LeTID test set-up designed by WAVELABS in cooperation with Fraunhofer CSP allowing quantitative LID reliability tests. Image: Wavelabs.
In a PVEL survey of 2018, light-induced degradation (LID, LeTID) was identified as the defect that causes the greatest concern among investors implying severe financial risks . One reason is that the defect is still relatively new and not entirely understood due to its complexity. To reduce these risks, the LeTID Norm consortium is working on a standard to test the LeTID sensitivity. To this end, the consortium brings together the experience of research cell manufacturers, research institutes, test facilities and PV power plant operators. Thus, the proposed test standard is based on a better understanding of the defect that is causing LeTID combined with practical applicability of the test procedure.
Light-induced degradation – the current scientific knowledge
The phenomenon of illumination leading to a loss of solar cell efficiency has been under investigation for more than 40 years. Several mechanisms causing such a degradation have been studied, including the activation of boron-oxygen-defects (BO), the dissociation of iron-boron-pairs (FeB), the degradation due to copper (Cu-LID), sponge-LID, and light and elevated temperature-induced degradation (LeTID). It is well known that all these defects are activated by charge carrier injection i.e. by illumination or current injection equivalently .
While most of these mechanisms are activated within minutes (FeB) or days (BO, Cu-LID and sponge-LID) during operation, it takes years until the LeTID degradation reaches its maximum . Due to the significantly different timescales it is relevant to determine LeTID apart from the other LID mechanisms to estimate the overall losses during operation. A separation of LeTID is feasible as this degradation can only be observed above 50-60°C implying testing times of the order of weeks. Quite generally, the kinetics strongly accelerate with increasing temperature . However, high temperatures over 75°C reduce the degradation extent, since the regeneration which occurs subsequently to the degradation is even more accelerated (see Figure 1). At a relatively low temperature of 25°C, a degraded cell exhibits a recovery of the degraded cell parameters under illumination. This recovery differentiates from the regeneration observed at elevated temperature, as it results in an instable state, which degrades again at an elevated temperature treatment .
Figure 1. Typical LeTID degradation and regeneration behavior of the normalised open circuit voltage (Voc) of solar cells during illumination equivalent to one sun at 75°C, 100°C, 115°C and 135°C
Investigations have also shown that the cell process strongly affects the degradation. The higher the temperature of the firing process step (the last high temperature step in cell manufacturing), the stronger the degradation . Slower cooling rates after reaching the peak temperature during firing step can reduce LeTID . Furthermore, pre-annealing before the firing step or post-annealing after the firing step can reduce LeTID . It was also shown that thinner wafers  and gettering steps for metallic impurities reduce LeTID . During the last years, several publications showed that a high hydrogen content introduced into the silicon from the silicon nitride passivation layer of a PERC cell leads to faster and stronger LeTID [10,11].
At the moment, there is no common model for the cause and description of LeTID. Due to the strong influence of hydrogen on LeTID the UNSW has presented a “three-bucket/four-state model”, which assumes that hydrogen is the only LeTID causal agent . Schmidt et al. assume that 3d transition metal impurities are the main causal agent. In this model, the assumed state after firing is that the interstitial metal impurities are paired with hydrogen atoms and are assumed as recombination inactive .
Within the LeTID Norm project a model has been developed assuming that 3d transition metal impurities dissolving from metal-silicon-precipitates paired with hydrogen cause the degradation (see Figure 2). In this model, the well-known property of Co, Ni and Cu (all common impurities in PV wafers and cells in typical concentrations up to or more than 1013 cm-3) of the formation of small metastable platelets of the type MSi2 even after the fastest cooling to room temperature  are used to explain the low recombination activity after firing in spite of the presence of the metal impurities. This model covers all currently known facts about LeTID. Further investigations will be necessary to confirm or disprove these three models.
Figure 2. Schematic representation of the LeTID model suggested by the LeTID Norm consortium
 PVEL survey, 2018.
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