Characterisation and diagnosis of silicon wafers and devices

Abstract

Monitoring the fabrication process of a solar cell covers a number of different measurements, including mechanical, optical, and electronic properties. This chapter focuses on the measurements of the minority carrier lifetime that provide information about the properties of silicon materials, surface passivations, and the emitter diffusions. There are three basic approaches to measure the lifetime, depending on the way that an excess of carriers is created in a semiconductor. One of the approaches, the transient decay method-in which a photogeneration is terminated abruptly and the rate, at which carriers disappear- is measured, together with excess electron concentrations. Another approach is the steady-state method, in which a constant generation rate of known value is maintained, and the effective lifetime is determined from the balance between generation and recombination values. The lifetimes vary considerably depending on the process history of their samples, measurement conditions, and they can degrade or recover when the samples are exposed to light or annealed at certain temperatures. The measured, apparent lifetime can also be affected by nonrecombination mechanisms, such as carrier trapping. Show more