Location and Properties of Carrier Traps in mc-Si Solar Cells Subjected to Degradation at Elevated Temperatures

Multi-crystalline Si (mc-Si) solar cells subjected to carrier-induced efficiency degradation at elevated temperatures are studied using deep-level transient spectroscopy (DLTS), capacitance-voltage, and electroluminescence (EL) measurements. Commercially available passivated emitter and rear cell (PERC) mc-Si solar cells are investigated after short annealing in the dark (at T = 200 degrees C for 20 min), after degradation by a constant forward current at 70 degrees C, and after regeneration annealing at 200 degrees C for 20 min. The degradation is detected in situ by EL imaging of the surface of the cell. Several n(+)p mesa-diodes, selectively fabricated on the front surface of the solar cell, allow the characterization of locations with various levels of degradation and density of extended defects. The degree of the degradation correlates with the local active boron concentration. At all stages, traps associated with extended defects (i.e., dislocations, grain boundaries, precipitates, etc.) are detected by DLTS, and these traps are not affected by changes in the degree of degradation. Two minority-carrier traps with energy level positions in the bandgap at Ec -0.19 eV and Ec -0.34 eV are detected only in the degraded solar cells in concentrations that correlate with the local degree of cell degradation.