Comparison of photovoltaic module luminescence imaging techniques: Assessing the influence of lateral currents in high-efficiency device structures

Imaging Lateral Drift Kinetics to Understand Causes of Outdoor Degradation in Silicon Heterojunction Photovoltaic Modules

“…However, we measure threefold lower luminescence intensity from the nonilluminated region for this weathered cell (on average 2.4 ± 0.8 times lower for cells in the degraded vs control modules at this laser fluence). This lower contactless EL amplitude occurs despite similar luminescence intensities in the illuminated region (on average 1.0 ± 0.4 between weathered vs control modules) . This results in a difference in a luminescence pattern between the weathered and control modules, where the weathered module has a higher luminescence ratio of illuminated to nonilluminated regions, and the higher ratio is primarily caused by the lower contactless EL.…”

“…Partial illumination of photovoltaic cells induces lateral currents that have important practical implications for laser‐based luminescence screening of photovoltaic modules. In particular, line‐scan photoluminescence (PL) and contactless electroluminescence (EL) are increasingly employed for module quality inspection, and both techniques rely on subcell illumination patterns rather than full, homogeneous illumination . Line‐scan PL is designed to measure the luminescence signal from cell regions coincident with the laser, making this technique particularly useful for in‐line screening .…”

“…The luminescence away from the photoexcitation is known as the contactless EL signal . Contactless EL imaging is a particularly appealing alternative for in‐line, outdoor, and remote (drone‐based) screening as the signal does not contain luminescence from voltage‐independent carriers, and (unlike traditional EL) it can be performed without electrical contact to the module …”

“…Importantly, the magnitudes of lateral currents during these measurements will be affected by the degree of degradation in the cells’ series resistance, shunting, and extent of nonradiative recombination . The resulting spatial convolution between the cell quality and the inhomogeneously photogenerated voltage introduces challenges for interpreting luminescence images collected under partial illumination because the images can no longer be assumed to be proportional to the bulk carrier lifetimes as they would be under homogeneous illumination .…”

“…We next investigate the effect of module degradation on lateral currents by comparing the luminescence images and μPCD transients for cells within a pair of field‐weathered and control HIT modules. We provide additional characterization of these modules in the studies given by Sulas et al and Jordan et al To highlight the different degrees of carrier spreading in the weathered module compared with the control, Figure a,b shows the steady‐state luminescence images for representative cells under CW excitation within a 1.2 cm 2 area. For both cells, we observe luminescence from both the illuminated and nonilluminated cell regions, indicating that lateral currents redistribute the carriers after laser excitation to some extent in both cases.…”