Photovoltaic Cell Reverse Bias & Hotspot Formation Under Partial Shade
Core Question: Why does shading just 5% of a single solar panel cause up to 80% power loss and trigger catastrophic cell hotspots?
Executive Technical Summary
Solar cells within a module are connected in series. When one cell is shaded while the rest are in bright sun, the shaded cell is forced into reverse bias, behaving as a dissipative resistor that absorbs string power and can reach temperatures above 150°C.
Detailed First-Principles Explanation
A normal illuminated silicon cell produces approx. +0.5V DC. When a leaf, chimney shadow, or bird dropping blocks light to a single cell, the unshaded cells in the series string push current through it. The shaded cell enters reverse breakdown, absorbing 6 to 8 volts. This 12x voltage multiplication concentrates thermal energy, threatening glass fracture, EVA delamination, and electrical fire unless bypass diodes switch into conduction.
Assuming microinverters or optimizers eliminate all cell hotspots; while they isolate panel-to-panel mismatch, internal cell-level shading within a module still triggers bypass diode activation and localized thermal stress.