Contacting bare solar cells for STC measurements

Accuracy in the measurement of solar cell performance attains more and more economic significance additionally to its importance in technology development. Measurement of bare (without additional interconnectors on the bus bars) wafer based silicon solar cells directly in production with a high relative accuracy is important for cell sorting and a high absolute accuracy is necessary for valuation of the product. The calibration of the references used to set the cell testers focuses on the precision of ISC measurement, but for the valuation the maximum power at standard testing conditions is the decisive number. Therefore the fill factor determination is a critical point which depends on the specific implementation of the four wire measurement technique. The paper discuses some contacting geometries in detail and proposes how to measure cell fill factor without interference of the bus bar resistance, because the bus bar is the interface to the cell ambience.