Carrier density imaging (CDI): a spatially resolved lifetime measurement suitable for in-line process-control

Carrier density imaging (CDI) is introduced as a new, spatially resolved carrier lifetime measurement technique in solar cell production. CDI provides the actual local lifetimes as compared to standard lifetime mapping techniques (e.g. MW-PCD) which yield the differential lifetime only. Most important, CDI is an extremely fast measurement technique: A measurement on a 100×100 mm² wafer under low-level injection conditions can be performed on a timescale of seconds whereas a standard MW-PCD map needs about 2 hours for a measurement with identical resolution even if high injection conditions are chosen. The combination of a spatially resolved and fast measurement predestines CDI for in-line process control. It is possible to achieve actual lifetime maps with overall measurement times on the order of seconds on as cut samples as well as on emitter-diffused wafers without any surface passivation. Improvements for further reduction of measurement time are discussed.