Electric properties and carrier multiplication in breakdown sites in multi-crystalline silicon solar cells

Abstract

This paper studies the effective electrical size and carrier multiplication of breakdown sites in multi-crystalline silicon solar cells. The local series resistance limits the current of each breakdown site and is thereby linearizing the current-voltage characteristic. This fact allows the estimation of the effective electrical diameters to be as low as 100 nm. Using a laser beam induced current (LBIC) measurement with a high spatial resolution, we find carrier multiplication factors on the order of 30 (Zener-type breakdown) and 100 (avalanche breakdown) as new lower limits. Hence, we prove that also the so-called Zener-type breakdown is followed by avalanche multiplication. We explain that previous measurements of the carrier multiplication using thermography yield results higher than unity, only if the spatial defect density is high enough, and the illumination intensity is lower than what was used for the LBIC method. The individual series resistances of the breakdown sites limit the current through these breakdown sites. Therefore, the measured multiplication factors depend on the applied voltage as well as on the injected photocurrent. Both dependencies are successfully simulated using a series-resistance-limited diode model.

Study of Low Voltage Prebreakdown Sites in Multicrystalline Si Based Cells by the LBIC, EL, and EDS Methods

“…Among various types of shunts, those associated with local breakdown of solar cells under reverse bias can be mentioned [5][6][7][8][9]. It was shown that such breakdown sites can be revealed by reverse-bias electroluminescence (EL) [5][6][7][8][9][10] and their classification was carried out [6,11]. The correlation of EL images with those obtained by the electron beam induced current (EBIC) method sometimes allows finding a link between these sites and extended defects [6,11].…”

“…The reason for this misunderstanding is determined by the manifold nature of shunts, especially in multicrystalline Si, and by a variety of extended defects and impurities in this material. Among various types of shunts, those associated with local breakdown of solar cells under reverse bias can be mentioned [5][6][7][8][9]. It was shown that such breakdown sites can be revealed by reverse-bias electroluminescence (EL) [5][6][7][8][9][10] and their classification was carried out [6,11].…”