On the spectral response of PV modules

The spectral response of silicon wafer based and thin-film photovoltaic (PV) modules is studied using simulation and experimental methods. Circuit simulations show that the module spectral response (SR) depends on (1) the SR of the cells, (2) the shunt resistance Rshunt of the cells, and (3) the bypass diodes of the module. For realistic Rshunt values, the module SR is significantly higher than the minimal SR of the individual cells (which would be the module SR in the case of infinite Rshunt). Round-robin tests using different experimental methods (partial illumination and full-area illumination) to determine the SR of a wafer-based module and a thin-film silicon module were conducted. Both SR methods are found to agree reasonably well. However, circuit simulations indicate that if only one cell, or a few cells, within the module have significantly different characteristics but not known, the results may differ considerably. The partial illumination method can access the SR of the individual cells within a module, but it possibly requires a long measurement time in order to measure the SR and Rshunt of each cell for confirming the SR of the whole module. In contrast, full-area illumination methods measure the module SR directly, but they cannot access the cell SRs if problematic cells exist. An uncertainty analysis of the full-area illumination method is conducted, which reveals that?if the calibrated reference cell is chosen properly?the calibration uncertainty of the reference cell itself is the main source of uncertainty.