\text{sc,STC,r}}}{I_{\text{sc,STC,f}}}, \frac{P_{\text{mpp,STC,r}}}{P_{\text{mpp,STC,f}}}\right) \tag{1}\]  
where \(I_{\text{sc,STC,r}}\) and \(I_{\text{sc,STC,f}}\) are the short-circuit current in standard test conditions (STCs) for the rear and front surfaces, respectively, and \(P_{\text{mpp,STC,r}}\) and \(P_{\text{mpp,STC,f}}\) are the maximum power-point value of the rear and front surfaces in STCs, respectively. While in mo-PVs, \(\varphi_{\text{mod}}\) is naturally equal to zero, its value ranges between zero and one in bi-PVs. Typical values are between 0.60 and 0.95 [28]. Therefore, the bifaciality factor collects the differences between the front and rear surfaces and permits the transformation of the rear incident irradiance into an equivalent front incident irradiance. The resulting equivalent irradiance, \(G_{\text{eq}}\), equal to the sum of the front and \"equivalent front\" components, can be used as input in the single diode model as if it were obtained with single-side illumination, as for mo-PVs. However, the evaluation of \(\varphi_{\text{mod}}\) is complex because it depends on the wafer and cell structure.