EVALUATING SILICON BLOCKS AND INGOTS WITH QUASI-STEADY-STATE LIFETIME MEASUREMENTS

Recent progress has been made in identifying key features of multicrystalline blocks that can be used to predict the potential cell efficiency. Many of these studies use Quasi-Steady-State PhotoConductance (QSSPC) performed on wafers cut from different positions within the blocks. In these previous studies, the final efficiency of solar cells has been found to be primarily a function of the elements O and B, Fe, and the grain structure. All of these key factors have distinct “signatures” that can be characterized by minority-carrier lifetime tests and lifetime response to light soaking. These studies require calibration of the minority-carrier-injection level for precise measurements. The QSSPC method has recently been extended to map the lifetime in silicon blocks directly. This paper presents progress towards obtaining an absolute calibration for QSSPC block measurements in order to enable device physics studies on blocks as they exist in the production line. Detailed characterization of blocks of silicon prior to sawing provides data on silicon ingot growth and improves decisions on wafering. Results are presented for the determination of Fe concentration and trapping based on lifetime measurements. The combination of lifetime, Fe, and trapping data will provide much better prediction of wafer yield than lifetime data alone.