Characterisation and Diagnosis of Silicon Wafers and Devices

This chapter focuses on characterization and diagnosis of silicon wafers and devices. Emphasis is placed on measurements of the minority carrier lifetime, and their interpretation to extract relevant information about the properties of the silicon material, the surface passivation, and the emitter diffusions. The correlation between the effective lifetime measured at the various stages of fabrication and the final device characteristics is discussed. Direct measurements of the open-circuit voltage under variable illumination conditions are shown to provide most of the parameters needed by the process engineer to assess the ultimate potential efficiency of the solar cells, including diode saturation currents and ideality factors, shunting resistance, intrinsic fill factor, and pseudo conversion efficiency. This chapter begins with a discussion on measurement of the bulk lifetime and surface passivation of silicon wafers. It also describes the lifetime testing methods, variability of the carrier lifetime, lifetime instabilities, surface component of the effective lifetime, emitter component of the effective lifetime, and carrier trapping effects. The relationship between device voltage and carrier lifetime is established. The chapter also explains applications to process monitoring and control of silicon solar cells.