Series resistance contribution of majority carriers in CELLO impedance analysis: Influence of wafer thickness variation

• CELLO obtains spatially resolved time constants describing photocurrent extraction.
• Local ohmic losses can be analyzed by light with varying penetration depth.
• For PERC cells, majorities-related time constants are large, possibly dominant.
• Majorities’ ohmic losses at point contacts are obtained by local laser illumination.
• Losses depending on wafer thickness and laser penetration depth yield thickness map.

Local series resistance measurements are an essential tool for optimization of silicon solar cells and cell concepts. This holds especially for nearly all advanced cell concepts where the minority carrier properties are improved at the expense of the majority ones (e.g., by a lower-doped emitter or point contacts at the back side). For such solar cells, frequency-dependent CELLO measurements allow an efficient series resistance characterization since they can e.g. distinguish ohmic losses originating in the volume of a solar cell from the total ohmic losses, thereby making it possible to identify wafer thickness variations. The thickness information allows to improve the reliability of fitting parameter results obtained from a CELLO impedance analysis and to identify other thickness-variation-related problems that may occur in the fabrication process.

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