First released in 2009, Tau Science’s IRIS™ product detects electrical shunt defects in solar cells that become hotspots in reverse bias– a leading cause of PV cell performance problems and module degradation.
IRIS detects cell hotspots in less than 500ms using proprietary time-resolved thermography. Data is synthesized to produce a map of ‘severity’ which described the likelihood of causing a critical shunt defect. These maps, generated at inline speeds, show excellent correlation to thermographs of the same cells after a full 15 seconds under reverse bias conditions (see image). Cells that reach more than 80C in reverse bias may be a reliability concern for module manufacturers.
Defective cells can overheat when shaded
When a cell in a module becomes shaded by, for example, a leaf, bird, or building, it is put into reverse bias. Under these condition the illuminated cells generate current and push it through the shunted regions of the shaded cell, generating heat. Depending on the size and distribution of the shunts, the overheating can lead to degradation of the cell and ultimately the module.
Hotspot Causes in Conventional Cells:
- Crystalline defects in the substrate
- Metal decoration of edge cracks
- PN junction punch-through from over firing
- Solder: high resistance
Small Print Geometry Cells, Additional Causes:
- Conductive bridging
- Junction overlap, printing errors
Thin Film Cells, Additional Causes:
- Poor film uniformity
- Edge shunts from scribing due to redeposition, etc.
- Cell damage*- abrupt power loss
- Module Fire (rare)
- Loss of hermetic seal due to:
- Cracked glass
- Backsheet bubbles
Long Term Effects
- Moisture Intrusion (EVA is hydrophilic)
- Corrosion- Progressive power loss
*Series Multiplier effect- One degraded cell = degraded module