Measurement Uncertainty in Production Solar Cell and Module Power
Cassidy Sainsbury, Harrison Wilterdink, Ronald Sinton
Sinton Instruments, Boulder, CO, United States

The IV test and measurement step is one of the only places in a production line where every cell or module is measured quantitatively. These measurement results are used for the sale and marketing of products, usually by power rating at the standard test conditions (STC Watts).  Production line power rating uncertainties are determined by two major components: (1) the uncertainty of the reference module used to calibrate the system, and (2) the reproducibility of the test system. Only the reproducibility part is under control of the production facility; making it in their favor to monitor, maintain, and improve production reproducibility. However, it is disturbingly common to find production facilities, and sometimes research organizations, making qualification decisions based on the statistically weak metric of maximum-minimum (range) as their qualifying metric for tools. Although range is easily computed and comprehended, it is not a good statistical measure. For the purpose of this paper, a Sinton Instruments’ FMT-500 in-line module tester was used for an emulated qualification with 9 commercial multicrystalline silicon modules. We show that reproducibility can be calculated easily with sound statistics that can be used by production facilities. Using the standard deviation of a reasonable sample size provides results with less time take out of production and provide results that can be compared directly between separate reproducibility studies. Using a statistically strong metric like standard deviation provides more insight into the quality of data and potential contributors to the metric under measure, reproducibility in this case, than the often-used range metric (maximum and minimum). Range uses two points of many collected and does not give an accurate value for tester reproducibility.