Light And Elevated Temperature Induced Degradation Research Articles

Solubility and Diffusivity: Important Metrics in the Search for the Root Cause of Light- and Elevated Temperature-Induced Degradation

Light- and elevated temperature-induced degradation (LeTID) is a detrimental effect observed under operating conditions in p-type multicrystalline silicon (mc-Si) solar cells. In this contribution, we employ synchrotron-based techniques to study the dissolution of precipitates due to different firing processes at grain boundaries in LeTID-affected mc-Si. The synchrotron measurements show clear dissolution of collocated metal precipitates during firing. We compare our observations with degradation behavior in the same wafers. The experimental results are complemented with process simulations to provide insight into the change in bulk point defect concentration due to firing. Several studies have proposed that LeTID is caused by metal-rich precipitate dissolution during contact firing, and we find that the solubility and diffusivity are promising screening metrics to identify metals that are compatible with this hypothesis. While slower and less soluble elements (e.g., Fe and Cr) are not compatible according to our simulations, the point defect concentrations of faster and more soluble elements (e.g., Cu and Ni) increase after a high-temperature firing process, primarily due to emitter segregation rather than precipitate dissolution. These results are a useful complement to lifetime spectroscopy techniques, and can be used to evaluate additional candidates in the search for the root cause of LeTID.

System performance loss due to LeTID

If not adequately suppressed, Light and elevated Temperature Induced Degradation (LeTID) has been shown to cause severe degradation of multicrystalline (mc-Si) silicon solar cells and modules with passivated emitter and rear cell (PERC). Within this work, the system performance of LeTID-sensitive mc-Si modules is investigated when operated in temperate and mediterranean climates, and a correlation to predict the observed field performance based on accelerated laboratory testing is presented. Severe degradation induced by LeTID of up to 7% in maximum output power is detected after one thousand hours of laboratory testing, which is shown to correspond to ~ 3 years of field installation time in Cyprus. In contrast, LeTID-sensitive modules installed in Germany show a degradation of 2.5% within the same time period. Due to lower irradiance values and module temperature in Germany the LeTID rate is lower than in Cyprus. A significant dependence of system performance loss and energy yield due to LeTID on installation site is shown. Hence, the loss in system performance due to LeTID affects one-to-one the levelized cost of electricity (LCOE) compared to reference without LeTID. Furthermore, it is shown that LeTID in the field can be suppressed by applying Hanwha Q CELLS’ Q.ANTUM technology, independently of installation site.

Recovery of LeTID by low intensity illumination: Reaction kinetics, completeness and threshold temperature

Light (and elevated Temperature) Induced Degradation (LeTID) can seriously limit the efficiency of mc-Si PERC solar cells if no countermeasures are taken. Within this paper, the subsequent regeneration reaction, also induced by light at elevated temperatures, is investigated systematically in the low intensity regime (up to 4 suns) as a possible countermeasure. As might be expectable, degradation and regeneration reaction are found to speed up with increasing treatment temperature; however, completeness of regeneration is also found to be noticeably reduced at high temperatures (up to 380°C). This enforces the introduction of reverse reactions and gives rise to a 3-state reaction model for LeTID explaining the observed threshold temperature above which LeTID elimination does not work anymore. Furthermore, it is found that a treatment at high temperatures not only fails to eliminate LeTID, but also slows down the regeneration in a later low temperature treatment.

Firing temperature profile impact on light induced degradation in multicrystalline silicon

Light- and elevated temperature-induced degradation (LeTID) in multicrystalline silicon can reduce the efficiency of solar cells significantly. We analyse the influence of the firing temperature profile on the degradation behaviour of neighbouring mc-Si wafers, varying peak temperatures above 800°C (measured) as well as heating and cooling ramps. The degradation intensity is determined by the normalized defect concentrations Nt* using spatially resolved and lifetime calibrated photoluminescence images. Wafers which were fired in a standard industrial fast firing furnace with steep ramps suffer from significant LeTID whereas samples that were subjected to the same or even higher peak temperatures but with slower heating and cooling rates hardly degrade. A spatially resolved analysis of Nt* over the whole wafer area shows that at the beginning of the experiment, the degradation is restricted to low-lifetime areas around dislocation clusters. After several hours, a very strong degradation is observed also in initially good grains. The possible roles of metallic impurities and hydrogen in-diffusion are discussed.

Evolution of LeTID Defects in p-Type Multicrystalline Silicon During Degradation and Regeneration

While progress has been made in developing engineering solutions and understanding light- and elevated temperature-induced degradation (LeTID) in p -type multicrystalline silicon (mc-Si), open questions remain regarding the root cause of LeTID. Previously, lifetime spectroscopy of multicrystalline silicon (mc-Si) passivated emitter and rear cell semifabricates in the unaffected and the degraded states enabled identification of the effective recombination parameters of the responsible defect. To gain further insight into the root cause of LeTID, in this paper, we measure the injection-dependent lifetime throughout degradation and regeneration and perform lifetime spectroscopy at several time points. Our analysis indicates that the change in lifetime during most of the process can be described by a corresponding change in the concentration of a single responsible defect. We also explore further exposure to light and temperature after nearly complete regeneration and a subsequent dark anneal to demonstrate that the behavior is no longer consistent with LeTID and the same defect is not detected by lifetime spectroscopy at maximum degradation. We consider our results in the context of the proposed hypotheses for LeTID and conclude that both hydrogenation and precipitate dissolution during firing are consistent with our results.

Degradation of multicrystalline silicon solar cells and modules after illumination at elevated temperature

In this work the performance stability of rear side passivated multicrystalline silicon solar cells and modules under carrier injection at different temperatures is investigated. Compared to most other tests the degradation procedure were extended to significantly longer time periods and include relevant module temperatures in field. Severe power degradation levels of above 10% can be detected after several hundred to thousand hours (corresponding ~5–20 years depending on the location) which cannot be explained by B–O complex formation or FeB pair dissociation. After detection of this light induced degradation at temperatures above 50°C the common abbreviation LID was renamed and state more precisely Light and elevated Temperature Induced Degradation (LeTID). A high number of cells and modules degraded in laboratory and outdoor using material from different wafer suppliers confirm the relevance of this effect. LeTID is a multicrystalline silicon bulk phenomenon leading to a highly injection dependent lifetime characteristic after degradation and features a regeneration phase after degradation. The time constant of this degradation mechanism accelerates with increasing temperature, however, the time span for degradation and regeneration of thousands of hours at relevant temperatures between 60 and 85°C demands for a solution on wafer material or processing side. LeTID can be significantly reduced by adapting the cell process and processing sequence.