Novel evaluation of intra-grain defects in polycrystalline silicon solar cells using light emission

121

Extensive investigations on industrial multicrystalline silicon solar cells have shown that, for standard 1 X cm material, acid-etched texturization, and in absence of strong ohmic shunts, there are three different types of breakdown appearing in different reverse bias ranges. Between À4 and À9 V there is early breakdown (type 1), which is due to Al contamination of the surface. Between À9 and À13 V defect-induced breakdown (type 2) dominates, which is due to metal-containing precipitates lying within recombination-active grain boundaries. Beyond À13 V we may find in addition avalanche breakdown (type 3) at etch pits, which is characterized by a steep slope of the I-V characteristic, avalanche carrier multiplication by impact ionization, and a negative temperature coefficient of the reverse current. If instead of acid-etching alkaline-etching is used, all these breakdown classes also appear, but their onset voltage is enlarged by several volts. Also for cells made from upgraded metallurgical grade material these classes can be distinguished. However, due to the higher net doping concentration of this material, their onset voltage is considerably reduced here.

mentioning

confidence: 99%

Understanding junction breakdown in multicrystalline solar cells

Breitenstein

Bauer

Bothe

et al. 2011

121

“…Therefore, intense research into the reasons and the physical understanding of breakdown were undertaken, especially for (multi-) crystalline silicon solar cells. [1][2][3][4][5][6][7][8] Like in other silicon-based electronic devices, breakdown causes the emission of visible light, the reverse biased electroluminescence (ReBEL). 9,10 It is therefore possible to investigate and locate the occurring high currents not only with (Lock-in) thermography 2 (LIT) but also with CCD cameras offering higher spatial resolution.…”

mentioning

confidence: 99%

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

Schneemann

Kirchartz

Carius

et al. 2015

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.

“…[1][2][3][4][5][6][7][8] More recently, the observation of ReBEL in multi crystalline silicon solar cells and the concern about the long-term stability of those devices brought the issue back into the focus of research. 9,10 Imaging of ReBEL provides additional information compared with forward biased electroluminescence (EL) imaging, 11 which is a well-established quality control tool in Si photovoltaics. 12 Forward biased EL emission is essentially homogeneous over the area of the solar cell with small variations due to locally varying minority carrier lifetimes and due to resistive and optical effects.…”

mentioning

confidence: 99%

Measurement and modeling of reverse biased electroluminescence in multi-crystalline silicon solar cells

Schneemann

Kirchartz

Carius

et al. 2013

Approach to the physical origin of breakdown in silicon solar cells by optical spectroscopy J. Appl. Phys. 108, 123703 (2010) Calibrated microscopic measurements of electroluminescent emission spectra of reverse biased multi-crystalline silicon solar cells in a wide range of photon energies E (0.8 eV E 4 eV) are reported. The observed spectra originating directly from point-like sources exhibit a broad maximum around 0.8 eV followed by a high photon energy tail. A model for intraband emission accurately fits microscopically measured spectra obtained from single point sources. Furthermore, we do not find significant features from interband recombination. From the fits to the intraband transition model, we extract an effective charge carrier temperature of around 4000 K for all investigated spots. The analysis also yields the different depths of the sources, which are shown to be consistent with the dimension of the space charge region. From the areas around the point sources, we observe indirect emission of internally reflected light. Due to the multiple paths through the wafer, this indirect emission exhibits a maximum at a photon energy slightly lower than the band gap energy E g . We demonstrate that global, non-microscopic measurements are strongly influenced by this indirect radiation and therefore prone to misinterpretation. V C 2013 AIP Publishing LLC.[http://dx