Light and elevated temperature induced degradation in multicrystalline silicon wafers studied by hyperspectral photoluminescence imaging

Light and elevated temperature induced degradation (LeTID) has become a profound problem for the new PERC cells, reducing performance by as much as 16% during operation. The mechanism that governs the degradation is not fully understood, but some effect caused by hydrogen in the bulk of the cell is one of the suggested causes. High performance multicrystalline silicon (hpmc-Si) p-type wafers of different processing are investigated for LeTID using a hyperspectral photoluminescence (HSPL) imaging setup. As cut, phosphorus diffusion gettered (PDG) and gettered hydrogen bulk passivated wafer samples, with and without surface passivation, were processed with light soaking and elevated temperatures, with imaging at 90 K at intervals of the processing. LeTID was only observed in the samples with both surface passivation, PDG and hydrogen bulk passivation. This supports the hypothesis that bulk hydrogen is involved with the LeTID mechanism and suggests that a high charge carrier density is required. Defect related luminescence (DRL), PL signals correlated with defects and impurities, did not appear or disappear as a result of LeTID but got reduced intensity as the band-toband (BB) signal, a PL signal correlated with cell performance, degraded. Both BB and DRL recovered with long time exposure to light and elevated temperate