Effect of the encapsulant temperature on the angular and spectral response of multi-junction solar cells

Multi-junction solar cells (MJSC) operating at working conditions under concentration are subjected to temperatures (T) for which the optical coupling provided by their anti-reflective coatings (ARC) has not been optimized. High temperatures and wide ray angles produced by the concentrator on the optical interface of the cell can significantly modify the ARC performance. This effect is especially pronounced for ARCs adapted to silicone encapsulant because the silicone refractive index (n) is significantly sensitive to temperature, modifying the optimal design thickness and material composition. This effect is magnified for tilted rays whose optical path length through the ARC layer is most modified. In this work, an absolute external quantum efficiency (EQE) characterization system is adapted to perform angular and temperature spectral response analysis, allowing to quantify the impact of the optical mismatch caused by the increase of encapsulant temperature for each of their junctions. The intricacies of this upgraded characterization technique are explored, providing insight on important unexpected measurement variables such as finger orientation with respect to the incident ray bundle. A significant spectral mismatch between junctions due to the change in silicone temperature has been observed, leading to short-circuit current (I_SC) losses as high as 6% with respect to the design conditions (T=25°C) for rays impinging the cell with tilt angles of 70° and more realistic operation temperatures of 65°C. The losses arise from current mismatch between subcells produced by variations in the optical coupling. Lessons from this analysis may be taken into account by future CPV system designers.