Distribution of radiative crystal imperfections through a silicon ingot

Abstract

Crystal imperfections limit the efficiency of multicrystalline silicon solar cells. Recombination through traps is more prominent in areas with high density of crystal imperfections. A method to visualize the distribution of radiative emission from Shockley Read Hall recombination in silicon is demonstrated. We use hyperspectral photoluminescence, a fast non-destructive method, to image radiatively active recombination processes on a set of 50 wafers through a silicon block. The defect related emission lines D1 and D2 may be detected together or alone. The D3 and D4 seem to be correlated if we assume that an emission at the similar energy as D3 (VID3) is caused by a separate mechanism. The content of interstitial iron (Fei) correlates with D4. This method yields a spectral map of the inter band gap transitions, which opens up for a new way to characterize mechanisms related to loss of efficiency for solar cells processed from the block.

Defect related radiative recombination in mono-like crystalline silicon wafers

“…Since the discovery and description of the original D1–D4 lines, several others have been identified. A strong, localised emission with similar peak energy as D3 (VID3) has been studied by Flø et al . The same paper describes two possible emissions at 0.68 and 0.74 eV.…”

“…In this work, we have analysed 3 of these wafers labelled (position from the bottom of the block) A‐108 (42.4 mm), A‐78 (54.0 mm) and A‐45 (66.3 mm). The as‐cut wafers were positioned on an aluminium surface cooled by liquid nitrogen to 90 K, as previously reported for multicrystalline wafers . The wafers were scanned using a hyperspectral camera (Specim HySpex SWIR) with a spectral range 900–2500 nm (1.34–0.49 eV) with a resolution of 6 nm.…”

“…This is a statistical‐mathematical method for deconvolving complex signals composed of several individual emissions. The method has been used with success together with hyperspectral imaging on silicon wafers and solar cells in a number of previous studies . The strength of the method lies in the ability to easily extract possible components of a very complex and highly dynamic signal, over a given range of spaxels (pixels with a spectral third dimension) in the hypercube.…”

“…This applies very well to the convoluted PL spectra from solar cells or wafers, where the DRL signals may emanate from small areas containing individual defects, while being dominated by a large band‐to‐band (BB) signal from large areas with small amounts of defects. For a more thorough description of MCR we refer to Flø et al . As the method is based on a pure mathematical approach, it is preferable to check the validity of the results by first imaging the score plot from MCR, and then validating by checking for the identified high‐intensity areas or pixels, i.e.…”

“…In this work, we utilise a spectrally resolved photoluminescence (SPL) method of cooled mono‐like wafers, to study the distribution of these mechanisms in the bottom, middle and top of a pilot‐scale mono‐like silicon ingot. This method has previously been employed with success for multicrystalline material . The purpose is to study the lateral and spectral distribution of these mechanisms in order to gain fundamental insight into the distribution of impurities – possibly in combination with dislocations.…”