Results of the application of a combination of synchrotron radiation based analytical techniques, X-ray Beam Induced Current (XBIC) and microprobe X-ray Fluorescence (ν-XRF) to the analysis of shunts and lifetime limiting defects in solar cells are reported. XBIC, a new lifetime measurement technique similar to the Laser Beam Induced Current (LBIC) technique, uses a focused X-ray beam to generate minority charge carriers, which are then collected by the p-n junction of the solar cell. The X-ray beam is focused down to a spot size varying from approximately 1×1 νm to 5×5 νm, depending on the settings of focusing mirrors and slits. The sample stage is moved by computer-controlled step motors with sub-micron accuracy. Since the X-ray Beam Induced Current, which characterizes the minority carrier diffusion length in the spot where the X-ray beam hits the sample, and the X-ray Fluorescence signal, which characterizes the chemical nature of the precipitates under the beam, are measured at the same time, the chemical nature of the defects and impurities and their recombination activity can be studied simultaneously, in situ, and with a micron-scale resolution.
We present the results of the applications of these techniques to low lifetime regions in fully processed solar cells. The solar cells were pre-characterized by LBIC and thermography, and regions of interest (containing shunts) were selected. An ν-XRF scan in this area of low lifetime revealed the presence of silver and titanium far from the contact strip, suggesting a process-induced defect.