Outdoor validated mismatch driven mirror-cell hotspots in half-cell silicon heterojunction modules

Half-cell photovoltaic modules reduce resistive losses but can exhibit hotspot mirroring under partial shading, where an unshaded cell in the parallel-connected mirror substring dissipates power when the bypass diode conducts. We present outdoor infrared thermography of a 120-half-cell silicon heterojunction (SHJ) module at fixed high- and low-voltage operating points: pronounced heating appears in an unshaded mirror cell at the low-voltage point with bypass activation, whereas no pronounced heating is observed at the high-voltage point without bypass activation. To clarify the circuit origin and triggering conditions, we developed a SPICE-based equivalent-circuit model parameterized from an encapsulated SHJ coupon cell. The model reproduces the measured I–V characteristics and shows that bypass activation imposes a small negative voltage across the diode-protected parallel-substring unit (i.e., the two parallel substrings protected by one bypass diode). Simulations with realistic cell-to-cell variability reveal that a slight short-circuit-current mismatch (ΔI sc ) can drive a mirror cell into strong reverse bias and localize dissipation, while a parametric study indicates ΔI sc dominates dissipated power and shunt-resistance variations are secondary. These results provide outdoor-validated, circuit-level guidance for mitigating hotspots in half-cell modules beyond shaded-cell-only evaluation.