Superior low-light-level performance of upgraded metallurgical-grade silicon modules

The industry is becoming critically sensitive to solar energy delivered in kilowatt-hours rather than in kilowatt at illumination peak intensity. This is because when comparing systems or modules, it is more relevant to compare the energy delivered during an entire day than the energy delivered during peak illumination, which happens for very few hours on the same day. For that reason, low-light-level performance is an important parameter that greatly influences the total energy yield of a PV system. This is especially important in low annual insolation regions such as northern Europe or the northeast United States. Low-light-level performance can vary significantly even within a particular PV technology. In this contribution, results of low-light performance of three modules are presented. The first module uses Calisolar upgraded metallurgical-grade (UMG) Si solar cells, the second module uses standard monocrystalline Si cells, and the third module uses standard electronic-grade (EG) Si cells. The modules were first tested at NREL's Outdoor Testing Facility. The low-light-level performance of the three modules indicated a markedly higher module output for the module with Calisolar UMG cells. Because angle of incidence, temperature, and spectral variations can significantly influence these data, these modules were also measured using a Spire indoor solar simulator. Measurements at 200, 400, 600, 800, and 1000 W/m² corroborated our outdoor tests and the superior performance of the UMG-based modules. We observed that the shunt resistance of the module with Calisolar UMG cells is higher than that of the other two modules, which can explain the higher module output. Thus, as the light intensity decreases, the light IV curve moves toward the lower part of the diode characteristics where R_shunt and J₀₂ (which describes recombination in the space charge region) dominate. In this low-light regime, a decrease of R_shunt drastically reduces the V_OC and fill factor. Cells with higher R_shunt are less affected. Large-scale system outputs have also confirmed higher performance module output in low-light conditions for modules using Calisolar cells.