**2.8. Potential-induced degradation**

reverse current path in the still active cell part. The cell may reverse bias and the full current will be able to flow along the localized path as a consequence of the missing cell area. This can

Snail tracks are discolorations of the silver fingers on solar cells. A significant example can be seen in **Figure 3**. The effect looks like a snail has passed across the front glass of the PV module. The discoloration takes place on cell cracks that are not visible at the edge of the solar cell. This typically happens between 3 months and 1 year after installation of the PV modules [18]. Discoloration speed is initially dependent on seasonal and environmental conditions, such that snail tracks seem to spread faster during summer months and in hot climates [5]. PV modules affected by snail tracks have been compared with reference modules under laboratory conditions [19] and results showed a 40% reduction in maximum power under standard conditions with a 25% lower yield than expected when measured over a 30-day period.

A typical and very common failure in silicon PV modules is burn marks. This failure occurs due to part of the module becoming very hot and can be because of ribbon breakage, solder bond failure, or localized heating from reverse current flow or other hot spots [5]. Burn marks can produce power losses and serious safety problems. They are usually located on or closed

Hot spots are areas in a photovoltaic module that have very high operating temperatures when compared to surrounding areas. This may be due to interconnection failures, defects in the cell, dispersion of characteristics between modules of a generator and between cells of the same module connected in series, potential-induced polarization in modules manufactured with novel techniques, or when a cell generates less current than other cells connected in series as a consequence of intermittent cloud cover or partial shading [20]. As a consequence, the cell becomes polarized (the voltage between the terminals becomes negative) and starts to

to the metal contacts of the PV solar cells, such as it can be seen in **Figure 4**.

dissipate the power generated by the other serial cells in the form of heat.

cause hot spots and subsequently burn marks [17].

**2.6. Snail tracks**

98 Solar Panels and Photovoltaic Materials

**2.7. Hot spots and burn marks**

**Figure 3.** Example of a snail track over a PV cell.

Potential-induced degradation gives rise to power losses owing to the presence of eddy currents in the PV modules. Its effect can potentially reduce the power of the equipment [22].

The principal cause of these currents is reported to be voltage gaps between the ground and the module. In photovoltaic systems without a grounding system, this effect occurs when modules have a non-zero voltage, which is normally negative especially under high ambient humidity and/or temperatures and high voltage conditions [23].

#### **2.9. Disconnected cells and string interconnected ribbons**

Cell strings can become disconnected if string interconnected ribbons are weak, which may be caused by large deformations, by the quality of the welds during the production process, or by weak connections between the string and the ribbon. Small distances between cells can also contribute to interconnected ribbon breakage [5].

The consequences of this may be a broken cell interconnected ribbon and a subsequent decrease in maximum power point current [24] or a shunt by a cell interconnected ribbon and a subsequent decrease of open circuit voltage.

**Figure 4.** Example of a burn mark on a PV cell.
