Optimization of the IV Measurement of Advanced Structures

Abstract

M. Debucquoy, F. Dross and J. Poortmans imec Kapeldreef 75, 3001 Leuven, Belgium ABSTRACT: The classification of solar cells and the comparison of cells from different origins is typically based on the cell efficiencies. As a consequence, the efficiency and the related parameters of short circuit current, open-circuit voltage and fill factor need to be measured precisely. This requires an IV setup with a small bias error and a good stability over time. In this study, an IV setup is gradually improved by the stepwise implementation of several adaptations. The effect of these improvements on the measured solar cell parameters is tracked and after completion of the improvement process, the measured parameters are compared to the values measured at a calibration lab to quantify the bias error of the final setup. Keywords: Calibration; Qualification and Testing; Solar Cell Efficiencies 1 INTRODUCTION An IV measurement is probably the most critical characterization for photovoltaic devices, and sources of errors and inaccuracies should be tracked intensively. First and second, the illumination beam has to be stable over time and uniform in space, in order to prevent possible erroneous readings of the short circuit current. Third, the temperature of the cell needs to be controlled and accurately measured. In best case, the temperature should be read directly at the device under test. Errors in the temperature readout can lead to errors in the measured open circuit voltage. Fourth, the contacting of the cell needs to be reproducible and preferably performed with several current (force) and voltage (sense) pins, with only a limited distance between both, to minimize the influence of the resistivity of the busbar on the measurement result. Fifth, when applying a working reference cell to adjust the illumination level of a solar simulator, one should ensure that the working reference cell has a design close-enough to the cell under measurement. Indeed, different types of Si solar cells might have different spectral responses, and a match for the current of one, will not necessary give accurate values for the other one. Sixth, solar simulators with only a Xenon lamp are very widely used. They result in a suboptimal spectral match with the AM1.5G spectrum in the spectral region above 750 nm. These last two aspects can result in errors in short circuit current. In this work, several of these potential sources of error are addressed one-by-one. Starting from an existing IV setup, improvements are implemented and the effects of these improvements on the measured short-circuit density J