Photoluminescense for Solar Cell Crack Detection

In the manufacturing process for photovoltaic (PV) cells and modules, a significant percentage of both single- and multi-crystalline silicon wafers and solar cells used in the industry today contain microcracks that are difficult or impossible to detect with the human eye or currently available machine vision systems. These cracks can propagate through the cells, resulting in power loss and/or cell breakage, due to mechanical and thermal stresses during cell fabrication and module assembly, as well as during diurnal thermal cycling after module installation. Electroluminescent techniques have been considered for crack detection, but material defects, such as impurities at grain boundaries in multicrystalline material, make it difficult to identify a high percentage of microcracks with a low percentage of false positives. In this program photoluminescent imaging will be investigated as a means of crack detection. In this technique the front surface of a solar cell is illuminated with photons from a spectrally narrow source. These photons will generate photoluminescence resulting from two recombination paths, one with photons having energy values near 1.1 eV which will be imaged in the near infrared, and one at 3.2 eV which is imaged in the ultraviolet. Illuminating the sample with spectrally narrow sources will allow us to take advantage of the varying depth of penetration of the various wavelengths into silicon. The samples will be imaged as a function of depth allowing us to discriminate between various classes of defects. This technique is exciting due to its potential for high speed non-contact measurements. Commercial Applications and Other Benefits: Identifying and removing microcracked silicon wafers from the production line has clear benefits to solar cell and module manufacturers. Automating the inspection and reject-part segregation processes reduces the cost of inspection and rework labor in cell and module production lines while increasing module yield, which will reduce the cost of finished modules. The detection of microcracks in wafers and cells, with subsequent removal of damaged materials from the end product, will also increase the lifetime (mean time to failure) of installed modules. This, in turn, will increase the total energy production over the effective lifetime of the PV systems, thereby reducing still further the energy generation cost. In order to advance these goals, Spire plans to integrate automated photoluminescent crack detection systems into its cell testing equipment and module assembly lines.