Infrared microscopy investigation of the crystal structure of ribbon growth on substrate (RGS) solar cells

Abstract

Ribbon Growth on Substrate (RGS) silicon is a multicrystalline ribbon material where the silicon wafers are casted directly out of the melt. The advantage referring to the standard block-casting of multicrystalline silicon is the avoidance of the time and energy intensive block-casting itself and the avoidance of material loss due to wafer wire sawing. Thus, the RGS process allows a cost effective silicon wafer production in combination with a very high production speed in the order of one wafer per second. The crystallization and casting characteristics limit the grain sizes of RGS wafers to values between 0.1 mm and 1 mm and the solar cell parameters are affected by a variety of crystal defects.Due to its bandgap, silicon is transparent for infrared light with wavelengths > 1200 nm. However, crystal defects like grain boundaries, cracks and precipitates can scatter and absorb this light and therefore the shadowing can be investigated. The non-destructive method of Infrared Microscopy makes it possible to observe the actual spatial distribution and properties of certain defects in a three-dimensional way. The comparison of Infrared Microscopy, Electron Microscopy, EDX and etch pit density determination on the one hand and spatially resolved measurements like Light Beam Induced Current (LBIC) of the processed RGS solar cell on the other hand, were investigated. Defects found by Infrared Microscopy can be linked to oxygen and carbon impurities at grain boundaries but grain sizes and etch pit densities of grains have the strongest correlation to the local mean Internal Quantum Efficiency (IQE).