Solar cells in modules are reverse biased when they are shaded. This can lead to diode breakdown and eventually to the occurrence of hot spots, which may, in the extreme case, destroy the module by thermal degradation. We observed at least three different types of diode breakdown in multicrystalline silicon solar cells. One of them is found to be related to the recombination activity of defects. This type is indicated by a slow increase in the reverse current with reverse bias and a relatively low breakdown voltage around -10 V. The local breakdown voltage depends significantly on the level of contamination of the material. When the solar cell is reverse biased, the breakdown sites emit bright light which shows a broad spectral distribution in the visible range with a maximum at 700 nm

The local prebreakdown behavior of a damage etched multicrystalline silicon solar cell produced from virgin grade feedstock was characterized. At the position of micrometer-scaled prebreakdown sites, which correlate with recombination active defects found along grain boundaries, micro-x-ray fluorescence mappings revealed the presence of Fe precipitate colonies. These measurements represent direct evidence that transition metal clusters lead to decreased breakdown voltage and cause soft diode breakdown

In the last fifteen years the measurement of the spatially resolved carrier lifetime has emerged as a valuable tool for the characterization of silicon wafers and solar cells. In most of the available measurement methods, the spatial resolution is constrained to the order of several 10 to 100 µm by the diffusion length of the charge carriers. In this paper we introduce a contactless quantitative technique to determine the Shockley-Read-Hall lifetime with a spatial resolution of 1µm. This technique is based on high injection microphotoluminescence spectroscopy and allows a quantitative analysis of microscopic defects such as grain boundaries and metal precipitates by virtue of the high spatial resolution

Metallic impurities are detrimental to many silicon devices and limit the efficiency of multi crystalline silicon solar cells. Therefore they are a major subject of ongoing research. Photoluminescence spectroscopy is a promising technique for detecting precipitated metals in silicon because of its sensitivity to the minority carrier density and to specific types of defects; however the impact of impurities on the defect luminescence could not be clarified yet. In this letter we examine the role of micron-sized iron and copper precipitates in direct bonded wafers by micro-photoluminescence spectroscopy. Both kinds of precipitates are detectable by means of the reduced band-to-band luminescence. An element-specific effect on the defect luminescence is observed. The results are confirmed by X-ray fluorescence spectroscopy

Contact formation with silver (Ag) thick film pastes on boron emitters of n-type crystalline silicon (Si) solar cells is a nontrivial technological task. Low contact resistances are up to present only achieved with the addition of aluminium (Al) to the paste. During contact formation, Al assisted spiking from the paste into the silicon emitter and bulk occurs, thus leading to a low contact resistance but also to a deterioration of other cell parameters. Both effects are coupled and can be adjusted by choosing proper Al contents of the paste and temperatures for contact formation. In this work the microscopic electric properties of single spikes are presented. These microscopic results, i.e. alterations of the local emitter doping density, the pronounced local recombination activity at the interface between spikes and Si and its influence on the charge collection efficiency, are used to explain the observed dependencies of global cell parameters on the Al content of contact pastes