## 3 Results and Discussion
### Influence on Mounting Structure  
An additionalfactor contributing to non-uniform rear illumination, in addition to the diffuse fraction, is the structural configuration of the mounting system itself. To elucidate its impact, bPV modules (bPV1 and bPV2) were positioned adjacently with varying frame obstruction patterns, as illustrated in Figure 4. Distinct _I-V_ curves were then generated by altering the tilt angle (and in-plane irradiance). In this case, the ground behind bPV modules was a metallic grid (see Figure 4a), and its albedo was found to be 0.09. Figure 9a presents a representative instance of bifacial devices exhibiting current mismatch on the rear side, attributed to non-uniform illumination, a phenomenon previously observed even under controlled indoor test conditions [41]. Notably, even minor discrepancies in mounting between bPV1 and bPV2 result in discernible deviations in the _I-V_ and _P-V_ curves. Independently, on the tilt angle, bPV1 shows a worse performance than bPV2, which is more evident at 0\({}^{\circ}\) and 30\({}^{\circ}\) (see Figure 9b). While bPV1 shows an almost flat _I-V_ profile in the range _I\({}_{sc}\)-I\({}_{mp}\)_, three different current levels with two steps (or kinks) are observed in bPV2 in the same range: the first one approximately at 10 V and the second one slightly above 20 V (see Figure 9a). Figure 8: Bifacial power gain of bPV by two \(P_{mp}\) estimation methods: Procedure 4 of IEC 60891 [35] (red crosses) and Equation (5) (black squares). Chronological samples (**a**) versus diffuse fraction ratio (**b**). BPG mean of low diffuse fraction samples is added to the legend. Figure 9: Curves of two identical bifacial modules (bPV1 and bPV2) with different backlight obstruction. _I-V_ (**a**) and _P-V_ (**b**) curves. As both bPV1 and bPV2 are bifacial modules of the same model, and their \(I\)-\(V\) curves are traced within a 1.5 min difference, the current discrepancies are only justified by discrepancies in the secondary face contribution. Even if the kinks are more evident in bPV2, their effect is more severe in bPV1, a single deep step, rendering it nearly akin to a conventional device, as the bifacial contribution is largely neglected. Consequently, bPV1 can serve as a reference to estimate the performance reduction in both bPV2 and an ideally placed bifacial module: its current at MPP provides an approximation to that of a conventional device of identical technology; the difference with bPV2 at MPP indicates the additional power/current under the tested conditions; and the discrepancy under the short-circuit conditions facilitates the estimation of the potential current (and power) in the absence of structural obstructions. These approximations are in agreement with the experimental results in similar outdoor setups [21], where an imitated structural beam completely blocks a row of cells in the back face and makes the \(I\)-\(V\) curve coincide with that of a conventional module, i.e., a practical cancellation of the bifacial contribution [21]. Despite the presence of these deviations, which attenuate its bifacial contribution, bPV2 exhibits a slight increase in maximum power across all tested conditions (see Figure 9b and Table 5). The variation in current attributed to the kinks in Figure 10a falls within a similar range (0-3% of \(I_{sc}\)), as reported in indoor measurements characterized by non-uniform back radiation [41]. To discern these differences more clearly, the variation (bPV2-bPV1) for all curves is depicted in Figure 10. The values at MPP are denoted by x-markers for bPV1 and o-markers for bPV2. Notably, the changes in voltage at MPP are negligible, indicating that any variation in power is primarily attributable to the changes in current. This can be derived by (8) and (9), where an apostrophe signifies the presence of bifacial contribution (either bPV2 or the ideal scenario of homogeneous illumination without kinks), while its absence denotes the scenario where bifacial contribution is entirely neglected (bPV1).