LeTID in multicrystalline PERC cells

January 2021 Spatial evaluation of the degradation and regeneration kinetics attributed the LeTID defect in hydrogenated multicrystalline silicon wafers has been performed. Two approached to study the effect of the charge carrier concentration throughout the degradation and subsequent regeneration of the carrier lifetime has been used; Different illumination intensities during the processing, as well as the different minority carrier lifetime in different areas of a multicrystalline wafer. An improved equation describing the kinetics of the reactions governing both the degradation and the regeneration has been proposed. A better fit to measurement has been obtained using the new equation, where we assume that the charge carriers (electrons) are one of the reactants. Thus, participation of charge carriers in both the degradation and the regeneration mechanism is therefore proposed. The experiments also showed that a rapid process for LeTID mitigation will result in a less stable regeneration.

A stronc characterization package combining spectral PL and TEM has been established. A joint project application was written and submitted (unsuccessfully). Many articles inn scientific journals has been written. At least 3 additional articles will be published in 2021. Through defect kinetics insights into both the defect reaction has been obtained. This information is useful in designing industrially relevant post-processes to suppress degradation in solar cells based on multicrystalline silicon.

The LETuP project addresses the root causes for light and elevated temperature inducted degradation (LeTID) in multi-crystalline Si passivated emitter and rear cells (mc-PERC). LeTID can reduce the performance of mc-PERC solar cell by as much as 16%rel. Even though LeTID is visible in cells and wafers, its origin traces back to the atomic level. Thus to find the mechanism of LeTID requires atomic scale characterization and modeling in close collaboration with investigation of silicon wafer quality as well as solar cell processing. In LETuP project, cells with different magnitude of LeTID effect will be prepared by varying gettering, passivation and firing process. In addition, the kinetics of degradation and regeneration will be investigated by varying temperatures and/or charge injections. High resolution imaging tools will be used to characterize the samples with the defects at different recombination states present; initial state, degrading state, and recovering state. When the interest regions is identified, more advanced atomic scale characterization tools will be carried out such as transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) as well as quantum modeling e.g. using density functional theory (DFT). A strong national and international collaboration between wafer and cell processing, characterization as well as theory will be carried out to improve current possibilities of analysis and thus contribute to a new level of quantitative and qualitative understanding of the LeTID mechanisms. The outcome of LETuP is expected to attract significant industrial interest both Si material providers and cell producers. The project partners, IFE, SINTEF and NMBU have a long track record within both fundamental and applied research with established competence ranging from the macroscopic scale down to the atomic scale, i.e. from silicon wafer, solar cell processing and characterization down to TEM.