Predicting multi-crystalline solar cell efficiency from life-time measured during cell fabrication

Many studies have been published on the use of minority-carrier lifetime testing at each step in a solar cell fabrication sequence. This paper illustrates one case of particular importance, multi-crystalline wafers with phosphorus diffusions. It is often stated that since the grains can have widely disparate lifetimes, high-resolution lifetime mapping must be used with sophisticated analysis in order to predict the cell efficiency. This paper demonstrates that the simple Quasi-Steady-State area-averaged lifetime measurement method can give similar results to a more sophisticated analysis with high-resolution mapping data. The phosphorus diffusion can maintain a constant junction voltage during the measurement, effectively averaging the grain lifetimes in a manner mimicking the eventual solar cell in operation. The limitations for this result are described using a special case and a simplified model.