Quality characterization of silicon bricks using photoluminescence imaging and photoconductive decay

Imaging techniques can be applied to multicrystalline silicon solar cells throughout the production process, which includes as early as when the bricks are cut from the cast ingot. Photoluminescence (PL) imaging of the band-to-band radiative recombination is used to characterize silicon quality and defects regions within the brick. PL images of the brick surfaces are compared to minority-carrier lifetimes measured by resonant-coupled photoconductive decay (RCPCD). RCPCD is a transient photoconductive decay technique that monitors the recombination of excess carriers using a frequency of about 420 MHz. Carriers are excited by nanosecond laser pulses of long-wavelength light in the range of 1150 nm. The low frequency and long penetration depth of light promote measurement of carriers away from the surface such that lifetimes of up to 100 μs are measured in upgraded-metallurgical-grade silicon, and up to 200 μs in electronic-grade silicon bricks. PL intensity shows correlation to lifetime in addition to the valuable spatial information from top to bottom of the brick and defect regions throughout the brick.