Contacting bare solar cells for STC Measurements
Abstract : 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. Keywords: Calibration
Introduction :
With new solar cell production facilities exceeding the 100 MW limit, measurement uncertainties on the order of 1% for the output power of the cells produced are of high economic interest. Thus not only the accurate calibration of the irradiance at Standard Testing Conditions (STC,[1]) in cell testers, but also high precision in determining the fill factor (FF) on the bare cell (without interconnectors) is quite important. At the cell calibration laboratory of Fraunhofer ISE (ISE CalLab) the analysis and reduction of uncertainty components involved in the solar cell measurement at STC in testing laboratory as well as in industrial production control is a central topic. Contacting the cell with four wire technique on the bus bar is in the cell calibration community well established. However, the design of the front contact grid is not standardized and on the contrary part of each manufacturer’s specific cell design. Also the contact probe geometry usually differs between different measurement labs, because there is no generally agreed practice. But the contact probe geometry can have a strong influence especially on the measured fill factor (compare [2]). This paper discusses different solutions for contacting the bus bar of bare solar cells. The number and position of the probes for current measurement influences the potential distribution on the bus bar, which depends on the bus bar conductivity as well. The position of a single voltage probe just measures one point of this distribution. The error depends on the geometry of the contacting unit and can reach values above 1 %. The aim is to design the contact bar of the measurement system in a way that the uncertainty introduced by the contact geometry is independent of the bus bar conductivity. We investigated different possibilities in detail as a basis for precise calibration of bare solar cells. These considerations give an understanding of the influence of the contact geometry on FF measurement of bare solar cells, which is meant to form a basis for standardized contact geometry to achieve comparable FF measurements between different testing labs and industrial producers.
-
Most popular related searches
Customer comments
No comments were found for Contacting bare solar cells for STC Measurements. Be the first to comment!